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  • richardmitnick 1:49 pm on June 28, 2022 Permalink | Reply
    Tags: "Young and cool and on edge — an unstable protoplanetary disk", Astrobites, , , , , , If the cloud is initially rotating the contraction of the gas will magnify that rotation due to the conservation of angular momentum., The very young (less than hundred thousand years) star L1527 IRS in the Taurus molecular cloud at a distance of 137 parsecs., Today’s authors present a detailed analysis of a particular protoplanetary disk — one that is gravitationally unstable., When a cloud of gas in space has enough mass the gravitational forces from all the gas overwhelm the gas pressure keeping the cloud puffed up and it collapses under its own gravity to form a star.   

    From astrobites : “Young and cool and on edge — an unstable protoplanetary disk” 

    Astrobites bloc

    From astrobites

    Jun 28, 2022
    Konstantin Gerbig

    Title: Formation of dust clumps with sub-Jupiter mass and cold shadowed region in gravitationally unstable disk around class 0/I protostar in L1527 IRS

    Authors: Satoshi Ohashi, Riouhei Nakatani, Hauyu Baobab Liu, Hiroshi Kobayashi, Yichen Zhang, Tomoyuki Hanawa, and Nami Sakai

    First Author’s Institution: RIKEN Cluster for Pioneering Research, 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan

    Status: Published on ArXiv, 17 Jun 2022

    When a cloud of gas in space has enough mass the gravitational forces from all the gas overwhelm the gas pressure keeping the cloud puffed up and it collapses under its own gravity to form a star. If the cloud is initially rotating the contraction of the gas will magnify that rotation due to the conservation of angular momentum – imagine spinning on a desk chair, and pulling your legs in towards your body. The rotation also drives material towards the equatorial plane ultimately resulting in a so-called protoplanetary disk — a flattened disk of leftover gas and dust orbiting the newly-formed star.

    The protoplanetary disk that birthed the planets in our solar system is long gone, so we need to look to stars much younger than our Sun, to study these planetary nurseries. Today’s authors present a detailed analysis of a particular protoplanetary disk — one that is gravitationally unstable.

    Remember the gravitational instability that formed the star from a cloud? Well, the disk can be unstable to its own gravity, too, when the pressure and rotational forces are too small to prevent collapse. This can occur if the disk is very massive and also very cool. Gravitational instability in disks is one possible way of manufacturing giant planets. It causes the disk to fragment into many small blobs of gas, which then collapse into planets. Thus, understanding how gravitational instability begins is an important piece of the puzzle in understanding the formation of the diverse range of planetary systems discovered over the last twenty years.

    1
    Figure 1: ALMA images (Band 7,4,3) and JVLA image (Q band) of the L1527 protoplanetary disk viewed edge-on. The clumps detected in the Q band are marked with black crosses in the other panels. The white ellipse in the bottom left of each panel indicates the image resolution. Figure 2 in the paper.

    Observing an edge-on protoplanetary disk

    Two excellent tools for observing protoplanetary disks are the Atacama Large Millimeter Array (ALMA) [above] and the Jansky Very Large Array (JVLA).

    Both use an array of dishes that look at the target in unison, acting as one massive telescope [interferometry]. Both can observe at different wavelengths, called bands, which can be combined to produce a more complete picture of the disk.

    2
    Figure 2: Sketch of L1527’s disk, as viewed from Earth. The hot regions (red) on the near side are obscured by the flared outer disk (blue), so the near side appears slightly hotter in temperature maps. Figure 7 in the paper.

    The disk observed by today’s authors is around the very young (less than hundred thousand years) star L1527 IRS in the Taurus molecular cloud at a distance of 137 parsecs. The disk is viewed nearly edge-on, and its host star is still accreting and the disk has not yet fully formed. Each band penetrates the disk to a different depth, so the observation will look very different depending on the filter. Figure 1 shows three ALMA images (bands 3,4,7) and one JVLA image (Q band). Viewed head-on, the center of the disk is expected to be symmetrical in temperature, so any temperature asymmetry in this region can be used to measure the disk inclination. The regions closest to the host star receive the most radiation, so they are the hottest. However, since the disk is flared (= it becomes thicker with distance to the star) , the inner regions on the near side should be mostly obscured, whereas the inner regions on the far side are better visible. This is sketched in the schematic in Fig. 2 [above]. Because of this asymmetry, the near side appears hotter in Fig. 1 [above]. The authors used this to put the disk inclination at around 5 degrees, with an additional warping potentially being present.

    Assessing gravitational instability

    A gravitationally unstable disk is characterized by a distinctive spiral structure. The problem is we can only view this disk edge-on, so we can’t see the spiral structure—much like how the Milky Way’s spiral structure isn’t visible from Earth.

    The author’s resolved this issue by assessing the stability of the L1527 disk using Toomre’s stability analysis with measured values for temperature and surface density. They find that the disk is expected to be gravitationally unstable. The left panel of Fig 3. shows the spiral structure typical to a gravitationally unstable disk in the face-on view. If we were to look at this model disk from an edge-on, 90 degree rotated view, we’d see two high density regions flanking the center of the disk (right panel of Fig. 3). This almost reproduces the shape of the Q-band observation (right most panel, Fig. 1) so the authors conclude that L1527’s disk is indeed likely to be gravitationally unstable.

    3
    Figure 3: Model of a gravitationally unstable disk. If a massive disk cools enough such that its gas pressure cannot withstand the gas’ self-gravity, it starts to fragment and form a spiral structure (left panel, face-on view). The spiral structure projects two clumps on the edge-on view (right panel). Figure 17 in the paper.

    One caveat of this assessment is that the surface density—which is a crucial quantity in Toomre’s stability analysis—has to be inferred indirectly by combining dust temperature measurements and opacity models that have some uncertainty attached to them (opacity is the ability of material to block photons). However, if truly unstable, the L1527 disk would be one of the youngest systems to be subject to gravitational instability, suggesting that this young star could have giant planets forming around it much sooner than expected.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 4:37 pm on June 27, 2022 Permalink | Reply
    Tags: "(almost) No Time (for stars) to Die", Astrobites, , , ,   

    From astrobites : “(almost) No Time (for stars) to Die” 

    Astrobites bloc

    From astrobites

    Jun 27, 2022
    Sahil Hegde

    Title: Constraints on the Explosion Timescale of Core-Collapse Supernovae Based on Systematic Analysis of Light Curves

    Authors: Sei Saito, Masaomi Tanaka, Ryo Sawada, Takashi J. Moriya

    First Author’s Institution: Astronomical Institute, Tohoku University, Sendai 980-8578, Japan

    Status: Accepted to ApJ [open access]

    Despite their being some of the most luminous and notable astronomical phenomena – with the first recorded observations dating back thousands of years (e.g. SN 185) – describing successful supernova (SN) explosions is still an open question in astronomy. Broadly, supernovae (SNe) can be classified into Types I and II based on the presence of hydrogen in their spectra. The prevailing explosion mechanism across these two classes (except for Type Ia SNe) is believed to be a process known as core collapse. For stars with masses greater than 6-8 times the mass of the Sun, eventually the stellar core evolves to a point at which nuclear fusion is unable to provide sufficient pressure support to balance the star’s gravitational contraction. At this point, the stellar core collapses until it reaches huge densities – material the mass of the Sun is squished into a region the size of a city – and the contraction is stopped. The outer layers of the star and the core rebound off of the newfound pressure support in the neutron star core and drive an outward propagating shock wave. As this shock passes through the layers of the parent star, disintegrating heavy elements like iron, it loses energy, and astronomers struggle to definitively explain how to maintain a sufficiently energetic shock that is able to propagate to the surface of the star. In other words, in many early models, the shock stalls and fails to reach the stellar surface!

    There are a variety of theories that have been proposed to explain this issue, the most popular of these being that absorption of neutrinos behind the shock wave is able to rejuvenate the failing shock. However, because we cannot directly observe the shock propagating through the star, we must instead rely on proxies, such as the light curve produced by the SN, to distinguish between various models. SN light curves, such as those shown in Figure 1, show a sharp brightening and dimming around a characteristic peak luminosity across the spectrum (though the specific features of the curve depend on the SN type).

    1
    Figure 1: The observed light curve (colored points) in various bands (different colors) with interpolation fit overlaid (colored curves) for an example SN, SN 2004ex. (Adapted from Figure 2 in the paper.)

    For SNe with minimal hydrogen envelopes – a group known as Stripped Envelope SNe (SESNe) – it is believed that the peak brightness (which is the energy released by the star) is directly connected to the energy released in the radioactive decay of Nickel-56 to Iron-56, so the mass of Nickel-56 in the explosion is crucially related to the observed light curve. Therefore, any good model to describe how to manufacture successful SN explosions needs to also be able to produce the requisite amount of Nickel-56 to explain our observations.

    It also turns out that the amount of nickel that is generated in the explosion is inversely related to the explosion timescale and various theoretical models for the explosion produce a variety of predictions for this explosion timescale and thus the nickel mass. Today’s authors seek to contextualize this range of theoretical predictions by placing a bound on the explosion timescale directly from observations of SNe.

    Crunching the Numbers

    To compile observational data, the authors of today’s paper use photometric data of SNe from the Open Supernova Catalog, which contains over 800 SESNe with data in several filters. Of these, roughly 400 had enough observations in every filter that interpolated light curves, such as the example given in Figure 1, could be constructed. Because the Open Supernova Catalog also provides the luminosity distance (the relationship between the absolute and apparent magnitude) of these sources, the authors are able to fit a blackbody spectrum to the estimated spectral energy distribution across the various photometric bands. This allows them to sum up the flux over the observed wavelength range, ultimately yielding the bolometric (wavelength-summed) light curves for these SNe, that they will later use in conjunction with analytic models to estimate various properties of the explosion. From this, they measure the peak bolometric magnitude, Mpeak, and a characteristic decline timescale that describes the time it takes the luminosity to drop by 0.5 mag from Mpeak. Ultimately, this process, combined with their data cleaning steps, yields 82 bolometric light curves, depicted in Figure 2.

    2
    Figure 2: The final 82 bolometric light curves generated by the authors’ processing pipeline. Different colors correspond to different types of SNe. (Adapted from Figure 5 in the paper.)

    How Do We Use These Measurements?

    As we discussed earlier, the key measurement to be made, the nickel mass, is directly connected to the observed light curves and thus to Mpeak. The amount of nickel that can be produced in the supernova explosion also depends on the amount of material available from the start – namely, the properties of the parent star. Therefore, the authors also compute how much material is ejected in the explosion, which can be estimated from the decline timescale measured from the light curve. The decline timescale is set by the velocity of the ejected material, its composition, and its mass, so they can make some reasonable assumptions about the velocity and the composition to estimate the ejecta mass.

    To connect these observations to an estimate of the explosion timescale, the authors run a series of hydrodynamical and nucleosynthesis calculations to generate predictions of the nickel mass and ejecta mass for various explosion timescales. In particular, they model the explosion with a one-dimensional hydrodynamical simulation and use the nucleosynthesis code to track the formation of nickel. These simulations can be described by the mass of ejected material and the explosion timescale, so they compute the nickel mass produced for various reasonable values of the explosion timescale and ejecta masses that span the observed range.

    3
    Figure 3: Upper bound on modeled relationship between nickel mass MNi and ejecta mass Mej for various explosion timescales (blue curves). Gray points represent observational estimates for this relationship for two types of SNe. A dashed line representing MNi = 0.2 Mej is shown for reference. (Adapted from Figure 10 in the paper.)

    So How Much Time is This Really?

    In Figure 3, the authors show the results of the modeled relation between nickel mass and ejecta mass for various explosion timescales (blue curves). Due to the uncertainties in the models, these are upper limits so the models account for the observations (gray points) that fall below the lines. From this, the authors argue that the curves corresponding to 0.1-0.3 second explosion timescales account for a majority of the observed nickel masses, whereas the 1 second timescale bound allows for <50% of the data. This comparison places constraints on future models of core collapse SNe, dictating that observed nickel masses require very rapid timescales for these explosions to occur. Evidently, like Mr. Bond, these massive stars have (almost) no time to die.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 8:09 am on June 25, 2022 Permalink | Reply
    Tags: Astrobites, , , , , Guest post "Planets on a Magnetic Roller Coaster Ride", If the planet is hosted by a magnetically active star atmospheric outflow could be accelerated leading to faster atmospheric depletion., Mimicing transit observations of the Lyman-alpha line., The authors choose 4 characteristic points and model the outflow of the planet’s atmosphere using a 3D magnetohydrodynamics code., the ESO Belgian robotic Trappist National Telescope at Cerro La Silla Chile altitude 2400 meters.., The hypothetical planet has the properties of TRAPPIST-1e so its size is close to the Earth., The main conclusion is that the shape and velocity of the gas leaving the planet’s atmosphere varies with the conditions of the stellar wind and thus with the orbital position of the planet., The observation of the Lyman-alpha absorption line during a transit of such a planet would show highly temporal variations., The TRAPPIST-1 star and planet system., These planets are probably not a good place to find life.   

    From astrobites : Guest post “Planets on a Magnetic Roller Coaster Ride” 

    Astrobites bloc

    From astrobites

    Jun 24, 2022

    This guest post was written by Fabienne Nail, a first year PhD student at the University of Amsterdam.

    Title: Stellar Winds Drive Strong Variations in Exoplanet Evaporative Outflows and Transit Absorption Signatures

    Authors: Laura M. Harbach, Sofia P. Moschou, Cecilia Garraffo, et al.

    First Author’s institution: University of Southampton, Southampton, UK

    Status: Published in The Astrophysical Journal [open access]

    Aliens, Magnetic Fields, and Atmospheric Loss

    How cool would it be to find aliens? Maybe they exist, and maybe there are already countless interstellar graffiti with the meaning “I was here,” but we, the human species, are unable to see them. Indeed, it seems that we are well on our way to finding signs of extraterrestrial life: we are getting better at understanding the atmospheres of exoplanets, a key in the search for extraterrestrial life. There is just one thing that astronomers tend to steer clear of… magnetic fields.

    Magnetic fields make things just super complicated. However, their impact on the evolution and habitability of a planet is enormous. For example, consider atmospheric loss. Did you know that the Earth’s atmosphere is losing about 3kg of hydrogen per second? It is important to understand how quickly the atmosphere is depleted in an exoplanet to get a time scale on which the emergence of life would be possible. Or could you imagine living on a planet without an atmosphere?

    So far, the focus in exoplanet research has been on hydrodynamic escape. Planets absorb the radiation of the central star high in their atmospheres.. The atmospheric gas heats up, expands, and escapes the planet’s gravity. But reality is much more complicated! For example, what about highly energetic particles from the host star’s stellar wind that hit the upper atmosphere of the planet? The work presented here takes a step in the right direction towards unraveling the effects of stellar winds on atmospheric loss.

    Magnetic Roller Coaster

    The authors’ model the outflow of a hypothetical planet that is orbiting a star which is less massive than the sun (only 8% of the solar mass). The atmosphere of this planet is considered to be hydrogen-rich, as they aim to mimic the planet’s teenage years. As a prototype, the researchers decided to take the properties of TRAPPIST- 1.

    _______________________________________
    The TRAPPIST-1 star and planet system; the ESO Belgian robotic Trappist National Telescope at Cerro La Silla, Chile.

    _______________________________________

    This system is promising to find extraterrestrial life, as it contains 3 planets that are orbiting in the star’s habitable zone. The hypothetical planet has the properties of TRAPPIST-1e, so its size is close to the Earth. The stellar wind is nothing else than ionized particles (plasma) moving along a magnetic field. To simulate the interaction of this particle storm and the planet’s atmosphere, the authors include the magnetic and plasma environment along the orbit of the planet based on prior predictions of the TRAPPIST-1 system.

    2
    Figure 1: Stellar wind conditions to which the planet is exposed during its orbit, derived from simulations of the host star TRAPPIST-1. The boundary of the global magnetosphere (GM) is representative of the planet’s environment. The strong variations in magnetic field strengths (top) and their influence on the velocity of the plasma (middle) and its density (bottom) are clearly visible.

    The variations of this environment along one orbit can be extreme – like a ride on a roller coaster (see Figure 1)! A reduction in the magnetic field strength slows down and compresses the local plasma. In addition, the geometry of the magnetic field changes along the orbit, which means that the plasma is pulled in different directions. Based on the properties of the plasma environment, the authors choose 4 characteristic points and model the outflow of the planet’s atmosphere using a 3D magnetohydrodynamics code. This kind of code can simulate the behavior of plasma under magnetic forces.

    Stellar winds blow away gas from the planet’s atmosphere, and a tail is formed around the planet, like that of a comet. The shape of this tail is determined by the strength and geometry of the magnetic field in the planet’s environment. You might be wondering if it is possible to observe the tail? And can we see the impact of the stellar wind?

    The authors try to find an answer to these questions and mimic transit observations of the Lyman-alpha line. This line is often used to trace atmospheric escape, as it has its origin from a neutral hydrogen transition and hydrogen is quite abundant in the planetary wind. When the planet is transiting the star, a part of the stellar light is absorbed by hydrogen atoms in the planet’s atmosphere. This absorption takes place at the characteristic Lyman-alpha wavelength of 1215.67 Angstrom. If you are observing the transiting system, you can compare the depth of this absorption line in your spectra before, during, and after the transit. The comparison allows conclusions about the amount and velocity of hydrogen in the planet’s atmosphere at different orbital positions. You see, this tool is pretty useful to characterize the planet’s tail.

    3
    Figure 2: Illustration of the evaporating planet passing the stellar disk from left to right. The stellar disk is represented by a Solar Dynamics observation in the 1600 Angstrom band. The different cases show the shape of the planetary tail for different orbital positions, i.e., different stellar wind conditions.

    Stellar Wind Conditions – The Engines of the Roller Coaster

    As can be seen in Figure 2, the simulations show that for random orbital phases, the tail of the planet can have completely different shapes. Sometimes the outflow is narrower around the planet (case 2), and sometimes it looks more like long hair blown away by the wind while riding a super-fast roller coaster (case 1). The authors found that the variations occur in periods of a few hours. Moreover, the stellar wind changes over a few orbital periods from our perspective. This makes it super difficult to make good predictions for observers. It is generally difficult to interpret the spectra correctly if you have no idea what to expect.

    The main conclusion is that the shape and velocity of the gas leaving the planet’s atmosphere varies with the conditions of the stellar wind and thus with the orbital position of the planet. The interaction with the stellar wind makes the planetary outflow highly asymmetric.

    How does this affect a transit observation?

    The observation of the Lyman-alpha absorption line during a transit of such a planet would show highly temporal variations. We should be careful in its interpretation, especially when we stack several observations of different time stamps. However, the shape and position of the Lyman-alpha absorption line profile allows us to draw conclusions about the outflow’s characteristics.

    How does this affect the evolution of the planet?

    If the planet is hosted by a magnetically active star, atmospheric outflow could be accelerated, leading to faster atmospheric depletion. These planets are probably not a good place to find life.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 1:35 pm on June 23, 2022 Permalink | Reply
    Tags: "(Re)discovering Gravity", Astrobites, , , “Symbolic regression”: This algorithm tries out different types of equations trying to recreate what the graph network already knows but only by using simple combinations of mathematical operators, , Converting the knowledge acquired by their graph neural network into a form more appropriate for human consumption., , Newton’s law of universal gravitation., The authors' network “learned” to emulate the motion of the solar system’s components., The list of topics once assumed to be the exclusive domain of human creativity which later fell to the cold computation of neural networks is long and getting longer., To start the authors first had to teach a computer how to move the planets., Using machine learning to discover a law of physics.   

    From astrobites : “(Re)discovering Gravity” 

    Astrobites bloc

    From astrobites

    Jun 23, 2022
    Ben Cassese

    Title: Rediscovering orbital mechanics with machine learning

    Authors: Pablo Lemos, Niall Jeffrey, Miles Cranmer, Shirley Ho, and Peter Battaglia

    First Author’s Institution: Department of Physics and Astronomy, University of Sussex,Brighton, BN1 9QH, UK

    Status: Under review [preprint on arXiv]

    The list of topics once assumed to be the exclusive domain of human creativity which later fell to the cold computation of neural networks is long and getting longer. Today’s authors might have just scrawled another onto the quickly filling page: they used machine learning to discover a law of physics.

    Now, this was a law we already knew about, and for a long time: Newton’s law of universal gravitation. However, even though this (re)discovery is a topic covered in most high school physics curricula, this is a thrilling first step into using machine learning to help formulate new equations to describe our universe. Understanding how they coaxed a computer to play “scientist” and deliver this result is worth a deeper dive, so let’s dig into the paper!

    Computer Coaching

    To start the authors first had to teach a computer how to move the planets. To do this, they gave a program called a graph neural network the locations of every major object in the solar system over thirty years and asked it how they’d move in the next three. The program went off and fiddled with different “weights” dictating how each object affected the others, then made a guess about their trajectories. If it noticed that nudging the weights in one direction made the guesses better, it’d keep moving them that way until its guesses were as close to right as they could get.

    After lots of the machine-equivalent of hard work and studying, their network “learned” to emulate the motion of the solar system’s components. To be clear, it didn’t “know” anything about gravity, space, or any of the various theories crafted to explain our changing night sky which have been proposed over the millennia. Even so, this ignorance did not limit its performance: if you fed this computer the positions and velocities of the planets at a certain time, its fans would whir and its lights would blink and eventually it’d tell you their accelerations thirty minutes after that given time to within 0.2% of their actual value. Everything else in this paper aside, that’s an impressive feat. Although graph neural networks have been trained in the past to replicate simulated astronomical data, this is one of the first times this type of code has managed to ingest information about the real world, then accurately predict what would happen next.

    1
    Fig 1: Comparing the true (solid) and predicted (dashed) paths of two planets over 6 months. Here, the predictions come from just their graph network alone. You can see that the trajectories begin initially very accurately, but diverge as time goes on. This is expected since the model was designed to predict only 30 minutes into the future, and small errors build up over time. Source: Fig 2 in the paper.

    Several Steps Back

    It’s worth stepping back and considering this stage of their project, not from an astrophysical or machine learning perspective, but with a more philosophical bent. What is our goal in all of science? It’s surprisingly difficult to answer this question with specificity beyond generic thoughts such as “to learn/understand our universe”, even (and possibly especially) for scientists. You might say something like “to find patterns which allow us to predict the future and explain the past”, and that sounds solid enough that we can run with it here.

    But, doesn’t this neural network allow us to do just that? Sure, it’s a “black box” made up of thousands of weights and complicated linear algebra, so we can’t exactly write it out on a page or even understand how it comes to its conclusions. But, whatever it’s doing, it’s clearly doing it well. In principle we could use this network to predict eclipses and moon phases, meaning it works just as well as scientific theories from 2,000 years ago.

    That isn’t very satisfying though. All of our scientific intuition tells us that a “theory” is something which can be written down, a mixture of mathematics which can be complex but is ultimately understandable and ideally elegant. Although the neural network can functionally do many of the things we’d like from a scientific theory, it certainly doesn’t meet this expectation.

    Translating from Computer to Human

    Acknowledging this, the authors move to the second phase of their process, which was to convert the knowledge acquired by their graph neural network into a form more appropriate for human consumption. To do this, they turned to a technique called symbolic regression, also known as automated equation discovery. This mildly intimidating sounding technique is indeed complicated enough that they leave its explanation to other papers, but luckily the big picture is digestible for non-specialists. Essentially, this new algorithm tries out a bunch of different types of equations, trying to recreate what the graph network already knows but only by using simple combinations of mathematical operators. For every guess, it checks how well its equation matches the expected result, then does some self-reflection and assesses how “complex” its guess was. It’s looking for something that’s both accurate and simple: in the end, it made over 100 million guesses, and decided about 7 were worth showing to the authors.

    And what do you know! The guess it was happiest with, the one which best balanced accuracy with simplicity, is exactly Newton’s law of gravitation.

    3
    Figure 2: The top 7 equations suggested by symbolic regression. The cyan bar represents an equation more complex than many of the others, but also much more accurate- this is Newton’s law, independently recovered. Source: Adapted from Fig 5 in the paper.

    With this equation in hand, the authors went the extra mile and plugged it back into their graph neural net so that it could estimate the masses of everything in the solar system. It did a phenomenal job, correctly weighing the planets to within 1.6%. With the correct masses and underlying law, their network could now predict the motions of the planets as well into the future. Their program had not only “learned” physics, it then applied it to build an accurate model of our solar system.

    What now?

    This article contains all the ingredients for sensationalist but hollow hot takes- “AI can replace scientists” or “machines learn physics” are both somewhat correct here, but boiling away the nuance both hides the limitations and cheapens the excitement behind these methods. Beginning with the limitations, the authors candidly admit several. For one, although this program “learned” some new physics, it had to be “taught” some important other laws to get there: the authors had to hard-code in Newton’s second and third laws, as well as rotational equivariance. For another, the final equation it spits out is the product of balancing accuracy with a somewhat arbitrary metric of “complexity”. If someone wanted a super simple formula, for example, they could tweak that metric and cause the algorithm to consider, then dismiss, Newton’s law.

    As for excitement though… These authors pulled off something of a magic trick. They trained a computer to rediscover one of the greatest scientific achievements of classical physics, and they did it with algorithms which in theory can be applied to some of the greatest outstanding questions in modern physics. We’re probably never going to give a computer some telescope images and get in return a simple equation describing dark matter, but it’s very possible that someday this technique will help us reach that goal.

    The authors summarize it well themselves: “Ultimately we believe our approach should be viewed as a tool which can help scientists make parts of their discovery process more efficient and systematic, rather than as a replacement for the rich domain knowledge, scientific methodology, and intuition which are essential to scientific discovery.” This is an exciting new tool, and I’m excited to see how it’ll be modified and applied in the future.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 8:12 pm on June 22, 2022 Permalink | Reply
    Tags: "Galactic Dust Skeletons? They Might be Closer Than You Think!", Astrobites, , , , ,   

    From astrobites : “Galactic Dust Skeletons? They Might be Closer Than You Think!” 

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    From astrobites

    Jun 22, 2022
    H Perry Hatchfield

    Title: The Radcliffe Wave as the gas spine of the Orion Arm

    Authors: C. Swiggum , J. Alves , E. D’Onghia , R.A. Benjamin , L. Thulasidharan , C. Zucker , E. Poggio , R. Drimmel , J.S. Gallagher III , A. Goodman

    First Author’s Institution: Department of Astrophysics, University of Vienna

    Status: Submitted to A&A Letters [open access]

    There are a lot of perks to living in the Milky Way Galaxy, but from our perspective stuck in the barred spiral galaxy’s disk, it can be difficult to make sense of the true 3-D structure of the dust, gas and stars around us.

    1
    Credit R. Hurt (SSC/Caltech)/NASA/JPL-Caltech/

    Over the last century, a lot of effort has gone into mapping out the structure of our home galaxy, both the large-scale spiral structure as well as our local solar neighborhood.

    With the advent of the Gaia mission, we’re living in an extremely exciting time for the study of Galactic structure.

    Gaia has been able to measure parallax distances for OVER A BILLION stars with completely unprecedented accuracy, including hundreds of thousands of nearby stars nearby to our Sun.

    This dataset is revolutionizing our understanding of the Sun’s neighborhood, as researchers map out the nearby positions and motions of stars and the stuff between them, building towards a full 3-D understanding of our place in the Galaxy. But this mapping process has not been without surprises!

    2
    Figure 1. The clouds that make up the Radcliffe Wave, shown as filled blue dots as they appear on the plane of the sky. The highly-linear, wave-like structure is completely disguised by its 2-D projection. figure 1 from Alves et al. 2020.

    A kiloparsec-scale “spine” of dense gas resembling an undulating wave was recently discovered, made up by many of the most nearby star forming regions in the solar neighborhood. This structure, called the Radcliffe Wave, was hidden in plain sight, disguised by its misleading 2-D projection on the sky (see Figure 1).

    4
    Radcliffe Wave map. Credit: Roberto Mura.

    5
    The nearby circa one-sixth outer sector of the galaxy, thus clearly showing the Local Arm (Orion Arm) and neighboring arms – as well as the Great Orion Nebula (as a very luminous feature of the less bright Orion molecular cloud complex) and broad-clouds North America Nebula (and Pelican Nebula) which is an intrinsic part of the Radcliffe wave.

    But in a spiral galaxy with supposedly beautiful arching arms, what’s this linear spine of gas and dust doing in the middle of everything? Could it somehow be the skeletal structure of our Galaxy’s spiral arms? Today’s paper aims to place the Radcliffe wave in the context of the Milky Way’s Galactic spiral arm structure, solving an intriguing problem in this exciting era of mapping our home Galaxy.

    6
    Figure 2. A top-down portrayal of the distribution of young stars (in blue colorscale), with nearby clouds of dust and gas overlaid in red, and other signatures of recent star formation shown as cyan circles. The shape of the Orion arm is clearly visible in the distribution of young stars, and the line of red dots represents the Radcliffe Wave, aligned with but offset from the young stars’ spiral pattern. Image credit: adapted from figure 1 from today’s paper.

    The authors of today’s paper consider the combination of multiple powerful datasets to figure out how this spine of clouds fits into our picture of Galactic structure. In addition to the 3-D dust map that describes the positions of dust clouds in the Radcliffe Wave, they consider the placement of young stars in the Solar neighborhood (from the paper covered in this great astrobite!), along with other signatures of recent star formation such as masers and open clusters. Using a map of the distribution of young O, B and A type stars, with positions determined by the Gaia mission’s early data release 3, shows quite clearly how recent star formation follows a spiral pattern (see Figure 2), which is thought to be caused by the compression caused by the passage of the Galaxy’s spiral arms!

    7
    Figure 3. A face-on view of two other spiral galaxies (Messier 74 and Messier 83), demonstrating the presence of kilosparsec-scale linear dust features, potentially resembling the Radcliffe Wave in our Milky Way. Image credit: Figure 3 in today’s paper.

    The clouds associated with the Radcliffe wave certainly appear aligned with the over-abundance of young stars representing the Galaxy’s spiral structure, specifically the nearby “Orion arm”. Strangely enough, the Radcliffe wave and the Orion arm structures are offset from each other by a few hundred parsecs, perhaps suggesting that the wave will go on to form a future population of young stars as the next stage of the spiral wave’s passage through the Galactic disk.

    But why is the Radcliffe wave so linear while the Orion arm appears curved? It turns out mostly to be a matter of perspective. We’re used to thinking of spiral arms as smoothly curved structures, but the authors point out that other spiral galaxies show segmented, “linear” dust features that are at least as long as the Radcliffe wave, as highlighted in Figure 3, aligned with their arching spiral arms. While several mysteries remain with regard to how such a long, coherent structure formed, and why it has its characteristic undulating shape, the authors have shown clearly that the Radcliffe wave appears to be a spine of dense gas associated with the Milky Way’s spiral arms, an exciting step in understanding the true nature of our Galaxy’s skeleton!

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 11:59 am on June 22, 2022 Permalink | Reply
    Tags: "How to turn two galaxies into ten", Astrobites, , , , , ,   

    From astrobites : “How to turn two galaxies into ten” 

    Astrobites bloc

    From astrobites

    Jun 21, 2022
    Roan Haggar

    Title: A trail of dark matter-free galaxies from a bullet dwarf collision

    Authors: Pieter van Dokkum, Zili Shen, Michael A. Keim, Sebastian Trujillo-Gomez, Shany Danieli, Dhruba Dutta Chowdhury, Roberto Abraham, Charlie Conroy, J. M. Diederik Kruijssen, Daisuke Nagai, Aaron Romanowsky

    First Author’s Institution: Yale University, New Haven, CT, USA

    Status: Published in Nature [open access], available on arXiv

    The nature of Dark Matter is one of the biggest mysteries in physics. All we really know about this strange type of matter is that it doesn’t experience any forces other than gravity. This makes it totally invisible, and allows it to pass through other matter, such as stars, planets and us, almost completely unnoticed. Despite this, evidence from the past several decades means that we know Dark Matter exists, and that there is a lot of it – about six times more than all of the visible matter in our Universe! Additionally, we know that most large astronomical structures, such as galaxies and galaxy clusters, are embedded in large, spherical haloes of Dark Matter.

    One of the most striking pieces of evidence for Dark Matter is the Bullet Cluster, shown in Figure 1. This image is actually the result of two galaxy clusters that have crashed into each other, and are now side-by-side. In pink, the hot, visible gas in these clusters is shown, which has been pulled towards the middle of the two clusters by drag forces. Using weak gravitational lensing to study the light from galaxies in the background of this image, we can infer the location of the matter in these clusters, which is shown by the blue areas.

    The fact that the distributions of hot gas and mass don’t overlap tells us that, whilst the regular matter in these clusters gets stirred up and pulled into the centre, there is some extra cluster material that is flying by unimpeded – our old friend, Dark Matter!

    2
    Figure 1: The Bullet Cluster, shown as three separate overlaid images. The galaxies of the two colliding clusters (as well as background galaxies) are shown in optical (visible) wavelengths. The two pink regions near the centre of the image show the X-rays emitted by the hot cluster gas, showing that these two clouds have collided and been dragged to the central region. The two blue circular regions show where the majority of the clusters’ mass is located, indicating that much of their mass is made of non-visible matter. Credit: Chandra/Magellan/NASA/STScI/ESO.

    This same process can, in principle, occur on smaller scales. Today’s paper provides evidence of two dwarf galaxies having previously experienced a collision similar to the Bullet Cluster, and goes on to discuss how this could explain some of the strangest types of galaxies that we observe in our Universe.

    A glancing blow

    This paper looks at the NGC1052 galaxy group, and in particular, two dwarf galaxies in this group, called DF2 and DF4. These galaxies are notable because they are ultra-diffuse galaxies, which contain very few stars but have a diameter similar to other galaxies, meaning they are very faint. Many ultra-diffuse galaxies, including DF2 and DF4, also contain very little Dark Matter, and the reason why they differ to other galaxies in this way is still unknown.

    But, we may be approaching an answer! The authors suggest that a collision between two galaxies, the predecessors of DF2 and DF4, occurred about 8 billion years ago. Figure 2 shows what such a collision would have looked like, and what the result of it would be at the present day.

    3
    Figure 2: Schematic showing collision between DF2 and DF4, nearby to the galaxy NGC1052, which lies at the centre of a galaxy group. In this scenario, a galaxy (progenitor 1) collided with a member of the NGC1052 group (progenitor 2) about 8 billion years ago. This removed the stars from the Dark Matter haloes of progenitors 1 and 2, which then became galaxies DF2 and DF4 respectively. Parts of these galaxies were pulled away by the collision, becoming other, small galaxies between these two. Adapted from Figure 1 in today’s paper.

    Such a collision could result in the positions and speeds of DF2 and DF4 that are consistent with observations: that they are separated by a distance of 2.1 Mpc (about 7 million light years) and are moving away from each other at 358 km/s. Additionally, this collision could cause the Dark Matter haloes and stellar components of these galaxies to separate, similarly to what occurred in the Bullet Cluster, leaving two galaxies with very little Dark Matter remaining: DF2 and DF4. The authors also predict that parts of these galaxies would be stripped away, leading to an arc of around 10 smaller galaxies, as well as two “dark galaxies”, made almost entirely of Dark Matter!

    A cosmic mess

    To back up their claims, the authors present observational data of DF2 and DF4, as well as the region of space between them. The image of this region is shown in Figure 3, which bears an uncanny resemblance to Figure 2! Amazingly, the observations match exactly what is predicted by a Bullet Cluster-like collision between two galaxies: a chain of small, faint galaxies located on a line between DF2 and DF4 can be seen, as well as a companion galaxy nearby to each of these two, which have both been found to be almost entirely made up of Dark Matter.

    4
    Figure 3: Image of NGC1052 group, and nearby galaxies. The central galaxy NGC1052 is highlighted with a dotted red circle, the ultra-diffuse galaxies DF2 and DF4 with solid green circles, and two galaxies composed almost entirely of Dark Matter (RCP32 and DF7) are shown by blue squares. Several other faint galaxies are marked in small white squares, along a line between DF2 and DF4, mirroring the prediction in Figure 2. Adapted from Figure 3 in today’s paper.

    This work provides an origin story for the galaxies in this region of the Universe, and explains their unusual characteristics – specifically, why some have such little Dark Matter, and some have much more than average. Similar collisions might be able to explain other galaxies with an atypical Dark Matter content. Crucially though, the formation of these systems will depend on the properties of the galaxies’ Dark Matter haloes. Studying galaxies that have been torn apart like this could help us to constrain the properties of dark matter, and get one step closer to finally understanding this elusive portion of the Universe.

    __________________________________
    Dark Matter Background
    Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM, denied the Nobel, some 30 years later, did most of the work on Dark Matter.

    Fritz Zwicky.
    Coma cluster via NASA/ESA Hubble, the original example of Dark Matter discovered during observations by Fritz Zwicky and confirmed 30 years later by Vera Rubin.
    In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

    Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

    Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.
    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science).

    Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970.

    Vera Rubin measuring spectra, worked on Dark Matter(Emilio Segre Visual Archives AIP SPL).
    Dark Matter Research

    Super Cryogenic Dark Matter Search from DOE’s SLAC National Accelerator Laboratory (US) at Stanford University (US) at SNOLAB (Vale Inco Mine, Sudbury, Canada).

    LBNL LZ Dark Matter Experiment (US) xenon detector at Sanford Underground Research Facility(US) Credit: Matt Kapust.

    Lamda Cold Dark Matter Accerated Expansion of The universe http scinotions.com the-cosmic-inflation-suggests-the-existence-of-parallel-universes. Credit: Alex Mittelmann.

    DAMA at Gran Sasso uses sodium iodide housed in copper to hunt for dark matter LNGS-INFN.

    Yale HAYSTAC axion dark matter experiment at Yale’s Wright Lab.

    DEAP Dark Matter detector, The DEAP-3600, suspended in the SNOLAB (CA) deep in Sudbury’s Creighton Mine.

    The LBNL LZ Dark Matter Experiment (US) Dark Matter project at SURF, Lead, SD, USA.

    DAMA-LIBRA Dark Matter experiment at the Italian National Institute for Nuclear Physics’ (INFN’s) Gran Sasso National Laboratories (LNGS) located in the Abruzzo region of central Italy.

    DARWIN Dark Matter experiment. A design study for a next-generation, multi-ton dark matter detector in Europe at The University of Zurich [Universität Zürich](CH).

    PandaX II Dark Matter experiment at Jin-ping Underground Laboratory (CJPL) in Sichuan, China.

    Inside the Axion Dark Matter eXperiment U Washington (US) Credit : Mark Stone U. of Washington. Axion Dark Matter Experiment.
    __________________________________

    See the full article here .


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    Please help promote STEM in your local schools.


    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 9:57 am on June 20, 2022 Permalink | Reply
    Tags: "CMB": Cosmic Microwave Background radiation, "Hubble’s Law", "UR:: A Crisis in Cosmology? A new perspective on the 'Hubble Constant Tension' ", Astrobites, , , ,   

    From astrobites : “UR:: A Crisis in Cosmology? A new perspective on the ‘Hubble Constant Tension’ “ 

    Astrobites bloc

    From astrobites

    Jun 20, 2022

    The Undergraduate Research series is where we feature the research that you’re doing. If you are an undergraduate that took part in an REU or similar astro research project and would like to share this on Astrobites, please check out our submission page for more details. We would also love to hear about your more general research experience!

    Sahil Ugale
    Department of Physics
    Mithibai College
    The University of Mumbai

    As a third-year undergraduate student majoring in physics, Sahil Ugale conducted this research along with collaborators as part of a remote research internship at National Astronomical Observatory of Japan (NAOJ) under the supervision of Dr. Maria Dainotti. This research paper was published on 29 January 2022 in the journal MDPI-galaxies.

    Cosmologists agree that the universe is expanding as we observe stars at increasing distances having larger redshifts, known as “Hubble’s Law”. Plotting the apparent recession speed against distance, the gradient of this line gives the “Hubble constant”. However, if we look at a large enough distance, we can see that the expansion rate doesn’t follow a straight line, but rather a curve, suggesting the value of Hubble’s constant has changed over time.

    In the local universe, measuring the Hubble constant (H0) from observations of nearby Cepheids and SNe Ia gives 74.03 ± 1.42 kms^-1Mpc^-1, which does not match values calculated for the early universe using Planck data of the Cosmic Microwave Background radiation (CMB) and the best fit lambda-cold-dark-matter model ΛCDM (67.4 ± 0.5 kms^-1Mpc^-1).

    This difference of 4 to 6 σ in the Hubble constant found between these different, independent methods is significant and known as “Hubble Constant Tension”. This tension raises important questions – is there something wrong with the observations? Or is there something wrong with our best fit cosmological model ΛCDM, despite it predicting so much data correctly?

    Our team focused on observations of Type Ia supernovae (SNe 1a) and Baryon Acoustic Oscillations (BAOs) in the nearby universe. We use the Pantheon sample – a compilation of 1048 spectroscopically confirmed SNe Ia from different surveys – divided into three redshift bins of increasing value. We then consider a SNe 1a + BAO set, combining the SNe data with one BAO data point in each redshift bin (so the same number of BAOs are in different bins). Using this binned redshift analysis, we investigated whether H0 really remains constant over the redshift span range of the probes (SNE + BAO) considered.

    This research extends on the previous analysis published last year in The Astrophysical Journal. This considered only the variation of H0 in the SNe Ia Pantheon sample (for both the flat ΛCDM and w0waCDM models), but not any other cosmological parameter or any other probe.

    In the current analysis, we extend this to vary two cosmological parameters with H0: the total matter density parameter (Ω0m) in the ΛCDM model, and the equation of state evolution parameter (wa) in the w0waCDM model, using both the SNe and SNe + BAO data. This can be seen in Figure 1. Including BAOs in the cosmological computations allows us to check if the trend of H0 in the previous research is still evident when we include other probes. This analysis is not intended to constrain Ω0m or any other cosmological parameter, but rather to investigate the reliability of the trend of H0 with redshift.

    2
    Figure 1: The Hubble constant vs redshift (z) for the two different models. The red color shows the SNe 1a only probe, while the blue represents SNe + 1 BAO. The left panel shows the ΛCDM model and the right panel shows the w0waCDM model.

    Our new study paves the way for understanding how combined probes (SNe + BAOs) still show the evolution of the H0 by redshift and how simulations on GRB cosmology are progressing to obtain Ω0m uncertainties comparable to SNe Ia results. Using GRBs, SNe Ia, and BAOs together as cosmological probes in the near future has proven to be not only feasible, but also necessary, since the redshift range that GRBs cover is much larger than that of SNe Ia. As a result of this characteristic, GRBs will certainly be able to give new insights into the nature of the early universe and pose new constraints on future measurements of H0.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 1:54 pm on June 12, 2022 Permalink | Reply
    Tags: "Meet the AAS Keynote Speakers: Prof. Jocelyn Bell Burnell" A Very Special Introduction to a Unique Scientist- Dame Susan Jocelyn Bell Burnell, Astrobites, , , , ,   

    From astrobites : “Meet the AAS Keynote Speakers: Prof. Jocelyn Bell Burnell” A Very Special Introduction to a Unique Scientist- Dame Susan Jocelyn Bell Burnell 

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    From astrobites

    Jun 11, 2022
    Graham Doskoch

    Becoming proficient with a sledgehammer is not typically part of doing a PhD in astronomy. Then again, neither is discovering an entirely new class of astronomical objects. In 1967, a graduate student at Cambridge University named Jocelyn Bell did both. Her story is one of both unexpected discovery and her struggles against the all-too-expected sexism she faced in the astronomy community.

    _______________________________________________________

    Women in STEM – Dame Susan Jocelyn Bell Burnell Discovered pulsars

    Dame Susan Jocelyn Bell Burnell discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.

    Dame Susan Jocelyn Bell Burnell at work on first plusar chart 1967 pictured working at the Four Acre Array in 1967. Image courtesy of Mullard Radio Astronomy Observatory.

    Dame Susan Jocelyn Bell Burnell 2009

    Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com

    Biography

    British astrophysicist, scholar and trailblazer Jocelyn Bell Burnell discovered the space-based phenomena known as pulsars, going on to establish herself as an esteemed leader in her field.Who Is Jocelyn Bell Burnell?
    Jocelyn Bell Burnell is a British astrophysicist and astronomer. As a research assistant, she helped build a large radio telescope and discovered pulsars, providing the first direct evidence for the existence of rapidly spinning neutron stars. In addition to her affiliation with Open University, she has served as dean of science at the University of Bath and president of the Royal Astronomical Society. Bell Burnell has also earned countless awards and honors during her distinguished academic career.

    Early Life

    Jocelyn Bell Burnell was born Susan Jocelyn Bell on July 15, 1943, in Belfast, Northern Ireland. Her parents were educated Quakers who encouraged their daughter’s early interest in science with books and trips to a nearby observatory. Despite her appetite for learning, however, Bell Burnell had difficulty in grade school and failed an exam intended to measure her readiness for higher education.

    Undeterred, her parents sent her to England to study at a Quaker boarding school, where she quickly distinguished herself in her science classes. Having proven her aptitude for higher learning, Bell Burnell attended the University of Glasgow, where she earned a bachelor’s degree in physics in 1965.

    Little Green Men

    In 1965, Bell Burnell began her graduate studies in radio astronomy at Cambridge University. One of several research assistants and students working under astronomers Anthony Hewish, her thesis advisor, and Martin Ryle, over the next two years she helped construct a massive radio telescope designed to monitor quasars. By 1967, it was operational and Bell Burnell was tasked with analyzing the data it produced. After spending endless hours pouring over the charts, she noticed some anomalies that did not fit with the patterns produced by quasars and called them to Hewish’s attention.

    Over the ensuing months, the team systematically eliminated all possible sources of the radio pulses—which they affectionately labeled Little Green Men, in reference to their potentially artificial origins—until they were able to deduce that they were made by neutron stars, fast-spinning collapsed stars too small to form black holes.

    Pulsars and Nobel Prize Controversy

    Their findings were published in the February 1968 issue of Nature and caused an immediate sensation. Intrigued as much by the novelty of a woman scientist as by the astronomical significance of the team’s discovery, which was labeled pulsars—for pulsating radio stars—the press picked up the story and showered Bell Burnell with attention. That same year, she earned her Ph.D. in radio astronomy from Cambridge University.

    However, in 1974, only Hewish and Ryle received the Nobel Prize for Physics for their work. Many in the scientific community raised their objections, believing that Bell Burnell had been unfairly snubbed. However, Bell Burnell humbly rejected the notion, feeling that the prize had been properly awarded given her status as a graduate student, though she has also acknowledged that gender discrimination may have been a contributing factor.

    Life on the Electromagnetic Spectrum

    Nobel Prize or not, Bell Burnell’s depth of knowledge regarding radio astronomy and the electromagnetic spectrum has earned her a lifetime of respect in the scientific community and an esteemed career in academia. After receiving her doctorate from Cambridge, she taught and studied gamma ray astronomy at the University of Southampton. Bell Burnell then spent eight years as a professor at University College London, where she focused on x-ray astronomy.

    During this same time, she began her affiliation with Open University, where she would later work as a professor of physics while studying neurons and binary stars, and also conducted research in infrared astronomy at the Royal Observatory, Edinburgh. She was the Dean of Science at the University of Bath from 2001 to 2004, and has been a visiting professor at such esteemed institutions as Princeton University and Oxford University.

    Array of Honors and Achievements

    In recognition of her achievements, Bell Burnell has received countless awards and honors, including Commander and Dame of the Order of the British Empire in 1999 and 2007, respectively; an Oppenheimer prize in 1978; and the 1989 Herschel Medal from the Royal Astronomical Society, for which she would serve as president from 2002 to 2004. She was president of the Institute of Physics from 2008 to 2010, and has served as president of the Royal Society of Edinburgh since 2014. Bell Burnell also has honorary degrees from an array of universities too numerous to mention.

    Personal Life

    In 1968, Jocelyn married Martin Burnell, from whom she took her surname, with the two eventually divorcing in 1993. The two have a son, Gavin, who has also become a physicist.

    A documentary on Bell Burnell’s life, Northern Star, aired on the BBC in 2007.


    Dame Susan Jocelyn Bell Purnell at Perimeter Institute Oct 26, 2018.
    _______________________________________________________

    The physics-minded daughter of an architect who helped design the Armagh Planetarium, Bell was exposed to astronomy at a young age through books like Fred Hoyle’s Frontiers of Astronomy, which she read cover to cover. Bell says that when she made the mental connection between the rotation of galaxies and her lessons on circular motion in school, “I suddenly thought, yeah, I like physics, I can be an astronomer.” She liked physics enough to obtain her undergraduate degree in physics from the University of Glasgow. The university caught her attention because of its astronomy classes, but when she found that they focused largely on positional astronomy, Bell changed her focus to astrophysics.

    In the 1960s, like previous decades, women attempting to study astronomy faced hostility and exclusion. For instance, ten years earlier, Margaret Burbidge – a coauthor, along with Fred Hoyle and two others, of a groundbreaking paper on stellar nucleosynthesis – had been rejected from multiple positions because of her gender. Bell did not expect to be treated any better. There were only two options for graduate school in astronomy in Britain at the time, Cambridge University and Jodrell Bank Observatory at the University of Manchester.

    U Cambridge Campus
    The University of Cambridge (UK)

    Bell had spent a summer at Jodrell Bank, but “the grad students there had said, you know, they won’t take a woman [as a PhD student],” Bell remembers. Thinking she would be rejected by Cambridge, she applied to Jodrell Bank nonetheless but never heard back – a snub which she interpreted as “their way of not taking a woman.” While considering options for study in Australia, Bell decided to apply to Cambridge anyway – and was accepted.

    At Cambridge, graduate students were expected to join a research group quickly. Bell’s research experience at Jodrell Bank gave her a head start over the others. Interested in quasars, she joined the group led by Antony Hewish. Quasars were a hot topic of research in the 1950s and ’60s. Today we know them to be supermassive black holes with accretion disks and energetic relativistic jets. When they were first discovered, however, they appeared more star-like; the term “quasar” comes from the descriptor “quasi-stellar radio source”. More and more quasars were being detected, but much about them remained mysterious. Astronomers hoped that more discoveries could lead to answers. The Cambridge quasar group planned to search for quasars by looking for scintillation, “twinkling” from radio waves passing through the solar wind. First, though, they needed to build a telescope.

    At first glance, the Interplanetary Scintillation Array may appear to be a jumble of wooden posts and wires. Unlike the dishes of most radio telescope, it’s an array antenna, consisting of thousands of thin dipole antennas spread over several acres of fields. Constructive interference enhances the radio signal, which allowed the Cambridge quasar group to look for scintillating sources.

    Building the IPS Array was a difficult task – both technically and physically. Bell was in charge of the electrical wiring, installing spark plugs and transistors. Most of the 1000 or so posts were left to the men in the group to hammer, but Bell “did enough that I could swing a sledgehammer – not one of the normal qualifications of a PhD.” She laughs. “I was playing field hockey at the time, and I could hit the ball from one end of the pitch to the other, which my teammates did not appreciate.”

    The IPS Array was completed in two years, and Bell became the first and primary operator. She spent three weeks debugging and six months performing observations. “The telescope was a transit instrument with all the wooden poles,” she explains. “You couldn’t steer it in right ascension, but you could steer it in declination, and observe different strip declination strips of the sky.” The IPS Array’s data was written out by a chart recorder onto pieces of paper, plotting the intensity of the signal recorded by the telescope. “I think I ended up with five kilometers of that chart paper, if I remember, after six months observing,” Bell remembers.

    Bell became skilled at distinguishing between signals from quasars and radio interference. Occasionally, however, she noticed a burst she couldn’t classify as either, marking it with a question mark. After seeing the same “piece of scruff” again and again, she pored through the shoe boxes containing data from earlier in her observing run. Bell and Hewish noticed that the source appeared in the same position in the sky. At her advisor’s suggestion, she increased the time resolution of the chart recorder – and saw that the scruff was actually a sequence of regularly spaced pulses. This raised eyebrows: could it be a satellite in a strange orbit? Had Bell made a mistake with the wiring?

    Two things convinced the team that the scruff was a real astronomical source. First, another radio telescope at Cambridge also picked up the signal. Second, Bell found three other bits of scruff that looked similar, at different positions in the sky. These sources were dubbed “pulsars” due to the train of pulses that appeared at high time resolution. We now know that pulsars are the remains of some of the most massive stars, some spinning hundreds of times per second. Their powerful magnetic fields create beams of radio waves that sweep across the cosmos like lighthouses, creating the illusion of short pulses as the pulsar rotates.

    While Bell found the first four pulsars and paved the way for the discovery of over 3,000 more, she did not stay in pulsar astronomy – nor did she initially receive the credit she deserved. Bell’s results were eventually published, but when the media covered her work, she was depicted as the “human interest” part of the story, while Hewish received most of the credit and eventually, unbelievably, a Novel Prize. Even her PhD thesis was ultimately on the quasars she had begun to study from her first weeks at Cambridge. Bell recalls, “My supervisor said it was too late to change the title of the thesis. . . From what I now know of university systems, I’m pretty certain he was wrong. . . But I was determined that the pulsars would go in somewhere, so they went in an appendix to the thesis.”

    Not long after graduating, the newly-minted Dr. Bell married – becoming Dr. Jocelyn Bell Burnell – and had a child. The family moved every five or six years because of her husband’s job. “Both of those things enormously compromised my career,” she says. Forced to continuously find new positions, “I went from radio astronomy to gamma ray astronomy to X-ray astronomy to infrared and millimeter wave astronomy.” It would be about 20 years before Dr. Bell Burnell could pick a job she described as “my first choice, rather than fitting with somebody else’s. I became head of the physics department to fairly new university called the Open University and set up a group studying energetic binary stars at whatever wavelength was most useful. That group remains and has done very well.”

    Prof. Bell Burnell’s work on pulsars led to the creation of an entire subfield of astronomy, and she continues to follow cutting-edge pulsar research, but her talk at AAS 240 will be on another subject which has been intertwined with her career: the prospects for women in astronomy. She mentions that data on the demographics of members of the International Astronomical Union shows that there has been a steady increase in the gender balance of the field. There are plenty of ongoing concerted efforts to address systemic inequity, including some led by Prof. Bell Burnell herself. In 2018, she was awarded the Special Breakthrough Prize in Fundamental Physics and chose to donate the more than $2 million she received to establish the Bell Burnell Graduate Scholarship Fund, administered by the Institute of Physics. It’s “for people from underrepresented groups. So in physics that in Britain that tends to mean women, people of color, people with disabilities, etc. The program’s now in its second year.” Prof. Bell Burnell mentions that she actually met with a group of recipients of the scholarship earlier today. “They’re in all branches of physics, but there are two or three in astrophysics.”

    Still, Prof. Bell Burnell emphasizes a point that is clear to this day: in astronomy, like other fields, “the USA doesn’t do too well on gender, and Britain does even worse.” Astronomy may be far past the era when even prominent scientists like her and Burbidge were treated as second-class citizens simply because they were women, but there is a long, hard road to anything like true gender equality. In the meantime, I ask Prof. Burnell Bell for her advice – both about what she would have told herself back in the 1960s, and what she would tell graduate students today. She smiles. “Hang in there. You will survive.”

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 12:55 pm on June 12, 2022 Permalink | Reply
    Tags: "BAO": Baryon Acoustic Oscillations, "Stare into the Void", Astrobites, , , , , , , The Dark Energy Survey   

    From astrobites : “Stare into the Void” 

    Astrobites bloc

    From astrobites

    Jun 11, 2022
    Kayla Kornoelje

    Title: Measurements of cosmic expansion and growth rate of structure from voids in the Sloan Digital Sky Survey between redshift 0.07 and 1.0
    ___________________________________________________________________
    Apache Point Observatory
    SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft).

    Apache Point Observatory near Sunspot, New Mexico Altitude 2,788 meters (9,147 ft).
    ___________________________________________________________________

    Authors: Alex Woodfinden, Seshadri Nadathur, Will J. Percival, Slađana Radinović, Elena Massara, Hans A. Winther

    First Author’s Institution: Waterloo Centre for Astrophysics, University of Waterloo

    Status: Submitted to ArXiv [12 May 2022]

    Pitch: BAOs are Cool

    Cosmologists love Baryon Acoustic Oscillations (BAO), and here’s why you should too. Back when the universe was just a baby made out of hot plasma, a battle between gravity and radiation was raging on. Small overdensities in the hot plasma fought to collapse under gravity, while tightly coupled radiation provided pressure that resisted this collapse. The outcome of this battle were oscillations of matter that spread out across the plasma, almost like ripples in a pond. As the universe began to cool, radiation no longer provided support against gravitational collapse, and overdensities of matter contained within these ripples could finally collapse into galaxy clusters. These ripple-like patterns of galaxy clusters are known as BAOs, and they’re extremely important to cosmologists because their size and properties depend on the details of our universe’s composition. Precise measurements of these BAOs can provide constraints on important cosmological parameters such as the amount of baryonic matter, dark matter, and dark energy in the universe. Additionally, monitoring the size of BAOs over a wide range of redshifts can be used to measure how the expansion rate of the universe changes across cosmic time. Given everything that we can learn from studying BAOs, it’s no wonder why cosmologists have been using them as cosmological probes for decades. Have I sold you on BAOs yet?

    Pitch 2: So are Cosmic Voids

    If so, I have another cosmological probe to sell you on: cosmic voids. If BAOs represent overdense regions in the universe, it’s no surprise that there are regions with extremely low densities as well. These cosmic voids are around a tenth of the average density of the universe, and they’re massive, making up around 90% of our universe. And, as it turns out, these voids are just as exciting to test parameters of cosmology as BAOs are! The properties of voids are sensitive to everything from Dark Energy and “modified gravity” [MOND], to structure growth and galaxy formation. Who knew you could get something from studying nothing!

    _____________________________________________________________________________________
    The Dark Energy Survey

    Dark Energy Camera [DECam] built at DOE’s Fermi National Accelerator Laboratory.

    NOIRLab National Optical Astronomy Observatory Cerro Tololo Inter-American Observatory(CL) Victor M Blanco 4m Telescope which houses the Dark-Energy-Camera – DECam at Cerro Tololo, Chile at an altitude of 7200 feet.

    NOIRLabNSF NOIRLab NOAO Cerro Tololo Inter-American Observatory(CL) approximately 80 km to the East of La Serena, Chile, at an altitude of 2200 meters.

    Timeline of the Inflationary Universe WMAP.

    The The Dark Energy Survey is an international, collaborative effort to map hundreds of millions of galaxies, detect thousands of supernovae, and find patterns of cosmic structure that will reveal the nature of the mysterious dark energy that is accelerating the expansion of our Universe. The Dark Energy Survey began searching the Southern skies on August 31, 2013.

    According to Albert Einstein’s Theory of General Relativity, gravity should lead to a slowing of the cosmic expansion. Yet, in 1998, two teams of astronomers studying distant supernovae made the remarkable discovery that the expansion of the universe is speeding up.
    Saul Perlmutter (center) [The Supernova Cosmology Project] shared the 2006 Shaw Prize in Astronomy, the 2011 Nobel Prize in Physics, and the 2015 Breakthrough Prize in Fundamental Physics with Brian P. Schmidt (right) and Adam Riess (left) [The High-z Supernova Search Team] for providing evidence that the expansion of the universe is accelerating.

    To explain cosmic acceleration, cosmologists are faced with two possibilities: either 70% of the universe exists in an exotic form, now called Dark Energy, that exhibits a gravitational force opposite to the attractive gravity of ordinary matter, or General Relativity must be replaced by a new theory of gravity on cosmic scales.

    The Dark Energy Survey is designed to probe the origin of the accelerating universe and help uncover the nature of Dark Energy by measuring the 14-billion-year history of cosmic expansion with high precision. More than 400 scientists from over 25 institutions in the United States, Spain, the United Kingdom, Brazil, Germany, Switzerland, and Australia are working on the project. The collaboration built and is using an extremely sensitive 570-Megapixel digital camera, DECam, mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, high in the Chilean Andes, to carry out the project.

    Over six years (2013-2019), the Dark Energy Survey collaboration used 758 nights of observation to carry out a deep, wide-area survey to record information from 300 million galaxies that are billions of light-years from Earth. The survey imaged 5000 square degrees of the southern sky in five optical filters to obtain detailed information about each galaxy. A fraction of the survey time is used to observe smaller patches of sky roughly once a week to discover and study thousands of supernovae and other astrophysical transients.
    _____________________________________________________________________________________

    MOND [Modified Newtonian dynamics]

    MOND

    MOND Rotation Curves with MOND Tully-Fisher

    Mordehai Milgrom, MOND theorist, is an Israeli physicist and professor in the department of Condensed Matter Physics at the Weizmann Institute in Rehovot, Israel http://cosmos.nautil.us


    _____________________________________________________________________________________
    1
    Figure 1: Measurements of the growth rate of structure (ƒσ₈) compared to the values measured by the cosmic microwave background (blue strip). The red marks represent the measurements from the void-galaxy correlation, the gray marks represent measurements from BAOs, and the green marks represent void-galaxy correlation measurements using different analysis techniques. (Figure 6 in the paper)

    But most important for today’s discussion is the information encoded within the void-galaxy cross-correlation function. While it sounds like a mouthful, this function really just describes the properties, such as the density and peculiar velocities, of galaxies surrounding voids. This statistical property is particularly important as it can characterize both red-shift-space distortions (RSDs), and the Alcock-Paczyński (AP) effect. Astronomers use an object’s velocity to determine its redshift, and RSDs are simply biases in our redshift measurement due to an additional peculiar velocity component. Since peculiar velocities are dependent on local gravitational interactions, accurately characterizing RSDs can tell us a lot about the properties of matter in our universe, such as the growth rate of structure. The AP effect, on the other hand, is a distortion of the shape of a distribution of galaxy clusters. If we assume galaxy clusters are distributed like a sphere around voids, the AP effect can make the distribution of clusters appear flattened or elongated if astronomers make incorrect assumptions about the geometry of the universe. Analysis of these distortions provides astronomers with measurements of what is known as the Alcock-Paczyński distance ratio, which is the ratio between the comoving angular diameter distance and the Hubble distance. This is particularly exciting for the study of cosmic voids, as they should be able to constraint this distance ratio even more precisely than BAOs can. All in all, cosmic voids are a novel probe of the properties of our universe. Not sold yet? Well, let’s let the results of today’s paper do the talking then!

    The Proof is in the Cosmic Void

    The authors of today’s paper examined the cross-correlation of galaxies and cosmic voids to unravel the wealth of cosmological information packed within them. After careful and rigorous treatment of both selection and systematic errors that can arise from the analysis of cosmic voids, the authors found exciting results. First, the authors found that cosmic voids alone can constrain the value of the growth rate of structure just as precisely as BAOs can (Figure 1). While this is not a novel result, this precision points to the fact that voids are a great tool to use in combination with BAOs to get better measurements of cosmological parameters than either of them can do alone. But, perhaps even more compelling is the fact that the author’s results also confirm that cosmic voids do beat BAOs when it comes to measuring the AP distance ratio (Figure 2). Such results demonstrate the importance of voids as cosmological probes, and support a powerful new avenue for cosmologists to explore to unlock the mysteries of our universe. And, as the cherry on top of it all, there are new galaxy surveys such as DESI and Euclid coming up in the near future, which will probe much larger volumes of the universe over a larger redshift range. Watch out world, precision cosmology is about to get even more precise!

    2
    Figure 2: Measurements of the AP distance ratio Dₘ/DH. Red points represent measurements from void-galaxy correlations, and gray points represent those from BAOs. The green and blue bands are expected values from the CMB (blue) and the CMB combined with other cosmological probes (green). (Figure 7 in the paper)

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 4:05 pm on June 9, 2022 Permalink | Reply
    Tags: "To Gaia and Beyond!", Astrobites, , , , , , ESA Gaia spacecraft, , , Solar System and Stellar Astronomy, , Spectrophotometry, The era of Galactic Seismology – exploring the response of the Galaxy to internal and external perturbations – has just begun.   

    From astrobites : “To Gaia and Beyond!” 

    Astrobites bloc

    From astrobites

    Jun 9, 2022
    Maryum Sayeed

    Title: Microarcsecond Astrometry: Science Highlights from Gaia

    Authors: Anthony G. A. Brown

    First Author’s Institution: Leiden Observatory, Leiden University

    Status: Published in Annual Review of Astronomy and Astrophysics [open access]

    1
    Figure 1: Map of the total flux measured in G_{\textrm{BP}}, G, and G_{\textrm{RP}} bands from all sources measured by Gaia in Galactic coordinates. Source: Gaia Collaboration (2018)

    The European Space Agency’s space mission Gaia, launched in 2013, has proved indispensable for all subfields in astronomy. From studying nearby solar system objects, to quasars and galaxies, Gaia proves to be the tool that does all.

    With the release of Gaia DR3 just around the corner, astronomers are (im)patiently anticipating new content that will not only improve on current astrometric measurements, but will yield unparalleled numbers of spectra, radial velocities, abundances, and more.

    This article briefly summarizes science results enabled by Gaia DR1 & DR2 thus far, with a specific focus on solar system and stellar astronomy.

    Gaia is an astrometry mission, meaning that it collects accurate positions, parallaxes, and proper motions for all sources to magnitude 20.7 in G band, and provides multi-color photometry and radial velocities for stars brighter than G ~ 17 mag. Gaia delivers incredible precision on its measurements. Positions for 1.1 billion sources are available up to G ~ 20, and parallaxes and proper motions are available for 2 million sources with sub-milliarcsecond (mas) precision. To put this into context, the angular diameter of Proxima Centauri – the closest star to the Sun – is 1 mas, which is also approximately the size of an astronaut on the surface of the moon, or the head of a pin on Earth as seen from the International Space Station.

    Finally, Gaia has also provided radial velocity for 7 million sources (out to G ~ 12), light curves for 550 000 variable stars, and astrophysical parameters (ie. effective temperature, radius, extinction) for ~160 million sources.

    Gaia truly is the one-stop shop for all your astrometric needs.

    Solar System

    Solar system science benefited tremendously from Gaia. Gaia DR2 provides astrometry and G-band photometry for ~14 000 solar system objects with high precision. Combining its astrometry and spectrophotometric data enables high-precision studies of solar system objects. In the context of the solar system, the major impact of Gaia has been the availability of all sky maps of star positions, parallaxes and proper motions, which allows astronomers to study occultations of stars by solar system objects. Orbit predictions even over a short period of time have increased the reliability of hazard predictions for near-Earth objects. For instance, using Gaia DR2 stellar positions and past occultation campaigns, Pluto’s shadow trajectory on Earth in 2016 was well predicted, and its atmospheric pressure was well studied. In general, Gaia DR2 astrometry enables much more precise predictions of stellar occultations by Kuiper Belt objects which in turn motivates astronomers to organize large-scale observation campaigns.

    Exoplanets

    While it has been predicted that by the end of its mission Gaia could increase the number of known exoplanets by a factor of three (21000 +/-6000, Perryman et al. 2014 [The Astrophysical Journal]), it is already making significant contributions to the characterization of known planets and their host stars. However, characterization of exoplanets requires precise knowledge of properties of its host star, motivating the popular saying Know Thy Star, Know Thy Planet. NASA’s Kepler mission observed a patch of the sky for ~4 years and discovered thousands of exoplanets.

    Thanks to Gaia, there have been multiple efforts to derive fundamental stellar parameters of stars in the Kepler field. Since precise stellar radii and masses can be calculated to ~5-10 % errors, precise stellar parameters enable astronomers to investigate exoplanet demographics as a function of stellar age, abundance, and evolutionary state, while also potentially probing their formation history. Gaia is truly bringing other worlds closer to home.

    Observational HR Diagrams

    3
    Figure 2: Example of a Hertzsprung-Russell diagram of the Orion region where the absolute magnitude on the y-axis is plotted against the color on the x-axis. The orange points represent a younger population of stars in the region as compared to the older gray points. Source: Figure 5 in the paper.

    Gaia’s precise measurements of stellar positions & parallaxes enable us to study stellar populations. Tools like an Hertzprung-Russell (HR) diagram allow us to study the various stages of stellar evolution. Many studies have provided HR diagrams of various populations, such as the Orion region (Figure 5 in the paper) or the Kepler field of view (Figure 8 in Berger et al. 2020 [ The Astronomical Journal]), that have revealed interesting substructures. Incredible insights into processes in the stellar interior have been made possible by a basic tool such as an HR diagram of Gaia data. For instance, Jao & Feiden 2020 [The Astronomical Journal], found a gap in their HR diagram of Gaia stars that they attribute to non-equilibrium fusion in the stellar core, and mixing between the core and the envelope.

    Similarly, Gaia has enabled many studies of moving groups and cluster characterization. One example is the discovery of a young stellar population found in the Scorpio-Centaurus-Lupus-Sky region via a simple selection on parallax and cuts in an HR diagram (Figure 1 in Villa Velez+2018 [Research Notes of the AAS]). Gaia also enabled the construction of a large all-sky sample of white dwarfs. In fact, an observational HR diagram in Gaia DR2 release revealed bifurcation of the white dwarf sequence attributed to varying compositions of white dwarfs (Figure 13 in this paper [A&A Gaia Data Release 2 special issue]).

    Galactic Archaeology

    Lastly, Galactic Archaeology was one of the many subfields that was revolutionized by Gaia. Helmi et al. 2020 provides a wonderful introduction of the field, as well as progress facilitated by Gaia DR2. A major discovery made possible by Gaia was that a large fraction of the halo stars near the Sun were debris stars from Gaia-Enceladus (or Gaia sausage), the last big merger event experienced by the Milky Way.

    Furthermore, Gaia’s velocity and positions of stars in three dimensions revealed that the Milky Way is in fact not in equilibrium. Our Galaxy was recently disturbed by an intruder – most likely the Sagittarius dwarf – that produced waves in the Galactic plane. Without precise information on stellar velocity and positions, this detail into our galaxy’s history would not have been possible.

    Other substructures in the Milky Way have also been better characterized by Gaia, such as stellar streams. Pre-Gaia, such streams were a faint overdensity of stars stretched out in position that were hard to disentangle or even see against the stars in front of and behind them. However, with Gaia measurements, we can filter out foreground stars based on their large parallaxes and/or proper motions, to isolate a stellar stream and even probe clumpy dark matter distributions. There are still more exciting discoveries to be made in Galactic archaeology and beyond; in fact, the era of Galactic Seismology – exploring the response of the Galaxy to internal and external perturbations – has just begun.

    Looking Ahead

    June 13th, 2022 is a date circled in many astronomers’ calendars. With unwavering excitement and trepidation, astronomers across subfields will finally have access to new Gaia data. Multiple cities, such as Chicago, NYC, and Aarhus, are also planning week-long Gaia sprints which involve exploring and playing with the new data with like-minded astronomers in similar research areas. Gaia DR3 promises to improve measurements on previous sources, but also provide new content. The full list of the data products are listed on Gaia’s webpage, but some exciting products include spectra from Radial Velocity Spectrograph (with resolution of ~11 500), catalog of binary stars, astrometry for 100 000 solar system objects, reflectance spectra of ~5000 asteroids, chemical abundances for ~2.5 million stars, and much, much, much more. The collective excitement over a new data release is inspiring, yet humbling, and like its predecessor, Gaia DR3 will advance our knowledge of the Universe on all scales, across the sky.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.


    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
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