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  • richardmitnick 9:34 am on March 18, 2020 Permalink | Reply
    Tags: "An Iced Cosmic-Ray Macchiato", Astrobites, , , , ,   

    From astrobites: “An Iced Cosmic-Ray Macchiato” 

    Astrobites bloc

    From astrobites

    1
    Artist’s impression of the shower of particles caused when a cosmic ray, a charged particle often produced by a distant astrophysical source, hits Earth’s upper atmosphere. [J. Yang/NSF]

    Title: Bottom-up Acceleration of Ultra-High-Energy Cosmic Rays in the Jets of Active Galactic Nuclei
    Authors: Rostom Mbarek and Damiano Caprioli
    First Author’s Institution: University of Chicago

    Status: Published in ApJ

    Our universe is littered with particles of unbelievably high energy, called cosmic rays. The most extreme of these particles carry the same amount of energy as a professional tennis serve, like the Oh-My-God Particle detected nearly 30 years ago. The catch: we don’t know exactly what processes can pack so much energy into a single particle. The authors of today’s article discuss how these particles might gain their energy in a way analogous to your morning trip to Dunkin’™.

    Cosmic Rays at a Glance

    Cosmic rays are atomic nuclei that have been accelerated to high energies in astrophysical environments, such as supernova remnants or active galactic nuclei. Although they might seem like a great tool in the multi-messenger astronomy toolbox, astronomy with cosmic rays is no simple task, as these particles get deflected by extragalactic magnetic fields.

    2
    Cosmic rays (red) consist of individual protons and nuclei of heavier elements. They are deflected by magnetic fields along their cosmological odysseys and can’t be used to point back to the place of their origin. [IceCube Neutrino Observatory]

    Despite efforts to pinpoint the origins of cosmic rays, especially those of the highest energies, we’ve come up empty-handed (check out these bites for previous studies: Galactic cosmic rays, cosmic-ray anisotropy).

    Even though we can’t measure where they come from, we do know their energies, and a variety of cosmic-ray experiments detect millions of these particles every year. Many of them are thousands to millions of times more energetic than the particles in the largest terrestrial particle accelerator, the Large Hadron Collider, but we don’t know how the highest energy cosmic rays get their energy.

    Cosmic-Ray Acceleration: Old News

    Many theories of cosmic-ray acceleration tend to revolve around the idea of Fermi acceleration. In this scenario, objects such as supernova remnants can create shocks, consisting of material moving together with supersonic speeds, and these shocks can accelerate particles to high energies. As a shock wave propagates, particles bounce back and forth across the shock boundary. Over time, successive bounces across the shock front lead to a net transfer of energy to the particles.

    While Fermi acceleration does a good job of explaining cosmic rays with moderate energies and has been a staple of models for decades, it has a few pitfalls, and many argue that it can’t provide the whole story for cosmic-ray acceleration at the highest energies.

    A Cosmic Cup o’ Joe

    The authors of today’s paper propose a new way of looking at cosmic-ray acceleration: the espresso mechanism. Why espresso? Because instead of gradually gaining energy over time, particles gain their energy from a single shot.

    3
    In the “espresso mechanism”, particles gain tremendous amounts of energy from entering a jet for a short period of time. Here, a particle with initial momentum and energy pi, Ei enters a jet with characteristic Lorentz factor Γ and leaves the jet with an energy equal to roughly Γ2Ei. [Caprioli et al. 2018]

    Consider an object with a jet, such as an active galaxy. If a low-energy cosmic ray enters the jet (or steam), then it can be shot down the barrel of the jet and get kicked out at much higher energy. In many cases, particle energies can increase by a factor Γ2, where Γ is the Lorentz factor (this reflects how fast the jet is moving). For some jets, this means particles can exit nearly 1,000 times as energetic as they were when they entered the jet.

    4
    In realistically modeled jets, material tends to clump in some regions, and these regions of overdensity (color scale in figure) cause the jet to locally move faster or slower. [Mbarek & Caprioli 2019]

    While this espresso scheme sounds great in principle, many previous calculations have relied on spherical cow treatments of jets, when in reality they are remarkably dynamic and complex structures.

    That’s where the authors of today’s paper come into play. These authors take a simple treatment of the espresso mechanism and complexify it by performing a full magnetohydrodynamic (MHD) simulation of ultrarelativistic jets. This takes factors like small-scale fluctuations of jet speed and jet density into account, to give a more accurate picture of the dynamics of jets.

    By simulating the full structure of jets, the authors find that complex environments don’t weaken the promises of espresso acceleration. In fact, the very imperfections that manifest in realistic jets can help with particle acceleration. What’s more, jet perturbations allow particles to receive double or even triple shots of energy.

    Throughout the paper, the authors describe the emergent spectra of espresso-accelerated cosmic rays. In doing this, they find that espresso acceleration is consistent with current measurements of ultra-high-energy cosmic rays in terms of energy, chemical composition, and spatial distributions, an accomplishment which no other model of cosmic-ray acceleration can boast.

    6

    Sample particle trajectories (black curves) are overlaid on top of slices of the jet, with jet velocity represented by the color in the top panels. Bottom panels show the amount of energy gained along the particle paths, showing that particles can leave jets with much more energy than they entered with. [Mbarek & Caprioli 2019]

    In light of all of this, it’s probably safe to say that the future of cosmic-ray science will be very caffeinated.

    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 2:58 pm on February 22, 2020 Permalink | Reply
    Tags: "Through the Lens: Milky Matter Magnifies Magellanic Motion", Astrobites, , , , , , ,   

    From astrobites: “Through the Lens: Milky Matter Magnifies Magellanic Motion” 

    Astrobites bloc

    From astrobites

    Feb 22, 2020
    Luna Zagorac

    Title: First Results on Dark Matter Substructure from Astrometric Weak Lensing
    Authors: Cristina Mondino, Anna-Maria Taki, Ken Van Tilburg, and Neal Weiner
    First Author’s Institution: Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY 10003, USA

    Status: pre-published on arXiv

    There is about five times more invisible Dark Matter than its luminous counterpart in the universe—but how do we go about detecting something that can’t be directly imaged?

    Fritz Zwicky discovered Dark Matter when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

    Fritz Zwicky from http:// palomarskies.blogspot.com

    Coma cluster via NASA/ESA Hubble

    Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)


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


    Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

    The LSST, or Large Synoptic Survey Telescope is to be named the Vera C. Rubin Observatory by an act of the U.S. Congress.

    LSST telescope, The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

    Dark Matter Research

    Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

    Scientists studying the cosmic microwave background [CMB]hope to learn about more than just how the universe grew—it could also offer insight into dark matter, dark energy and the mass of the neutrino.

    [caption id="attachment_73741" align="alignnone" width="632"] CMB per ESA/Planck

    Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al

    Dark Matter Particle Explorer China

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

    LBNL LZ Dark Matter project at SURF, Lead, SD, USA


    Inside the ADMX experiment hall at the University of Washington Credit Mark Stone U. of Washington. Axion Dark Matter Experiment

    One way is to look for the gravitational effects of dark matter clumps on images of normal matter along the same line of sight. This type of effect is called gravitational lensing.

    Gravitational Lensing NASA/ESA

    In today’s paper, the authors specifically look for the effects of weak lensing from low-mass structures consisting entirely of dark matter.

    Weak gravitational lensing NASA/ESA Hubble

    The foreground dark matter structure creates a lens that bends the light coming towards an observer from some background luminous source. Unlike strong lensing, weak lensing doesn’t impact a single background source, but instead serves to preferentially align several background sources along some field. For more information on different types of lensing and how they work, check out this bite.

    Why Use Weak Lensing?

    Alignments of foreground and background sources that lead to weak lensing are much more common than those leading to strong lensing. Because low-mass dark matter structures are predicted to exist in the Milky Way, they should be both common in observational data sets and detectable through microlensing signatures. Furthermore, because such structures are completely devoid of normal matter, they pose a “pristine testing ground” for probing the microphysics of dark matter without the interference of normal, luminous matter.

    How to Look For Weak Lensing?

    1
    Figure 1: Diagram of gravitational lensing of sources i by lens l. Note the blue monopole pattern of the angular displacement \Delta \theta_{il}. This is not constant in time, leading to the red dipole pattern lensing corrections \Delta \mu_{il} to the sources’ proper motions \mu_i. This dipole pattern is universal, and is what the authors look for. Figure 1 in the paper.

    The authors use a template approach, which is similar to the one used when detecting astrophysical signals with LIGO. Figure 1 shows the dipole pattern of velocity corrections of background stars which stems from weak lensing. The exact shape and size of the template depend on the angular position \mathbf{\theta}_t, angular scale \beta_t, and effective lens velocity direction \hat{\mathbf{v}}_{t} of the dark matter lens. The details of the matched filter to the lens-induced velocity vector profile also include information about the density profile of the dark matter lens. This means that finding the correct shape of velocity corrections in the data and comparing its magnitude with the theoretical template model can inform the size, position, and density profile (and subsequently, mass) of the dark matter lens.

    Where to Look For Weak Lensing?

    The researchers looked to the Milky Way to provide the dark matter lenses, and extra-galactically to the Large and Small Magellanic Clouds (LMC, SMC) to provide the luminous matter to be lensed.

    Milky Way NASA/JPL-Caltech /ESO R. Hurt. The bar is visible in this image

    Large Magellanic Cloud. Adrian Pingstone December 2003

    smc

    Small Magellanic Cloud. NASA/ESA Hubble and ESO/Digitized Sky Survey 2

    They used the second data release from Gaia and chose the LMC and SMC data for their large stellar number densities and low proper motion dispersions, both intrinsic and instrumental.

    ESA/GAIA satellite

    This left the authors with a high signal-to-noise ratio, thus best equipping them to look for signatures of weak lensing.

    In order to look for the tell-tale dipole template motion, the authors cleaned the data up a bit. First, they subtracted overdense stellar clusters, as they generally move coherently and independently from the bulk stars in the Magellanic Clouds. Additionally, they subtracted the large-scale proper motion of the clouds themselves. Finally, they removed stars which are in the line of sight, but not bound to the clouds.

    3
    Figure 2: Average stellar proper motion per 0.03° pixels in the RA (left) and DEC (right) across the Large Magellanic Cloud. The top panel shows the proper motion in the original Gaia data sample after the removal of dense clusters; the bottom shows it after further background motion subtraction and removal of outlier stars. Figure 7 in the paper.

    What did the authors find?

    In performing their analysis, the authors produced exclusions on the fraction of dark matter present in lensing sources as a function of lens mass (see Figure 3). They also noted that the current analysis is statistics-limited, with their figure of merit being largest for relatively faint stars, such as the majority of those present in the Magellanic Clouds. Thus, the statistics in their analysis will improve with additional integration time, which is currently at 22 months for Gaia DR2. Furthermore, having a larger sample of stars, better resolution of binaries, and accurate modeling of telescope systematics will all lead to improvements over time, yielding promising prospects for the use of their method on future data releases from Gaia and other astrometric surveys.

    4
    Figure 3: Constraints from the Magellanic Cloud velocity template analysis on the fractional dark matter abundance f_l of compact objects with mass M_l with a given density profile. The three linewidths represent compact object radii r_{l}=10^{-3}, 0.5, \text { and } 1 \mathrm{pc}. The constraint for the smallest radius is equivalent to the one for point-like objects. Above the diagonal line at the bottom right, at least one subhalo eclipses the data sample with 90% confidence level (CL). Figure 5 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 3:05 pm on February 18, 2020 Permalink | Reply
    Tags: "The TESS Mission’s First Earth-Like Planet Found in an Interesting Trio", Astrobites, , , ,   

    From astrobites: “The TESS Mission’s First Earth-Like Planet Found in an Interesting Trio” 

    Astrobites bloc

    From astrobites

    18 February 2020
    Haley Wahl

    1
    Artist’s illustration of what the exoplanet TOI 700 d might look like. [NASA’s Goddard Space Flight Center/Chris Smith (USRA)]

    Title: The First Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System
    Authors: Emily A. Gilbert, Thomas Barclay, Joshua E. Schlieder, et al.
    First Author’s Institution: University of Chicago

    Status: Submitted to AJ

    2
    Artist’s impression of TESS observing planets orbiting a dwarf star. [NASA Goddard SFC]

    Since the discovery of the first planet outside of our solar system in 1992, the field of exoplanets has been booming with interesting finds. From the diamond planet orbiting a neutron star to the giant pink planet orbiting a star in the constellation of Virgo, telescopes all over the world have been racing to find the latest gem. Of particular interest are Earth-like planets. A team led by a graduate student at the University of Chicago report the first Earth-sized planet in the habitable zone found by the TESS mission, and its surroundings were quite a surprise to astronomers.

    NASA/MIT TESS replaced Kepler in search for exoplanets

    Searching for Planets in All the Right Places

    Some of the biggest questions we humans like to ask are, “Is there life out there in the universe?” and “Are there other solar systems out there with planets just like ours?” To answer these questions, astronomers have built larger and more advanced telescopes to try to find planets outside of our own neighborhood, specifically those similar to our own world. The Kepler mission was launched in 2009 specifically to search for these kinds of planets: Earth-sized planets in Earth-like orbits around Sun-like stars in order to study how common they are in the universe.

    NASA/Kepler Telescope, and K2 March 7, 2009 until November 15, 2018

    The mission has made many amazing discoveries, such as an exoplanet with the density of Earth, a planet in a binary star system, and the first Earth-sized planet in the habitable zone of its star that orbits around an M-dwarf star that is about half the size of the Sun. Kepler’s extended mission, K2, focuses on low-mass stars and has led to the discovery of hundreds of small planets, some even in the habitable zone of their stars. Together, Kepler and the K2 mission have found over 3,000 new exoplanets.

    Exoplanets are very small and very far away, so it is very difficult to find them. Astronomers use four methods: the transit method (which looks at how much a star dims as a planet goes in front of it, or eclipses it), the wobble method (which looks at how a planet and a star move around a common center of mass), direct imaging (which means taking a picture of the planet, straightforward but very difficult and limiting), and microlensing (which happens when light from a distant star bends around a star/planet system).

    Planet transit. NASA/Ames

    Radial Velocity Method-Las Cumbres Observatory

    Direct imaging-This false-color composite image traces the motion of the planet Fomalhaut b, a world captured by direct imaging. Credit: NASA, ESA, and P. Kalas (University of California, Berkeley and SETI Institute

    Gravitational microlensing, S. Liebes, Physical Review B, 133 (1964): 835

    TESS, or the Transiting Exoplanet Survey Satellite, was launched in 2018 and was designed to search for small planets around the Sun’s nearest neighbors using the transit method. In today’s paper, we discuss the first results of Earth-sized planets found in the habitable zone of an M-dwarf star, planets contained in an odd planetary trio.

    3
    The light curves observed by TESS. The pink line represents how much the star was expected to be dimmed by the eclipsing planet and the blue is the actual data. [Gilbert et al. 2020]

    The Host Star

    Understanding the properties of the host star is key in determining the habitability of the planets around it. M-dwarf stars are smaller and dimmer than the Sun but are much more common in the universe. The star that the team found the planets around is called TOI 700. To determine its fundamental properties such as mass and temperature, the team used three different methods in order to validate their results. After using known relations, checking their spectral energy distributions against known spectra, and using spectroscopy, they concluded that the star has an effective temperature of 3,480 K (which is about ⅔ of the Sun’s temperature) and a mass and radius that is about half that of our Sun. They found no flares from the star in the observations over five years, which points to a low amount of magnetic activity, making the system more likely to be habitable.

    Goldilocks (Zone) and the Three Planets

    By analyzing how much the star dimmed as the planets went around it, the team determined that it hosts three planets. From the inner to the outer planet (respectively called TOI 700 b, TOI 700 c, and TOI 700 d), they have radii of 1.01 ± 0.09, 2.63 ± 0.4, and 1.19 ± 0.11 Earth radii. Figure 1 shows how much the planets dim the light of the star. TOI 700 b and d are likely Earth-sized while TOI 700 c is a sub-Neptunian-type planet. TOI 700 d receives about 90% of the energy that the Earth does from the Sun, which places it in the habitable zone of the star. After finding these planets, the team performed tests with many different software packages in order to verify their discoveries. Each of the three planets passed these tests with a false-alarm probability (the probability that the signal is due to something else like instrument noise) of less than 1%. The masses of the planets were determined to be ~1.07 Earth masses, ~7.48 Earth masses, and ~1.72 Earth masses for planets b, c, and d respectively.

    So How Does One Make a Neptune Sandwich…?

    The fact that the largest planet is in the middle of this system is a bit puzzling. Usually planets in a given system have similar sizes — and in the case of our solar system, the inner planets are small and rocky, while the outer planets are larger and gaseous. In this system, the low-density gas planet is sandwiched between the higher-density rocky planets with similar masses. Figure 2 shows the planet in comparison to other systems. The team postulates that this could have come from the two inner planets forming faster and accreting significant gaseous envelopes and the outer one forming more slowly and accreting less gas, then the innermost planet loses its envelope somehow. It is also possible that the middle planet formed outside the outermost planet but migrated inward somehow, but how this could happen isn’t clear. This strange system may be difficult to explain, but it provides a rich laboratory for exploring the formation mechanisms of complex multi-planet systems.

    4
    Figure 2: The TOI 700 planets compared to other known systems. The bottom axis shows flux (or energy received by the planets) compared to Earth’s and the top axis shows distance. [Gilbert et al. 2020]

    Thirty years ago, we did not even know planets could exist around other stars. Now, we know of thousands — and some of those planets are possibly habitable. New exoplanet discoveries like this one are shaking up the field of planetary formation and causing us to rethink our ideas of what stars could host planets and how planets form. As time goes on, new telescopes like James Webb Space Telescope will come online and further expand our understanding of exoplanets. Who knows what kind of weird extrasolar planets we will find next!

    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 11:57 am on February 13, 2020 Permalink | Reply
    Tags: "More Clues to the Environment in Which FRBs Originate?", Astrobites, , , , , FRB 121102 (the repeating FRB “the repeater”)., FRB 191108   

    From astrobites: “More Clues to the Environment in Which FRBs Originate?” 

    Astrobites bloc

    From astrobites

    Feb 12, 2020
    Haley Wahl

    Title: A bright, high rotation-measure FRB that skewers the M33 halo
    Authors: Liam Connor, Joeri van Leeuwen, et. al.
    First Author’s Institution: Anton Pannekoek Institute, University of Amsterdam, Amsterdam, The Netherlands
    1
    Status: Submitted to MNRAS, open access on arXiv

    Fast radio bursts (FRBs) are one of the hottest topics in astronomy right now. First discovered by Dr. Duncan Lorimer in 2007, these intense millisecond-long bursts of radio emission have continued to captivate scientists across the planet because they keep defying our expectations with discoveries like the repeater. Now, with the discovery of an interesting property of a new FRB just outside a major galaxy, we may be getting one step closer to finally solving one of the many puzzles of FRBs.

    More Questions Than Answers

    Our questions about FRBs seem to fall into two categories: What causes the bursts? And how can they be put to use? Each time the community moves toward an answer on one of these questions, a new discovery throws a wrench in it. For example, astronomers thought FRBs were single events but a discovery in 2016 showed that they can actually repeat. This opens new questions, like whether the repeaters and non-repeaters come from the same mechanism. In another case, we thought FRBs only came from dwarf galaxies until one was localized to a massive spiral galaxy. This finding opened more questions about the types of environments that could produce FRBs in very different galaxies. The authors of today’s article present a newly discovered FRB with a very high rotation measure that may give clues to the kind of environment FRBs originate from.

    Understanding Rotation Measures

    When light passes through a medium, it gets Faraday rotated, which is a rotation of the orientation of the light by a magnetic field. How much that light gets rotated as it travels is called its rotation measure (RM). Rotation measure depends on the strength of the magnetic field between us and the source, the density of the material that the light is going through, and the distance to the source. RMs can be used to understand the environment an FRB traveled through. Most of the RMs from FRBs are -100 rad m^-2 to -120 rad m^-2 (a negative RM simply denotes the fact that the magnetic field is pointing away from the observer) but newly discovered FRB 191108 has an RM of almost 500 rad m^-2.

    Caught Between a Galaxy…and Some Interstellar Medium

    FRB 191108 was detected with the Apertif Radio Transient System on the Westerbork Synthesis Radio Telescope in the Netherlands.

    Westerbork Synthesis Radio Telescope, an aperture synthesis interferometer near World War II Nazi detention and transit camp Westerbork, north of the village of Westerbork, Midden-Drenthe, in the northeastern Netherlands

    It lies close to the halo of the galaxy M33 (see Figure 1) and is within the intergalactic medium of the Andromeda Galaxy.

    Andromeda Galaxy Adam Evans

    The total observed rotation measure is a combination of the RM from the Milky Way, the RM from the medium between galaxies, and the RM of the host itself. Each of these different components has its own electron density, magnetic field, and thickness so they all contribute to the RM differently. What the authors find from the RM is that it points to an extragalactic contribution of 525 rad m^-2, which would require the magnetic field between galaxies to be 1000x greater than they are. It’s possible that ionized material surrounding the two galaxies could be the cause but because other sources around M33 have RMs of <100 rad m^-2, it is not likely (see Figure 2 for a comparison). Therefore, the RM has to come from somewhere else.

    2
    Figure 1. The location of FRB 191108. The blue cross denotes the most likely location, the red line indicates where the authors believe with 90% certainty the FRB came from, and the circles are the beam size.

    Dense Plasma Environment?

    One of the only explanations the authors find plausible is that the high RM is due to magnetized plasma in the host galaxy. It is possible that the burst originated from an area of very dense, magnetized material. FRBs have been seen to exist in many different environments and some of the RMs of FRBs that have been found point to a high contribution of material from the host galaxy that the burst goes through. FRB 121102 (the repeating FRB, “the repeater”) has an RM that is 100 times greater than FRB 191108 and it has been localized to an environment that is extreme and dynamic. It also has a persistent radio source counterpart to it. The fact that the authors don’t find a radio counterpart to this FRB and don’t find it to repeat means it’s formation environment is likely different from the repeater.

    2
    Figure 2. The RM of sources near M31 and M33. The x-axis shows how far away they are from the galaxies and the y-axis shows the RM. The fact that the RM is so different from the others around the galaxies show that it cannot be caused by plasma around the galaxies (if it was, others along that line of sight would see it).

    Has this discovery pointed to an answer to the question of what kind of environment FRBs originate from, or does the differences between FRB 191108 and FRB 121102 only raise more questions? Only by finding more FRBs will we get closer to answering these questions!

    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 11:38 am on February 12, 2020 Permalink | Reply
    Tags: "Unlocking the secrets of chaotic planetary systems", Astrobites, , , ,   

    From astrobites: “Unlocking the secrets of chaotic planetary systems” 

    Astrobites bloc

    From astrobites

    Feb 11, 2020
    Spencer Wallace

    Title: Fundamental limits from chaos on instability time predictions in compact planetary systems
    Authors: Naireen Hussain, Daniel Tamayo
    First Author’s Institution: Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON

    Status: Accepted for Publication in MNRAS, preprint on arxiv

    It shows up in nearly every field of study – from weather forecasting, to physics, to economics – even sociology – and of course, astronomy. Chaos theory is the study of systems whose seemingly random behavior is the result of an extreme sensitivity to initial conditions. (For an excellent, more in-depth explanation of chaos, check out this astrobite). Chaos is a subject that commonly comes up when trying to understand the long-term stability of planetary systems.

    It turns out that certain arrangements of planets are inherently unstable – that is – if you place them in a certain configuration and let them orbit their star for long enough, the gravitational interactions between the planets will fling some (or sometimes all) of the bodies clear out of the system. Unfortunately, determining how and when this will happen is not possible to work out on paper. Or at least, no one has been clever enough to figure it out yet.

    Fortunately, computers make this problem somewhat tractable. By gradually evolving a collection of massive bodies over many tiny time steps, it is possible to get an incredibly accurate estimate of where and how these bodies will be moving sometime in the future (or the past, for that matter). Given enough computing power, you can simply take a planetary system and evolve it forward in time and see what happens. Does it stay stable? Do any planets get ejected? Using this technique, astronomers can try placing extra bodies in known planetary systems and see if things remain stable. If not, this sometimes can rule out the presence of additional, undetected planets.

    Searching for chaos

    As mentioned above, these types of systems are sometimes chaotic. If so, this means, by definition, that the outcome of whether the system is stable not, and how long it takes to become unstable, is highly sensitive to the initial conditions. The authors of today’s paper want to examine how reliable the estimates of instability timescales from these simulations actually are. If the initial conditions are tweaked just slightly, does this timescale change? And if so, is there an underlying pattern?

    For this study, the authors ran a large suite of N-body simulations of compact, three planet systems (loosely inspired by the well-known TRAPPIST-1).

    A size comparison of the planets of the TRAPPIST-1 system, lined up in order of increasing distance from their host star. The planetary surfaces are portrayed with an artist’s impression of their potential surface features, including water, ice, and atmospheres. NASA

    The TRAPPIST-1 star, an ultracool dwarf, is orbited by seven Earth-size planets (NASA).

    ESO Belgian robotic Trappist National Telescope at Cerro La Silla, Chile

    ESO Belgian robotic Trappist-South National Telescope at Cerro La Silla, Chile, 600 km north of Santiago de Chile at an altitude of 2400 metres.

    For each simulation, the orbits and sizes of the planets were varied and the planetary configuration was then evolved for a billion or so orbits to test whether or not the system became unstable. If it did, the timescale for instability was recorded and the same configuration was run again with the initial conditions tweaked ever so slightly to probe the underlying chaotic behavior. If chaos was indeed influencing the outcome, each slight modification to the initial conditions should result in a measurably different instability time.

    2
    Figure 1: The two types of instability timescale distributions found by tweaking the initial conditions of the simulations. In a small number of cases the distribution is sharply peaked (left), while in most cases the instability times follow a log-normal shape.

    After doing this, the authors found that the distribution of instability times for a given configuration of planets fell into two broad categories. This is shown in Figure 1. In some cases, the distribution was very sharply peaked around a single value. Otherwise, the distribution had a log-normal shape.

    The road to dynamical instability

    The difference between these two types of results can be explained by comparing the instability timescale to the Lyapunov timescale, which is how long it takes for chaotic behavior to emerge in a given system. For the sharply peaked distribution, the planets become unstable before chaos sets in. This results in an instability time that is not sensitive to slight changes in the initial conditions. For the broadly peaked distributions, chaos occurs well before the instability. Two example sets of simulations are shown in Figure 2, which demonstrate this difference.

    3
    Figure 2: The Lyapunov time as a function of the instability time for a number of simulations (top). The symbols indicate whether the instability timescales follow a peaked or a log-normal distribution. A representative example of the change in the orbital properties of a planet in each of the two cases is shown at the bottom for the peaked (left) and log-normal (right) results. The ‘shadow trajectories’ indicate how this quantity changes after the initial conditions are slightly tweaked.

    Most interestingly, the log-normal distributions all have a width of ~0.4 dex, regardless of the differences in the initial conditions. The instability time distribution has the same size and shape if the instability time is short or long, or whether the planets are arranged randomly or placed in mean motion resonances. Mean motion resonances occur when the orbital periods are integer multiples of each other. This can act to substantially stabilize or destabilize an orbital configuration, which makes it even more surprising that the instability time distribution shape is not sensitive to this. The remarkable similarity between these distributions across a wide range of configurations is shown in Figure 3. The only requirements here are that the planets start off in a compact configuration and the Lyapunov time be shorter than the instability time.

    4
    Figure 3: The width of the log-normal distribution of instability times for a wide number of simulations. The top panel shows little difference between systems whose planets are arranged randomly and those which begin in mean motion resonances. The bottom panel demonstrates that there is little difference between systems that become unstable quickly and systems which take many millions of orbits for instability to occur.

    It is not terribly surprising that a seemingly random process can give rise to a reproducible pattern. To quote the authors of the paper, “While individual steps in a drunkard’s random walk might be unpredictable, the cumulative effect of many steps approaches a well-defined statistical distribution.” In addition to indicating that the mean instability time in a chaotic system like this can be estimated by running only a small fraction of the simulations required to fill out the entire probability distribution, it hints at a fundamental underlying truth connecting these results produced by an extremely complicated process. The authors do not attempt to speculate on what this similarity tells us about chaotic planetary systems, but it provides a tantalizing clue about the underlying mechanisms that drive this rather abstruse process.

    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 10:51 am on February 9, 2020 Permalink | Reply
    Tags: "Where Are All the Baryons?", Astrobites, , , ,   

    From astrobites: “Where Are All the Baryons?” 

    Astrobites bloc

    From astrobites

    Feb 6, 2020
    Jason Hinkle

    Title: Probing the missing baryons with the Sunyaev-Zel’dovich effect from filaments
    Authors: Anna de Graaff, Yan-Chuan Cai, Catherine Heyman, and John A. Peacock
    First Author’s Institution: Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, UK; Leiden Observatory, Leiden University, Leiden, The Netherlands
    1

    Status: Published in Astronomy & Astrophysics, open access on arXiv

    The Missing Baryon Problem

    All of the material we see around us is made up of atoms, also known as baryonic matter. From studies of Big Bang Nucleosynthesis and the Cosmic Microwave Background (CMB), we know that baryons make up only 5% of the Universe. The rest is made of still largely unknown forms of matter and energy we call dark matter and dark energy. We have a problem though with the 5% of the Universe we know about: we don’t know where all the baryons are.

    Baryons in galaxies and galaxy clusters make up only ~20% of all baryons in the Universe. The existence of another ~30% of the baryons can be inferred from the Lyman-alpha forest. Cosmological simulations suggest that the rest of the baryons (roughly 50%) reside in the WHIM (warm-hot intergalactic medium). The WHIM is composed of filaments and sheets of warm-hot gas that connect galaxies and galaxy clusters. Because emission from this gas is faint, finding baryons in the WHIM is difficult. Previous work has relied on studies of absorption lines of distant quasars and X-ray absorption. Using these methods, we have found just 20% of the total baryon budget in the WHIM, far less than the theoretically expected 50%. Thus, summing up all the known baryons gives us only 70% of all the baryons in the Universe.

    Finding the WHIM

    An alternative technique to measure baryons in the WHIM is known as the thermal Sunyaev- Zeldovich (tSZ) effect. The tSZ effect measures the change in energy of photons from the CMB caused by interactions (Compton scattering) with hot particles along our line of sight. In order to apply this method, this paper utilizes maps of galaxies from the Sloan Digital Sky Survey to create both a sample of galaxy pairs that are likely to be connected by a filament and a control sample of pairs of galaxies that are physically unrelated, but closely separated on the sky. The authors also use the Planck map of the Compton y-parameter, which quantifies the strength of the tSZ effect.

    Because the tSZ signal from the WHIM is weak, the authors add together the signals of over a million pairs of galaxies, rotating and scaling each particular pair as needed. In order to measure the tSZ signal from the filament, rather than the galaxies themselves, a model is used to subtract the contribution of hot gas in the galaxy halos. Figure 1 shows the summed signal, the galaxy halo models, and the residual signal from the filament.

    1
    Figure 1: (a) Sum of tSZ signal for ~1 million galaxy pairs; (b) modeled galaxy halo signal; (c) residual between the data and model. The filament is marked by the dashed box in (c). The axes represent distances parallel (r_{\parallel}) and perpendicular (r_{\perp}) to the axis of galaxy separation in h^{-1} Mpc scaled to the mean galaxy separation of 10.5 h^{-1} Mpc. The color is proportional to y, which quantifies the tSZ effect. Figure 1 in the paper.

    The authors recognize that this signal may not be from the WHIM, but rather from unrelated sources, leftover galaxy halo signal, or the cosmic infrared background. To confirm that these signals are in fact caused by particles in the WHIM, the authors ran the same tests on their control sample, created a simulated tSZ signal map for the halos, and estimated dust contamination. They found that the only potential contamination is from galaxy halos, contributing less than 20% of the observed filament signal.

    Baryons: Present and Accounted-For?

    Confident that the signals from the filaments were real, the authors needed to estimate how many baryons they had found. Using constraints on the gas temperature and density from gravitational lensing and previous studies, they claim to have found an additional 11 ± 7 % of all baryons. Figure 2 summarizes our current knowledge of the distribution of baryons in the Universe. When interpreting the final results, it is important to note that this paper’s galaxy pair sample is incomplete and that deeper surveys may be able to find even smaller filaments that this work missed.

    2
    Figure 2: Left panel – known distribution of baryons as of 2012 (Shull et al. 2012). Right panel – census of baryons after this work, with the newly discovered baryons shown in red (Figure 8 in the paper).

    If we now add up all the known baryons, we are still missing 18 ± 16 %. Further studies of quasar absorption lines, X-ray absorption, and the thermal SZ effect to include more galaxies may help to find more baryons in the WHIM, especially at different temperatures. While it is clear that there is still work to be done and more baryons to find, at least now we can be confident we are looking in the right place.

    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 5:51 pm on February 8, 2020 Permalink | Reply
    Tags: "Polarime-trying to Map Magnetic Fields in the Orion Nebula", Astrobites, , , , , HAWC+ instrument onboard NASA’s airborne observatory NASA/DLR SOFIA, Orion Molecular Cloud 1 (OMC-1) is part of the Orion Nebula   

    From astrobites: “Polarime-trying to Map Magnetic Fields in the Orion Nebula” 

    Astrobites bloc

    From astrobites

    Feb 8, 2020
    Ashley Piccone

    Title: HAWC+/SOFIA Multiwavelength Polarimetric Observations of OMC-1
    Authors: David T. Chuss, B-G Andersson, John Bally, Jessie L. Dotson, C. Darren Dowell, Jordan A. Guerra, Doyal A. Harper, Martin Houde, Terry Jay Jones, A. Lazarian, Enrique Lopez Rodriguez, Joseph M. Michail, Mark R. Morris, Giles Novak, Javad Siah, Johannes Staguhn, John E. Vaillancourt, C. G. Volpert, Michael Werner, Edward J. Wollack, Dominic J. Benford, Marc Berthoud, Erin G. Cox, Richard Crutcher, Daniel A. Dale, L. M. Fissel, Paul F. Goldsmith, Ryan T. Hamilton, Shaul Hanany, Thomas K. Henning, Leslie W. Looney, S. Harvey Moseley, Fabio P. Santos, Ian Stephens, Konstantinos Tassis, Christopher Q. Trinh, Eric Van Camp, Derek Ward-Thompson
    First Author’s Institution: Department of Physics, Villanova University

    Status: accepted to The Astrophysical Journal, open access on arXiv

    OMC-what?

    The Orion Molecular Cloud 1 (OMC-1) is part of the Orion Nebula, and one of the most massive star-forming regions in the solar neighborhood.

    1
    Orion Molecular Cloud Complex-1 .© Graeme Healey Photography

    Orion Nebula M. Robberto NASA ESA Space Telescope Science Institute Hubble

    The gas and dust within OMC-1 act as a nursery for young stars, providing them with the necessary materials to develop. As such a close and large stellar nursery, OMC-1 is an easily accessible and important laboratory for studying the still-mysterious conditions that surround and encourage star formation. Today’s paper contributes to our understanding of star formation by determining OMC-1’s magnetic field and dust properties using polarimetry (more on this technique later!).

    OMC-1 is a particularly interesting target for magnetic field and dust measurements because of the variation in structure across the cloud, which is shown in Figure 1. In front of OMC-1, there is an HII region ionized by a relatively young group of stars, the Trapezium cluster. The west side of OMC-1 hosts the Kleinman-Low (KL) Nebula and the Becklin-Neugebauer (BN) object. The KL Nebula is a clump of molecular gas and dust with a bunch of massive stars inside, of which the BN object is the brightest. In the infrared, the KL Nebula appears to be exploding because stellar winds from the massive stars heat up the surrounding gas. The southeast region of OMC-1 contains the Orion Bar, a photodissociation region that is cold, neutral, and creates the divide between HII and molecular gas. These features contribute to a complex magnetic field structure within OMC-1 that today’s authors map with polarimetry measurements.

    2
    Figure 1. The OMC-1 region, with overlaid magnetic field lines. Blue shows the KL Nebula and BN object, gray shows the HII region ionized by the Trapezium cluster, and purple shows the Orion bar.

    So what is this polarimetry I speak of?

    We’ve all heard of polarized sunglasses, which block sunlight and reduce glare. Thinking of light as a wave, it travels in one direction and oscillates in the two planes perpendicular to that direction of travel. Polarized sunglasses block out one of these planes of vibration, and allow only half of the light to travel through the lenses.

    Polarization measurements in astronomy work much the same way. For today’s paper, we are looking at the infrared light that is emitted from dust, but stars and other sources can emit polarized light too. For dust, the primary concepts of polarization remain the same as the case of blocking light with sunglasses. However, instead of blocking the light, dust actually emits light that has one plane of vibration brighter than the other from the start. To understand the reason behind this, assume that the dust has an egg shape. Because there is more surface area along the long axis of the egg than the short axis, we get more emission traveling in the direction of the long axis. This creates a net polarization of the signal: we get more infrared light in the direction that is parallel to the long axis of the dust. Lots of theories suggest that dust aligns its long axis perpendicular to the magnetic field, so by measuring the direction of the polarization, we can infer the direction of the magnetic field!

    Flying High

    Today’s authors used the HAWC+ instrument onboard NASA’s airborne observatory (the Stratospheric Observatory for Infrared Astronomy, or SOFIA) to look at the infrared emission of the dust in OMC-1.

    NASA SOFIA High-resolution Airborne Wideband Camera-Plus HAWC+ Camera

    NASA/DLR SOFIA

    They measured the total flux and polarization at four different wavelengths, and the results of their measurements can be seen in Figure 2. Interestingly, they found that at the smaller wavelengths (upper two panels), the magnetic field direction near the BN/KL objects, represented by the white star, is radically different than the surrounding region. Today’s authors also discovered that the magnetic field direction in the Orion Bar differs significantly from elsewhere in OMC-1, and the magnetic field strength and dust temperature are highest near the BN/KL explosion location.

    3
    Figure 2. Polarimetry measurements at 53, 89, 154, and 214 microns. The star symbol represents the location of the BN object, while the Orion Bar can be seen at the lower left. Colors represent total intensity, with red the highest and blue the lowest. Lines represent magnetic field direction.

    Sweeping (up the dust) Conclusions

    So why the change in magnetic field direction and strength across the OMC-1 region? Today’s authors propose some interesting explanations. Remember how the KL nebula appears to be exploding from stellar winds? Well, it’s possible that this explosion has compressed the magnetic field opposite the material that it spits out, creating the distinctly different direction of the magnetic field that we see at shorter wavelengths. And the reason we don’t see the same compression at longer wavelengths? Longer wavelengths are emitted by the colder dust (Wein’s Law) that is likely to be outside of the explosion range! Today’s authors also provide an explanation for the change in magnetic field direction that is present in the Orion Bar: the magnetic field of the bar may run parallel to its long side. When the vector of the magnetic field along the bar is added to the vector of the magnetic field in the surrounding region, it is likely to cancel itself out.

    These insights on the magnetic field structure of OMC-1 demonstrate the power of polarimetry in astronomy, and the HAWC+ instrument on SOFIA will continue to make similar measurements more prevalent for molecular clouds. Because molecular clouds act as stellar nurseries, learning about their properties (like the direction and strength of their magnetic field) provides us with a better understanding of star formation processes.

    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 4:21 pm on February 6, 2020 Permalink | Reply
    Tags: "Where the Solar System Ends", Astrobites, , , , , , The Heliopause, Voyager missions   

    From astrobites: “Where the Solar System Ends” 

    Astrobites bloc

    From astrobites

    Feb 6, 2020
    Briley Lewis

    Title: Voyager 2 plasma observations of the heliopause and interstellar medium
    Authors: John D. Richardson, John W. Belcher, Paula Garcia-Galindo, Leonard F. Burlaga
    First Author’s Institution: Kavli Institute for Astrophysics and Space Research; Massachusetts Institute of Technology

    Status: Published in Nature Astronomy [closed access]

    Where does the solar system actually end? We could say it’s where the Sun’s gravity stops being strong enough to hold onto things. This would make it the edge of the Oort Cloud, the loosely bound sphere of rocky and icy bits left over from the solar system’s formation, extending almost 3 light-years from the Sun.

    Oort Cloud, The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA, Universe Today

    Or, we could say it’s where the energetic particles from the Sun (the solar wind) stop flowing away from us, blocked by the pressure of all the other gas that’s between stars, the interstellar medium

    Today, we’ll focus on the latter: the Heliopause, the boundary where the solar wind meets the interstellar medium (ISM), which marks the edge of the Heliosphere, the bubble of gas surrounding the Sun.

    Both the solar wind and the ISM are made of plasma, the 4th state of matter. In a plasma, some of the electrons have been stripped off the atoms, leaving charged particles (ions) to move around. There are a few different parts of the heliosphere, and the Voyager missions, launched in the 1970s, have traveled through all of them.

    NASA/Voyager 1


    NASA/Voyager 2

    After traveling far beyond the planets, the two Voyager missions first encountered a region where the solar wind slows down below the speed of sound, known as a termination shock. Their next big milestone would be the Heliopause (see Figure 1 for illustration). This is an important and unique part of our solar system to understand; it is where our solar system and our star interact with the surrounding galaxy. There’s a lot we can learn here about how forces and magnetic fields in the plasma of the interstellar medium confine and influence the solar wind. The heliopause is even important for understanding how life comes about in solar systems – it’s what protects us from dangerous cosmic rays and other high energy radiation that could be disastrous for life.

    1
    Figure 1: Illustration (not to scale) showing the planets and the different features of the Heliosphere. (Image from Encyclopedia Britannica)

    Voyager 1 crossed the heliopause first in 2012, at 121.7 Astronomical Units, (AU) meaning Voyager 1 was 121.7 times further from the Sun than the Earth is. Voyager 2 finally reached this milestone in late 2018, at 119 AU, passing through a slightly different flow of the solar wind than Voyager 1 did, as shown in Figure 2. Although Voyager 1 gave us the first information on the heliopause, it passed through a weird spot, where the solar wind seemed to be flowing more slowly and in ways we wouldn’t expect. It also didn’t get to take all its measurements, since its plasma instrument was broken. Since 2012, astronomers have been waiting for Voyager 2 to reach this milestone, so that they can take new measurements and understand another perspective of the heliopause, including the speed and direction of the plasma’s flow, its temperature, and its density in that region.

    3
    Figure 2: Diagram of the Voyager 1 and 2 trajectories, illustrating the different paths they took towards the outer reaches of the solar system. (Image from NASA/JPL)

    So, what did Voyager 2 see out there? As it approached the heliopause, it entered a “boundary layer” – a region where the density and magnetic field increase as the solar wind encounters the ISM. Voyager 1 also traveled through this layer, and observed something unusual: the flow of the solar wind was stagnated, traveling much more slowly than expected. Voyager 2 saw very different velocities of the solar wind near the boundary, and although we’re not sure why these two observations were so different, the authors think they might be due to instabilities in the boundary layer; the heliosphere isn’t a perfect bubble, instead its edges might have swirls and uneven patches. It took the spacecraft 8 days to cross this boundary region, but the actual heliopause is so sharply defined that it only took 1 day to cross (as shown in Figure 3)!

    4
    Figure 3: Data from Voyager 2’s plasma instruments, showing the steep drop in current once the spacecraft reached the heliopause. The rise in current before the heliopause happens as V2 crosses the boundary layer (Figure 1 in the paper).

    After the heliopause, Voyager 2 was officially out in the “very local interstellar medium” (VLISM). The VLISM isn’t perfectly smooth either; Voyager 2 observed variations in the speed, flow direction, density, and temperature of the plasma out there, and found that the further it gets from the heliopause, the more dense the VLISM gets. This makes sense, since it’s cooler out there (around 7500 K, a bit hotter than the Sun’s surface) than it is closer to the heliopause, where gas gets compressed as the solar wind presses into the ISM, and the plasma is observed to be much hotter – around 30,000 K! This is actually hotter than expected, suggesting that the plasma is getting more compressed or heated in other ways. Voyager 2 also passed through an interesting region where the current in the plasma spiked up (illustrated in the data in Figure 4); the authors think this is a shock, a sudden change in pressure and density.

    5
    Figure 4: Measurements of the current in the VLISM (very local interstellar medium) flow as measured by Voyager 2. The heliopause is marked “HP” and the spike at Day 418 shows when V2 may have passed through a shock. (Figure 5 in the paper)

    Having direct measurements of all this plasma and matter beyond the heliopause is important, because it’s literally the stuff that is between ALL the stars! Most of the universe (besides dark matter, of course) is made of plasma, and after more than 30 years of traveling and waiting, now we have the chance to directly observe it.

    Originally created as a mission to study Jupiter and Saturn up close, the Voyager probes ended up flying by Jupiter, Saturn, Uranus, Neptune, and 48 of their moons. Now, they’re continuing past the planets, past the heliopause, and into interstellar space. Both Voyagers are now out there observing the VLISM – and we can look forward to getting info on all this stuff between stars as long as the spacecraft stay alive and communicating with Earth.

    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 9:22 am on February 6, 2020 Permalink | Reply
    Tags: "Investigating Early Populations of Galaxies with the Best Telescopes in the Universe", 30 meter class optical telescopes, Astrobites, , , , ,   

    From astrobites: “Investigating Early Populations of Galaxies with the Best Telescopes in the Universe” 

    Astrobites bloc

    From astrobites

    Feb 5, 2020
    Lukas Zalesky

    Title: Early Low-Mass Galaxies and Star-Cluster Candidates at z ~ 6-9 Identified by the Gravitational Lensing Technique and Deep Optical/Near-Infrared Imaging
    Authors: Shotaro Kikuchihara, Masami Ouchi, Yoshiaki Ono, Ken Mawatari et al.
    First Author’s Institution: Institute for Cosmic Ray Research, The University of Tokyo

    Status: Submitted to ApJ

    In the coming years, we will see the launch of one of the most powerful space-based telescopes ever built, the James Webb Space Telescope (JWST), and we will see a new class of colossal ground-based observatories built with primary mirrors exceeding 30 meters in diameter.

    NASA/ESA/CSA Webb Telescope annotated

    ESO/E-ELT, 39 meter telescopeto be on top of Cerro Armazones in the Atacama Desert of northern Chile. located at the summit of the mountain at an altitude of 3,060 metres (10,040 ft).

    TMT-Thirty Meter Telescope, proposed and now approved for Mauna Kea, Hawaii, USA4,207 m (13,802 ft) above sea level, the only giant 30 meter class telescope for the Northern hemisphere

    Giant Magellan Telescope, 21 meters, to be at the Carnegie Institution for Science’s Las Campanas Observatory, to be built some 115 km (71 mi) north-northeast of La Serena, Chile, over 2,500 m (8,200 ft) high

    However, despite all of our technical ingenuity, the most powerful telescopes in the universe are in fact galaxy clusters. As the most massive gravitationally bound structures, galaxy clusters severely distort their local spacetimes and can magnify substantial areas of the sky through the phenomenon of gravitational lensing.

    Gravitational Lensing NASA/ESA

    Cluster lenses allow astronomers to observe many distant sources in unprecedented detail that would otherwise be too faint to study (e.g., Fig. 1). Indeed, the possibility of discovering and characterizing some of the earliest and most distant galaxies observable was a primary motivation for conducting a deep survey of six galaxy clusters known to be powerful lenses. This project, dubbed The Hubble Frontier Fields, involved hundreds of hours of observations with the Hubble Space Telescope and the Spitzer Space Telescope.

    NASA/ESA Hubble Telescope

    NASA/Spitzer Infrared Telescope. No longer in service.

    By combining our best telescopes with those that nature provides, astronomers uncovered hundreds of distant galaxies from times as early as one billion years after the Big Bang. In this astrobite, we cover a work that uses this rich sample of galaxies to trace the growth of stellar mass across the first few billion years of the universe.

    3
    Figure 1 – Galaxy cluster Abell 370, pictured above, is one of the six Hubble Frontier Fields. Among the population of orange cluster member galaxies are bluer background galaxies, magnified and distorted into giant arcs by gravitational lensing. Credit: NASA, ESA, the Hubble SM4 ERO Team, and ST-ECF.

    Hundreds of Magnified Galaxies

    In this paper, the authors exploit the power of gravitational lensing to magnify and reveal intrinsically faint sources at great distances, sources that would otherwise be impossible to study. The team begins by identifying high-redshift galaxies through the Lyman break method, (a.k.a., the “dropout” technique). UV radiation from distant galaxies is absorbed by neutral intervening gas, causing high redshift sources to appear faint in blue filters – thus, high redshift galaxies can be identified quickly by their colors. Combining all available imaging of the Hubble Frontier Fields, the team uses the Lyman break method to find a total of 357 magnified galaxies at 6 < z < 9, when the universe was less than a billion years old.

    In order to study the full sample of galaxies in detail, the authors required carefully manicured images of their targets; at these distances, all galaxies appear extraordinarily faint, and low-mass galaxies typically go undetected. To alleviate this, the authors first correct for the magnification introduced by the cluster lenses using magnification maps provided by the Hubble Frontier Fields science team. Afterwards, the team creates image stacks of sources binned according to their apparent magnitudes, rewarding the team with high signal-to-noise (S/N) images which provide accurate photometry of the even the faintest galaxies at their sample.

    Galaxy Evolution Metrics

    Nearly all physical parameters of galaxies affect the way they emit light. This means that it is possible to recover the properties of galaxies by modeling their SEDs, or spectral energy distributions. To characterize each source, the team builds SED models that account for redshift, age, ionization state, stellar mass, stellar metallicity, dust content, and contribution from nebular emission. The constraints provided by high S/N photometric measurements across ten filters exceeds the number of free parameters, ensuring well-constrained models.

    4
    Figure 2 – In both plots, the red data points are from this work. Left: GSMF measured here, along with the best-fit Schechter function shown as a black line. Open circles and downward arrows are poorly constrained data points. Right: GSMD obtained by integrating the GSMF. The black and blue lines illustrate competing models of stellar mass build-up derived in previous works.

    The authors use the inferred properties from their models to trace fundamental aspects of galaxy evolution, in particular the growth of stellar mass in galaxies during the first few billion years of the universe. A key observable in this regard is the galaxy stellar-mass function (GSMF; Fig. 2 left), which describes the average number-density of galaxies in the universe as a function of stellar mass. Integrating the GSMF gives the galaxy stellar mass density (GSMD; Fig. 2 right), which is the average density of stellar mass throughout the universe. Measuring these quantities across redshifts quantifies the growth of galaxies’ mass in stars throughout cosmic time, providing a simple benchmark for all theories of galaxy evolution.

    While the GSMF has been well studied at low redshift, much is unknown about the GSMF in the early universe, especially at low masses. Fortunately, the high magnification due to lensing reveals some galaxies studied here with masses as low as Mstellar ~ 10^6 M☉, an extraordinary measurement at these distances. Consequently, this work provides some of the first constraints on the physical characteristics of such low mass galaxies during this epoch. At higher masses, the GSMF they measure is mostly consistent with previous works. However, at the highest redshifts, the authors find a greater abundance of massive galaxies than previous works. Furthermore, the authors measure higher stellar mass to (intrinsic) UV luminosity ratios. These pieces of information reveal that these massive star-forming galaxies favor a duration of star formation lasting ~ 100Myr, rather than a shorter, dramatic buildup of stars on shorter timescales suggested by other astronomers. In other words, the evolution of star formation rates is mostly smooth during these time periods. Regardless, further work is needed to ultimately constrain these evolutionary trends, given the current uncertainties on the massive galaxies in their sample.

    Globular Cluster-type Sources Beyond z ~ 6…?

    The mass-size relation in galaxies encodes fundamental differences in galaxy types. The last piece of this work involves assessing the masses and physical sizes of these distant galaxies and comparing these characteristics to those of more local sources. The authors combine the physical sizes obtained in a previous work with their inferred stellar masses and make an exciting discovery. Two sources, magnified by factors of ~ 20 and 80, have stellar masses (Mstellar < 10^7 M☉) and physical sizes (R < 40kpc) that make them comparable to globular clusters observed in the Milky Way (Fig. 3). The authors conclude that these sources could be members of a dominant class of low-mass galaxies expected to exist at these redshifts and could even be related to modern day globular clusters, which are known to have populations of old stars. These sources are particularly interesting, as it is thought that low-mass galaxies such as these likely evolve into Milky Way-sized galaxies we see today. Future telescopes, like the James Webb Space Telescope, may be able to obtain spectroscopic observations of these compact sources and reveal even more insight into their physical qualities.

    5
    Figure 3 – Galaxy size plotted against mass, with various types of galaxies color-coded accordingly (see this helpful astrobites page for a glossary of galaxy types). Galaxies at 6 < z < 7 are shown as the orange-to-red data points, and their colors indicate their magnification (μ). Other data points are from previous works. The blue highlighted square, where two galaxies identified in this work reside (along the top-right edge), indicates the region in parameter space occupied by globular clusters (GCs) and ultra-compact dwarf galaxies.

    Gravitational lensing by galaxy clusters provides a unique window to the high-redshift universe. Thanks to these cosmic lenses, the authors were able to study the growth of stellar mass in galaxies during early times in the universe and probe some of the lowest mass systems ever detected beyond z = 6. This work highlights some of the great science that is possible when we combine the power of the best telescopes humans have made with these otherworldly lenses. Indeed, this combination ensures a truly remarkable view of the cosmos.

    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 5:34 pm on February 4, 2020 Permalink | Reply
    Tags: "Why Are There so Many Sub-Neptune Exoplanets?", Astrobites, , , ,   

    From astrobites: “Why Are There so Many Sub-Neptune Exoplanets?” 

    Astrobites bloc

    From astrobites

    4 February 2020

    1
    Artist’s illustration of a Neptune-like planet. A new study explores why Neptunes are so rare when their smaller cousins, sub-Neptunes, are very common. [NASA Ames/JPL-Caltech/T. Pyle]

    Title: Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis
    Authors: Edwin S. Kite, et al.
    First Author’s Institution: University of Chicago

    Status: Published in ApJL

    In the few decades since the discovery of the first exoplanet in 1992, we’ve realized that our own solar system is just plain weird. We have no hot-Jupiter gas-giant planets whizzing around our star in a matter of days, nor do we have any sub-Neptune planets, the most common type of planet in the galaxy. Critically, our lack of sub-Neptunes severely hinders our understanding of the transition between Earth-like and Neptune-like planets.

    The Kepler Space Telescope operated 2009—2018 and discovered over 2,600 exoplanets, nearly 1,000 of which were classified as sub-Neptunes. But Neptune-like planets are considerably rarer, despite being only slightly bigger. This “radius cliff” (Fig. 1) separates sub-Neptunes (radii 3 R⨁). What could cause such a steep dropoff? Today’s authors explore this question.

    2
    Figure 1. The exoplanet radius distribution. The two gray bands represent two different studies. The radius cliff is denoted near 3 R⨁ with the dashed line. [Kite et al. 2019]

    Previous Model Shortcomings

    Gas-giant planets are made primarily of … well … gas. Specifically, most of this gas is molecular hydrogen, H2. Smaller gas giants like Neptune and Uranus have a larger fraction of helium and methane than planets like Jupiter or Saturn, but their atmospheres are still primarily hydrogen (Fig. 2). Previous attempts to explain the sub-Neptune radius cliff have therefore focused on atmospheric hydrogen loss and accretion.

    3
    Figure 2: Atmospheric composition of gas-giant planets in our solar system. Jupiter and Saturn primarily boast hydrogen atmospheres, which turns into metallic hydrogen under the high pressures deep in the atmosphere. Neptune and Uranus are both smaller and colder, leading to a mantle of ices, and their atmospheres contain more helium and methane than the larger two planets. [NASA/Lunar and Planetary Institute]

    In one model, researchers proposed that larger atmospheres (when considered with the same core mass) are easier to strip away. However, this model can’t explain both the wide range of sub-Neptune masses and the radius cliff. Other researchers have looked instead at the flip side — accreting more atmosphere just as the protoplanetary disk dissipates. As the disk disappears, the planet’s source of atmosphere disappears, cutting off the planet’s growth. But this model depends closely on the properties of the disk and how long it lives, so the model is likely not the universal solution to the radius cliff problem. What, then, can explain it?

    An Active Core

    One critical assumption that both of the previous explanations incorporated is that they left the planetary core chemically and thermally inert. That is, it does not interact at all with the atmosphere. Our own experiences on Earth, from the wavy lines radiating from the road on a hot day to the very existence of the water and carbon cycles, suggest that an inert core may not be a valid assumption (even though Earth is structured differently than a gas giant).

    Today’s authors throw out the assumption that the core does not interact with the atmosphere. Additionally, the deep atmospheres of gas giants insulate and slow down the cooling of their cores, which results in a magma ocean that directly touches the atmosphere. The authors then look at how the solubility of H2 in magma depends on various atmospheric properties such as pressure and temperature.

    The immense pressures at the magma–atmosphere interface mean that the properties of the gasses no longer follow the Ideal Gas Law. Instead, they behave non-linearly. In such high-pressure situations, the H2 molecules are so squished together that they begin to repel each other and can no longer be compressed. In this case, the only place the H2 can go is down, into the magma. Furthermore, the H2 no longer dissolves linearly (as in Henry’s Law) due to the high pressures. Therefore, as more and more gas accretes onto the atmosphere, more and more H2 dissolves into the magma, and the planet’s overall radius growth stalls. The authors call this non-linear solubility property of highly-pressurized H2 the “fugacity crisis,” where fugacity refers to a gas’s tendency to dissolve into an adjacent liquid.

    The authors find that sub-Neptunes with radii of 2–3 R⨁ are so numerous because the atmospheres of planets that size reach the pressures required to force the H2 into the magma ocean. Then, once the magma ocean saturates, planetary radius growth can resume. However, planets that have enough gas to reach beyond the saturation point are much rarer, simply because they require much more gas. Hence, the radius cliff!

    4
    Figure 3: Same as Figure 1, but with the authors’ simulations added. The black line represents the case of an inert, impermeable core. The blue line shows the case where H2 dissolves linearly with pressure into the magma, as in Henry’s Law. The red line, and the focus of the authors’ work, incorporates non-linear effects and reproduces the radius cliff. [Kite et al. 2019]

    Toward a More Complete Description of Sub-Neptunes

    Today’s paper shows the importance of revisiting assumptions and considering additional factors to explain interesting phenomena. Even though the authors reproduced the radius cliff separating sub-Neptunes from Neptunes, they note that further research remains. Essentially no laboratory data exists about the true solubility of H2 in magma at the temperatures and pressures that exist in the depths of sub-Neptunes because no Earth-bound container can hold that magma. The authors instead extrapolated from lower temperature and pressure measurements. Additionally, the magma–atmosphere interface likely isn’t a hard boundary but rather more fuzzy, which would likely change how the H2 dissolves into the magma. Finally, the authors note that different core compositions would also likely change the interactions of the magma with the atmosphere.

    On the plus side, the authors’ non-linear H2 dissolving model makes a number of predictions, including how steep the radius cliff is, non-dependence on planetary disk conditions, and the ratios of molecules present in sub-Neptune atmospheres. Future data from TESS will allow astronomers to test this latest hypothesis and bring us a step closer to understanding how planets form.

    NASA/MIT TESS replaced Kepler in search for exoplanets

    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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