Tagged: Astrobites Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 10:28 am on March 22, 2017 Permalink | Reply
    Tags: Astrobites, , , , , One mechanism to rule all magnetic bodies   

    From astrobites: “One mechanism to rule all magnetic bodies” 

    Astrobites bloc

    Astrobites

    Mar 21, 2017
    Ingrid Pelisoli

    Title: A common origin of magnetism from planets to white dwarfs
    Authors: Jordi Isern, Enrique García-Berro, Baybars Külebi, Pablo Lorén-Aguilar
    First Author’s Institution: Institut de Ciències de l’Espai (CSIC), Spain
    1
    Status: Published in ApJ [open access]

    If you have ever used a compass, you know the Earth has a magnetic field.

    1
    Earth’s magnetic field. LBNL

    That’s lucky for us, because this field protects us from highly energetic particles that could make life on Earth quite difficult, like it is on Mars (although NASA might have a solution). The Sun has a magnetic field too.

    3
    Sun’s magnetic field. NASA

    The explanation for these fields is a dynamo effect: in short, ionised matter circling inside the Sun and the Earth generates the field. This explanation holds for most stars in which we detect a magnetic field. White dwarf stars, the most common end-point of stellar evolution (which makes them extremely useful in understanding the history of the Galaxy and even of the Universe), seemed to be an exception. They can present unusually high magnetic fields, up to a 100 million times the field of the Sun! The explanation for such colossal fields is still an open question. The authors of today’s paper present a possible solution by cleverly making use of already well-known astrophysical mechanisms.

    Magnetism in white dwarfs: how common is it?

    The fraction of magnetic white dwarfs is also open to discussion. It may be as high as 20%, but as low fields can be difficult to spot, we cannot be sure. Observations also indicate that this fraction is larger for cool white dwarfs, suggesting that the field is somehow amplified during the white dwarf’s evolution, which is basically a cooling process. Another observational fact is that the average mass of magnetic white dwarfs is higher than that estimated for non-magnetic white dwarfs. A good explanation for the origin of white dwarf magnetism should be able to explain these facts as well. As you will see, today’s paper fits the bill!

    One, two, three possible scenarios

    There are three proposed explanations in the literature for the observed magnetism in white dwarfs. The first hypothesis is that the observed fields are simply the remnants of those of their progenitors. Specifically, white dwarf magnetic fields could be the left-over “fossil fields” of main sequence Ap/Bp stars (which have stronger magnetic fields than classical A/B type stars). As the magnetic flux must be conserved throughout the star’s evolution (assuming mass loss doesn’t carry away a significant portion of the flux), the amplification of the magnetic field can be accounted for by the contraction of the star into a white dwarf. However, the fraction of observed Ap/Bp stars is not enough to explain the number of observed white dwarfs with high magnetic fields.

    In the second scenario, magnetic white dwarfs occur as the result of the evolution of binary systems. In this case, the magnetic field is amplified by a dynamo either in the common envelope phase or in the hot corona produced by the merger of two white dwarfs. Again, population synthesis models suggest that the number of white dwarfs produced by this channel cannot explain what we find observationally.

    Last but not least, in the third scenario, the magnetic field is generated inside the convective envelope formed as a white dwarf cools down. The problem with this explanation is that it cannot account for the strength of the observed fields. Therefore, we need a more efficient mechanism!

    The missing ingredient

    Another characteristic of most white dwarfs is that, as they cool, their nucleus will eventually undergo a phase transition and crystallise, releasing energy without changing the star’s temperature significantly. This process provides extra energy that can boost the dynamo effect and lead to higher fields. A similar effect occurs for the Earth and Jupiter (where the convective dynamos are powered by cooling and chemical segregation in their interiors) as well as for T Tauri stars and rapidly rotating M dwarfs. The authors estimated the dynamo energy density for a number of white dwarfs with known fields taking that into account, as shown on Figure 1. They noted that this boost in energy is sufficient to explain the observed fields in most white dwarfs with hydrogen-dominated atmospheres. This implies that the magnetic fields observed in planets, non-evolved stars and white dwarfs all share a common origin!

    4
    Figure 1: Magnetic field intensity as a function of the dynamo energy density. Black symbols represent Earth and Jupiter, T Tauri are shown in cyan, M dwarfs in magenta, white dwarfs with hydrogen dominated atmospheres in red and hydrogen deficient in blue. The top panel shows the magnetic field as a function of the present energy density of the dynamo, while the bottom panel shows it as a function of the maximum energy density. The solid line is a known relation between the magnetic fields of the Earth, Jupiter, T Tauri and M dwarf stars; the dotted lines add an additional deviation of a factor of 3 from it. The dashed lines represent where non-DA stars cluster. [Figure 3 from the paper.]

    As the crystallisation happens when the white dwarfs are relatively cool, this could also explain why magnetic white dwarfs usually show low temperature. Moreover, the amount of energy released during crystallisation is larger for more massive white dwarfs, so this mechanism naturally explains why magnetic white dwarfs are more massive than average.

    Another cool thing about this mechanism is that it doesn’t exclude other possibilities. On the contrary, it alleviates one of the major drawbacks of the other two hypotheses, which didn’t predict a sufficient number of magnetic white dwarfs. As a bonus, the authors offer an explanation for the fact that most white dwarfs that do not fit into their mechanism are hydrogen-deficient: they are indeed formed by the merger of two white dwarfs, as suggested by scenario two. During their coalescence, the temperatures reached are so high that the hydrogen in the outer layers is burned. So, using already known mechanisms, the authors may have finally solved the mystery of the highly magnetic white dwarfs: no need to reinvent the wheel!

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 10:33 am on March 20, 2017 Permalink | Reply
    Tags: , Astrobites, , , , , , , , Sardines in Space   

    From astrobites: “Sardines in Space: The Intensely Densely-Packed Planets Orbiting Kepler-11” 

    Astrobites bloc

    Astrobites

    Title: A Closely-Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11
    Authors: Jack J. Lissauer, Daniel C. Fabrycky, Eric B. Ford, et al.
    Lead Author’s Institution: NASA Ames Research Center, Moffett Field, CA, 94035, USA

    Status: Published in Nature 2011 [open access]

    The dawn of the Kepler Space Telescope data has unearthed a treasure trove of new and unusual celestial objects. Among these new discoveries is the planetary system Kepler-11. The system contains six transiting planets that are packed incredibly close around the Sun-like star, much like sardines are packed very closely in cans. The first five of these planets fall within the orbit of Mercury, and the sixth one falls well within the orbit of Venus. Few systems like this have been discovered; most planetary systems have a much larger separation between the planets, yet this system has its planets arranged in an extremely packed, yet extraordinarily still stable, way.

    1
    Figure 1: This figure from the NASA website is a visual representation of the Kepler-11 system, overlaid with the orbits of Mercury and Venus.

    When a single planet orbits a star, its period follows Kepler’s Laws to a tee; however, when other planets are introduced in the system, the orbiting bodies tend to perturb each other’s orbits. Their periods differ slightly according to the gravitational perturbations, and this variation is called a transit timing variation (TTV). Since Kepler-11 has five planets orbiting in extreme proximity to one another, it is the perfect illustration of measurements from transit-timing variations.


    Planet transit. NASA/Ames

    The photometric Kepler data marked the discovery of this system. The transits for each of the planets appeared separately in the light curve of the system. The light curve is just a measurement of the brightness of the star over time, so when a planet passes in front of the star, the brightness decreases, causing the dip in the light curve. The shape varies with each planet based on differences in size of the planet and orbital radius. From this data, it is possible to measure the radius of the transiting planet. This team followed up their photometric data with spectroscopic analysis from the Keck I telescope. This additional data allowed for the precise measurements of transit-timing variations, which yielded mass measurements for the inner five planets.

    For the first five planets, the TTVs were successfully measured, and with this information, the research team found the densities of the inner five planets, which yielded a surprising result. These planets, despite being densely packed, are not made of very dense material. Kepler-11b is both closest to the Sun and densest, but only with an overall density of 3.31 g/cm3. For comparison, Earth has an overall density of about 5.5 g/cm3. The densities of the planets orbiting Kepler-11 are depicted in Figure 2.

    2
    Figure 2: This shows the mass versus radius of the planets in the Kepler-11 system. The planets orbiting Kepler-11 are represented by the filled in circles. The other marking on the graph indicate planets in our solar system, shown for comparison. Figure 5 from today’s paper.

    While transit timing variations worked like a charm for the inner five planets, the sixth planet (Kepler-11g) was too distant from the others for this method to work well, so to confirm this planet, another method was employed. This team used several simulations to rule out alternate scenarios, which include chance alignment of the Kepler-11 system with and eclipsing star or with another star-planet system. This analysis successfully confirmed Kepler-11g , but because no TTVs could be measured for this particular planet, its mass and radius remain unknown.

    Even though this system has been more closely studied than most, the measurements have raised nearly as many questions as they have answered. The inner five have small inclinations and eccentricities, which implies some planetary migration process. However, since the periods of these planets are not in resonance, slow and convergent migration theories—which would naturally force the planets into resonant orbits—seem unlikely to be at play in this system. Formation of such a system is still a bit of a mystery. After all, such low-density planets are unusual and do not completely fit within the current understanding of planet formation.

    Kepler-11 continues to be one of the more intriguing planetary systems discovered, and its formation is not fully understood. Even though this system has been more closely studied than most, the measurements have raised nearly as many questions as they have answered. Systems like this extend our understanding of astrophysics, perhaps in a bit of an unexpected way; these closely packed planets have so much more to teach us about their system formation.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 2:22 pm on March 14, 2017 Permalink | Reply
    Tags: Astrobites, , , , ,   

    From astrobites: “Gemstones askew in the heavens” 

    Astrobites bloc

    Astrobites

    Title: Planck’s Dusty GEMS. III. A massive lensing galaxy with a bottom-heavy stellar initial mass function at z=1.5
    Authors: R. Cañameras, N. P. H. Nesvadba, R. Kneissl, M. Limousin, R. Gavazzi, D. Scott, H. Dole, B. Frye, S. Koenig, E. Le Floc’h, and I. Oteo

    First Author’s Institution: Institut d’Astrophysique Spatiale, Université Paris-Sud, France
    1
    2
    Status: Accepted for publication in Astronomy & Astrophysics, open access

    Introduction

    In today’s article I want to take a closer look at gravitational lenses and open a window on some of the interesting science astronomers are using these objects for.

    A gravitational lens results from a chance alignment of two galaxies, one near and one far.


    Gravitational Lensing NASA/ESA


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

    To understand how it works, we need to take a brief tour through Einstein’s General Theory of Relativity – but don’t panic! I’ll keep it light. In his landmark theory, now over a century old, Einstein outlined the mechanism by which gravity acts. Rather than being a fixed background against which events happen – a sort of cosmic stage – space itself can be warped and stretched by the presence of mass. The more massive the object, the stronger the distortion. The upshot is that anything following a straight path through space, such as a light ray, finds itself travelling a curved path instead when it passes near a mass – as if a force was acting directly upon it. It’s this apparent force that we call gravity. This effect was used in 1919 by Sir Arthur Eddington to confirm the predictions of General Relativity, to much excitement and confusion; during an eclipse, Eddington measured the angle through which the Sun’s gravity deflected the light of distant stars, showing that this matched the theory. In special circumstances, the same effect can focus the deflected light rays, just like a traditional lens (see figure 1).

    3
    How gravitational lensing works. Large concentrations of mass (such as a galaxy or cluster of galaxies) can deflect light from a background object, focusing it on the observer’s position. Credit: NASA/ESA

    In more recent times, the study of galaxy-sized gravitational lenses has been particularly fruitful – as we’ve documented before. The immense mass of a galaxy is sufficient to focus the light from a more distant partner (as in figure 2), magnifying the background galaxy and allowing us to study it in much more detail than would otherwise be possible. On the other hand, the precise pathways through which the lens deflects light can encode information about the foreground galaxy (the one doing the lensing), telling us about its structure. This is the approach pursued by the authors of today’s featured paper.

    This one’s a real gem

    The system studied by the authors, ‘the Ruby’, is particularly unusual. Part of the GEMS sample (Gravitationally Enhanced subMillimetre Sources – another one for the collection of dubious astronomy acronyms), it is one of only a handful of lenses where the foreground galaxy is not local. For those who speak redshift, it’s at z=1.525, which means the Sun hadn’t even formed yet when the light from the background galaxy shot past it (and indeed wouldn’t for another few billion years). In most other lenses the foreground galaxy is close enough that it doesn’t have time to change appreciably while its light travels to us. That’s the key point here: this particular system offers an opportunity to understand the structure of a massive galaxy at a much earlier cosmological epoch.

    Right, so why is that interesting? Well, one of the principal limitations to our understanding of galaxy formation is the brute fact that we can never really watch a galaxy evolve over time: we only ever see a snapshot of a particularly galaxy at a particular time in its life. We can try and connect galaxies in the local universe to those which are more distant, identifying common features and trends which offer clues to how galaxies evolve in the general sense. We can run simulations which approximate the important physics and test how galaxies evolve in those, too. But we’re always limited by the leap from distant galaxies to local ones. Anything we can use to try and connect the two is of interest.

    Galaxies like the Ruby’s foreground lens are thought to be the precursors of the most massive local galaxies. They are characterised by rapid star formation in the early universe followed by a long, passive period where they form few stars. This rapid burst of activity leads to important differences in the stars hosted by those galaxies from those in our own Milky Way. In particular, as outlined here, local massive galaxies seem to have formed an excess of dwarf stars, which are dim and long-lived. These galaxies are more massive than we might guess simply from how bright they are, since they have extra stars which don’t contribute much to the total luminosity. But hang on … if a galaxy happens to be a gravitational lens, we can figure out how massive it is from the way it deflects light from the background galaxy!

    Lens modelling

    The lensing arcs/rings actually comprise several images of the Ruby which have taken different paths through the lens. The authors try to match points within these images to common points of origin. This tells them the precise way in which the lens deflects the Ruby’s light, which provides the information necessary to build a model for the mass distribution of the lens. Put another way, they reconstruct both what the Ruby looked like before its light passed through the lens and what distribution of mass is required of the foreground galaxy in order to create the images actually seen. This is depicted in a bit more detail in figure 3.

    3
    Modelling the lens. The left panel shows the best-fit model (grey pixels) with red contours representing the data (observations taken with ALMA). The right panel shows the rotational profile of the background galaxy (the Ruby) as viewed through the lens. The important thing to take away is that blue points in different images of the source (the annotated numbers identify separate images) correspond to the same point of origin in the background galaxy, and the same for red points. This information is used to reconstruct the lens galaxy’s mass distribution. The orange and blue lines correspond to features of the model geometry, but are not crucial here. Figure 3 from the paper.

    Following this method, the authors infer that the foreground galaxy is indeed more massive than might be expected from its brightness alone, indicative of an excess of dim dwarf stars. This is consistent with results from galaxies in the nearby universe, which have undergone billions of years of additional evolution, forging a crucial link between local galaxies and their antecedents. It’s clear that gravitational lensing observations have much to offer this field. Let’s hope the best is yet to come!

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 9:05 am on March 10, 2017 Permalink | Reply
    Tags: Astrobites, , , How long do quasars shine?,   

    From astrobites: “How long do quasars shine?” 

    Astrobites bloc

    Astrobites

    Title: Statistical detection of the He II transverse proximity effect: evidence for sustained quasar activity for > 25 million years
    Authors: Tobias M. Schmidt, Gabor Worseck, Joseph F. Hennawi, J. Xavier Prochaska, Neil H. M. Crighton
    First Author’s Institution: MPIA, Heidelberg, Germany
    1
    Status: Submitted to ApJ, open access

    In the deep center of every massive galaxy, extremely massive but invisible black holes reign supreme. How these supermassive black holes (SMBHs) grew to boast of their 107-109 Msun masses still eludes us today. These massive beasts are awakened when surrounding matter spirals in and falls into them, creating active galactic nuclei (AGN) as luminous as our Milky Way.

    2
    UCSB

    In this state, they spew out radiation from the radio to the X-rays. When the accretion of matter is particularly high, the AGN becomes very luminous and is called a quasar. (Here is a handy guide on AGN taxonomy.)

    We believe that part of the reason SMBHs grow so massive is that they are fed by a voluminous amount of accreting material during the bright quasar phase. Even if some SMBHs appear dormant (i.e. no accretion of matter), it is believed that they must have gone through at least one quasar period in the past. Thus the quasar lifetime can shed light on the growth and evolution of SMBHs. Current estimates of the quasar lifetime are not very tight — they span a couple of orders of magnitudes from 106 to 108 years. These assume that black holes go through the quasar phase once, although it is possible that a black hole undergoes multiple quasar outbursts. Multiple episodic lifetimes would then sum up to the net quasar lifetime.

    This paper examines the episodic lifetime of quasars using singly-ionized helium (He II) as the probe. At redshift z~3, most of the Helium in the Universe is singly-ionized. The last electron in He II can be knocked free by the powerful radiation from quasars. As a quasar ionizes its surrounding He II, one can imagine a sphere of ionized He II around the quasar that expands outward with the ionizing radiation. The longer the quasar shines, the larger this sphere becomes.

    What happens when there is a foreground quasar at almost the same redshift close to a background quasar? In this case, when light from the background quasar passes through the ionized He II sphere of the foreground quasar, it will not be absorbed by the He II near the foreground quasar, since the He II would already be ionized (by the foreground quasar). On Earth we see an increased flux transmission in the spectrum of the background quasar, at the ionization wavelength of He II. This is known as the proximity effect. We’re particularly interested in the transverse proximity effect, which is the proximity effect across the plane of the sky.

    To find this, the authors carried out an intensive imaging and spectroscopic campaign in the vicinity of 22 background quasars at z~3 with 4m- and 8m-class telescopes including the Large Binocular Telescope (LBT), the Very Large Telescope (VLT), and the New Technology Telescope (NTT).


    Large Binocular Telescope, Mount Graham, Arizona, USA


    ESO/VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level


    ESO/NTT at Cerro La Silla, Chile

    They ended up with a final sample of 66 foreground quasars, the largest that has ever been used in such studies. There were multiple foreground quasars near any one background quasar. The authors first searched for the proximity effect in individual background quasars (an example is shown in Figure 1), but failed to detect any signal. But don’t lose heart, as stacking the spectra can reveal the hidden signal.

    2
    Fig. 1 – Spectra of four background quasars in this study, where the stars mark the locations of the foreground quasars. Values on top indicate the rate at which the foreground quasars emit ionizing photons. The spectra had been truncated to only focus on foreground quasars with high ionizing radiation. The transverse proximity effect appears as a spike at the location of the stars relative to the continuum. Except for the lower right panel, none of the spectrum here and in the rest of the sample indicates strong proximity effect. [Figure 6 in paper]

    The authors stacked their spectra at the positions of foreground quasars whose ionizing radiation passes a certain cut — this gives them an average He II transmission profile, as shown in Figure 2. The stacked profile shows a clear transmission spike with 3σ significance. In order to ionize the He II gas, the quasar had to shine for at least the transverse light-crossing-time between the foreground and background quasar. The authors stacked quasar spectra for a range of transverse separations from the background quasar. The maximum separation where the proximity effect persists gives a lower limit on the quasar lifetime. The proximity effect is found to persist up to > 25 mega light years for their sample, translating to a minimum 25 Myr for the quasar lifetime.

    3
    Fig. 2 – Stacked spectra of foreground quasars whose ionizing radiation passes a certain cut (indicated top left). The red line indicates the position of the foreground quasars, where an increase in He II transmission can be seen. The thin black line is an estimate of the mean He II spectrum in the intergalactic medium, while the bottom panel shows the number of foreground quasars that are used in the stacking process. [Figure 7 in paper]

    Since the authors ran out of foreground quasars at larger separations and implemented a cut on the quasar ionizing radiation, the intrinsic quasar lifetime could actually be longer than 25 Myr. To probe longer lifetimes, one would need a larger sample of foreground and background quasars. More sophisticated modeling would also provide richer interpretation of the data beyond a simple lifetime constraint. In any case, this study is the first to be able to detect the proximity effect in any ion (previous studies have detected the proximity effect in neutral Hydrogen). It also places stronger constraints on the episodic lifetime of quasars than any past studies of the same nature.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 3:17 pm on March 9, 2017 Permalink | Reply
    Tags: Astrobites, , , , , , Intra-cluster medium (ICM), Same ol’ same ol’? Galaxy Clusters across Cosmic Time,   

    From astrobites: “Same ol’ same ol’? Galaxy Clusters across Cosmic Time” 

    Astrobites bloc

    Astrobites

    Mar 9, 2017
    Gourav Khullar

    Title: The Remarkable Similarity of Massive Galaxy Clusters from z~0 to z~1.9
    Authors: Michael McDonald, Steve W. Allen, Matt Bayliss et al.
    First Author’s Institution: Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, USA
    1
    Status: Submitted to The Astrophysical Journal [open access]

    Introducing…Galaxy Clusters and X-rays!

    We have come a long way since the 1930s, when the words ‘galaxy cluster‘ were posited for the first time by Fritz Zwicky, in relation with the presence of dark matter in the Coma cluster.
    [Please do not forget the contributions of Vera Rubin.]


    Coma Cluster via Hubble

    Developments in multi-wavelength astrophysics have allowed us to probe different components of a cluster with different telescopes. For example, star-forming galaxies of galaxy clusters are observed using optical telescopes because starlight in these galaxies loves emitting photons with the roughly the same energy that we see from the sun. Some of these galaxies are super-red, have no star-formation and a ton of dust, that is best seen from infra-red and radio telescopes. Today’s story takes us to the intermittent space between different galaxies inside a cluster, called the intra-cluster medium (ICM) and its emissions.

    The ICM of a galaxy cluster is filled with gas or plasma that comprise free electrons and protons. This medium reaches temperatures of the order of 10^7 to 10^8 K, that emits light in the form of X-rays. This happens due to a phenomena called free-free emission of electrons, or Bremsstrahlung. X-ray observations of galaxy clusters are a crucial element in understanding how the cluster gas evolves with time, and how they influence the formation and evolution of massive galaxies in clusters. Moreover, the effect of active galactic nuclei (AGN) that heat up cluster environments after firing up from individual member galaxies can also be analyzed through X-ray studies, using telescopes like XMM-Newton and Chandra.


    ESA/XMM-Newton

    </a
    NASA/Chandra

    Looking for Distant Galaxy Clusters

    2
    Fig 1. Picture of an SPT Cosmic Microwave Background [CMB] map (created by Bradford Benson at Fermilab and University of Chicago, USA). This is a small patch of 50 sq. degrees with CMB anisotropies seen clearly. Small bright point sources are dusty galaxies that come out in these maps. Similarly, the dark spots are shadows on the CMB caused by inverse-comptonization of CMB photons by galaxy clusters.

    This is easier said than done. We know a lot about close-by galaxy clusters by pointing an X-ray telescope to the sky, but finding X-ray emitting clusters that are extremely far-away is a tough job. This was made easy by the advent of sub-mm (or Cosmic microwave background (CMB)) telescopes, like the South Pole Telescope (SPT) or Planck.


    CMB per Planck


    South Pole Telescope (SPT)


    ESA/Planck

    These telescopes discover galaxy clusters that cast a shadow on the background CMB, by a phenomena called the Sunyaev-Z’eldovich effect (look at this bite for details!) This makes cluster detection in CMB telescopes a distance (or redshift) independent activity, that gives us a better look at far away clusters.

    Let’s make this slightly easier on us. If I was to summarize my chain of thought in the last two paragraphs, I would do so with the following steps:

    Step 1: Study the CMB and look for shadows in the maps. These shadows are galaxy clusters that are distorting the background CMB light.

    Step 2: Use an X-ray telescope to point to these shadows – you will see the X-ray ICM gas of these clusters.

    Step 3: Make a list of these clusters, and study the X-ray ICM gas as a function of their distance, or redshift.

    Step 4: Party.

    Today’s paper is exactly that!
    Evolution of the ICM

    2
    Fig 2. Plotted here is Mass of Cluster vs redshift for the clusters considered in today’s paper. The orange background is an evolution map, incorporating the physics of galaxy cluster evolution. This implies that clusters at high redshift (the black stars) could very well be the ancestors of nearby clusters (the blue squares) which are much more massive and fall within the orange band. No image credit.

    McDonald et al. present the first ever X-ray analysis of 8 galaxy clusters (with masses of ~2 to 4 x 10^14 solar masses) at redshifts greater than 1.2 that were detected with the SPT telescope, that add to the thermodynamic studies done by the same collaboration for low-redshift (or nearby) clusters. This allows them to discuss the evolution of cluster ICM from redshift 1.9 to redshift 0 i.e. from a time when the universe was 3 billion years old, to now! What they are looking for are signs of similarity between far away and nearby clusters. Not just looking alike, but whether distant clusters are younger versions of the nearby clusters. We call this property self-similarity – young less massive clusters accrete matter, cool down and evolve into massive clusters.

    They find that centres of clusters, called cool cores, show no significant evolution in the density of the ICM gas when comparing far away clusters with close ones. As we go further out – about 20% of the ‘defined’ cluster radii – we see that far away and nearby clusters have self-similar densities. Based on their analysis, they propose a scenario where the cool cores formed at redshifts > 1.5 and their size, mass and density roughly remained constant. The rest of the cluster around them merrily continued accreting matter, and grew in their size and mass. This is possible if there is a gigantic AGN at the centre of these clusters, that is reheating all the cool gas that would have fallen to the centre of these clusters. This cooling and reheating seems to be tightly regulated, just like a thermostat on a fixed temperature. This explains the preservation of density around the cool cores, but not the rest of the massive cluster.

    3
    Fig 3. (a) Absolute gas density as a function of radius for the 8 new galaxy clusters studied in today’s paper. At low radii i.e. near centre of clusters, there is considerable difference in the density profiles, with a big scatter. As one goes outwards, the outskirts of the clusters look remarkably self-similar. (b) A similar result is seen when comparing clusters across different redshifts (or epochs).

    Wait..what?!

    This is huge. The work in today’s paper indicates that far-away clusters could very well be progenitors to nearby clusters, if given enough time to evolve into massive structures. The cluster centres seem to stand the test of time, unfazed by the chaos around them. This is irrespective of how disturbed or relaxed the shapes of these clusters are, or how many galaxies are merging into these clusters.

    4
    Fig 4. Photon assymetry (A, a tracer of disturbance in the clusters) vs electron density in galaxy clusters considered in today’s paper. The black stars are the 8 new clusters added to the analysis sample. This plot shows that there is no bias in the new sample, and the clusters span the typical range of these numbers, distributed uniformly.

    A study like this makes us reach regimes where we can connect the physics of central cluster environments to its macro-surroundings, that we find so hard to replicate in hydrodynamical simulations at the moment. With the advent of new X-ray, CMB and optical telescopes, the precision with which we can make these claims only gets better!

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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.

     
    • MIKE EYE 3:33 pm on March 9, 2017 Permalink | Reply

      Great inspiration for Sci-Fi novelists, thank you! And the findings seem to flow with the way Eye see it. Starlight pumps outward in waves right, and clusters can become entangled regardless of how many light years away they are from one another, is that generally correct on a basic layman’s level? Or am I totally off?

      Like

    • richardmitnick 3:42 pm on March 9, 2017 Permalink | Reply

      Sci-Fi and Science constantly feed each other. I am sure that the incandescent bulb and the cell phone were once science fiction.

      What you propose works if Quantum Mechanics works. The problem is that Quantum Mechanics is short on peer reviewed evidence.

      Like

  • richardmitnick 8:43 am on March 8, 2017 Permalink | Reply
    Tags: A Volcanic Hydrogen Habitable Zone, Astrobites, ,   

    From astrobites: “A Volcanic Hydrogen Habitable Zone” 

    Astrobites bloc

    Astrobites

    Title: A Volcanic Hydrogen Habitable Zone
    Authors: Ramses Ramirez and Lisa Kaltenegger
    First Author’s Institution: Cornell University

    Status: Accepted in The Astrophysical Journal Letters, open access

    The search for life beyond the solar system has long focused on the habitable zone (HZ). This is the region around a star where a planet with the right properties could maintain liquid water on its surface for a substantial period of time. The classical inner edge of the HZ was set using the runaway greenhouse effect, in which a positive feedback loop causes oceans to evaporate creating an oven-like world similar to Venus. The classical outer edge of the HZ was set using the maximum greenhouse effect from carbon dioxide, which is the distance at which adding carbon dioxide to a planet’s atmosphere starts cooling the planet (due to scattering the light or condensation). There have been many other calculations of the HZ edges using different assumptions, such as a nearly desert planet and planets with different masses. In this paper, the authors try to use volcanoes to expand edges of the HZ. They calculate the HZ edges for atmospheres with significant amounts of hydrogen gas produced by volcanoes, another powerful greenhouse gas.

    Hydrogen-induced Greenhouse Warming

    An atmosphere with significant greenhouse warming due to hydrogen is difficult to maintain. Hydrogen gas escapes atmospheres quickly. However, early in the Earth and Martian geological histories, volcanic outgassing of hydrogen may have exceeded atmospheric escape of hydrogen. This means that both planets might have had significant amounts of hydrogen in their atmosphere. Different conditions in the mantle could make this hydrogen outgassing occur over a much longer timescale and therefore giving the planet a longer hydrogen-induced greenhouse effect. Using this volcanic outgassing as their source of hydrogen, the authors used a 1D atmospheric climate model to compute the edges of the HZ for an atmosphere composed of nitrogen, water vapor, carbon dioxide, and hydrogen for stars with temperatures between 2,600 K and 10,000 K. A variety of atmospheric concentrations were tested up to 50% hydrogen. (30% hydrogen is the highest concentration they could reasonably acquire by assuming different geologies, but 50% was included as an extreme outlier.) Because these models depended on so many variables, many assumptions were necessary in the model too, such as plate tectonics, the carbon-silicate cycle, an oxygen-reduced (i.e., oxygen-poor) mantle, and a constant albedo.

    New Habitable Zone Results

    Adding hydrogen into planetary atmospheres moved both the inner and outer edges of the HZ outward. The outer edge moved farther than the inner edge, which widened the HZ. The incident stellar flux (the amount of energy hitting the planet per second per square meter) needed to maintain liquid water on the surface at the outer edge of the HZ decreased by 25%, 44%, and 52% when the atmosphere was 5%, 30%, and 50% hydrogen, respectively. This moved the classical HZ edge from 1.67 AU to 1.94 AU, 2.23 AU, and 2.4 AU, respectively. The HZ expanded much more for the hotter stars than the cooler stars. The inner edge of the HZ, on the other hand, shifted only a tiny bit: 0.1% outward for 1% hydrogen and 4% outward for 50% hydrogen.

    1
    Figure 1: The outer edge of the habitable zone. The x-axis is the amount of energy received from the star per second per area, and the y-axis is the temperature of the star. The stellar temperature is important because it changes the distribution of energy hitting the planet (e.g., a higher proportion of the incident energy is in the infrared as the stellar temperature decreases). The dashed line is the classical outer edge of the HZ. The solid line is the empirical outer edge using evidence suggesting that early Mars had liquid oceans. The red lines are the outer edges of the HZ for atmospheres with different concentrations of hydrogen. For reference, the Sun’s effective temperature is 5,780 K (where Mars is).

    Conclusions

    It is expected that terrestrial planets are born with oxygen-reduced mantles. A planet with a reduced mantle is more likely to have an extended period of hydrogen outgassing and therefore a longer hydrogen-induced greenhouse effect. Over time, however, the mantles become oxidized. Some research has suggested that smaller planets’ mantles (like Mars’s) stay reduced, while larger planets’ mantles oxidize quickly. This suggests that hydrogen outgassing might only be relevant for smaller planets. On the other hand, more massive planets can hold onto hydrogen more easily due to their gravity and higher likelihood to have a strong magnetic field. The relationship between planetary mass and the effectiveness of hydrogen outgassing on habitability remains unclear.

    In our own solar system, Earth’s mantle may have become oxidized only about 100 million years after formation. Mars’s mantle, though, may have stayed reduced for a billion years. Two meteorites (called ALH84001 and NWA Black Beauty) from 4 billion years ago support this idea. Therefore, hydrogen outgassing from volcanoes could have contributed to a warm, wet, early Mars.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 10:10 am on March 7, 2017 Permalink | Reply
    Tags: Astrobites, , , Baby Galaxies Blowing Bubbles, ,   

    From astrobites: “Baby Galaxies Blowing Bubbles” 

    Astrobites bloc

    Astrobites

    Mar 7, 2017
    Christopher Lovell

    Title: MODELING OF LYMAN-ALPHA EMITTING GALAXIES AND IONIZED BUBBLES AT THE ERA OF REIONIZATION
    Authors: Hidenobu Yajima, Kazuyuki Sugimura, Kenji Hasegawa

    First Author Institution: Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, Miyagi, Japan
    1
    Submitted to the Astrophysical Journal, Open access

    About four hundred thousand years after the Big Bang, the universe settled into a pretty dull period in its history. There were no stars or galaxies, just one massive expanse of neutral hydrogen, sitting in the dark. This period in the universe’s history, known appropriately as the Dark Ages, came abruptly to an end when the first stars were born and began to shine, dumping loads of high energy photons into their surroundings. These photons created ‘bubbles’ of ionised hydrogen around the stars, which slowly grew as more photons were pumped out by the stars. The bubbles surrounding the first stars were pretty small, but later, as stars began to group together into the first galaxies, these bubbles were blown much bigger by the combined photons from all the stars in the galaxy. Over time the bubbles from neighbouring galaxies began to overlap, until eventually all of the hydrogen in the universe was ionised (see Figure 1). This process is known as reionisation (Astrobites has written plenty about reionisation in the past – for more background, go check out some of these articles), and it’s a key period in the universe’s history.

    The subject of today’s bite are these ionised bubbles, the baby galaxies that blew them, and how much they contributed to reionisation. We will see that there is a close relationship between the properties of a galaxy and the size of the bubble it can blow. The size of the bubble also affects how easily we can see the galaxy. Finally, we’ll also learn about two upcoming observatories that it’s hoped will be able to see both the bubbles and their galaxies at earlier times than ever before.

    2
    Figure 1: A timeline showing the beginning of the Dark Ages (at recombination), and its end when the first stars and galaxies were born, ionising nearby hydrogen. These ionised bubbles soon grow and overlap, until the majority of the Hydrogen in the universe is ionised – this period is known as the Epoch of Reionisation (source: Nature 468).

    Who blew all the bubbles?

    One burning question researchers would like answered is ‘What kinds of galaxies contributed the most to reionisation?’ Many researchers in the field assert that it was small galaxies; they tend to allow their ionising photons to escape much easier than massive galaxies as they have less gas to get in the way. There are also far more small galaxies than big ones: more galaxies, more high energy photons, more reionisation! Unfortunately, such small galaxies are typically harder to detect than their big cousins since they’re less luminous.

    3
    Figure 2: size of ionised Hydrogen bubbles (RHII ) plotted against the luminosity of the Lyman-alpha emission (LLyα ). The bigger the bubble, the stronger the emission. This relationship doesn’t change much with redshift.

    That’s not to say that finding small galaxies is impossible. In the early universe, galaxies tend to be creating lots of new stars, and these young stellar populations emit light with a strong hydrogen spectral line, known as Lyman Alpha. Using Lyman Alpha, Astronomers hope to be able to see the small galaxies that contribute to reionisation in a big way.

    Unfortunately, as it’s so energetic, Lyman alpha radiation is absorbed by neutral hydrogen. So how can we detect it before the universe was ionised? The trick is to choose galaxies that have blown large bubbles. Galaxies with large enough bubbles allow any newly emitted ionising radiation from the galaxy to travel far enough uninhibited through the bubble to become redshifted. Redshifted Lyman Alpha radiation doesn’t have enough energy to ionise the neutral hydrogen outside the bubble, so it can happily continue travelling all the way to our telescopes on Earth, 12 billion light years away.

    So now the question is, what galaxies blow the biggest bubbles? The authors of today’s paper use a simulated model of the early universe to investigate this. Figure 2 shows the predicted size of ionised bubbles against the luminosity of Lyman-alpha. There’s a strong correlation between bubble size and luminosity. So… what galaxies emit the most Lyman Alpha? The bottom left panel of Figure 3 shows the relationship between Lyman Alpha luminosity and stellar mass. There is a clear correlation between the size of a galaxy and the amount of Lyman Alpha radiation it’s pumping out.

    4
    Figure 3: The relationships between bubble size and Lyman Alpha luminosity (y axis, top and bottom respectively) with stellar mass and star formation rate (x axis, left and right respectively). The different coloured lines are for different redshifts. The biggest galaxies emit the most Lyman Alpha, and therefore blow the biggest bubbles. The link between bubble size and star formation rate is not as strong.

    What does this all tell us? For a start, the model seems to suggest that we won’t be able to see the very smallest galaxies at very high redshifts using Lyman Alpha. All is not lost, however: thanks to two upcoming observatories, we may still be able to see the most energetic LAEs and their bubbles at redshifts of around 10, much higher than we’ve ever seen them before (The most distant LAE found to date is at z ~ 8.6).

    The first of these new observatories will be the James Webb Space Telescope (JWST), an enormous space based telescope scheduled to launch in 2018.


    NASA/ESA/CSA Webb Telescope annotated

    It will be capable of detecting Lyman Alpha radiation out to very high redshifts: the horizontal line in figure 2 shows the expected sensitivity of the instrument, within range of the most luminous Lyman Alpha emitters at z ~ 10 according to the model.

    The second of these enormous observatories to come online will be the Square Kilometer Array (SKA), a truly enormous radio telescope array based in both South Africa and Australia.

    It will be able to ‘see’ neutral hydrogen, leaving the ionised hydrogen bubbles to stand out like holes in a cheese. The vertical dashed line in figure 2 shows the smallest bubble size that it’s hoped the SKA will be able to see, again well within the range of the biggest bubbles at z~ 10.

    Combining these observatories, the yellow region in figure 2 represents those galaxies with bubbles are that are big enough to be observed with the SKA, and that allow enough Lyman alpha escape to be picked out by JWST. If the model is correct, these will be the most distant LAEs observed, and the first ever detection of ionised bubbles. But the smaller galaxies, thought to be responsible for the majority of reionisation, will have to wait for future generations of humongous space and ground based telescopes to be detected.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 11:20 am on February 27, 2017 Permalink | Reply
    Tags: Astrobites, , , , Constraints and Conundrums, , , GPI   

    From astrobites: “Constraints and Conundrums” 

    Astrobites bloc

    Astrobites

    Feb 27, 2017
    Mara Zimmerman

    Title: Constraints on the Architecture of the HD 95086 Planetary System with the Gemini Planet Imager
    Authors: Julien Rameau, Eric L. Nielsen, Robert J. De Rosa et al.
    Lead Author’s Insititution: Université de Montréal
    1
    Status: Accepted for publication in The Astrophysical Journal Letters open access

    HD 95086 is one of the more well studied and characterized systems; it hosts planetary, planetesimal, and dust components, which make it quite the intriguing subject to study.

    2
    An artist’s impression of a young star surrounded by debris rings and a vast dust halo. Credit: NASA/JPL-Caltech

    Its planet HD 95086 b was directly imaged by GPI in late 2013;

    GPI blocNOAO Gemini Planet Imager on Gemini South
    NOAO Gemini Planet Imager on Gemini SouthGemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile
    Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile

    the planet is about 4 times the mass of Jupiter and orbits its star at a distance of about 56 AU. The disk in the system is characterized by three components— a 55 K cool component, a 75 K warm component at – each of those two corresponding to a planetesimal belt– and a possible hot component at 300 K, which could suggest activity in the habitable zone of the star (Su et al. 2015). There is also quite a large gap in HD 95086’s disk, extending from about 8 to 80 AU. This system presents a perfect playground for the study of evolution and formation of unusual systems.

    3
    Figure 1: The direct images of HD 95086, taken over several epochs, are shown in this figure

    This paper focuses on constraining the orbit of HD 95086 b based on re-analyzation of old images in combination with their newer images. HD 95086 b was originally imaged in late 2013 (Galicher et al. 2014). Overall, their images spanned four epochs, from December 2013 to March 2016, and provided astromteric measurements for HD 95086 b. From these measurements, the authors found that the orbit of HD 95086 b is face-on and circular. Examples of the direct imaging on the system are shown in Figure 1.

    To constraint the orbit of HD 95086 b, the authors used Monte Carlo (MC) techniques that were more efficient than a traditional Markov Chain Monte Carlo. The technique generated parameters from a probability density function, then fit the parameters of the orbits through each epoch. The probability of the orbits generated was then evaluated by comparing the remaining epochs against a uniform random variable, and the orbits were then accepted or rejected by the program. In Figure 2, the generated orbits are shown with the data points. Using the constraints for planet b in the system, the researchers constrained the HD 95086 system as a whole.

    3
    Figure 2: The model fitting for HD 95086, overlaid with the measurements, which are color coded by date taken, is shown in this image. The gray areas indicate the approximate location of the planetesimal belts in the system.

    In an earlier publication, (Su et al. 2015), several possibilities for the system architecture were presented, and in this paper, these possibilities are explored with the new analysis of data. The authors rule out several scenarios and present the most likely scenarios from their models. The scenarios are as follows:

    Scenario A: HD 95086 b is the only planet in the system and has carved out the large gap in the disk through an eccentric orbit of about 0.7. Verdict: Ruled out with 95% confidence. Astrometric measurements from this paper do not indicate such a high eccentricity

    Scenario B: HD 95086 b has a slight eccentricity of about 0.3, and another more massive, more eccentric planet resides at 16 AU. Verdict: Neither ruled out nor confirmed. The 16 AU planet, if it were there, is undetectable by GPI, and this scenario is not constrained by the observations so a verdict for this one can’t quite be reached.

    Scenario C: Two other planets, slightly larger than HD 95086 b at about 7 Jupiter masses, orbit at 12 and 26 AU. All three planets in this scenario have low (0.3 or less) eccentricities. Verdict: Needs reconfiguration. With a low eccentricity at 26 AU, the planet would have been detected in the observations, but the inner planet at 12 AU would not have been. Its possible that the inner planet is more massive, so parts of this scenario could work with the observations. However, this scenario with two additional planets is unlikely.

    Scenario D: In addition to HD 95086, three large Jupiter-like planets, of all the same mass, orbit at 11, 19, and 34 AU, respectively. Verdict: Ruled out. The third planet, projected at 34 AU, would have been visible in observations if it were there, and this scenario would necessitate all three extra planets to account for the disk configuration.

    Certainly, these aren’t the only possibilities, but with the new analysis and data, the scenarios have been considerably confined. It is likely that HD 95086 has two massive planets that have sculpted out the gap in the disk, with one at a closer separation than is currently detectable by direct imaging. Another possibility is that three or four planets are present in the system, but their eccentricities and mass vary greatly, which could explain why they have not been detected by current observations.

    In HD 95086, planets have clearly disrupted the disk and the planetesimals, but the exact nature of this contact remains unknown, though now these researchers have found that a two-planet system of moderate eccentricity or possibly an inhomogeneous mix of three or fours planets. The interactions between the planets and the disk can reveal so much about how these systems form and evolve. There’s still some mystery left in the HD 95086 system, but the scientific sleuths of this papers have thoroughly narrowed down the possibilities.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 11:03 am on February 20, 2017 Permalink | Reply
    Tags: , Astrobites, , , , , Helioseismology, The Sun In A Distant Mirror   

    From astrobites: “The Sun In A Distant Mirror” 

    Astrobites bloc

    Astrobites

    Title: A Distant Mirror: Solar Oscillations Observed on Neptune by the Kepler K2 Mission
    Authors: P. Gaulme et al.
    First Author’s Institution: Department of Astronomy, New Mexico State University
    1
    Status: Published in The Astrophysical Journal Letters, open access

    1
    Figure 1: Snapshot of the line-of-sight velocity variations on the Sun’s surface, measured from the Doppler shift of atmospheric absorption lines. (Image: GONG/NSO/AURA/NSF)

    How can we learn what lies beneath the surface of a star? One approach, called asteroseismology, is to study a star’s vibrations to infer its internal structure. The inside structure of the Sun in particular can be studied in great detail, because its vibrations are actually apparent on its surface (Fig. 1, see also: Helioseismology): The visible pattern of surface quivers shows the imprint of global acoustic oscillations, which are caused by resonant waves traveling through the Sun on peculiar paths, probing various depths. The same must be happening in faraway stars, yet due to their distance only the variability of their overall properties can be measured, for instance changes in brightness or temperature, which are however similarly caused by the stars’ intrinsic oscillations.

    An impressive instrument that has been built specifically to measure the brightness of stars with extreme precision over time is the Kepler space telescope.

    NASA/Kepler Telescope
    NASA/Kepler Telescope

    Its primary aim is to detect the transits of exoplanets, but its features make it suitable for asteroseismology as well. The authors of today’s paper investigate how Kepler would see a Sun-like star from far away, by pointing it at Neptune.

    Methods

    The key idea of the paper is to determine the oscillation characteristics of the Sun by analyzing the intensity variations of the sunlight reflected by Neptune. This kind of measurement would allow a novel check of the calibration of widely used scaling relations, by observing the reference star – the Sun – with the same instrument as the actual target stars. These scaling relations are equations that connect the measurable (asteroseismic) quantities with the fundamental stellar properties, for example mass and radius.

    The main parameters of interest are the oscillation frequency of the Sun at maximum amplitude (\nu_{max}), which corresponds to the dominant “5-minute oscillation”, and the mean frequency separation between overtones (\Delta\nu), which are weaker oscillations that are also excited. The paper’s authors split up into seven teams to independently measure these parameters, all using slightly different methods of analysis. The underlying data, namely the light curve and its power spectrum (a decomposition of the light curve into frequency components), are treated just like the data of any other Kepler target (Fig. 2).

    2
    Figure 2: Left: The full Neptune light curve taken with Kepler, showing the intensity variations of the reflected sunlight over 49 days in 1 minute intervals. Right: The gray and black lines are the raw and smoothed power spectrum of the Neptune light curve. The solid red line is the best-fit model, which includes several noise components indicated by the dashed red lines. The main signal due to the 5-minute oscillations of the Sun appears at the bottom right of the plot, around 3100 μHz. For comparison, the green line shows simultaneous VIRGO data (see text). The blue peaks are caused by Neptune’s rotation. (Figure 1 from the paper.)

    Results

    Surprisingly, all teams consistently overestimate the mass and radius of the Sun significantly by about 14% and 4%, assuming the standard solar reference values. However, this discrepancy can be explained by comparison with another, simultaneous light curve that was taken with the dedicated Sun-observing instrument VIRGO (on board the SOHO satellite).

    ESA/NASA SOHO
    ESA/NASA SOHO

    The true value of \nu_{max} was larger than usual during the time of observations, simply due to the random nature of the Sun’s oscillations.

    In addition, the teams attempted to determine not just the frequency spacing, but also the heights and widths of the individual overtones (“peak-bagging”), to create a complete model of the observed oscillation spectrum. The results are rather uncertain due to noise, but the findings of the teams are generally consistent, and they agree well with the VIRGO measurements, after differences in the technical design have been taken into account (e.g. different bandpasses).

    The successful indirect detection of the Sun’s acoustic oscillations in intensity measurements, with Neptune as “a distant mirror”, is a marvelous technological achievement. Not only that, but the lessons learned from this experiment will help further explore the limits of high-precision asteroseismology with Kepler.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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 2:54 pm on February 17, 2017 Permalink | Reply
    Tags: A lander for Europa, Astrobites, , , ,   

    From astrobites: “Move over Philae, we have a new lander in town” 

    Astrobites bloc

    Astrobites

    Feb 17, 2017
    Amber Hornsby

    Title: Europa Lander Study 2016 Report
    Co-chairs of Science Definition Team (SDT): J. Garvin, A. Murray and K. Hand.
    First Author’s Institution: NASA Goddard Spaceflight Center, Greenbelt, Maryland, United States
    nasa-goddard-bloc-large
    Status: open access

    Europa is the smallest of the Galilean moons orbiting Jupiter, but it’s potentially one of the most exciting bodies in our diverse Solar System. Why? With oceans of liquid water hiding underneath its icy crust, Europa is an essential target in the search for life beyond our pale blue dot. To uncover the secrets of this mysterious moon we must take one giant leap for aliens everywhere and get to grips with its poorly understood surface.

    Today’s bite will be going beyond the extremely cool science goals of a recently proposed Europa lander, and focusing on one big question. How do we successfully land on a moon?

    Previous landings

    Since the beginning of the space race, we’ve sent probes to study the dense Venetian atmosphere, rovers to traverse the rusty terrain of Mars and have even bounced on the duck-shaped comet 67P/Churyumov–Gerasimenko. There have been many triumphs and failures during our attempt to explore the Solar System (thankfully someone has been keeping score – see Interplanetary Lobbing), but how good are we at landing on moons? So far we’ve had many successful landings on our own Moon, however we have only managed to land on one other moon – Titan (Saturn’s giant moon). Do not fear, as we have also landed on Mars, Venus and even two asteroids. On to our next concern – how do we get to Europa?

    Woah! We’re going to Jupiter *

    2
    Figure 2: Suggested Earth-Jupiter journey. Launch in 2025, Deep Space Manuever (DSM) in 2016, Earth Gravity Assists (EGA) in 2027, Jupiter Orbit Insertion (JOI) in 2030. (Figure 10.4 of report)

    Getting to Jupiter is relatively easy, but putting something in orbit around the giant planet presents more of a challenge. In 1979 it took Voyager 1 546 days to reach Jupiter, but sadly a spacecraft travelling that fast will just fly right on by. To place a spacecraft in orbit around a planet we must take a slower, indirect journey which requires gravity assists. If launched in October 2025, the Carrier and Relay Orbiter (CRO) will travel out towards the asteroid belt before being drawn back towards the Earth because of the gravitational pull of the Sun (DSM – Deep Space Manuever). By December 2027, a close encounter with the Earth (EGA – Earth Gravity Assist) will propel the CRO out to Jupiter’s orbit by July 2030 (JOI – Jupiter Orbit Insertion). Once the JOI is complete, the CRO will require several more gravity assists from the Galilean moons to achieve an orbit around Europa, adding a further 18 months to the journey. Now we’re in orbit, how do send a lander to the surface?

    Woah! Now to the surface *

    3
    Figure 3: Landing on Europa via the use of a sky crane (Figure 10.6 in report)

    There are two common methods used to place a lander on an extra-terrestrial body which require the use of airbags and/or lander legs. Often scientists make use of the atmosphere and deploy a parachute to slow the craft down – clearly not an option for atmosphere-less Europa. Next a lander will either employ airbags for protection upon impact (limiting the payload) or will use rocket thrusters to control the descent before landing on legs (could pollute the local environment), but neither method was suitable for Curiosity – the most recent successful mission to Mars. Instead NASA used a sky crane to lower the rover to the surface and scientists intend to implement this method again on Europa. There are several reasons for this, but an important one is that it allows for a hazard detection/avoidance system to be put in place. Landing on Europa must utilize a fully automated process because it takes too long to communicate with the CRO. This means the lander must be able to adjust its own course to ensure a successful touchdown on a surface we know so little about, which introduces the next issue – where do we land?

    Mysterious surface of Europa

    Multiple flybys of Europa have been undertaken courtesy of Pioneer 10 & 11, the Voyager probes and most recently the New Horizons mission, however it was the Galileo satellite in 2003 which has provided the most detailed images to date. As groundbreaking as these images are, we would struggle to place a lander in a safe place of scientific interest without more information.

    5
    Figure 4: Some of the best images we have of Europa taken by the Galileo satellite. Credit: NASA

    Introducing the Europa Multiple-Flyby Mission! NASA recently selected nine instruments for a future mission to Europa, which could be launched as early as 2020, including cameras and spectrometers for high resolution images of the surface and an ice penetrating radar to determine the thickness of the icy crust. This mission intends to perform 45 flybys of Europa, allowing scientists to identify candidate sites for the lander. If you’re interested in planetary science this could be quite an exciting opportunity, as the SDT intends to host an open, inclusive and publicly visible landing site selection process (see Figure 6.7 of the report).

    Conclusion

    Having visited comets, planets and other moons, landing on Europa presents one of the best opportunities for discovering extra-terrestrial life. As with all space missions, a Europa lander will require thousands of hours of work by dedicated scientists for the next 8 years and that’s just to get it to the launch pad. If this little spacecraft can survive a launch, several gravity assists and the small matter of a soft landing, it has the potential to test new technology, revolutionise astrobiology and improve our understanding of an unexplored moon. The mission to land on Europa is still in the early stages of development, but combined with the more mature Europa Multiple-Flyby Mission, it seems likely the proposed lander could become a fully fledged, physical craft in the near future.

    • I’m really sorry, but I’ve been revisiting the 90s

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    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.

     
c
Compose new post
j
Next post/Next comment
k
Previous post/Previous comment
r
Reply
e
Edit
o
Show/Hide comments
t
Go to top
l
Go to login
h
Show/Hide help
shift + esc
Cancel
%d bloggers like this: