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  • richardmitnick 2:26 pm on June 27, 2017 Permalink | Reply
    Tags: Astrobites, , , , , The Grass Might Be Redder on the Other Side   

    From astrobites: “The Grass Might Be Redder on the Other Side” 

    Astrobites bloc

    Astrobites

    27 June 2017
    Shang-Min Tsai

    1
    Artist’s impression of a habitable exoplanet covered in vegetation. A new study explores a spectral signature that might indicate the presence of life on an exoplanet. [Ph03nix1986]

    Title: Natural and Artificial Spectral Edges in Exoplanets
    Authors: Manasvi Lingam, Abraham Loeb
    First Author’s Institution: Harvard-Smithsonian Center for Astrophysics

    Status: Submitted to ApJL, open access

    The explosion in the number of discovered exoplanets — especially some interesting systems with terrestrial planets in the habitable zone — has attracted a lot of attention. We are moving one step closer to the ultimate question: are we alone? Today’s paper looks at certain distinctive spectral features that could be caused by “extraterrestrial plants”, or even crazier: advanced civilizations.

    The Sky Is Blue and All the Leaves Are Green

    Ever wondered why most plants look green? The first answer you might get is because of chlorophyll, the green pigments responsible for photosynthesis. Plants carry out photosynthesis to convert water and CO2 into sugar and oxygen, using energy from the Sun. But one might further ask: why is chlorophyll green? Well, chlorophyll absorbs light primarily in the range from ~450 nm (blue) to ~650 nm (red). It operates in the visible spectrum range but is not as efficient in green light. So in visible light, the photons at green wavelength are reflected the most, producing the color that we see. This causes the small bump of the leaf reflectance near 500 nm (0.5 μm) in Figure 1. Note the sharp jump of reflectance starting around 0.7 μm and going into the infrared. This so-called “red edge” can be a useful feature for detecting vegetation on planets, since few substances in nature have such high reflectivity in that wavelength range. The strength of the red-edge feature is used on Earth to monitor the growth of vegetation (such as crops). Imagine if our eyes were a little more sensitive toward red; we would see the world very differently with plants turning red (and much brighter)!

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    Figure 1. The reflectance R for silicon-based solar cells (black) and plants (red), shown as a function of wavelength λ. The peaks in the reflectivity in the UV region of silicon and at 0.7 μm of plants are the distinct “spectral edges”. [Lingam & Loeb 2017]

    We Need More Energy!

    In today’s paper, the authors also boldly explore the possible “artificial spectral edges” — that is, advanced civilizations modifying the planet surface such that it changes the observable spectra as well. It is conceivable to assume that advanced civilizations would come up with a method to handle energy crises. One possible way is to harness a significant amount of energy from the star by constructing large arrays of solar cells. This is particularly relevant for tidally-locked planets around M-stars, such as Proxima b, where the dayside is permanently illuminated. The solar cells are made of semiconductors (typically silicon), which have an energy gap between the valence band and the conduction band. Photons with energies less than the band gap are scattered, causing high reflectance, similar to plants but at a shorter wavelength in UV. The authors explored a hypothetical scenario in which planets are covered with mega-scale arrays of solar cells, showing the reflectance for silicon-based solar cells in Figure 1.

    2
    Figure 2. Schematic illustration of terraforming on tidally locked exoplanets. Photovoltaic arrays on the day side are used to harness stellar energy, which is redistributed as heat and light on the night side. [Lingam & Loeb 2017]

    Another similarity between natural vegetation and solar cells is that, on tidally locked planets, they most likely are only situated on the day side (see the schematic in Figure 2). Therefore, as the fraction of vegetation or solar cells varies during the orbit, the changes of photometric flux in different wavelengths could be analyzed to characterize the spectral features. The authors calculated the change in the reflected light contrast to be within the sensitivity of future telescopes, like WFIRST (10-3 ppm) and LUVOIR (10-4 ppm), provided that (i) the coverage is large enough, (ii) the viewing angle is favorable, and (iii) the cloud cover is limited.

    Of course, this is not saying we are going to find extraterrestrial life tomorrow, but it is helpful to keep in mind the possible information hidden in the reflected light. After all, the last thing we want is to see the sign, yet miss it.

    See the full article here .

    Please help promote STEM in your local schools.

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    What do we do?

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

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 1:55 pm on June 26, 2017 Permalink | Reply
    Tags: , Astrobites, , , , , Fomalhaut debris disk   

    From astrobites: “A New Glow in the Eye of Sauron” 

    Astrobites bloc

    Astrobites

    Jun 26, 2017
    Mara Zimmerman

    Title: A Complete ALMA Map of the Fomalhaut Debris Disk
    https://arxiv.org/pdf/1705.05867.pdf
    Authors: Meredith Macgregor, Luca Matrá, Paul Kalas, et al.
    Lead Author’s Institution: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

    Status: Accepted to ApJ [open access]
    _________________________________________________________________________

    The Fomalhaut debris disk is one of the most recognizable circumstellar disks—mainly because images of it bear an uncanny resemblance to a certain tyrant of Middle Earth.

    1
    An image of the debris disk, showing optical detection of dust (Source: Kalas et al. 2005).

    The Fomalhaut system contains a dusty-debris disk, which gives the images their distinctive glowing eye appearance. Because of its proximity and unusual features, this debris disk is quite well studied, but, as the authors of this paper demonstrate, there is still much more we can learn from Fomalhaut. It has been shown to have a companion, Fomalhaut b, on a highly eccentric orbit. The interaction between Fomalhaut b and the debris disk is what makes this system so intriguing; when Fomalhaut b was first discovered, it was thought to be a massive planet, however new observations in today’s paper cast doubt on this hypothesis. Given Fomalhaut b’s lack of brightness in the optical and unusual brightness in the infrared, the authors of this paper suggest that Fomalhaut b is likely a giant dust cloud.

    In this paper, the authors use observations from the ALMA (Atacama Large Millimeter/submillimeter Array) observatory to reveal the structure of the Fomalhaut disk, and observe an unusual glow in the disk corresponding to the apocenter of the system.

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

    4
    Figure 2: These ALMA images of Fomalhaut show the apocenter glow. The difference in brightness between the North-west (apocenter) and South-east (pericenter) sides reflects the surface density distribution of the disk. The image on the left is just the ALMA image while the image on the right is corrected by the HST images. On the right, the apocenter glow is clearly visible.

    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 12:32 pm on June 20, 2017 Permalink | Reply
    Tags: Astrobites, , , , , Galactic Archaeology of the Sagittarius Stream   

    From astrobites: “Galactic Archaeology of the Sagittarius Stream” 

    Astrobites bloc

    Astrobites

    June 20, 2017
    Nora Shipp

    Title: The star formation history of the Sagittarius stream
    Authors: T.J.L. de Boer, V. Belokurov, S. Koposov
    First Author’s Institution: Institute of Astronomy, University of Cambridge, UK

    Status: Published in MNRAS, open access

    Small satellite galaxies and star clusters orbiting the Milky Way are often torn apart by its gravitational potential, leaving behind trails of stars stretched out across the sky (Figure 1). These stellar streams (more info in this astrobite!) contain information about the history of their parent object and can be studied to learn about the evolution of the Milky Way in an approach referred to as “Galactic Archaeology.” Like archaeologists who study ancient history on Earth, Galactic archaeologists search for present-day remnants of historical events. The spatial structure and the properties of the stars that make up these stellar streams tell a story of the parent object’s evolution and eventual destruction.

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    Figure 1. This diagram shows the stream of stars left behind as the Sagittarius dwarf galaxy is torn apart by the gravitational potential of the Milky Way. Long tails extend in either direction from the parent galaxy (labeled Sgr. core), and stretch across the northern and southern skies, wrapping in a full circle around the Milky Way. Source: David R. Law, UCLA

    To study the star formation history of a stellar stream, scientists compare data to theoretical models of stellar populations. Stellar evolution is one area of astrophysics that is fairly well understood, so detailed predictions can be made about how stars evolve throughout their lifetime. In particular, stars that form at the same time in the same environment will have the same composition and the same age, but will also have a wide range of masses that will trace out a distinctive curve through stellar brightness and color referred to as an isochrone (meaning equal age). (Figure 2 illustrates several isochrones as a stellar population evolves.) Comparing these isochrones to the observed groups of stars can determine how long ago these stars formed and their approximate composition, and can therefore tell us about how the population of stars and the environment in which they form changes throughout history.

    3
    Figure 2. These lines illustrate how a population of stars evolves with time. The y-axis is brightness and the x-axis is temperature which directly corresponds to color. Source: Casanellas et al. 2011

    In today’s paper, de Boer and collaborators study the star formation history of different regions of the Sagittarius stream. This stream is the largest and brightest known stream in the Milky Way system, and is the remnant of the Sagittarius dwarf galaxy, one of the largest satellite galaxies orbiting the Milky Way.

    4
    Image Credit: R. Ibata (UBC), R. Wyse (JHU), R. Sword (IoA). http://annesastronomynews.com/annes-picture-of-the-day-the-sagittarius-dwarf-elliptical-galaxy/.

    The authors are especially interested in a region where the stream is split into two – a brighter half and a fainter half. Do these two regions come from the same place? Did they form stars at the same time and in the same environment? And if they are in fact both remnants of the Sagittarius dwarf galaxy, what causes their difference in appearance?

    The authors observe stars belonging to the Sagittarius Stream and then determine the model that most accurately describes them. This method reveals the evolution of the stellar composition in the bright and faint regions over time and provides clues to several facts about the ancient history of the stellar stream. (Figure 3 shows the star formation as a function of time in the two regions.)

    5
    Figure 3. The star formation history of the Sagittarius Stream. The y-axis is the composition, and the x-axis is the age of the observed stars. The color indicates the star formation rate – red regions were times of rapid star formation and blue regions were slower. In the bright region (left), two times of rapid star formation are visible – at an age of about 7 billion years and an age of about 12 billion years. Star formation cuts off around 5 billion years. In the faint region (right), the overall star formation rate is much lower and stars stopped forming earlier. Source: Figures 5 and 9 in the paper.

    First, the authors confirm that both parts of the stream do in fact come from the Sagittarius dwarf:

    They discover that the stars from both the bright and faint regions follow a tight relation between age and composition, meaning that at any given time the stars being formed had a similar composition and formed in a homogeneous environment. They also find that this relationship is similar to that of stellar clusters known to belong to the Sagittarius dwarf, confirming that both the bright and faint parts of the stream were stripped from the galaxy.

    Second, they find evidence of when the destruction of the dwarf began:

    The ages of stars in the two parts of the stream span about 8 billion years and then have a cut-off that indicates that star formation in the Sagittarius dwarf stopped about 5 billion years ago. This sudden halt of star formation was probably caused by the Sagittarius dwarf being torn apart!

    Third, they discover differences between the two parts of the stream:

    Even though star formation in the Sagittarius dwarf appears to have ended only 5 billion years ago, the fainter part of the stream has very few stars formed less than 10 billion years ago. These stars also have a simpler composition. This means that this part of the stream comes from a region of the dwarf galaxy in which star formation stopped earlier before more complex stellar populations were able to develop. This could mean it was stripped earlier than the bright part of the stream and only from the outskirts of the galaxy.

    Observations of the stars in the Sagittarius stream were therefore able to provide clues to the history of star formation and the destruction of the Sagittarius dwarf galaxy. They revealed the origin of the Sagittarius stream, how many billions of years ago different parts of the stream were torn apart from their parent galaxy, and how the composition of these stars changed over the 8 billion year history of star formation in the Sagittarius dwarf galaxy. All of this is only the beginning of Galactic Archaeology, an exciting field that involves taking present-day artifacts of stellar systems and revealing secrets of their complex history spanning billions of years.

    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 11:38 am on June 19, 2017 Permalink | Reply
    Tags: Astrobites, , , , , Could we detect Europa-quakes?   

    From astrobites: “Could we detect Europa-quakes?” 

    Astrobites bloc

    Astrobites

    June 19, 2017
    Kerrin Hensley

    Title: The seismic noise environment of Europa
    Authors: Mark P. Panning, Simon C. Stähler, Hsin-Hua Huang, Steven D. Vance, Sharon Kedar, Victor Tsai, W. T. Pike, Ralph D. Lorenz
    First Author’s Institution: University of Florida
    1
    Status: Submitted to the Journal of Geophysical Research — Planets, open access

    Early images of Europa, Jupiter’s fourth-largest moon, revealed an icy surface scarred with reddish stripes. Later observations by the Galileo spacecraft of distortions of Jupiter’s magnetic field near Europa hinted at the presence of a global water ocean beneath the ice shell. But how thick is the ice shell? And just how deep is the ocean? A Europa lander equipped with a seismometer might be able to answer these questions. A seismometer measures how much the ground moves as a result of seismic activity like earthquakes or volcanic eruptions. In this paper, the authors use statistical models of seismic activity and thermodynamical models of planetary interiors to estimate the seismic noise on Europa. With these estimates, we can begin to set basic requirements for the instruments that could one day measure the seismic rumbles of Europa’s ice shell.

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    Figure 1. An image of Europa’s surface captured by Galileo. Credit: NASA/JPL-Caltech.

    Europa-quakes?

    Europa’s orbit around Jupiter is slightly elliptical, so the moon is subjected to tidal stresses of varying strength throughout its orbit. Models of the tidal stress on Europa indicate that it should induce cracks in the ice shell. These tidal cracking events, as well as turbulence of the ocean beneath the ice shell, will produce seismic noise. But how much?

    To estimate the amount of ambient seismic noise on Europa, the authors use a statistical model of seismic events to determine how much seismic energy is released through tidal cracking. On Earth and the Moon, seismic events follow the Gutenberg-Richter law: a relationship between the magnitude of a seismic event and the number of events of equal or greater magnitude. By scaling the parameters observed on the Moon, the authors create a catalog of seismic events on Europa. They then uniformly distribute the mock events across Europa’s surface, and at random depths down to 2 km below the surface. Using this catalog and a model of how seismic waves propagate from the sources to a hypothetical detector placed at one of the poles, they generate a seismic record from which they can derive the magnitude of the ambient seismic noise. A sample week-long noise record is shown in Figure 2.

    3
    Figure 2. A sample noise record (top) and event catalog (bottom) for one week in model time. Magnitude is logarithmic, so the noise record is vastly dominated by the few large events. The much more frequent smaller events make up the background noise. (Figure 3 from the paper.)

    The authors then use a signal processing tool to transform event catalogs and noise records like those shown in Figure 2 into a power spectrum (distribution of the power of a signal as a function of period) for the seismic noise. Figure 3 shows the noise estimates for four different models of Europa with different ice shell thicknesses and internal structures. In most cases, the seismic noise on Europa is below the sensitivity limit (dashed lines) of the detectors available today. However, some models show that the more sensitive instruments will be able to detect the average ambient seismic noise, and will thus certainly be able to detect much larger, but more infrequent, events.

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    Figure 3. Power spectra for noise models with four different sets of parameters. The Q value is related to how strongly the waves decay as they pass through the ice (low Q means a high rate of decay). This figure shows that for most models (solid colored lines), the noise level is below the sensitivity limit of most instruments (dashed lines), although the most sensitive instruments will be sufficient in some cases. (Figure 5 from the paper.)

    Probing the depths of Europa’s ocean

    Even in absence of larger tectonic events, the ambient seismic noise could be used to determine the depth of the ocean using a technique called autocorrelation. Autocorrelation links a particular event to other, related events in a series that are separated by a fixed interval. In this case, the related events are the sequential signals received as a seismic wave repeatedly reflects off the bottom of the ocean. In this work, the authors show that autocorrelation of the seismic background is a feasible, albeit challenging, way to glean information about Europa’s global ocean without depending upon larger and less frequent seismic events.

    Ultimately, the models are highly sensitive to input parameters and the results could depend strongly on conditions that haven’t yet been explored fully, like scattering of seismic waves by a layer of regolith. Europa enthusiasts itching to use seismology to crack the case will have to wait; there is no Europa lander mission currently underway, although NASA recently solicited input from the scientific community about possible instruments for a future mission. For now, we’ll have to settle for the promise of two missions set to launch in the 2020s: the European Space Agency’s JUpiter ICy moons Explorer (JUICE) and NASA’s Europa Clipper.

    ESA/Juice spacecraft

    NASA Europa Clipper

    Featured image courtesy of NASA/JPL-Caltech/SETI Institute.

    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 6:30 am on June 17, 2017 Permalink | Reply
    Tags: Astrobites, , , , , Planetesimals, , Watermelon Dust is the Best Dust: Forming Planetesimals Near the Snow Line   

    From astrobites: “Watermelon Dust is the Best Dust: Forming Planetesimals Near the Snow Line” 

    Astrobites bloc

    Astrobites

    June 16, 2017
    Michael Hammer

    Title: Planetesimal Formation near the snow line: in or out?
    Authors: Djoeke Schoonenberg and Chris Ormel
    First Author’s Institution: Anton Pannekoek Institute for Astronomy, University of Amsterdam
    1
    Status: Published in A&A [open access]

    How is it possible for planets to exist? Even though we know planets must have formed from planetesimals that are tens of kilometers in size, the most basic models of protoplanetary disks have trouble forming planetesimals from the micron to centimeter-sized dust that populates these disks. For dust particles to grow into planetesimals, they need to be able to clump together enough to reach roughly the same level of concentration as the gas in the disk – which can be difficult since there is 100 times more gas than dust.

    A few months ago, I wrote an Astrobite describing a simple model that naturally achieves the conditions needed to form planetesimals in the inner disk, thereby offering a way to form planets in the inner solar system like Earth and Mars. However, this model leaves out the gas giant planets in the outer solar system like Jupiter!

    Fortunately, beyond the snow line at about 2 AU, the disk will get cold enough for water vapor to condense into solid ice. It is already widely accepted that this extra ice will enhance the concentration of solids enough to form planetesimals and in turn, planets in the outer disk. Yet, the most detailed models of this scenario leave out enough relevant effects that we cannot reliably determine the impact of this extra ice.

    2
    Rough sketch which shows the sharp increase of solid surface density at the snow line, by a factor of ~3-4. [https://ay201b.wordpress.com/the-snow-line-in-protoplanetary-disks/]

    Djoeke Schoonenberg and Chris Ormel, the authors of today’s paper, set out to improve our understanding of whether the snow line can trigger the formation of planetesimals by creating a more rigorous model that better captures the dynamic structure of the disk and of the dust grains themselves.

    Model Upgrades with a Side of Fruit

    Schoonenberg and Ormel develop a steady-state model of how the ice beyond the snow line evaporates as it moves inwards and how some of the water vapor inside the snow line condenses as it moves outwards.

    My favorite part of their model is that they factor in the structure of individual “icy dust” grains. Many studies of protoplanetary disks often leave out what the mass in the disk is made of – instead, only tracking a distribution of density across the disk or nondescript particles. However, not only do Schoonenberg and Ormel describe each icy dust grain as 50% silicates (the typical composition of a regular dust grain) and 50% ice; but they also establish how the silicate “seeds” in each grain are divided up. As shown in Figure 1, a dust grain can either have a single silicate core like an avocado (their “single-seed” model), or it can have many smaller seeds evenly distributed through the grain like a watermelon (their “many-seeds” model).

    The authors run separate models for each type of dust grain. They then solve for the enhancement of ice and the solid-to-gas ratio to see if these values reach a high enough concentration to form planetesimals.

    3
    Figure 1. Structures of icy dust grains. In the avocado model, each grain has a single silicate core surrounded by a shell of ice. In the watermelon model, each grain has a bunch of smaller silicate seeds evenly distributed through the ice. When an avocado dust grain evaporates, it slowly loses only its ice before leaving its core behind. When a watermelon dust grain evaporates, it slowly loses both its ice and its silicate seeds together as it drifts inward. Adapted from Figure 1.

    Forming Planetesimals Early and with Watermelon

    Besides varying the structure of the dust grains, the authors also experiment with different disk viscosities, dust particle sizes, and a few other variables. In particular, they find that higher viscosities are best-suited for producing planetesimals. Since these higher viscosities are more likely to occur early on in a disk’s lifetime, this suggests that disks can form planetesimals right away!

    In the single-seed “avocado” model with the optimal higher viscosities, Schoonenberg and Ormel find that the concentration of ice can be enhanced by a factor of 3 to 5 just beyond the snow line (see Figure 2). Interestingly with the many-seeds “watermelon dust” model, they find that this enhancement can double! This occurs because in the single-seed model, only the extra ice from inside the snow line contributes to the enhancement. However, in the watermelon model, the small size of the watermelon seeds plays a key role. Since the seeds are so small, they get carried by the gas that makes up most of the disk. When some of the gas from inside the snow line moves outward, some of the watermelon seeds follow it outward. As a result, many of these watermelon seeds end up captured in icy dust grains and also contribute to the extra ice, which doubles the enhancement.

    Ultimately, the authors expect the ice enhancement in an actual disk to be in-between the results from the avocado and watermelon models due to the fact that both structures of dust grains are plausible, and also because other dust grains may be a “hybrid” of both fruits and have a large silicate core as well as additional smaller silicate seeds in the outer icy shell.

    4
    Figure 2. Solid-to-gas ratio near the ice line (blue vertical line). Solids are ice plus dust. The many-seeds model safely exceeds the threshold (orange horizontal line) to form planetesimals. Since the amount of solids is just an average at a given radius, the single-seeds model should also be able to form planetesimals. Adapted from Figure 5.

    Summary

    The ice enhancement of about ~7 that the authors find is ten times lower than the enhancement of 75 found with a simpler model, emphasizing the importance of considering the more intricate details of the problem! Thankfully, this enhancement is still high enough for the concentration of large dust grains to reach the concentration of gas and produce planetesimals (see Figure 2). More importantly, the authors find that the best conditions for forming planetesimals happen soon after a star and its disk are born, supporting observational evidence that giant planets in the outer disk can form quickly.

    Lastly, Schoonenberg and Ormel expect the cm-sized ice pile-up near the snow line to be detectable by radio (cm-wavelength) telescopes. Seeing this feature in a real disk would be the best test for finding out how well we understand planetesimal formation near the snow line.

    Featured Image Credit: Luis Calçada

    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 7:00 am on June 15, 2017 Permalink | Reply
    Tags: Astrobites, , , , Clouds over the sunlit arch, , , WASP-12 b   

    From astrobites: “Clouds over the sunlit arch” 

    Astrobites bloc

    Astrobites

    Jun 15, 2017
    Eckhart Spalding

    Title: High-temperature condensate clouds in super-hot Jupiter atmospheres
    Authors: H.R. Wakeford, C. Visscher, N.K. Lewis, T. Kataria, M.S. Marley, J.J. Fortney, A.M. Mandell
    First Author’s Institution: Planetary Systems Lab, NASA Goddard Space Flight Center

    Status: Published in MNRAS [open access]

    Introduction

    Having grown up on Earth, we tend to associate cloudy skies with brisk weather. Even gloomy weather. As Robert Frost said, if it’s the month of May and a “cloud comes over the sunlit arch” you’ll find yourself “two months back in the middle of March”. How different are clouds on alien worlds? Is there such a thing as searingly hot clouds, suspended high above in skies so bright they make your eyes ache when you shut them tight?

    If we want to study alien clouds outside our own solar system, “hot Jupiter” planets are our best test subjects. Hot Jupiters are gas giant planets which orbit very close to their host stars. Their atmospheres are relatively easy to characterize if they pass directly in front of their host star [transit], which allows light from the host star to pass through their extended atmospheres and provide us with transmission spectra.

    Planet transit. NASA/Ames

    In this light (no pun intended), the physics of clouds have experienced a recent surge of interest in the exoplanet field. Clouds play critical roles in an atmosphere’s energy balance: they can reflect light back into space, trap heat, remove absorbing compounds, influence wind structure and chemical reaction times, and exert feedback effects on atmospheric temperature-pressure profiles. Clouds also complicate measurements of chemical abundances, because clouds tend to dampen absorption features and can transmit photons as an unknown function of wavelength. (Remember when you got sunburned on a cloudy day?)

    Clouds in exoplanet atmospheres are a little bit like turbulence or magnetic fields in star formation– they add a rich level of physics to what would otherwise be a much simpler physical picture. But the first strong evidence of thick cloud decks on exoplanets dates only from 2014, and we still don’t know much about the nature of those clouds. (In fact we’re still working on Earth clouds. Earth is, after all, a little-known planet.)

    Today’s paper

    The authors of today’s paper push the boundaries of knowledge into the most scalding area of parameter space: the realm of sizzling “super-hot” Jupiters, with temperatures of more than 1800 K. Part of the authors’ motivation is to address a longstanding puzzle. The temperature in most planetary atmospheres should become colder at higher altitudes. But according to models, hot Jupiters with very high incident flux levels are expected to harbor high-altitude atmospheric absorbers like TiO or VO. If these compounds are present, they will cause a “temperature inversion” where temperature actually rises with altitude. Strangely, however, these compounds or temperature inversions have been difficult to find in hot Jupiters.

    2
    Fig. 1: The transmission spectrum of WASP-12 b. The y-axis represents the effective “thickness” of the atmosphere at a certain wavelength. Actual data are in the form of black data points, and pure condensate curves are in orange and green. The relative flatness on the left side of the plot indicates the presence of clouds. Before the James Webb Space Telescope (JWST) fills in the right side of this plot, the condensate models cannot be well distinguished. (Fig. 5 from today’s paper.)

    NASA/ESA/CSA Webb Telescope annotated

    The authors begin by considering the hottest condensates that may exist in a hot Jupiter atmosphere: calcium (Ca), titanium (Ti), and aluminum (Al). They assume cloud formation occurs when rising air reaches vapor pressure saturation, and that clouds stop forming when one of the constituent elements is depleted. They calculate the resulting relative cloud masses, play around with the atmospheric metallicities, and overlay temperature-pressure curves on condensation curves to see what types of clouds should form. Among their findings, Al compounds will form much more cloud material than Ti, and higher atmospheric metallicity levels allow the formation of more massive clouds.

    The authors turn to the specific case of the planet WASP-12 b. The transmission spectrum of WASP-12 b exhibits some water absorption but is otherwise fairly flat, which indicates the presence of clouds.

    3
    Fig. 2: Left: Solid colored lines represent temperature-pressure curves in different regions of WASP-12 b.

    4
    Hubble Finds a Star Eating a Planet. Artist’s concept of the exoplanet WASP-12b. Credit: NASA/ESA/G. Bacon

    NASA/ESA Hubble Telescope

    Dashed lines represent condensation curves for different compounds. Clouds will form between the temperature-pressure and condensation curves. For example, iron (Fe) clouds will exist on the night side at altitudes corresponding to pressures in bars (P) below log(P) = -1.5. Pressures accessible to transmission spectroscopy are between about log(P) of -1 to -4. (Note that pressure decreases as the y-axis increases.) Right: An alternative representation showing the effect of distance from the substellar point. Clouds will form on the nightside side of the lines. Vertical dashed lines show the regions between day and night sides probed by transmission spectroscopy. (Fig. 4 from today’s paper.)

    After doing some more model calculations, the authors overlay condensation curves on a 2D plot of temperature as a function of pressure and longitude. Interestingly, the clouds roasting in WASP-12 b’s atmosphere start to form right around the regions accessible to transit spectroscopy. At temperatures of less than 1900 K, WASP-12 b’s clouds can shroud the signatures of TiO or “hide” its constituents via condensation.

    There still is much work to be done in understanding clouds among the worlds that “favor fire“. Condensation nuclei and condensate growth are very poorly constrained, and more lab work is needed to determine the optics of different compounds. But there is light on the horizon (again no pun intended). Certain absorption bands including Ti-O vibrations are within the discovery space of JWST, which may allow us to distinguish between WASP 12 b clouds which hide TiO or remove Ti from the gas phase.

    That would bring us full circle: after predicting the compositions of clouds on super-hot Jupiters, today’s paper has left us with a test observation to disentangle the effect of clouds on WASP-12 b. That test will get us another step towards resolving the mystery of the missing thermal inversions.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

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    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 5:52 am on June 15, 2017 Permalink | Reply
    Tags: Astrobites, , , , CGM - CircumGalactic Medium, , , It’s a gas gas gas: understanding gas motions surrounding galaxies, Keck Baryonic Structure Survey (KBSS)   

    From astrobites: “It’s a gas gas gas: understanding gas motions surrounding galaxies” 

    Astrobites bloc

    Astrobites

    Jun 14, 2017
    Christopher Lovell

    Title: A comparison of observed and simulated absorption from HI, CIV, and SiIV around z ≈ 2 star-forming galaxies suggests redshift-space distortions are due to inflows
    Authors: M.L. Turner, J. Schaye, R. A. Crain, G. Rudie, C.C. Steidel, A. Strom, T. Theuns
    First Author’s Institution: MIT-Kavli Center for Astrophysics and Space Research, Massachusetts Institute of Technology
    1
    Status: Submitted to the Monthly Notices of the Royal Astronomical Society, Open Access

    1
    Figure 1: a schematic of the observational measurement. The light from the distant QSO (quasar) passes near to a galaxy and interacts with its associated gas. The absorption is dependent on many things: the position of the gas along the line of sight (LOS), the distance from the galaxy (Transverse Distance, TD), whether it’s infalling or outflowing, and even its rotation.

    For galaxies, gas is a pretty big deal. Without it, they’re unable to form new stars — which is pretty much their only job. Once they get hold of some though, stars of all shapes and sizes start forming. Some of these will be massive, rapidly burning up their fuel and going supernovae. If the supernovae is energetic enough, this can eject a load of precious gas and shut down the star formation again. Understanding this precarious galactic balancing act, between inflows and outflows of gas, is crucial for modelling the properties of galaxies.

    Flowing me, flowing you

    One way of picking apart this relationship is to look at where the inflows and outflows of gas meet. This area is approximately a few megaparsecs outside the galaxy, roughly 8 times the distance from the center of the galaxy as the edge of the disc of the Milky Way, and is known as the CircumGalactic Medium, or CGM. The gas in these regions doesn’t shine brightly of itself, so we have to infer its presence through other sources of light. One method takes advantage of Quasi-Stellar Objects (QSOs or quasars for short) behind the galaxy: as the light from a quasar passes through the CGM it gets absorbed by neutral hydrogen and metals (astronomer speak for any element heavier than helium) in the gas, and we can see this in its spectrum (see Figure 1). The pattern of absorption can tell us what metals are in the gas, and, importantly for this work, its direction of flow with respect to the galaxy.

    Today’s paper uses observations from the Keck Baryonic Structure Survey (KBSS), which studies the gas around 854 star forming galaxies at a redshift of two (around 3 billion years after the big bang) using background quasars. The authors compare the observations with mock spectra from the EAGLE simulation, a computer model of galaxy formation and evolution that matches various galaxy properties. The mock spectra are designed to mimic KBSS as closely as possible, so that comparisons can be made between the two. For this study they measure the optical depth (which roughly corresponds to the amount) of three elements: neutral hydrogen, ionised carbon and ionised silicon. Figure 2 shows two dimensional maps of these elements in both KBSS and the simulations – they look pretty similar!

    2
    Figure 2: 2D maps of the amount of neutral hydrogen (HI), ionised carbon (CIV) and ionised silicon (SiIV) surrounding galaxies. The left column shows the observations, the right column shows the best fitting simulated galaxies. The bottom left corner of each panel corresponds to the position of the galaxy, and the x and y axes represent the transverse / line of sight distance from the galaxy, respectively.

    Tangled spectra

    Unfortunately, these measurements can get messy. Not only can the gas be inflowing or outflowing, but also rotating around the galaxy (see Figure 1). The host galaxy can also be moving with respect to the gas. All of this introduces uncertainties in the measured positions and velocities of the galaxy and its gas. The authors attempt to disentangle all of these effects, and find that the uncertainties on measurements of the galaxy distance (its redshift) have little effect: it’s the velocity of the gas that is important. They then try to pick apart its direction.

    In a simulation you know the motion of the gas directly, rather than having to infer it from a spectrum. The authors find that in EAGLE most of the gas is infalling, and since the mock spectra in the simulation are similar to the observations, the authors tentatively suggest that the gas in the observations could also be infalling. They also find that the more massive the dark matter halo hosting the galaxy, the higher the rate of infall.

    Dark matter halo Image credit: Virgo consortium / A. Amblard / ESA

    The optical depth is also insensitive to the amount of ‘feedback’ from supernovae in the simulation, supporting evidence for the idea that the higher observed gas densities are due to infalling, rather than outflowing, gas.

    Leaving on a simulation

    Today’s paper is a classic example of how simulations can help us understand our observations of the universe. By unpicking subtle features in the light of distant objects, which by chance happen to align with a galaxy, we can reveal the complicated relationships between galaxies and their surrounding gas. The authors also note that future studies of even greater detail could pick out other elements in the spectrum, such as Oxygen. These results are another step toward the ultimate goal of building a comprehensive model of galaxy evolution.

    See the full article here .

    Please help promote STEM in your local schools.

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    What do we do?

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

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 12:46 pm on June 13, 2017 Permalink | Reply
    Tags: Astrobites, , , , , , intermediate Palomar Transient Factory (iPTF), iPTF17cw, These aren’t the bursts you’re looking for   

    From astrobites: “These aren’t the bursts you’re looking for” 

    Astrobites bloc

    Astrobites

    Jun 13, 2017
    Amber Hornsby

    Title: iPTF17cw: An engine-driven supernova candidate discovered independent of a gamma-ray trigger
    Authors: A. Corsi et al.
    First Author’s Institution: Department of Physics and Astronomy, Texas Tech University, Texas
    1
    Status: Submitted to the Astrophysical Journal (ApJ) , open access

    Using the intermediate Palomar Transient Factory (iPTF), a broadlined type Ic supernova (Bl-Ic SN) was accidentally uncovered during follow up observations of the newest gravitational wave in town – GW170104.

    2

    7

    Further investigation of iPTF17cw suggests it is the first discovery of a candidate relativistic BL-Ic SN discovered independently of a gamma ray trigger. Today’s bite presents the discovery, classification and follow-up observations of this fascinating supernova.

    Coincidence?

    On the 4th of January 2017, the ripples in space-time created by two black holes merging were detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO).


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

    This detection resulted in a call to scientists around the world for follow-up observations, with the aim of locating an elusive electromagnetic (EM) counterpart. Observations undertaken by the extensive iPTF, a fully-automated survey measuring a region almost 40 times the size of the full moon, revealed a candidate event.

    3
    Figure 2: The location of iPTF17cw, indicated by the star, superimposed on the likely origin of GW170104 as determined by LIGO. Black contours indicate the 90% credible region. Figure 1 in the paper.

    Dubbed iPTF17cw, the event originated outside of the 90% localisation area of the gravitational wave, shown above in Figure 1, allowing scientists to quickly rule out the two events as being related.

    Follow-up observations

    With the gravitational wave scenario ruled out, scientists moved towards the event being a SN. At the time of its discovery this SN had a magnitude of R = 19.5 mag, but was not visible during observations of the same field taken in December. Therefore several follow-up observations of the source were made to work out what had caused this burst of energy.

    4
    Figure 3: Light curves created after several follow up observations of iPTF17cw. Several similar supernovae light curves are also plotted. Figure 3 in the paper.

    The authors note several interesting features in their observations including the detection of crucial narrow emission lines, such as those created by hydrogen, allowing for a redshift of z=0.093 to be calculated. Several broadline features were also observed, causing the team to classify the supernova as a type Ic SN. Type Ic SNe are a subclass of SN described as engine-driven, meaning they’re commonly associated with Gamma Ray Bursts (GRBs).

    In the gap

    GRBs are short-lived, energetic explosions of gamma-rays which are the brightest known events in the universe. Their origin, however, is not well understood, but the most likely cause is a star collapsing to form a neutron star or black hole – a supernova! GRBs are relativistic because they eject particles at speeds comparable to the speed of light; as a result, they fade on timescales of a few days, whereas the afterglow of a SN is known for its longevity, shining brightly for hundreds of years.

    The link between GRBs and broadline type Ic supernovae has been established for over twenty years, yet it is still uncertain why some supernovae emit jets of energetic particles. This jet-like behaviour is observed in the X-ray and gamma ray regime, meaning beams must be directed towards Earth to be observed. Moreover, a small portion of SNe discovered to have relativistic jets lack GRB components. It is unclear whether these events could be a new population of events that exist somewhere in the gap between SNe and GRBs, or if we’ve just missed key observations due to technological barriers. To establish whether iPTF17cw is relativistic and associated with a GRB, the team moved towards multi-wavelength observations of the surrounding area.

    Multi-wavelength observations

    Using the Very Large Array (VLA) observations of the iPTF17cw were made over a 3 month period, yielding a faint point-like radio source at 6 GHz. Because of its point-like nature, it’s unlikely to be caused by star formation occurring in the host galaxy.

    NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA

    5
    Figure 4: VLA observations of iPTF17cw, red point-like source in the centre, at 6 GHz. Figure 6 in the paper.

    The team then investigated the X-ray regime with the Swift Satellite and Chandra X-ray observatory to search for counterparts, but only detected X-rays with Chandra.

    NASA/SWIFT Telescope

    NASA/Chandra Telescope

    This was not a confident detection, yet its location is consistent with the optical and radio counterparts of the burst. Next, the Fermi and Swift catalogues were searched to find possible gamma-ray counterparts. A candidate match, a gamma-ray burst lasting 30s called GRB 161228, was uncovered.

    Analysis of multi-wavelength data suggests that GRB 161228 and iPTF17cw are likely to be related. The rate of Fermi GRBs falling within the region of GRB161228 was estimated to be 0.05 per month, meaning a chance coincidence of the events occurring but not being related has a probability of around 5%.

    Is this a new kind of SN?

    Most of the conclusions drawn for this SN rely heavily on comparisons with previous events, such as similarities in light curve shown in Figure 3. There are clear agreements between the engine-driven SN1998bw and relativistic SN2009bb, which further suggest iPTF17cw and GRB161228 are related.

    6
    Figure 4: VLA observations of iPTF17cw at 14 GHz. No detection was found. Figure 6 in the paper.

    Finally, this SN was not detected at 14 GHz radio frequencies, suggesting that the SN is relativistic because it faded very quickly at this more energetic frequency. This 14 would put this SN in a rare category: relativistic and discovered independently of gamma-rays. Follow-up observations with the VLA are crucial to confirm iPTF17cw’s relativistic nature by confirming that the SN has also faded at lower frequencies.

    Thanks to the iPTF many more BL-Ic SNe are now being discovered, and a greater sample will greatly improve our understanding of this weird phenomenon of engine-driven SNe. In fact the team expect to collect a sample of BL-Ic SNe in a year as large as the sample that has been collected over the last five years. It is only a matter of time before scientists conclude if a GRB counterpart is required, or if we have an extra category of events existing in the SN – GRB gap.

    Further investigation of iPTF17cw suggests it is the first discovery of a candidate relativistic BL-Ic SN discovered independently of a gamma ray trigger. Today’s bite presents the discovery, classification and follow-up observations of this fascinating supernova.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

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    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 2:19 pm on June 9, 2017 Permalink | Reply
    Tags: , Astrobites, , , , , The Role of Gender in Asking Questions   

    From astrobites: “The Role of Gender in Asking Questions” 

    Astrobites bloc

    Astrobites

    Jun 9, 2017
    Stacy Kim

    In your experience, what usually happens just after a talk, when the floor is opened for questions? A moment of awkward silence? A flurry of hands in the air? A question asked without a raised hand? Whatever ensued, one thing was (statistically) quite likely: it was a male who asked the first question.

    If you’ve thought that a lot of the questions asked after a talk were by males, you’ve made a perceptive—and accurate—observation. In 2014, a team of astronomers headed by James Davenport created an informal survey through which AAS 223 attendees recorded the gender of the speaker and of every questioner. While the survey only included male and female gender identifications, the authors recognized that a gender binary was false and marginalized those who didn’t identify with the two choices, and solicited feedback on how to address this limitation. They found that about 34% of those who’d registered for AAS 223 were female, and among the talks given at parallel sessions, females were well represented (making up about 31% of the talks). However, although males made up about 66% of those who registered for the conference, they asked 76% of the questions.

    The authors found other intriguing correlations. It appears the gender of the speaker doesn’t affect the gender ratio of questioners—males asked 74% of the questions after a talk by a female speaker, and 77% of the questions after a talk by a male speaker. As for the gender of the session chair? The gender ratio of the session chairs roughly matched that of those that attended AAS 223: 29% were female, and 71% were male (see Fig. 1). In sessions with female chairs, males asked 66% of the questions—matching the male attendance rate—and asked 80% of the questions in sessions chaired by males. Under the caveat that the sample sizes were small, the authors concluded that the gender of the chairs made the biggest difference. Investigating the reason for this trend was not feasible given how small their data set was, and the authors only put forth one example explanation: male session chairs may tend to ask the speaker a question.

    1
    Figure 1. The impact of the genders of the speaker and chair on the gender ratio of questioners. The two bars on the left show the gender ratio of those who asked questions to female speakers (“FS”) and male speakers (“MS”), while the bars on the left are the ratios under female chairs (“FC”) and male chairs (“MC”). The solid black line denotes the gender ratio of AAS 223, while the dashed black line denotes the gender ratio of all questioners. The gender of the speakers did not make a large difference in the gender ratio of questioners, but the gender of the chairs did. Figure generated from data in Davenport et al. 2014.

    This effort inspired similar studies at different conferences. A group of astronomers in the UK led by Jonathan Pritchard performed a similar study at the UK’s equivalent of the AAS meetings: the 2014 National Astronomy Meeting (NAM2014). Sarah Schmidt led a similar study of questioners at a specialized conference: the 2014 and 2016 Cambridge Workshops on Cool Stellar Systems and the Sun (C18 and C19; also known as “Cool Stars” for short). Both groups also found that women asked 10% fewer questions than expected based on the gender ratio of attendees, suggesting that this phenomenon may be widespread in astronomy. However, both Prichard and Schmidt didn’t find that the gender of the chairs affected the gender ratio of questioners at NAM2014 nor C18 and C19 as much as it did at AAS 223.

    Prichard (and Schmidt, though the result was not statistically significant) found one intriguing new finding: males are disproportionately more likely to ask the first question (at NAM2014, they were 6x more likely to, asking 86% of first questions). However, females are increasingly likely to ask questions as more questions are asked. For the 4th-7th questions, the gender ratio of questioners were about equal to the attendance rates (see Fig. 2).

    2
    Figure 2. Females tend to ask more questions later. The fraction of male and female questioners are shown for the 1st, 2nd, etc. questions asked after each talk. The first question is predominantly asked by males (86%), but later questions approach the gender ratio of those who attended, which is shown by the green horizontal line (28% female). The blue horizontal line shows the overall gender ratio of all questions asked during the conference (18% female). Figure from Pritchard et al. 2014, who studied NAM 2014.

    Another intriguing finding in all three studies was that female speakers were asked more questions than male speakers. The reasons for this are also unclear—did male questioners think that female speakers needed to be interrogated more? Do female presenters tend to create an environment that’s more inviting of questions?

    Given the lack of data, it’s difficult to say why these trends exist with much confidence, but there are some possibilities that have been floated. Could it be because of seniority? It’s been shown that those who have more seniority are more willing to ask questions. Schmidt pointed out that the fraction of women who asked questions (~20%) matched the fraction of women members of AAS who were born before 1980. Prichard points out studies of the psychology of asking questions that show that males generally dominate in discussions. In addition, females may be discouraged from asking questions from fear of being perceived as dominating (even if they participate a similar amount as males), or possibly from differences in criticisms from teachers that, over time, cause females to be less confident, and males over-confident, in their respective abilities.

    These studies also indicate ways in which the format of question and answer sessions could be modified to encourage more questions from females: for chairs to make sure talks end on time so that there is sufficient time for questions; to allow at least 4 questions so females more comfortable with asking later can have a chance to ask; and not choosing the first person to raise a hand to give others a chance to ask, and giving priority to females for the first question, among others. With these small changes, we can get one step closer to a more inclusive environment for scientific exchange during conferences.

    Ultimately, more studies are needed in order to untangle the roots of the trends that have been uncovered. Davenport has continued to collect data on questioners at subsequent AAS conferences—check out his group’s latest results here. If you’d like to spearhead a similar study at a conference you’re attending, Davenport has also provided the resources to do so on his github page.

    See the full article here .

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    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 4:31 pm on June 3, 2017 Permalink | Reply
    Tags: Astrobites, , , , , It’s a tough life for a small galaxy   

    From astrobites: “It’s a tough life for a small galaxy” 

    Astrobites bloc

    Astrobites

    Jun 2, 2017
    Paddy Alton

    Title: The hELENa project – I. Stellar populations of early-type galaxies linked with local environment and galaxy mass
    Authors: A. Sybilska, T. Lisker, H. Kuntschner, A. Vazdekis, G. van de Ven, R. Peletier, J. Falcón-Barroso, R. Vijayaraghavan, and J. Janz
    First Author’s Institution: European Southern Observatory

    Status: Accepted for publication by Monthly Notices of the Royal Astronomical Society, [open access]

    It’s a jungle out there, and it’s all about survival of the biggest.

    As our understanding of how galaxies form has developed, we’ve come to realise that galaxies tend to start small and grow over time by gobbling up smaller galaxies: a kind of cosmic food chain. The biggest galaxies – the predators of this vicious ecosystem – can usually look after themselves. It’s pretty rare for them to encounter an equally big and bad galaxy and for the most part they barely pause as they tear unfortunate smaller galaxies to shreds.

    (Quite literally – no exaggeration here! When the galaxies are similar sizes it gets pretty messy…)

    In the end, most massive galaxies generally end up all looking fairly similar, because there’s not much that the galaxies around them can do to make them evolve differently to how they would if they were left alone. But for smaller galaxies it’s a very different story.

    The trials and tribulations of dwarf ellipticals

    Today’s featured paper is the first in a series all about such galaxies and the lives they lead. The hELENa project (that’s The role of Environment in shaping Low-mass Early-type Nearby galaxies) may not be doing the astronomy community any favours in terms of our reputation for egregious acronym abuse, but we shouldn’t let that detract from some great science.

    The authors are taking a closer look at the stellar populations of dwarf elliptical galaxies. These galaxies at first sight look a lot like the massive galaxies I’ve already mentioned, only smaller – but because they are smaller, they are much more susceptible to external sources of disruption. The stars they contain can bear the scars of these encounters, so we can learn a lot about the large scale physics of galaxy formation by taking a closer look.

    1
    The Virgo cluster of galaxies. In the left panel the locations on the sky of the residents of this galactic metropolis are marked with dots, colour-coded by galaxy type. The hELENa targets are shown with black squares, and the monstrous galaxy Messier 87 is given a special marker. The other panels show the sample superimposed on a map of the cluster’s density (computed two different ways in the middle and right-hand panels), so you can see that some of the sample lie in the cluster’s quiet outskirts, whilst others reside in the busy central regions. Figure 1 from the paper.

    Many galaxies can be found in clusters, giant gravitationally-bound agglomerations of galaxies. In some ways clusters are like cities for galaxies, with densely populated regions with lots of action in the centres and more relaxed suburban areas around the edge. The type of environment a dwarf galaxy finds itself in can matter a great deal. In particular, processes such as ram-pressure stripping can become important for galaxies falling into denser regions, tearing out their gas and abruptly curtailing star formation. Likewise, galaxy harassment (close encounters with other galaxies) can disturb the structure of small galaxies. In short, it’s not easy being a dwarf elliptical. In figure 1 (above) the target galaxies from this paper – all from the Virgo cluster – are shown along with the environment

    2

    The effect of environment

    The authors’ goal is to look for correlations between the stellar populations in their target sample and the environments their samples live in. Information about stellar populations is encoded in the light from these galaxies and can be accessed via spectroscopy. Briefly, the idea is that certain spectral features (things like absorption bands at particular wavelengths) get stronger or weaker in the light from stars depending on how old they are, whether they are more/less enriched in heavy elements, more/less massive etc. Measuring these features – in particular, measuring how they vary within each galaxy – can tell us lots of useful things about the history of the galaxy, for example how quickly it formed its stars. Some examples of these measurements are shown in figure 2 (below).

    3
    Some spectroscopic maps of a few target galaxies (a comprehensive version can be found in the paper’s Figure 2). Taking spectra from each pixel in the image, it’s possible to learn a lot about each galaxy. The three left-most columns show measurements of key spectral absorption features, colour-coded by strength. From these the age and chemical composition of the stars in each galaxy can be inferred. The other three columns give information about how bright parts of the galaxy are and the velocities of the stars.

    From these measurements the authors can estimate things like the ages of the stars in different parts of the galaxy, their chemical composition, whether the galaxy is rotating, and more.

    So what can be learnt from this bounty of information? The authors find a number of interesting effects, for example that the stars in galaxies residing in denser regions of the cluster tend to be older. This suggests that the effect of the denser environment is indeed to help shut off star formation earlier. Likewise, the properties of galaxies deep in the cluster seem to correlate with their size more tightly than those in the outskirts. This shouldn’t be too surprising: the effects of many encounters on these galaxies will drive them towards a common, average outcome. By contrast those galaxies which lie further out will have a variety of histories.

    The authors are able to confirm that whereas the properties of massive galaxies are principally determined by their size, dwarf ellipticals show a much wider variation in their properties. In other words, massive galaxies mostly ignore external effects and only their internal physics matters, but for dwarf galaxies the varied external encounters they have experienced are more important.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

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    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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