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  • richardmitnick 12:44 pm on August 21, 2017 Permalink | Reply
    Tags: Astrobites, , , , ,   

    From astrobites: “Sneaky pete baryons in gravitational lensing” 

    Astrobites bloc

    Astrobites

    Aug 21, 2017
    Suk Sien Tie

    Title: Flux-ratio anomalies from discs and other baryonic structures in the Illustris simulation
    Authors: Jen-Wei Hsueh, Giulia Despali, Simona Vegetti et al.
    First Author’s Institution: University of California, Davis

    Status: Submitted to MNRAS, open access

    When a photon perilously escapes being engulfed by gases in its galaxy, it embarks on a long journey to reach our telescopes. Along the way, the combined gravitational field of nearby galaxies and galaxy clusters lures the photon away from sticking to a straight path. Occasionally, its path gets very bent when it passes very close to a galaxy, so much so that when the photon reaches our telescope, we see multiple images of the galaxy where the photon originates. This phenomenon of light bending due to the gravity of matter is known as gravitational lensing.

    Gravitational Lensing NASA/ESA

    As you gaze at the Sun later today during the solar eclipse, remember Albert Einstein, Sir Arthur Eddington, and gravitational lensing. Nearly a century ago on May 29, 1919 when the Sun was completely eclipsed by the Moon, on the west coast of Africa and Brazil, Sir Arthur Eddington and his team proved Einstein’s theory of general relativity. On the day of the eclipse, the Sun was destined to pass by the Hyades cluster, and the darkness that ensued cause the stars to be visible. Eddington and his team measured the stars to have shifted in positions due to the Sun’s gravitational field by the amounts predicted by Einstein. This was also the first observation of gravitational lensing.

    In the context of today’s paper, gravitational lensing is a tool to detect dark matter substructures in the halo of the lensing galaxy. Dark matter is that mysterious stuff that makes up nearly 85% of the Universe mass, does not emit light, and interacts only through gravity. The ratio of fluxes between any pair of lensed images is sensitive to the underlying mass distribution of the lens galaxy. In the absence of dark matter substructures, the flux ratios of the images are well predicted using a smooth lens model. But it does not work as well if dark matter substructures are present, resulting in anomalous flux ratios. Hence, flux ratio anomaly is a telltale sign of dark matter.

    Or is 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 11:35 am on August 15, 2017 Permalink | Reply
    Tags: Astrobites, , , , , Detecting Exoplanet Life in Our Proximity,   

    From astrobites: “Detecting Exoplanet Life in Our Proximity” 

    Astrobites bloc

    Astrobites

    8.15.17
    Mara Zimmerman

    1
    This artist’s interpretation shows the planet Proxima Centauri b around its host star. Proxima Centauri is actually part of a trinary system which includes the close binary Alpha Centauri AB. You can see Alpha Centauri AB in between the planet and star, as two faint white dots in the background. [ESO]

    Title: Detecting Proxima b’s Atmosphere with JWST Targeting CO2 at 15 Micron Using a High-Pass Spectral Filtering Technique
    Author: I.A.G. Snellen, J.M. D’esert, L.B.F.M. Waters, T. Robinson, et al.
    First Author’s Institution: Leiden Observatory, Leiden University, The Netherlands

    Leiden Observatory


    Status: Accepted to AJ, open access

    Proxima Centauri b and the Significance of CO2

    This summer marks the one-year anniversary of the detection of an exoplanet orbiting our solar system’s nearest stellar neighbor, Proxima Centauri. This Earth sized exoplanet, with the oh-so-imaginative name Proxima Centauri b, lies in the habitable zone of its M-dwarf star. As we’ve previously discussed, having such an Earth-like planet in our stellar backyard is really exciting, and astronomers are keen to explore the system as thoroughly as possible for potential signs of life. The cover image above shows an artist’s rendition of this intriguing exoplanet.

    Since the discovery of this close planet, researchers have been studying methods that might detect the presence of life on Proxima Centauri b. Today’s paper approaches this by devising a method to detect carbon dioxide (CO2) in the planet’s atmosphere. The authors focus on this particular molecule because it is one of the four main biomarkers used in evaluating habitability of exoplanets; water, methane, carbon dioxide, and oxygen are primarily produced during biological processes, so their presence in an atmosphere can imply life. In addition to being a biomarker molecule, CO2 has many distinguishable features that are visible in the 15 micron band, which JWST is equipped to look at.

    Snellen and collaborators present a technique that can be performed with the soon-to-be-launched James Webb Space Telescope (JWST), which would reveal the presence of CO2 in the atmosphere of this nearby exoplanet.

    NASA/ESA/CSA Webb Telescope annotated

    The emission in the 15 micron band will be ideal for detecting CO2, since this molecule has over 100 features within this band.

    JWST and High-Pass Spectral Filtering Techniques

    JWST is equipped with several extremely sensitive instruments. One of the goals of this mission is to detect and characterize atmospheres of exoplanets. With this in mind, these authors suggest using the medium resolution spectrograph (MRS) mode of the Mid-Infrared Instrument (MIRI) to detect CO2 markers in the atmosphere of Proxima Centauri b.

    3
    Figure 1: This shows an example planet spectrum and high-pass filtered spectrum. The high-pass image has more distinguishable features, which allows for greater sensitivity in molecule detection. [Snellen et al. 2017]

    This new technique combines several methods to attain greater sensitivity. The cross-correlation requires a high spectral resolution to find the radial component of the planet orbital velocity, used to filter out the planet’s signal. The authors’ method cross-correlates the observed spectrum with template spectra; however, the spectral resolution is not high enough to achieve this with Proxima Centauri b, so the authors suggest a slight modification: use this method while targeting a specific feature in the spectrum. Proxima Centauri b is believed to be tidally locked, meaning that the same hemisphere always faces the star. This means that at certain alignments, Proxima b will show a contrast of up to 100 ppm with respect to the star. This contrast will be helpful in separating planet signatures from the flux of the star. The method does not require absolute flux calibration, but only depends on the relative flux of the star and the planets. However, one limitation is that the stellar spectrum must be precisely known beforehand, since the flux of the planet, even at its highest, is much smaller than the stellar flux.

    The authors used atmospheric models of Proxima Centauri b to show the limits of their detection method in extreme cases and expected results which are shown in Figure 2.

    4
    Figure 2: Planet model spectra, assuming a standard Earth atmospheric model for the temperatures and gas mixing ratios. The temperature and pressure relation is shown on the right column. The left column has the observed spectra given the temperature profiles. The upper panel has stratospheric temperatures that are equal to the tropopause temperature; the middle panel shows a model for clear sky conditions with an Earth-like temperature profile; the lower panel shows the spectrum for high, thick cloud coverage, again with an Earth-like temperature profile. [Snellen et al. 2017]

    A Step Forward in the Search for Extraterrestrial Life

    If we can detect exoplanet life or a habitable exoplanet so close to Earth, this could tell us more about how common life is in our universe and the origins of life in different environments. Until we find a way to accurately detect the presence of life, such as detecting CO2 in atmospheres, we won’t know the extent of life in all parts of the universe. Though we don’t know for certain yet, I for one hope that we can soon discover and say hello to our new alien neighbors, hopefully on Proxima b!

    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 8:19 am on August 15, 2017 Permalink | Reply
    Tags: Astrobites, , , , , , OGLE-Optical Gravitational Lensing Experiment, U Warsaw   

    From astrobites: “New twinkles in the sky” 

    Astrobites bloc

    Astrobites

    Aug 15, 2017
    Ingrid Pelisoli

    Title: Blue large-amplitude pulsators as a new class of variable stars
    Authors: P. Pietrukowicz, W.A. Dziembowski, M. Latour et al.
    First Author’s Institution: Warsaw University Observatory, Poland
    1
    Status: Published in Nature Astronomy [open access via arXiv]

    Has the Sun ever winked at you? It was probably too subtle for you to notice, but this definitely has happened. Our Sun’s brightness is constantly varying due to convective bubbles near its surface. Other stars similar to the Sun undergo the same process, forming a class of pulsating stars called solar-like oscillators. This is only one of many classes of pulsating stars. We are very lucky that stars pulsate and we are able to detect that: this is the only way we can explore the interior of stars [1]. The frequency of the pulsations we can detect depends directly on the internal structure of the star, so this method allows us to probe stellar interiors and check if what we obtain from our stellar evolution models is correct.

    Thus astronomers are constantly sweeping the sky in search of new pulsators. There are dedicated projects doing that, but sometimes data from projects with other scientific goals can be used. One project that has so far revealed the existence of almost 500,000 pulsating stars is the Optical Gravitational Lensing Experiment (OGLE).


    1.3 meter OGLE Warsaw Telescope at the Las Campanas Observatory in Chile

    As the name suggests, the goal of the project is to study dark matter though microlensing phenomena. Matter, be it dark or not, bends the light, focusing it and acting as a magnifying glass. Identifying regions where this magnification occurs is a way to study objects that emit little or even no light. To achieve that, photometric data was obtained for about a billion stars over the years, allowing also the search for pulsators. With such a huge amount of data, there’s a good chance of finding something new and rare. That’s exactly what the authors of today’s paper spotted: an entirely new class of variables, with some properties which are kind of hard to explain.

    The BLAPs

    The first object of this new class was discovered back in 2003. It showed variations of 0.24 magnitudes with a period of about half an hour. The shape of the light curve and the period were similar to the very common delta-Scuti stars, so at first the star was classified as that. The amplitude of the variation, however, is much higher than any known delta-Scuti is this period range. This motivated the astronomers to flag this object as a puzzling variable and schedule a spectroscopic follow-up so they could learn more about its properties. Modelling the obtained spectrum, the authors found the object had a temperature over 30,000K and surface gravity with a log g of 5.3. This temperature is way too high for delta-Scuti stars, and the log g indicates the object is not on the main sequence. These physical properties are similar to hot subdwarf stars, that get stuck into a core-He burning phase due to severe mass loss, usually credited to the presence of a companion. Hot subdwarfs do pulsate as well, but with amplitudes hundreds of times smaller than for the observed star. In short, its properties did not fit any known type of variable star.

    Studying OGLE data, the authors found another 13 objects with these same properties. The light curves for all targets are shown in Fig. 1. Besides the high amplitude, all of them have extremely blue colors, reflecting the fact that they are hot. Hence they were dubbed blue large-amplitude pulsators, or BLAPs, for short.

    3
    Figure 1: Light curves for the fourteen known BLAPs. The period for each is indicated. [Figure 1 from the paper.]

    More data!

    As is the usual approach in astronomy when we discover something we don’t quite understand, the authors set out to obtain more data for their newly found discoveries. In intermediate resolution spectra obtained for four stars, three of them showed in Fig. 2, they’ve found that the surface gravity is slightly smaller than previously thought, with a log g of around 4.6. That is still too high for hot main sequence stars (log g < 4.3), but now rules out that the objects are hot subdwarfs (5.3 < log g < 6.2). They’ve also noticed that the temperature changes by at least 13% throughout the pulsation period, and that the stars are moderately rich in helium.

    4
    Figure 2: Obtained spectra (black lines) for three of the BLAPs. They show strong hydrogen and helium lines. Fitted models are shown in red, and the obtained parameters are indicated. [Figure 5 from the paper.]

    Putting pieces together

    Analysing long-term OGLE data, the authors estimated the rate at which the period of the stars is changing, and found it to be quite low, indicating the pulsation is stable. This implies that the interior of the star is not changing significantly, so they must be evolving in a slow timescale. As the authors already ruled out the main sequence, two options are left: the objects could either be already burning helium in their cores (instead of hydrogen, as on the main sequence), or be red giants burning hydrogen in a shell outside their helium core. The first option requires that the object lost a lot of mass, so the high temperature is explained by us seeing almost inside the core, which is hot enough to burn helium. The second model doesn’t require such a severe mass loss, but the red giant would still have to be stripped somehow of its outer layers, so we can see close to the hot burning shell around the core. The two models predict slightly different luminosities, so we should be aiming at obtaining more accurate measurements of the gravity, allowing for an independent estimate of mass and luminosity to select between the two proposed models.

    However, even if we end up being able to tell which model best explains the BLAPs, the formation scenario is still a mystery. What led to the severe mass loss or stripped the giant of its outer layers? The easiest way to achieve that would be a binary companion. The authors found no evidence for it, but the possibility of a faint unseen companion cannot be ruled out. A search for these possible companions would be a good next step on understanding the BLAPs. Their small number in comparison to hot subdwarfs, for example, suggests they are rare episode in stellar evolution. Let’s not miss a chance to crack it!

    1. Ok, we could in principle also do that by studying neutrino emissions. Neutrinos interact so little that they can be emitted in the internal regions and leave the star basically unaltered, carrying information from the region they originated. However, their unwillingness to interact also means that they are really hard to catch, so this method is not nearly as effective as studying stellar pulsations.

    See the full article here .

    Previously covered here From Gemini Observatory, but this is from U Warsaw and is much more thorough .

    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 9:02 am on August 4, 2017 Permalink | Reply
    Tags: Astrobites, , , , ,   

    From astrobites: “The Generation Game” 

    Astrobites bloc

    Astrobites

    Aug 3, 2017
    Paddy Alton

    Title: Young LMC clusters: the role of red supergiants and multiple stellar populations in their integrated light and CMDs
    Authors: Randa S. Asa’d, Alexandre Vazdekis, Miguel Cervi ̃no, Noelia E. D. No ̈el, Michael A. Beasley, Mahmoud Kassab
    First Author’s Institution: American University of Sharjah, UAE
    1
    Status: Accepted for publication in MNRAS, open access

    An introduction to globular clusters

    Stars are a sociable bunch, by and large. They don’t like to be alone: some hang out in pairs, or sometimes in small groups of three or four. Others still are extreme extroverts, keeping company with hundreds of thousands of other stars, for example in densely packed globular clusters (GCs) – see figure 1 for an example.

    This isn’t be the first article on star clusters to feature on astrobites – and you can be pretty sure it won’t be the last. After all, we’ve been studying them since the seventeenth century and people are still publishing papers about them!

    2
    Figure 1. The globular cluster M80 – hundreds of thousands of stars bound together by gravity, 28000 light-years from Earth. (credit: Hubble Heritage Team)

    NASA/ESA Hubble Telescope

    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 1:13 pm on July 28, 2017 Permalink | Reply
    Tags: Astrobites, , , , , What are Mars’ moons made of?   

    From astrobites: “What are Mars’ moons made of?” 

    Astrobites bloc

    Astrobites

    Jul 28, 2017
    Kerrin Hensley

    Title: On the Impact Origin of Phobos and Deimos I: Thermodynamic and Physical Aspects
    Authors: Ryuki Hyodo, Hidenori Genda, Sébastien Charnoz, and Pascal Rosenblatt
    First Author’s Institution: Earth-Life Science Institute, Tokyo Institute of Technology

    Status: Accepted to The Astrophysical Journal, open access

    Where did Phobos and Deimos come from?

    Phobos and Deimos, Mars’ two small moons, were initially believed to be the result of interplanetary kidnapping. Many moons in the Solar System appear to be captured objects, and the featureless reflectance spectra of Phobos and Deimos hint that they might be D-type asteroids. However, captured objects tend to have highly eccentric orbits, and both Phobos and Deimos orbit Mars in a nearly circular fashion. More recently, it has been proposed that both moons are the result of a massive impact 4.3 billion years ago—instead of being captured from interplanetary space, they could coalesce from the debris disk generated by the impact. Past research [Nature Geoscience] has shown that the masses and orbits of Phobos and Deimos can be explained by this method. This theory could also explain the presence of Borealis basin, an extended low-altitude region spanning Mars’ north pole, which can be seen in Figure 1.

    1
    Topographical map of Mars. Borealis basin is the low-lying (blue) region in the northern hemisphere. It encompasses many officially-named regions, such as Vastitas Borealis and Utopia Planitia. Adapted from this image, which is made from data from the Mars Orbiter Laser Altimeter aboard Mars Global Surveyor.

    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 12:54 pm on July 27, 2017 Permalink | Reply
    Tags: Astrobites, , , Bald black holes, ,   

    From astrobites: “Bald Black Holes Wearing Wigs” 

    Astrobites bloc

    Astrobites

    Jul 27, 2017
    Lisa Drummond

    Title: No-Hair Theorem for Black Holes in Astrophysical Environments
    Authors: Norman Gürlebeck
    First Author’s Institution: University of Bremen
    1
    Status: Physical Review D, open access

    The No-Hair Theorem is a conjecture about the simplicity of black holes. They are called “bald” to reflect the paucity of information necessary to characterise their spacetime – only three parameters at most are needed. The theorem is formulated to describe isolated black holes. A realistic black hole will almost invariably be distorted by its astrophysical environment, e.g. a binary companion, surrounding plasma, accretion disks, or nearby jets. It seems possible that in these cases, the black hole could grow hair and the No-Hair Theorem would break down.

    However, this paper shows that the distortions from the surrounding environment do not imply the black hole now has hair. Although our observations of the spacetime an infinite distance from the black hole do change, this is solely due to the external neighbourhood around the black hole, not the distorted black hole itself. In the words of Norman Gürlebeck, the author of this paper:

    “Thus, even though the black hole might put on a wig it still looks bald.”

    The No-Hair Theorem remains valid in this more general context: even when engulfed by a complex environment, the black hole resists growing hair.

    2
    The No-Hair Theorem: physical information that enters a black hole is lost forever. No matter what matter contributed to the black hole forming in the first place, and no matter what kind of matter falls into the black hole, the black hole still has no hair! Source: http://gravitation.web.ua.pt/index.php?q=node/362

    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 2:25 pm on July 25, 2017 Permalink | Reply
    Tags: Astrobites, , , , ,   

    From astrobites: “Dark Matter in the Milky Way: ‘A Matter of Perspective’ “ 

    Astrobites bloc

    Astrobites

    Jul 25, 2017
    Nora Shipp

    Title: The core-cusp problem: A matter of perspective
    Authors: Anna Genina, Alejandro Benitez-Llambay, Carlos S. Frenk, Shaun Cole, Azadeh Fattahi, Julio F. Navarro, Kyle A. Oman, Till Sawala, Tom Theuns
    First Author’s Institution: Institute for Computational Cosmology,University, UK

    Status: Submitted to the Monthly Notices of the Royal Astronomical Society, Open Access

    Dark matter dominates the Universe around us, far exceeding the amount of everyday baryonic matter that makes up humans, the Earth, and the entire visible Milky Way. Our galaxy is embedded in an invisible cloud of dark matter, which contains smaller dark matter clouds that orbit around us like satellites. These satellites do not contain big spiral galaxies like the Milky Way and, although they may contain smaller galaxies, they are made up of almost entirely dark matter, which means that they are very sensitive to the precise nature of the dark matter particle.

    Today’s paper investigates whether two of the Milky Way’s largest satellite galaxies (Fornax and Sculptor, Figure 1) conflict with the leading theory of Cold Dark Matter (CDM), potentially requiring a complete reconsideration of our understanding of the evolution of the Universe.

    2
    Projected density plot of a redshift {\displaystyle z=2.5} dark matter halo from a cosmological N-body simulation. The visible part of the galaxy (not shown in the image) lies at the dense centre of the halo and has a diameter of roughly 20 kiloparsecs. There are also many satellite galaxies, each with its own subhalo which is visible as a region of high dark matter density in the image. http://en.wikipedia.org/wiki/User:Cosmo0

    Don’t get too excited, though. I will break the suspense and say that, as usual, the answer is “not yet” – we don’t know enough about these mini galaxies to throw away CDM. There is still a lot of work to be done if we want to break this paradigm.

    1
    Figure 1. The Fornax (left) and Sculptor (right) galaxies. (Source: ESO)

    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 5:49 am on July 22, 2017 Permalink | Reply
    Tags: Astrobites, , , , , I see skies of blue and clouds of white   

    From astrobites: “I see skies of blue and clouds of white” 

    Astrobites bloc

    Astrobites

    July 21, 2017
    Shang-Min Tsai

    Title: A Cloudiness Index for Transiting Exoplanets Based on the Sodium and Potassium Lines: Tentative Evidence for Hotter Atmospheres Being Less Cloudy at Visible Wavelengths
    Authors: K. Heng
    First Author’s Institution: Center for Space and Habitability, University of Bern
    1
    Status: Published on ApJL, open access

    Background

    Most astronomers hate clouds, except for those who study them. Not only our clouds on the sky ruin observing nights, clouds on other planets obscure everything underneath, too. When observing extra-solar planets during transit, different atoms, molecules or particles absorb light at certain wavelengths, making the apparent radii of the planets change as viewed with different colors — known as transmission spectroscopy. Although it is a powerful tool to infer the atmospheric composition of exoplanets, these clouds often hinder our efforts to understand such alien worlds. With clouds present at high altitude, only the atmosphere above the clouds can be seen and it is too thin to provide any spectral features (see this popular example). It turns out that a useful strategy for dealing with a problem is to avoid the problem. Valuable space telescope time could be saved if we could filter cloud-free objects from ground-based measurements. Today’s paper presents an index to quantify the degree of cloudiness for transiting planets.

    Aim

    The author first revisited how previous studies used the slope of transmission spectra caused by scattering (blue light is scattered more than red) to distinguish cloudy and dense atmospheres. Such dense atmospheres are more packed (with smaller scale height) and can produce flat spectra that are compatible with cloud-free hydrogen-dominated atmospheres. The author demonstrates that the scattering cross sections of gaseous molecules (e.g. hydrogen, nitrogen) and aerosols or condensates (cloud particles) have the same wavelength dependence. Measuring the spectral slope alone does not solve the ambiguity between clouds and atmospheric composition. With this motivation, the author proceeds to find a cloudiness index that does not depend on the spectral slope.

    Methods

    Sodium has large cross sections so it can produce a prominent spectral line without being abundant. It is, in fact, the first extrasolar atmospheric detection on HD 209458b. In a clear, cloud free atmosphere, the difference in transit radii between the line center and wing of sodium can be theoretically calculated. By measuring the actual difference in transit radii between the line center and wing (Δ Robs), the author constructs a dimensionless index (C) for the degree of cloudiness as the ratio of Δ R and Δ Robs. For an entirely cloud-free atmosphere, Δ R equals to Δ Robs and C = 1. Very cloudy atmospheres have C Gt 1. This cloudiness index is independent of the spectral slope, with the caveat that it is limited to planets with sodium or potassium line detections.

    2
    Figure 1: Cloudiness index plotted against equilibrium temperature (top), surface gravity (middle), planetary mass (bottom). The labels “W6,” “W17,” “W31,” “W39,” “H1,” “H12,” and “HD189” refer to WASP-6b, WASP-17b, WASP-31b, WASP-39b, HAT-P-1b, HAT-P-12b, and HD 189733b, respectively [from the featured paper].

    Application to data

    Figure 1 plots the cloudiness index (C) versus equilibrium temperature, gravity, and mass of the planets. The uncertainty of the cloudiness basically stems from estimating the scale height with the equilibrium temperature in the calculation, which is larger for colder planets. An interesting trend of decreasingC with increasing equilibrium temperature (a proxy for stellar flux) is seen in the top panel. If the trend is real, it implies that more irradiated planets tend to be less cloudy. Future measurements of sodium lines at higher resolutions and for a larger sample will confirm or debunk the trend. If the trend holds, the author suggests that we can weed out cloudy objects for the detailed survey of JWST, to avoid spending lots of time (and money) for uninformative, featureless spectra.

    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 12:07 pm on July 19, 2017 Permalink | Reply
    Tags: An unconventional solar fountain, Astrobites, , , , ,   

    From astrobites: “An unconventional solar fountain” 

    Astrobites bloc

    Astrobites

    Jul 19, 2017
    Amber Hornsby

    Title: The association of a J-burst with a solar jet
    Authors: D. E. Morosan et al.
    First Author’s Institution: School of Physics, Trinity College Dublin, Ireland
    1
    Status: Submitted to A&A, open access

    2
    Figure 1: Charged particles from the Sun interacting with the Earth’s magnetosphere excite particles in the atmosphere, creating dazzling light shows – the Aurorae. This image was taken on the International Space Station (ISS). Credit: NASA.

    Our local star, the Sun, is an active star. It regularly sends streams of highly energetic particles hurtling towards our home planet, causing dazzling auroral displays at the poles, and occasionally we notice emissions in the radio regime. Back in July 2013, an unusual and very bright burst of radio wave energy was observed by the Low Frequency Array (LOFAR) based in the Netherlands, with our Sun being the likely culprit.

    ASTRON LOFAR Radio Antenna Bank


    ASTRON LOFAR Map

    Today’s bite will illustrate how J-bursts, an unconventional type of jet from the Sun, differ from the so-called type III bursts commonly observed. Also, we will describe their likely origin and the proposed mechanism to explain their odd characteristics.

    Twist-it!

    3
    Figure 2: Twisting of magnetic fields from the rotation of the Sun stores energy in entangled field lines. When field lines reorganise themselves, they release bursts of energy. Credit: Addison Wesley.

    Being made of plasma (hot, ionised gas) means the surface of the Sun experiences differential rotation – the material at located towards the equator travels faster than material located at the poles. This leads to magnetic field lines, pointing from pole to pole, to become twisted over time leading to very strong localised fields and is the cause of dark regions on the Sun known as sunspots. Generally field lines do not want to be tangled, therefore they re-arrange themselves, resulting in energetic solar events such as: solar flares, jets and Coronal Mass Ejections (CMEs). These events causes particles to be accelerated along field lines as they travel away from the source of activity.

    A what-burst?

    Solar flares and jets are commonly associated with X-ray emission and type III radio bursts, with the main difference being the resulting direction of the electron as it travels away from the Sun. Electrons accelerated by the re-configuring magnetic field can travel up through the corona, the aura around the Sun only visible during a solar eclipse, or down towards the layer located just above the surface (photosphere) – the chromosphere. Different layers of the Sun can be seen below in Figure 3.

    4
    igure 3: An artistic view dissecting the internal layers of the Sun. The regions of interest in today’s bite are the outer two layers – the chromosphere and the corona. Credit: NASA

    It is the electrons accelerating towards the corona which result in type III bursts and are identified as rapidly varying bursts of radiation which last a few seconds. They are considered to be the radio signature associated with electrons travelling via the corona, into interplanetary space along magnetic field lines. Several typical type III radio bursts are visible as vertical bursts in the top panel of Figure 4. Generally type III bursts are the result of electrons escaping via the corona because they have access to open magnetic fields lines, but this is not always the case.

    5
    Figure 4: Multiple Type III radio bursts observed by LOFAR at frequencies in the range of 10 – 240 MHz. The top panel is a dynamical spectrum. It shows the strength of the signal of a certain frequency (vertical axis) as a function of time (horizontal axis). Figure 1 of paper (top two panels)

    Today’s peculiar burst contained no radio waves below 30 MHz (top panel of Figure 4), therefore the authors coined this particular burst a J-burst. It was observed using the Low Frequency Array (LOFAR) based in the Netherlands. For more details about LOFAR, check out this awesome astrobite.

    Where did it come from?

    Already suspected to be solar in origin, scientists turned to an observatory which has been observing the Sun since 2011 – the Solar Dynamics Observatory (SDO). The SDO is able to produce one high resolution image of the Sun every second, making it the best space-based eye we currently have on our local star when compared to the STEREO and SOHO observatories.

    NASA/STEREO spacecraft

    ESA/NASA SOHO

    A solar jet was observed by the SDO at a time and location coincident to the burst in question. Its evolution as a function of time is highlighted in Figure 5 by a white arrow. The jet glows in panel (b) but has faded by panel (d), lasting around 8 minutes in total.

    6
    Figure 5: Evolution of jet which caused J-burst as a function of time. It glows in panels (b) and (c), but is not visible in (a) and has faded by (d). Figure 4 of paper

    Scientists continued their investigations by plotting their LOFAR observations at different frequencies on top images taken by the SDO. Observations were extended to higher frequencies of 150 and 228 MHz via the Nançay Radioheliograph (NRH) based in France. The evolving frequencies as a function of time tell an interesting story about the journey of some electrons.

    Nancay Radioheliograph North arm (Meudon, Observatoire de Paris)

    7
    Figure 6: Radio contours taken from LOFAR observations overlaid on SDO images highlight how the frequency of radio waves observed evolves with time. Higher frequencies were taken from NRH data. Figure 2 of paper.

    What is going on?

    The J-burst appears at 11:06:24 UT over a wide range of frequencies in Figure 6(b), including two LOFAR sources at 72 and 78 MHz (white and yellow contours) are associated with the highlighted jet (pink contour). The 228 and 150 MHz sources in the bottom right of Figure 6 (a-d) appear to be unrelated to this J-burst as it lasts around 10s and does not drift in frequency like the J-burst does. The 150 MHz source (blue contour) visible in Figure 6(b) is a component of the J-burst, but it has faded by Figure 6(c). The white and yellow contours shift to the right from their original location in Figure 6(c) which suggests we are sampling an electron beam moving in a different direction.

    The radio sources 72, 78, 55 and 50 and 39 MHz move southwards from their initial location and decrease in frequency. By Figure 6(d) only the lowest frequency radio sources remain. The appearance and behaviour of the sources suggests an initial electron beam was accelerated to produce the burst in Figure 6(b) but eventually the electrons reached a region where they stop producing radio emission i.e they were no longer travelling upwards through the corona and had become trapped in a closed magnetic field loop – this could explain this lack of observations below 30 MHz shown in Figure 4.

    Conclusion

    An unusual burst of radio energy observed at frequencies above 30 MHz, a J-burst was likely caused by electrons, which are accelerated by the reconfiguration of the Sun’s magnetic field, becoming trapped within a closed magnetic field loop whilst travelling upwards through the corona. Even with a vast number of investigations and models of solar jets associated with radio emission, alongside a well-known classification system, there are still unanswered questions about their exact mechanism and the path followed by accelerated electrons in the solar corona. Today’s paper has explored these themes and arrived at some interesting conclusions, but there is a still a lot of work to be done.

    This discovery does highlight the usefulness of radio emission as a tool to study the Sun’s magnetic field as its affect on charged particles, but it also puts an emphasis on the need for an instrument with a broader range of frequencies to probe flaring events associated with particle accelerations. The authors required the use of a wide range of instruments in order to draw their conclusions – something which might not always be possible – therefore they highlight the European Solar Radio Array (ESRA) as a highly promising candidate mission for future funding.

    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 2:20 pm on July 17, 2017 Permalink | Reply
    Tags: Astrobites, , , , , White dwarfs   

    From astrobotes: “How well do we measure the radii of white dwarfs?” 

    Astrobites bloc

    Astrobites

    Jul 17, 2017
    Ingrid Pelisoli

    Title: Testing the white dwarf mass-radius relationship with eclipsing binaries
    Authors: S. G. Parsons, B. T. Gänsicke, T. R. Marsh et al.
    First Author’s Institution: Department of Physics and Astronomy, University of Sheffield, UK

    Status: Accepted to MNRAS [open access]

    Look outside your window. Can you see the Sun? If it’s night-time, just pick a random star instead. Our Sun one day will become a white dwarf star, and the chance that the random star you’ve picked will follow the same path is over 95%. White dwarfs are by far the most common final evolutionary state for a star. The famous supernovas actually only occur when a star is massive enough to burn elements heavier than helium in its core, and that is usually not the case. What happens instead is that the star can only produce elements up to carbon and oxygen, and then nuclear reactions in the core cease to occur. With no release of energy to counteract the gravitational force, the carbon-oxygen core will contract more and more until it becomes degenerate. This degenerate core is essentially the white dwarf, which becomes visible when the outer layers of the star are ejected on its final breadth of hydrogen burning in outer shells.

    What does it mean to be degenerate?

    Degenerate matter occurs in stars when the density is so high that all the electrons are cramped in lower energy states. But there’s a catch: due to the Pauli exclusion principle, only two fermions, e.g. electrons, can occupy the same energy level. So other electrons end up in higher energy states and cannot move to already filled lower energy levels. This makes degenerate matter very resistible to compression, and is in fact what keeps white dwarfs from collapsing.

    Degenerate matter has a really cool property: the pressure does not depend on temperature. This is because the kinetic energies of these electrons that cannot move to lower states of energy are quite high, and the rate of collisions between electrons and other particles is quite low, so the electrons essentially travel at the speed of light. The pressure on the gas depends on this speed. As the speed of light is the fastest the electrons can travel, adding heat will not change the pressure at all. This has an effect somewhat counter-intuitive. As the only way to increase pressure is by adding mass, when you increase pressure you also increase gravity and make the particles become spaced closer together, so the object becomes smaller. In other words, the more massive the white dwarf, the smaller its radius. As a consequence of this weird property of degenerate matter, there’s a relationship between a white dwarf’s mass and its radius, so that if you know one you can estimate the other and the other way around. But how accurate is this mass-radius relationship? Today’s paper authors decided to test!

    Obtaining mass and radius independently

    Despite being widely used, the mass-radius relationship remained untested observationally until this paper. That is because there aren’t many ways to estimate the mass of a white dwarf without relying on this relationship at some point. What the authors realised is that we can use eclipsing binary stars for that. Eclipsing binaries orbit in a plane which intersects our line of sight, allowing us to detect dips in light when the stars transit in front of each other. For these objects, we can combine photometric and spectroscopic measurements to estimate the mass and the radius independently.

    The shape of the eclipses of the white dwarfs by their companions, which in this paper are all main-sequence stars, gives us two pieces of information: the width and the duration of the eclipses. However, there’s an issue: we have three unknown quantities, namely the orbital inclination and the radius of each star. So we need one more piece of information to be able to determine all the unknowns.

    The authors suggest different methods, but what works best in their sample is to use the gravitational redshift estimated from spectroscopy. As the gravity in white dwarfs is really high (about 350,000 times the gravity of Earth!), light gets shifted a measurable amount towards the red when it exits the white dwarf, as it was delayed. The amount of redshift depends on the mass and radius of the white dwarf. Combining that with Kepler’s third law, we obtain a relationship between the white dwarf’s radius and the binary inclination. All we need is to estimate the radial velocity semi-amplitudes for both stars, hence the necessity of spectroscopy, which was also used to constrain the effective temperature of the white dwarf. Going back to Kepler’s third law with the estimated inclination, we can also estimate the mass of the white dwarf.

    So the combined fit of the light curve and the radial velocities of both stars, together with Kepler’s law and the known relation for gravitational shift, give us the mass and the radius without having to recur to the mass-radius relationship. Just the independent measure we needed!

    The results

    Figure 1 shows the comparison between the values the authors obtained with the theoretical models for different temperatures. Black lines are for the more common C/O core white dwarfs, while green lines are for white dwarfs with a He core. He-core white dwarfs are less massive and are formed when the outer envelope is lost by the progenitor star before helium is even ignited. They would be formed by lower mass stars that do not achieve conditions to burn helium, but the Universe is not old enough for them to have evolved off the main sequence just yet, so this objects are explained by some form of enhanced mass loss (such as binary evolution). As the analysed white dwarfs have different temperatures and core compositions, is difficult to define how the results agree with the theory based on this figure.

    1
    Figure 1: Comparison between mass and radius obtained from observations, represented by the red data points, and theoretical mass-radius relationships for different effective temperatures. Black lines assume a C/O core and green lines assume a He core. [Adapted from figure 9 in the paper.]

    Instead, the authors analyse what we see in Figure 2, which is the ratio between the radii estimated observationally and using the mass-radius relationship as a function of mass. Below 0.5 solar masses, the authors assume both a relationship assuming a He core and a C/O core. The first result is that white dwarfs with masses below 0.5 solar masses are more consistent with a He core, which agrees with our theories of stellar evolution. However, this is the first time we have direct observational evidence for that.

    2
    Figure 2: Ratio of the estimated white dwarf radii to theoretical predictions as a function of mass. Below 0.5 solar masses, black points assume a C/O core white dwarf and red points assume a He core white dwarf. Almost all radius measurement at this range are more consistent with He core. [Figure 10 in the paper.]

    Another interesting test the authors made was to compare surface gravities obtained from their fit to the spectra and derived from the mass and radius obtained from the light curves. Fitting spectra is the most widely used method to obtain the physical parameters of a white dwarf, so testing this independently is important to check we are basing our science in correct estimates. As Figure 3 shows, in most cases there’s excellent agreement between the results, but there are a few outliers where the spectral fit overestimates the gravity. In most of these cases, this disagreement can easily be explained by contamination from the companion. There’s one exception where the white dwarf dominates the spectrum so contamination from the companion is not enough to cause the discrepancy, but for this one the discrepancy is only at a 2-sigma level. Additional broadening mechanisms, such as magnetic fields, might be the cause for that.

    3
    Figure 3: Comparison between surface gravities estimated from spectral fits, and those computed from the mass and radius values obtained from the light curves. [Figure 13 in the paper.]

    Why does it matter?

    The authors came to conclude that both our mass-radius relationship and spectral modelling are quite good at estimating the properties of white dwarfs. This result is important not only for those directly interested in white dwarfs, but has also implications for cosmology. The radius of a white dwarf is related to its cooling rate, which depends on its surface area. As you can read on this bite, the cooling times of white dwarfs can be used to estimate the ages of different stellar populations. The mass-radius relationship also sets an upper limit to the mass of a white dwarf, which is an important result to the study of type Ia supernovae, which in turn are used to measure the expansion of the Universe.

    Now that we know that our theoretical models are doing a good job, we can keep doing cool science with them!

    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.

     
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