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  • richardmitnick 2:04 pm on April 24, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , Is TRAPPIST-1 Really Moonless?, Worlds Without Moons   

    From AAS NOVA: ” Worlds Without Moons” 

    AASNOVA

    American Astronomical Society

    24 April 2017
    Susanna Kohler

    1
    Many exoplanets are expected to host moons — but can planets in compact systems orbiting close to their host stars do so? [NASA/JPL-Caltech]

    Many of the exoplanets that we’ve discovered lie in compact systems with orbits very close to their host star. These systems are especially interesting in the case of cool stars where planets lie in the star’s habitable zone — as is the case, for instance, for the headline-making TRAPPIST-1 system.

    But other factors go into determining potential habitability of a planet beyond the rough location where water can remain liquid. One possible consideration: whether the planets have moons.

    Supporting Habitability

    2
    Locations of equality between the Hill and Roche radius for five different potential moon densities. The phase space allows for planets of different semi-major axes and stellar host masses. Two example systems are shown, Kepler-80 and TRAPPIST-1, with dots representing the planets within them. [Kane 2017]

    Earth’s Moon is thought to have been a critical contributor to our planet’s habitability. The presence of a moon stabilizes its planet’s axial tilt, preventing wild swings in climate as the star’s radiation shifts between the planet’s poles and equator. But what determines if a planet can have a moon?

    A planet can retain a moon in a stable orbit anywhere between an outer boundary of the Hill radius (beyond which the planet’s gravity is too weak to retain the moon) and an inner boundary of the Roche radius (inside which the moon would be torn apart by tidal forces). The locations of these boundaries depend on both the planet’s and moon’s properties, and they can be modified by additional perturbative forces from the host star and other planets in the system.

    In a new study, San Francisco State University scientist Stephen R. Kane modeled these boundaries for planets specifically in compact systems, to determine whether such planets can host moons to boost their likelihood of habitability.

    3
    Allowed moon density as a function of semimajor axis for the TRAPPIST-1 system, for two different scenarios with different levels of perturbations. The vertical dotted lines show the locations of the six innermost TRAPPIST-1 planets. [Kane 2017]

    Challenge of Moons in Compact Systems

    Kane found that compact systems have a harder time supporting stable moons; the range of radii at which their moons can orbit is greatly reduced relative to spread-out systems like our own. As an example, Kane calculates that if the Earth were in a compact planetary system with a semimajor axis of 0.05 AU, its Hill radius would shrink from being 78.5 times to just 4.5 times its Roche radius — greatly narrowing the region in which our Moon would be able to reside.

    4
    Image of the Moon as it transits across the face of the Sun, as viewed from the Stereo-B spacecraft (which is in an Earth-trailing orbit). [NASA]

    Kane applied his models to the TRAPPIST-1 system as an example, demonstrating that it’s very unlikely that many — if any — of the system’s seven planets would be able to retain a stable moon unless that moon were unreasonably dense.

    Is TRAPPIST-1 Really Moonless?

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

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

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

    How do these results fit with other observations of TRAPPIST-1? Kane uses our Moon as an example again: if we were watching a transit of the Earth and Moon in front of the Sun from a distance, the Moon’s transit depth would be 7.4% as deep as Earth’s. A transit of this depth in the TRAPPIST-1 system would have been detectable in Spitzer photometry of the system — so the fact that we didn’t see anything like this supports the idea that the TRAPPIST-1 planets don’t have large moons.

    On the other hand, smaller moons (perhaps no more than 200–300 km in radius) would have escaped detection. Future long-term monitoring of TRAPPIST-1 with observatories like the James Webb Space Telescope or 30-meter-class ground-based telescopes will help constrain this possibility, however.

    Citation

    Stephen R. Kane 2017 ApJL 839 L19. doi:10.3847/2041-8213/aa6bf2

    There are further referenced articles of interest on the full article.

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  • richardmitnick 12:36 pm on April 18, 2017 Permalink | Reply
    Tags: AAS NOVA, ,   

    From AAS NOVA: “Samples and Statistics: Distinguishing Populations of Hot Jupiters in a Growing Dataset” 

    AASNOVA

    American Astronomical Society

    astrobites

    18 April 2017
    Jamila Pegues

    Title: Evidence for Two Hot Jupiter Formation Paths
    Authors: Benjamin E. Nelson, Eric B. Ford, and Frederic A. Rasio
    First Author’s Institution: Northwestern University

    Status: Submitted to AJ, open access

    5
    Figure 1: A gorgeous artist’s impression of a hot Jupiter orbiting around its host star. [ESO/L. Calçada]

    Frolicking Through Fields of Data

    The future of astronomy observations seems as bright as the night sky … and just as crowded! Over the next decade, several truly powerful telescopes are set to launch (read about a good number of them here and also here).

    1
    NASA/TESS

    Giant Magellan Telescope, Las Campanas Observatory, to be built some 115 km (71 mi) north-northeast of La Serena, Chile

    TMT-Thirty Meter Telescope, proposed for Mauna Kea, Hawaii, USA

    ESO/E-ELT,to be on top of Cerro Armazones in the Atacama Desert of northern Chile


    LSST Camera, built at SLAC



    LSST telescope, currently under construction at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

    NASA/ESA/CSA Webb Telescope annotated

    NASA/WFIRST

    FAST radio telescope, now operating, located in the Dawodang depression in Pingtang county Guizhou Province, South China

    That means we’re going to have a LOT of data on everything from black holes to galaxies, and beyond — and that’s in addition to the huge fields of data from the past decade that we’re already frolicking through now. It’s certainly far more data than any one astronomer (or even a group of astronomers) wants to analyze one-by-one; that’s why these days, astronomers turn more and more to the power of astrostatistics to characterize their data.

    The authors of today’s astrobite had that goal in mind. They explored a widely-applicable, data-driven statistical method for distinguishing different populations in a sample of data. In a sentence, they took a large sample of hot Jupiters and used this technique to try and separate out different populations of hot Jupiters — based on how the planets were formed — within their sample. Let’s break down exactly what they did, and how they did it, in the next few sections!

    Hot Jupiters Are Pretty Cool

    First question: what’s a hot Jupiter, anyway?

    They’re actually surprisingly well-named: essentially, they are gas-giant planets like Jupiter, but are much, much hotter. (Read all about them in previous astrobites, like this one and this other one!) Hot Jupiters orbit perilously close to their host stars — closer even than Mercury does in our own Solar System, for example. But it seems they don’t start out there. It’s more likely that these hot Jupiters formed out at several AU from their host stars, and then migrated inward into the much closer orbits from there.

    As to why hot Jupiters migrate inward … well, it’s still unclear. Today’s authors focused on two migration pathways that could lead to two distinct populations of hot Jupiters in their sample. These migration theories, as well as what the minimum allowed distance to the host star (the famous Roche separation distance, aRoche) would be in each case, are as follows:

    Disk migration: hot Jupiters interact with their surrounding protoplanetary disk, and these interactions push their orbits inward. In this context, aRoche corresponds to the minimum distance that a hot Jupiter could orbit before its host star either (1) stripped away all of the planet’s gas or (2) ripped the planet apart.
    Eccentric migration: hot Jupiters start out on very eccentric (as in, more elliptical than circular) orbits, and eventually their orbits morph into circular orbits of distance 2aRoche. In this context, aRoche refers to the minimum distance that a hot Jupiter could orbit before the host star pulled away too much mass from the planet.

    The authors defined a parameter ‘x’ for a given hot Jupiter to be x = a/aRoche, where ‘a’ is the planet’s observed semi-major axis. Based on the minimum distances in the above theories, we could predict that hot Jupiters that underwent disk migration would have a minimum x-value of x = aRoche/aRoche = 1. On the other hand, hot Jupiters that underwent eccentric migration would instead have a minimum x-value of x = 2aRoche/aRoche = 2. This x for a given planet is proportional to the planet’s orbital period ‘P’, its radius ‘R’, and its mass ‘M’ in the following way:

    And this x served as a key parameter in the authors’ statistical models!

    Toying with Bayesian Statistics

    Next question: how did today’s authors statistically model their data?

    4
    Figure 2: Probability distribution of x for each observation group, assuming that each hot Jupiter orbit was observed along the edge (like looking at the thin edge of a DVD). The bottom panel zooms in on the top one. Note how the samples have different minimum values! [Nelson et al. 2017]

    Short answer: with Bayesian statistics. Basically, the authors modeled how the parameter x is distributed within their planet sample with truncated power laws — so, x raised to some power, cut off between minimum and maximum x values. They split their sample of planets into two groups, based on the telescope and technique used to observe the planets: “RV+Kepler” and “HAT+WASP”. Figure 2 displays the distribution of x for each of the subgroups.

    The authors then used the Markov Chain Monte Carlo method (aka, MCMC; see the Bayesian statistics link above) to explore what sort of values of the power laws’ powers and cutoffs would well represent their data. Based on their chosen model form, they found that the RV+Kepler sample fit well with their model relating to eccentric migration. On the other hand, they found evidence that the HAT+WASP sample could be split into two populations: about 15% of those planets corresponded to disk migration, while the other 85% or so corresponded to eccentric migration.

    Remember that a major goal of today’s authors was to see if they could use this statistical approach to distinguish between planet populations in their sample … and in that endeavor, they were successful! The authors were thus optimistic about using this statistical technique for a much larger sample of hot Jupiters in the future, as oodles of data stream in from telescopes and surveys like KELT, TESS, and WFIRST over the next couple of decades.

    Their success joins the swelling toolbox of astrostatistics … and just in time! Telescopes of the present and very-near future are going to flood our computers with data — so unless we’re willing to examine every bright spot we observe in the sky by hand, we’ll need all the help from statistics that we can get!

    See the full article here .

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  • richardmitnick 3:29 pm on April 7, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , KIC 10403228, The Search for Ringed Exoplanets   

    From AAS NOVA: ” The Search for Ringed Exoplanets” 

    AASNOVA

    American Astronomical Society

    7 April 2017
    Susanna Kohler

    1
    Are there exoplanets that have rings similar to Saturn’s (seen in this stunning Cassini image)? A team of scientists proposes how to hunt for them. [Cassini Imaging Team/Mattias Malmer]

    Are planetary rings as common in our galaxy as they are in our solar system? A new study demonstrates how we might search for ringed exoplanets — and then possibly finds one!

    Saturns Elsewhere?

    2
    Artist’s illustration of the super ring system around exoplanet J1407b. This is the only exoplanet we’ve found with rings, but it’s not at all like Saturn. [Ron Miller]

    Our solar system is filled with moons and planetary rings, so it stands to reason that exoplanetary systems should exhibit the same features. But though we’ve been in the planet-hunting game for decades, we’ve only found one exoplanet that’s surrounded by a ring system. What’s more, that system — J1407b — has enormous rings that are vastly different from the modest, Saturn-like rings that we might expect to be more commonplace.

    Have we not discovered ringed exoplanets just because they’re hard to identify? Or is it because they’re not out there? A team of scientists led by Masataka Aizawa (University of Tokyo) has set out to answer this question by conducting a systematic search for rings around long-period planet candidates.

    3
    The transit light curve of KIC 10403228, shown with three models: the best-fitting planet-only model (blue) and the two best-fitting planet+ring models (green and red). [Aizawa et al. 2017]

    The Hunt Begins

    Why long-period planets? Rings are expected to be unstable as the planet gets closer to the central star. What’s more, the planet needs to be far enough away from the star’s warmth for the icy rings to exist. The authors therefore select from the collection of candidate transiting planets 89 long-period candidates that might be able to host rings.

    Aizawa and collaborators then fit single-planet models (with no rings) to the light curves of these planets and search for anomalies — curves that aren’t fit well by these standard models. Particularly suspicious characteristics include a long ingress/egress as the planet moves across the face of the star, and asymmetry of the transit shape.

    After applying a series of checks to eliminate false positives, the authors are left with one candidate: KIC 10403228.

    3
    Schematics of the two best-fitting ringed-exoplanet models for KIC 10403228, and the possible parameters of the system. The planet crosses the disk of the star from left to right with a grazing transit. [Adapted from Aizawa et al. 2017]

    Rings or Not?

    Next, the authors apply a wide range of ringed-exoplanet models to KIC 10403228’s light curve. They find two different scenarios that fit the data well: one in which the ring is significantly tilted with respect to the orbital plane, and another in which it’s only slightly tilted.

    The authors conclude by testing a variety of other scenarios that could explain the anomalies in the light curve instead. They find that two other scenarios are plausible: 1) the star is in an eclipsing binary system, with the second star surrounded by a circumstellar disk, and 2) the star is part of a hierarchical triple, and the transits are caused by a binary star system as it orbits KIC 10403228.

    Though Aizawa and collaborators aren’t able to rule either of these other two scenarios out, they suggest that follow-up spectroscopy or high-resolution imaging may help distinguish between the different scenarios. In the meantime, their methodology for systematically searching for ringed exoplanets has proven worthwhile, and they plan to extend it now to a larger data set. Perhaps we’ll soon find other Saturn-like planets in our galaxy!

    Citation

    Masataka Aizawa (逢澤正嵩) et al 2017 AJ 153 193. doi:10.3847/1538-3881/aa6336

    See the full article here .

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  • richardmitnick 1:20 pm on March 24, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , Challenging the Model for Galactic Bulges, , Galactic bulges   

    From AAS NOVA: “Challenging the Model for Galactic Bulges” 

    AASNOVA

    American Astronomical Society

    24 March 2017
    Susanna Kohler

    1
    The Sombrero Galaxy (Messier 104), an example of a galaxy with a large, classical central bulge. A recent study examines how bulges might grow in the centers of galaxies similar to our own. [ESO]

    Galaxies of similar stellar mass to our own don’t all have the same bulge and black hole masses. So what determines how much mass will end up in the bulge and the black hole at the center of a Milky-Way-like galaxy?

    The Role of Mergers

    One theory is that major and minor mergers build up the bulge and black-hole masses for some galaxies. It’s often argued that massive, centrally concentrated “classical” bulges are caused by merger activity, whereas less massive, more disk-like “pseudobulges” might be caused by other means, such as violent disk instabilities in early gas-rich disks, or misaligned infall of gas throughout cosmic time.

    2
    Bulge mass (top) and BH mass (bottom) as a function of stellar halo mass. Red denotes galaxies with low-mass pseudobulges, black shows galaxies with higher-mass classical bulges. The grey shaded area in the bottom plot shows what would be expected if there were a 1:1 correlation between bulge mass and stellar halo mass. [Bell et al. 2017]

    A team of scientists led by Eric Bell (University of Michigan) set out to test the role of major and minor mergers in bulge formation by examining the stellar halos of a sample of 18 Milky-Way-mass galaxies — six with classical bulges expected to have grown through mergers and 12 with pseudobulges expected to have grown through a variety of other mechanisms.

    Halos as Historical Record

    Stellar halos offer a useful way of tracking the merger history of a galaxy. It’s believed that as major mergers with larger satellites occur, a galaxy’s stellar halo will increase in both mass and metallicity as it retains the stars of the satellite.

    Bell and collaborators first verify this picture in their sample by plotting the stellar halo metallicities against the stellar halo masses. This check reveals a strong correlation between the two properties that’s consistent with the outcomes from simulations — so the stellar halos indeed encode the merger history of the galaxies. This means that from their halos, we can infer the masses of the largest satellites accreted by these galaxies.

    Laboratories for Quiet Accretion

    The authors then search for any indication of correlation between the stellar halo mass and the galaxy’s bulge mass or black hole mass. They find that their galaxy sample has a wide range in stellar halo masses that don’t correlate significantly with the bulge-to-total ratio, bulge mass, or black hole mass of the galaxy. This is true not only for the pseudobulges, but also for the classical bulges.

    4
    The galaxy Messier 81 has a massive classical bulge but an anemic stellar halo containing only 2% of its total stellar mass. This galaxy may be a useful laboratory for studying quiet accretion events. [Subaru Telescope (NAOJ)/HST/R. Colombari/R. Gendler]


    NAOJ/Subaru Telescope at Mauna Kea Hawaii, USA

    This outcome suggests that not even the classical bulges form primarily via minor and major merging activity. Instead, the bulges all form from a variety of mechanisms: a few are likely created by mergers, but the remainder are probably caused by quieter means like secular evolution, disk instabilities or misaligned gas accretion.

    These findings challenge the classical models of massive bulge formation and suggest that more detailed simulations and observations are necessary to unravel how the bulges and black holes at the centers of Milky-Way-like galaxies are grown. In particular, the galaxies with massive classical bulges but without massive stellar halos (the galaxy M81 is suggested as an example) may be ideal laboratories for studying quiet growth mechanisms.

    Citation

    Eric F. Bell et al 2017 ApJL 837 L8. doi:10.3847/2041-8213/aa6158

    See the full article here .

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  • richardmitnick 2:02 pm on March 15, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , Corona and Jet, ,   

    From AAS NOVA: “A Connection Between Corona and Jet” 

    AASNOVA

    American Astronomical Society

    15 March 2017
    Susanna Kohler

    1
    Artist’s impression of an AGN according to the unified model. Credit: ESA/NASA, the AVO project and Paolo Padovani

    The structure immediately around a supermassive black hole at the heart of an active galaxy can tell us about how material flows in and out of these monsters — but this region is hard to observe! A new study provides us with clues of what might be going on in these active and energetic cores of galaxies.

    In- and Outflows

    In active galactic nuclei (AGN), matter flows both in and out. As material flows toward the black hole via its surrounding accretion disk, much of this gas and dust can then be expelled from the vicinity via highly collimated jets.

    2
    Top: The fraction of X-rays that is reflected decreases as jet power increases. Bottom: the distance between the corona and the reflecting part of the disk increases as jet power increases. [Adapted from King et al. 2017]

    To better understand this symbiosis between accretion and outflows, we examine what’s known as the “corona” — the hot, X-ray-emitting gas that’s located in the closest regions around the black hole. But because the active centers of galaxies are generally obscured by surrounding gas and dust, it’s difficult for us to learn about the structure of these inner regions near the black hole.

    Where are the X-rays of the corona produced: in the inner accretion flow, or at the base of the jet? How far away is this corona from the disk? And how does the corona’s behavior relate to that of the jet?

    Reflected Observations

    To address some of these questions, a group of scientists led by Ashley King (Einstein Fellow at Stanford University) has analyzed X-ray observations from NuSTAR and XMM-Newton of over 40 AGN. The team examined the reflections of the X-rays off of the accretion disk and used two measurements to learn about the structure around the black hole:

    the fraction of the corona’s X-rays that are reflected by the disk, and
    the time lag between the original and reflected X-rays, which reveals the distance from the corona to the reflecting part of the disk.

    3
    A visualization of the authors’ model for an AGN. The accretion disk is red, corona is green, and jet is blue. The corona shines on the disk, causing the inner regions (colored brighter) to fluoresce, “reflecting” the radiation. As the accretion rate increases from the top to the bottom panel, the jet power increases and the dominant reflective part of the disk moves outward due to the ionization of the inner region (which puffs up into a torus). [Adapted from King et al. 2017]

    King and collaborators find two interesting relationships between the corona and the jet: there is an inverse correlation between jet power and reflection fraction, and there is a correlation between jet power and the distance of the corona from the reflecting part of the disk the disk. These observations indicate that there is a relationship between changes in the corona and jet production in AGN.

    Modeling the Corona

    The authors use these observations to build a self-consistent model of an AGN’s corona. In their picture, the corona is located at the base of the jet and moves mildly relativistically away from the disk, propagating into the large-scale jets.

    As the velocity of the corona increases, more of its radiation is relativistically beamed away from the accretion disk, which decreases the fraction of X-rays that are reflected — explaining the inverse correlation between jet power and reflection fraction.

    At the same time, the increased mass accretion further ionizes the inner disk region, pushing the dominant reflection region to further out in the disk — which explains the correlation between jet power and the distance from corona to reflection region.

    King and collaborators show that this model is fully consistent with the X-ray observations of the 40 AGN they examined. Future X-ray observations of the strongest radio jet sources will help us to further pin down what’s happening at the heart of active galaxies.

    Citation

    Ashley L. King et al 2017 ApJ 835 226. doi:10.3847/1538-4357/835/2/226

    See the full article here .

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  • richardmitnick 12:42 pm on March 10, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , Building Magnetic Fields in White Dwarfs,   

    From AAS NOVA: “Building Magnetic Fields in White Dwarfs” 

    AASNOVA

    American Astronomical Society

    10 March 2017
    Susanna Kohler

    1
    Some white dwarfs have magnetic fields with strengths ranging from kilogauss to gigagauss. A new study examines how these fields form. [ESO/L. Calçada]

    White dwarfs, the compact remnants left over at the end of low- and medium-mass stars’ lifetimes, are often found to have magnetic fields with strengths ranging from thousands to billions of times that of Earth. But how do these fields form?

    Multiple Possibilities

    Around 10–20% of white dwarfs have been observed to have measurable magnetic fields with a wide range of strengths. There are several theories as to how these fields might be generated:

    The fields are “fossil”.
    The original weak magnetic fields of the progenitor stars were amplified as the stars’ cores evolved into white dwarfs.
    The fields are caused by binary interactions.
    White dwarfs that formed in the merger of a binary pair might have had a magnetic field amplified as a result of a dynamo that was generated during the merger.
    The fields were produced by some other internal physical mechanism during the cooling of the white dwarf itself.

    In a recent publication, a team of authors led by Jordi Isern (Institute of Space Sciences, CSIC, and Institute for Space Studies of Catalonia, Spain) explored this third possibility.

    2
    The inner and outer boundaries of the convective mantle of carbon/oxygen white dwarfs of two different masses (top vs. bottom panel) as a function of luminosity. As the white dwarf cools (toward the right), the mantle grows thinner due to the crystallization and settling of material. [Isern et al. 2017]

    Dynamos from Crystallization

    As white dwarfs have no nuclear fusion at their centers, they simply radiate heat and gradually cool over time. The structure of the white dwarf undergoes an interesting change as it cools, however: though the object begins as a fluid composed primarily of an ionized mixture of carbon and oxygen (and a few minor species like nickel and iron), it gradually crystallizes as its temperature drops.

    The crystallized phase of the white dwarf is oxygen-rich — which is denser than the liquid, so the crystallized material sinks to the center of the dwarf as it solidifies. As a result, the white dwarf forms a solid, oxygen-rich core with a liquid, carbon-rich mantle that’s Rayleigh-Taylor unstable: as crystallization continues, the solids continue to sink out of the mantle.

    By analytically modeling this process, Isern and collaborators demonstrate that the Rayleigh-Taylor instabilities in the convective mantle can drive a dynamo large enough to generate the magnetic field strengths we’ve observed in white dwarfs.

    3
    Magnetic field density as a function of the dynamo energy density. The plots show Earth and Jupiter (black dots), T Tauri stars (cyan), M dwarf stars (magenta), and two types of white dwarfs (blue and red). Do these lie on the same scaling relation? [Isern et al. 2017]

    A Universal Process?

    This setup — the solid core with an unstable liquid mantle on top — is exactly the structure expected to occur in planets such as Earth and Jupiter. These planets’ magnetic fields are similarly thought to be generated by convective dynamos powered by the cooling and chemical separation of their interiors — and the process can also be scaled up to account for the magnetic fields of fully convective objects like T Tauri stars, as well.

    If white-dwarf magnetic fields are generated by the same type of dynamo, this may be a universal process for creating magnetic fields in astrophysical objects — though other processes may well be at work too.

    Citation

    Jordi Isern et al 2017 ApJL 836 L28. doi:10.3847/2041-8213/aa5eae

    See the full article here .

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  • richardmitnick 4:45 pm on March 8, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , , The Discovery of Five Early Gamma-Ray Blazars   

    From AAS NOVA: “The Discovery of Five Early Gamma-Ray Blazars” 

    AASNOVA

    American Astronomical Society

    8 March 2017
    Susanna Kohler

    1
    An artist’s impression of a quasar, in which accretion onto a supermassive black hole powers a high-speed jet. A new study has discovered five such galactic nuclei from the early universe that are pointed toward us. [NASA/Dana Berry (SkyWorks Digital)]

    Due to a recent software improvement, the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope has discovered five gamma-ray blazars at high redshifts, opening a window to the early universe.


    NASA/Fermi LAT


    NASA Fermi Telescope

    Spotting Pointed Activity

    Quasars, the active centers of some distant galaxies, shine brightly across the electromagnetic spectrum. These luminous objects are fueled by material accreting onto a supermassive black hole, and the release of energy in this accretion launches powerful jets that, in the case of a type of quasar called a “blazar”, point along our line of sight — causing them to be relativistically boosted to look especially bright. The jet radiation dominates blazar broadband emission, especially at gamma-ray wavelengths.

    Fermi-LAT has detected thousands of blazars in gamma rays; in fact, blazars have been found to be the most numerous gamma-ray population in our sky. In spite of this, Fermi hasn’t found any gamma-ray blazars at a redshift greater than z = 3.1 — likely because at this distance, the blazars’ gamma-ray emission is redshifted to lower frequencies at which the LAT is less sensitive.

    A recent, spectacular set of improvements to Fermi data analysis, however, known as Pass 8, has substantially enhanced the sensitivity of LAT to gamma rays across the spectrum — with particular improvement at lower frequencies. Motivated by this increased sensitivity, a team of Fermi scientists has used the data from Pass 8 to search for especially distant gamma-ray-bright blazars.

    2
    Maps of the five high-redshift blazars detected by Fermi-LAT. [Ackermann et al. 2017]

    Window to the Early Universe

    The Fermi team began by selecting high-redshift radio-loud blazars from a known catalog of over a million quasars. They then searched for these ~1100 sources within the 92 months of LAT data produced by Pass 8.

    This systematic search led to the detection of five new gamma-ray sources consistent with the positions of blazars at redshifts greater than z = 3.1 — including NVSS J151002+570243, which now qualifies as the most distant gamma-ray blazar known, at a redshift of z = 4.31.

    Analysis of the sources’ spectral energy distributions verifies that they have all the properties expected of especially powerful blazars, confirming their identity. Modeling of their spectra suggests they harbor massive black holes in the range of hundreds of millions to tens of billions of solar masses. The properties of these sources allow the authors to estimate the space density of massive black holes hosted in jetted systems: roughly 70 per cubic gigaparsec.

    Though these five new gamma-ray blazars may constitute a small sample, they provide information that can be used to begin to constrain our models of how supermassive black holes formed in the early universe. They’re also a shining example of the remarkable benefit possible with clever software improvements!

    Citation

    M. Ackermann et al 2017 ApJL 837 L5. doi:10.3847/2041-8213/aa5fff

    See the full article here .

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  • richardmitnick 1:48 pm on March 3, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , , , Where Do Messy Planetary Nebulae Come From?   

    From AAS NOVA: ” Where Do Messy Planetary Nebulae Come From?” 

    AASNOVA

    American Astronomical Society

    3 March 2017
    Susanna Kohler

    1
    This Hubble image shows the planetary nebula NGC 5189. This nebula’s strong asymmetry suggests it was very likely formed by a triple stellar system. [NASA/ESA/Hubble Heritage Team (STScI/AURA)]

    If you examined images of planetary nebulae, you would find that many of them have an appearance that is too “messy” to be accounted for in the standard model of how planetary nebulae form. So what causes these structures?

    2
    Examples of planetary nebulae that have a low probability of having been shaped by a triple stellar system. They are mostly symmetric, with only slight departures (labeled) that can be explained by instabilities, interactions with the interstellar medium, etc. [Bear and Soker 2017]

    A Range of Looks

    At the end of a star’s lifetime, in the red-giant phase, strong stellar winds can expel the outer layers of the star. The hot, luminous core then radiates in ultraviolet, ionizing the gas of the ejected stellar layers and causing them to shine as a brightly colored “planetary nebula” for a few tens of thousands of years.

    Planetary nebulae come in a wide variety of morphologies. Some are approximately spherical, but others can be elliptical, bipolar, quadrupolar, or even more complex.

    It’s been suggested that non-spherical planetary nebulae might be shaped by the presence of a second star in a binary system with the source of the nebula — but even this scenario should still produce a structure with axial or mirror symmetry.

    A pair of scientists from Technion — Israel Institute of Technology, Ealeal Bear and Noam Soker, argue that planetary nebulae with especially messy morphologies — those without clear axial or point symmetries — may have been shaped by an interacting triple stellar system instead.

    Technion bloc

    3
    Examples of planetary nebulae that might have been shaped by a triple stellar system. They have some deviations from symmetry but also show signs of interacting with the interstellar medium. [Bear and Soker 2017]

    Departures from Symmetry

    To examine this possibility more closely, Bear and Soker look at a sample of thousands planetary nebulae and qualitatively classify each of them into one of four categories, based on the degree to which they show signs of having been shaped by a triple stellar progenitor. The primary signs the authors look for are:

    1. Symmetries
    If a planetary nebula has a strong axisymmetric or point-symmetric structure (i.e., it’s bipolar, elliptical, spherical, etc.), it was likely not shaped by a triple progenitor. If clear symmetries are missing, however, or if there is a departure from symmetry in specific regions, the morphology of the planetary nebula may have been shaped by the presence of stars in a close triple system.

    2.Interaction with the interstellar medium
    Some asymmetries, especially local ones, can be explained by interaction of the planetary nebula with the interstellar medium. The authors look for signs of such an interaction, which decreases the likelihood that a triple stellar system need be involved to produce the morphology we observe.

    4
    Examples of planetary nebulae that are extremely likely to have been shaped by a triple stellar system. They have strong departures from symmetry and don’t show signs of interacting with the interstellar medium. [Bear and Soker 2017]

    Influential Trios

    From the images in two planetary nebulae catalogs — the Planetary Nebula Image Catelog and the HASH catalog — Bear and Soker find that 275 and 372 planetary nebulae are categorizable, respectively. By assigning crude probabilities to their categories, the authors estimate that the total fraction of planetary nebulae shaped by three stars in a close system is around 13–21%.

    The authors argue that in some cases, all three stars might survive. This means that we may be able to find direct evidence of these triple stellar systems lying in the hearts of especially messy planetary nebulae.
    Citation

    Ealeal Bear and Noam Soker 2017 ApJL 837 L10. doi:10.3847/2041-8213/aa611c

    See the full article here .

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  • richardmitnick 2:52 pm on February 22, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , Don’t Underestimate Tiny Telescopes, , Kelt-North Telescope, The Kilogree Extremely Little Telescope (KELT) consists of two telescopes — one in Arizona and one in South Africa — that each have a 4.2-centimeter aperture   

    From AAS NOVA: ” A Planet Soon to Meet Its Demise” 

    AASNOVA

    American Astronomical Society

    22 February 2017
    Susanna Kohler

    1
    Artist’s impression of a transiting hot-Jupiter planet. The recently discovered KELT-16b orbits so close to its host that it zips around it in less than a day! [NASA/ESA/G. Bacon (STScI)]

    A tiny telescope has discovered a scalding hot world orbiting its star 1,300 light-years from us. KELT-16b may only be around for a few more hundreds of thousands of years, however.

    KELT North Kilodegree Extremely Little Telescope at WINER Observatory in Arizona, USA c J.Peppe
    KELT North Kilodegree Extremely Little Telescope at WINER Observatory in Arizona, USA

    The KELT-North telescope in Arizona. This tiny telescope was responsible for the discovery of KELT-16b. [Vanderbilt University]

    Don’t Underestimate Tiny Telescopes

    In an era of ever larger observatories, you might think that there’s no longer a place for small-aperture ground-based telescopes. But small ground-based telescopes have been responsible for the discovery and characterization of around 250 exoplanets so far — and these are the targets that are especially useful for exoplanet science, as they are more easily followed up than the faint discoveries made by telescopes like Kepler.

    The Kilogree Extremely Little Telescope (KELT) consists of two telescopes — one in Arizona and one in South Africa — that each have a 4.2-centimeter aperture. In total, KELT observes roughly 70% of the entire sky searching for planets transiting bright hosts. And it’s recently found quite an interesting one: KELT-16b. In a publication led by Thomas Oberst (Westminster College in Pennsylvania), a team of scientists presents their find.

    2
    Combined follow-up light curves obtained for KELT-16b from 19 transits. The best-fit period is just under a day. [Oberst et al. 2017]

    A Hot World

    KELT-16b is what’s known as a hot Jupiter. Using the KELT data and follow-up observations of 19 transits, Oberst and collaborators estimate KELT-16b’s radius at roughly 1.4 times that of Jupiter and its mass at 2.75 times Jupiter’s. Its equilibrium temperature is a scalding 2453 K — caused by the fact that it orbits so close to its host star that it completes each orbit in a mere 0.97 days!

    This short period is extremely unusual: there are only five other known transiting exoplanets with periods shorter than a day. KELT-16b is orbiting very close to its host, making it subject to extreme irradiation and strong tidal forces.

    Based on KELT-16b’s orbit, Oberst and collaborators estimate that the planet began a runaway inspiral by the age of 1 billion years. Now, at ~3.1 billion years old, KELT-16b is orbiting at a radius of just over 3 stellar radii above its host’s surface. The authors estimate that KELT-16b’s continuing inward spiral could end in the planet’s destruction by tidal forces in as little as another 550,000 years.

    3
    KELT-16b in context with other transiting-exoplanet discoveries on a diagram of planet radius vs. period. Only five other planets have been found with periods shorter than a day. [Oberst et al. 2017]

    What We Can Learn from KELT-16b

    This highly irradiated world makes for an especially useful target due to its short period (which means we can observe many transits) and bright host (which means follow-up observations are more convenient and have a large signal-to-noise ratio).

    In particular, with followup observations of KELT-16b from missions like Hubble, Spitzer, and eventually the James Webb Space Telescope, we can learn more about open questions in exoplanet atmospheric processes — like how heat is transferred vertically through the atmosphere, or what happens at the day-to-night terminator line on such a highly irradiated planet.

    In addition, by studying KELT-16b, we can hope to gain overall insight into hot Jupiter formation and migration. The ease of observing this planet and the wealth of information it can provide will likely make it one of the top-studied exoplanets. KELT-16b has a lot to teach us before it’s torn apart!

    Citation

    Thomas E. Oberst et al 2017 AJ 153 97. doi:10.3847/1538-3881/153/3/97

    See the full article here .

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  • richardmitnick 5:21 pm on February 20, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , , Structures in the Interstellar Medium   

    From AAS NOVA: “Featured Image: Structures in the Interstellar Medium” 

    AASNOVA

    American Astronomical Society

    20 February 2017
    Susanna Kohler

    1

    This beautiful false-color image (which covers ~57 degrees2; click for the full view!) reveals structures in the hydrogen gas that makes up the diffuse atomic interstellar medium at intermediate latitudes in our galaxy. The image was created by representing three velocity channels with colors — red for gas moving at 7.59 km/s, green for 5.12 km/s, and blue for 2.64 km/s — and it shows the dramatically turbulent and filamentary structure of this gas. This image is one of many stunning, high-resolution observations that came out of the DRAO HI Intermediate Galactic Latitude Survey, a program that used the Synthesis Telescope at the Dominion Radio Astrophysical Observatory in British Columbia to map faint hydrogen emission at intermediate latitudes in the Milky Way.

    Synthesis Telescope at the Dominion Radio Astrophysical Observatory in BC,CA
    Synthesis Telescope at the Dominion Radio Astrophysical Observatory in BC,CA

    The findings from the program were recently published in a study led by Kevin Blagrave (Canadian Institute for Theoretical Astrophysics, University of Toronto); to find out more about what they learned, check out the paper below!
    Citation

    K. Blagrave et al 2017 ApJ 834 126. doi:10.3847/1538-4357/834/2/126

    See the full article here .

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