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  • richardmitnick 8:03 pm on May 26, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , , The Carina Nebula   

    From AAS NOVA: ” Observations of a Windy Star” 

    AASNOVA

    American Astronomical Society

    26 May 2017
    Susanna Kohler

    1
    The Carina Nebula, as seen by the 1.5-m Danish telescope at ESO’s La Silla Observatory. Eta Carinae is the brightest star in the image. [ESO/IDA/Danish 1.5 m/R.Gendler, J-E. Ovaldsen, C. Thöne, and C. Feron]

    3
    2
    ESO LaSilla 1.5 meter Danish telescope

    4
    Eta Carinae. N. Smith / J.A. Morse (U. Colorado) et al. / NASA

    The incredibly luminous massive star Eta Carinae has long posed a challenge for astronomers to model. New observations are now in … so were our models correct?

    Dramatic Target

    The massive evolved star Eta Carinae, located ~7,500 light-years away in the constellation Carina, is the most luminous star in the Milky Way. Eta Carinae has a quite a reputation for drama: it has been very unstable in the past, exhibiting repeated eruptions that have created the spectacular Homunculus Nebula surrounding it. Its present-day wind has the highest mass-loss rate of any hot star we’ve observed.

    Picture of Stellar Wind

    5
    Top panel: February 2017 observations of Eta Carinae in continuum (left) and H-alpha. Middle panel: the normalized radial profile for H-alpha and continuum emission. Bottom panel: the full width at half maximum for H-alpha and continuum emission of Eta Carinae. The H-alpha is about 2.5 to 3 milliarcseconds wider than the continuum. [Adapted from Wu et al. 2017]

    In our goal to understand the late evolutionary phases of very massive stars, we’ve developed radiative-transfer models to explain the behavior of Eta Carinae. One of the most well-known models, developed by John Hillier and collaborators in 2001, describes Eta Carinae’s mass loss via stellar winds. With the right observations, this model is testable, since it predicts observable locations for different types of emission. In particular, one prediction of the Hillier et al. model is that the dense, ionized winds surrounding the star should emit in H-alpha at distances between 6 and 60 AU, with a peak around 20 AU.

    This nicely testable hypothesis is rendered less convenient by the fact that it’s hard to get resolved images of Eta Carinae’s H-alpha emission. Its distance from us — and the fact that it’s shrouded in the complex nebula it created — have thus far prevented us from resolving the H-alpha emission from this star. Now, however, a team of scientists from Steward Observatory, University of Arizona have changed this.

    Confirming the Model

    Led by Ya-Lin Wu, the team obtained diffraction-limited images of Eta Carinae using the Magellan adaptive optics system. The observations, made in both H-alpha and continuum, show that the H-alpha emitting region is significantly wider than the continuum emitting region, as predicted by the model. In fact, the measured emission implies that the H-alpha line-forming region may have a characteristic emitting radius of ~25–30 AU — in very good agreement with the Hillier et al. stellar-wind model.

    This confirmation is strong support of the physical wind parameters estimated for Eta Carinae in the model, like the mass-loss rate of 10^-3 solar masses per year. These parameters are enormously helpful as we attempt to understand the physics of strong stellar-wind mass loss and the late evolutionary phases of very massive stars.
    Citation

    Ya-Lin Wu et al 2017 ApJL 841 L7. doi:10.3847/2041-8213/aa70ed

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    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 12:53 pm on May 22, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , Rapid Rotation of a Heavy White Dwarf, SDSSJ0837+1856   

    From AAS NOVA: “Rapid Rotation of a Heavy White Dwarf” 

    AASNOVA

    American Astronomical Society

    22 May 2017
    Susanna Kohler

    1
    Artist’s illustration of a white dwarf. A recent study has measured the heaviest and fastest-rotating pulsating isolated white dwarf known. [NASA/ESA]

    New Kepler observations of a pulsating white dwarf have revealed clues about the rotation of intermediate-mass stars.

    NASA/Kepler Telescope

    Learning About Progenitors

    Stars weighing in at under ~8 solar masses generally end their lives as slowly cooling white dwarfs. By studying the rotation of white dwarfs, therefore, we are able to learn about the final stages of angular momentum evolution in these progenitor stars.

    Most isolated field white dwarfs cluster in mass around 0.62 solar masses, which corresponds to a progenitor mass of around 2.2 solar masses. This abundance means that we’ve already learned a good deal about the final rotation of low-mass (1–3 solar-mass) stars. Our knowledge about the angular momentum of intermediate-mass (3–8 solar masses) stars, on the other hand, remains fairly limited.

    2
    Fourier transform of the pulsations from SDSSJ0837+1856. The six frequencies of stellar variability, marked with red dots, reveal a rotation period of 1.13 hours. [Hermes et al. 2017]

    Record-Breaking Find

    A newly discovered white dwarf, SDSSJ0837+1856, is now helping to shed light on this mass range. SDSSJ0837+1856 appears to be unusually massive: it’s measured at 0.87 solar masses, which corresponds to a progenitor mass of roughly 4.0 solar masses. Determining the rotation of this white dwarf would therefore tell us about the final stages of angular momentum in an intermediate-mass star.

    In a new study led by J.J. Hermes (Hubble Fellow at University of North Carolina, Chapel Hill), a team of scientists presents a series of measurements of SDSSJ0837+1856 that suggest it’s the highest-mass and fastest-rotating isolated pulsating white dwarf known.

    3
    Histogram of rotation rates determined from the asteroseismology of pulsating white dwarfs (marked in red). SDSSJ0837+1856 (indicated in black) is more massive and rotates faster than any other known pulsating white dwarf. [Hermes et al. 2017]

    Rotation from Pulsations

    Why pulsating? In the absence of measurable spots and other surface features, the way we measure the rotation rate of a star is using asteroseismology. In this process, observations of a star’s tiny oscillations can reveal information about its internal structure and rotation.

    Hermes and collaborators used Kepler K2 observations spanning nearly 75 days — in addition to ground-based follow-up and spectroscopy — to estimate the white dwarf’s rotation period based on its observed internal pulsations. The resulting rotation rate, 1.13 ± 0.02 hours, is the fastest rotation period ever measured for an isolated pulsating white dwarf.

    Placing SDSSJ0837+1856 in the context of other white-dwarf rotation period measurements, the authors argue that there seems to be a connection between the highest-mass white dwarfs and the fastest rotators. More observations of this kind will help us to determine whether this is a general trend that tells us something significant about the angular momentum evolution of intermediate-mass stars.

    Citation

    J. J. Hermes et al 2017 ApJL 841 L2. doi:10.3847/2041-8213/aa6ffc

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    1

    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 11:50 am on May 17, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , Measuring Sirius: An Exercise in Patience   

    From AAS NOVA: ” Measuring Sirius: An Exercise in Patience” 

    AASNOVA

    American Astronomical Society

    1
    The white dwarf Sirius B — visible as a tiny spot to the bottom left of Sirius A in this Hubble image of the binary system — is 9,200 times fainter than its companion, making it very difficult to image! [Bond et al. 2017]

    NASA/ESA Hubble Telescope

    Sometimes important astronomical advances require the newest and fanciest observatories and technologies — but sometimes they just require decades of work and a lot of patience. Patience is finally paying off for a team of scientists who have been observing the Sirius star system for nearly 20 years.

    3
    Historical (black, blue and green) and Hubble (red) observations of the relative orbit of Sirius B around Sirius A. [Adapted from Bond et al. 2017]

    Bright Neighbors

    Located a mere 8.5 light-years away, the Sirius system consists of the main-sequence star Sirius A and its white-dwarf companion Sirius B. Sirius A is the brightest star in our sky, and Sirius B is the brightest and nearest white dwarf we’ve observed. The unusual proximity and brightness of these stars make them excellent targets for learning about stellar and white-dwarf astrophysics.

    In order to interpret our observations, however, we first need to pin down the basic information about these stars. In particular, we want to measure the precise masses and orbital elements for the system — but because the stars orbit each other only once every ~50 years, these properties take time to measure well!

    Toward this end, a team of scientists began an observing campaign in 2001 to regularly image the Sirius system using the Hubble Space Telescope. Now, 16 years later, they have enough data to make precise statements about the system.

    4
    Comparisons of white-dwarf theory with the observed parameters of Sirius B, both on the H-R diagram (top) and in a mass-radius plot of cooling white dwarfs (bottom). Sirius B’s measured parameters matches the theoretical models very well. [Bond et al. 2017]

    Precision Measurements at Last

    In a recent publication led by Howard Bond (Pennsylvania State University and Space Telescope Science Institute), the team details nearly two decades of precise photometric and astrometric measurements using Hubble. In addition, they supplemented these data by dredging through 150 years’ worth of historical observations of Sirius and critically analyzing 2,300 of these as well.

    The result? Bond and collaborators were able to make very precise measurements of the masses of Sirius A and Sirius B — 2.063 ± 0.023 and 1.018 ± 0.011 solar masses, respectively — and of their orbital elements. They find that the position of Sirius B on the Hertzsprung-Russell diagram is beautifully consistent with models based on cooling white dwarfs of Sirius B’s measured mass. Similarly, stellar models of Sirius A are nicely consistent with Bond and collaborators’ measurements if the star has a slightly low metallicity of ~85% that of the Sun.

    The high-precision measurements also allowed the authors rule out the possibility of a third body in the system — an idea that’s been tossed around for decades — unless the third body is smaller than 15–25 Jupiter masses.

    Bond and collaborators enumerate some open puzzles of the Sirius system, such as like conflicting signs that the two stars might have interacted, long ago. Though these puzzles remain unresolved, the painstaking decades of observations of Sirius have already revealed much about the system and improved our understanding of stellar evolution. What’s more, these measurements give us an ideal launching point for future studies of these two objects. In the case of the Sirius system, patience has definitely paid off.

    Citation

    Howard E. Bond et al 2017 ApJ 840 70. doi:10.3847/1538-4357/aa6af8

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    1

    AAS Mission and Vision Statement

    The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

    The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
    The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
    The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
    The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
    The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

    Adopted June 7, 2009

     
  • richardmitnick 1:00 pm on May 12, 2017 Permalink | Reply
    Tags: AAS NOVA, Are LIGO’s Black Holes Made From Smaller Black Holes?, , , , ,   

    From AAS NOVA: “Are LIGO’s Black Holes Made From Smaller Black Holes?” 

    AASNOVA

    American Astronomical Society

    1
    A still image from a simulation that shows a black-hole binary inside a globular cluster. A new study examines how we can tell whether the black holes detected by LIGO were formed hierarchically from mergers of smaller black holes. [Northwestern Visualization/Carl Rodriguez]

    The recent successes of the Laser Interferometer Gravitational-Wave Observatory (LIGO) has raised hopes that several long-standing questions in black-hole physics will soon be answerable. Besides revealing how the black-hole binary pairs are built, could detections with LIGO also reveal how the black holes themselves form?


    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


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


    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project


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

    Isolation or Hierarchy

    The first detection of gravitational waves, GW150914, was surprising for a number of reasons. One unexpected result was the mass of the two black holes that LIGO saw merging: they were a whopping 29 and 36 solar masses.

    2
    On the left of this schematic, two first-generation (direct-collapse) black holes form a merging binary. The right illustrates a second-generation hierarchical merger: each black hole in the final merging binary was formed by the merger of two smaller black holes. [Adapted from Gerosa et al., a simultaneously published paper that also explores the problem of hierarchical mergers and reaches similar conclusions]

    How do black holes of this size form? One possibility is that they form in isolation from the collapse of a single massive star. In an alternative model, they are created through the hierarchical merger of smaller black holes, gradually building up to the size we observed.

    A team of scientists led by Maya Fishbach (University of Chicago) suggests that we may soon be able to tell whether or not black holes observed by LIGO formed hierarchically. Fishbach and collaborators argue that hierarchical formation leaves a distinctive signature on the spins of the final black holes — and that as soon as we have enough merger detections from LIGO, we can use spin measurements to statistically determine if LIGO black holes were formed hierarchically.

    Spins from Major Mergers

    When two black holes merge, both their original spins and the angular momentum of the pair contribute to the spin of the final black hole that results. Fishbach and collaborators calculate the expected distribution of these final spins assuming that all the hierarchical mergers are so-called “major mergers” — i.e., the smaller black hole of the pair is at least 70% of the mass of the larger one.

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    Distribution of spins for 4th-generation mergers, with two different mass ratios (q = 0.7 and q = 1) and initial first-generation spins (non-spinning and maximally spinning). [Fishbach et al. 2017]

    The authors find that hierarchical major mergers result in a distribution of spins with a distinctive shape, peaking at a spin of a ~ 0.7 with relatively low contribution from spins below a ~ 0.5. Intriguingly, this distribution is universal — if you include several generations of mergers, the resulting spin distribution converges to the same shape every time. This is true regardless of the details of the hierarchical merger scenario, like the exact black hole mass ratio (as long as only major mergers occur) or the initial spin distributions.

    Testing the Model

    What does this tell us? Since the hierarchical merger model predicts a very specific distribution of spins for the black holes detected by LIGO, we can compare future LIGO detections to see if they’re consistent with this model.

    The authors calculate the statistics to show that after order ~100 LIGO detections, we should be able to tell whether these black holes are consistent with a hierarchical merger formation model or not. With luck, this could mean that we will have solved this mystery within a few years of advanced LIGO operations!

    Citation

    Maya Fishbach et al 2017 ApJL 840 L24. doi:10.3847/2041-8213/aa7045

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  • richardmitnick 11:25 am on May 8, 2017 Permalink | Reply
    Tags: AAS NOVA, , ,   

    From AAS NOVA: ” Escape for the Slow Solar Wind” 

    AASNOVA

    American Astronomical Society

    8 May 2017
    Susanna Kohler

    1
    This Solar Dynamics Observatory extreme ultraviolet image of the Sun reveals a coronal hole — a region of open magnetic field — surrounded by regions of closed magnetic field. A new study examines how plasma might escape from regions of closed magnetic field on the Sun. [SDO; adapted from Higginson et al. 2017]

    NASA/SDO

    Plasma from the Sun known as the slow solar wind has been observed far away from where scientists thought it was produced. Now new simulations may have resolved the puzzle of where the slow solar wind comes from and how it escapes the Sun to travel through our solar system.

    An Origin Puzzle

    The Sun’s atmosphere, known as the corona, is divided into two types of regions based on the behavior of magnetic field lines. In closed-field regions, the magnetic field is firmly anchored in the photosphere at both ends of field lines, so traveling plasma is confined to coronal loops and must return to the Sun’s surface. In open-field regions, only one end of each magnetic field line is anchored in the photosphere, so plasma is able to stream from the Sun’s surface out into the solar system.

    This second type of region — known as a coronal hole — is thought to be the origin of fast-moving plasma measured in our solar system and known as the fast solar wind. But we also observe a slow solar wind: plasma that moves at speeds of less than 500 km/s.

    The slow solar wind presents a conundrum. Its observational properties strongly suggest it originates in the hot, closed corona rather than the cooler, open regions. But if the slow solar wind plasma originates in closed-field regions of the Sun’s atmosphere, then how does it escape from the Sun?

    Slow Wind from Closed Fields

    A team of scientists led by Aleida Higginson (University of Michigan) has now used high-resolution, three-dimensional magnetohydrodynamic simulations to show how the slow solar wind can be generated from plasma that starts out in closed-field parts of the Sun.

    Motions on the Sun’s surface near the boundary between open and closed-field regions — the boundary that marks the edges of coronal holes and extends outward as the heliospheric current sheet — are caused by supergranule-like convective flows. These motions drive magnetic reconnection that funnel plasma from the closed-field region onto enormous arcs that extend far away from the heliospheric current sheet, spanning tens of degrees in latitude and longitude.

    The simulations by Higginson and collaborators demonstrate that closed-field plasma from coronal-hole boundaries can be successfully channeled into the solar system. Due to the geometry and dynamics of the coronal holes, the plasma can travel far from the heliospheric current sheet, resulting in a slow solar wind of closed-field plasma consistent with our observations. These simulations therefore suggest a process that resolves the long-standing puzzle of the slow solar wind.

    Citation

    A. K. Higginson et al 2017 ApJL 840 L10. doi:10.3847/2041-8213/aa6d72

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  • richardmitnick 12:40 pm on May 5, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , Curious Case of a Stripped Elliptical Galaxy, ESO/VLT MUSE Instrument   

    From AAS NOVA: “Curious Case of a Stripped Elliptical Galaxy” 

    AASNOVA

    American Astronomical Society

    5 May 2017
    Susanna Kohler

    1
    The observations of this elliptical galaxy in Abell 2670 reveal long ionized gas tails emanating from the disk. What caused this galaxy’s unusual appearance? [Adapted from Sheen 2017]

    An elliptical galaxy in the cluster Abell 2670 has been discovered with some unexpected features. What conditions led to this galaxy’s unusual morphology?

    2
    MUSE fields of view (1′ × 1′ for each square) are superimposed on a pseudo-color image of the elliptical galaxy in Abell 2670. The blue blobs lie in the opposite direction to the galactic center. [Sheen et al. 2017]

    Unexpected Jellyfish

    We often see galaxies that have been disrupted or reshaped due to their motion within a cluster — but these are usually late-type galaxies like our own. Such gas-rich galaxies are distorted by ram pressure as they fall into the cluster center, growing long tidal tails of stripped gas and young stars that earn them the name “jellyfish galaxies”.

    But early-type, elliptical galaxies have long since used up or cleared out most of their gas, and they correspondingly form very few new stars. It’s therefore unsurprising that they’ve never before been spotted to have jellyfish-like features.

    3
    Panels a and b show zoomed-in observations of some of the star-forming blobs with tadpole-like morphology. Panel c shows a schematic illustration of how ram-pressure stripping causes this shape. [Adapted from Sheen et al. 2017]

    New deep observations of an elliptical galaxy in the cluster Abell 2670, however, have revealed some unexpected structures for an early-type galaxy. Led by Yun-Kyeong Sheen (Korea Astronomy and Space Science Institute), a team of scientists now reports on the optical and spectroscopic observations of this galaxy, made with the MUSE instrument on the Very Large Telescope in Chile.

    ESO MUSE on the VLT

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

    Tadpole Blobs

    These observations reveal a number of features, including starbursts at the galactic center, 80-parsec-long tidal tails of ionized gas, disturbed halo features, and several blue star-forming blobs with tadpole-like morphology in the surrounding region. The blobs have stellar tails that point in the direction of motion of the galaxy (toward the cluster center) and streams of ionized gas that point in the opposite direction.

    All of these features are signs that this galaxy is being tidally stripped as it falls into the center of the cluster. The star-forming blobs, for example, are exhibiting classic ram-pressure-stripping behavior: as a galaxy falls into the cluster center, streams of ionized gas blow downwind, and stars (which don’t respond as easily to the force of the wind) are left behind in a stream pointing upwind.

    4
    An example of a tidal tail drawn out from a disrupted late-type galaxy. The disrupted galaxy in Abell 2670 is, in contrast, an early-type, elliptical galaxy that should be gas-poor. [H. Ford, JHU/M. Clampin, STScI/G. Hartig, STScI/G. Illingworth, UCO, Lick/ACS Science Team/ESA/NASA]

    Gas from a Merger?

    But if this is an elliptical galaxy, where did the gas come from for the tidal tails and the galactic-center star formation? To rule out the obvious, the authors first check that this galaxy really is an early-type elliptical. The galaxy’s color (reddened), morphology (elliptical and no sign of a stellar disk), and stellar velocities (no sign of stellar rotation) all confirm this.

    The authors therefore speculate that the galaxy recently underwent a “wet merger” — a merger with a companion galaxy that was gas-rich. Much of this gas was driven to the center of the elliptical galaxy in the merger, and it’s now responsible for the starbursts there.

    We’ll hopefully be able to draw stronger conclusions about this unusual galaxy after additional investigation into the amount of gas it contains and the galaxy’s star formation rate. In the meantime, this stripped elliptical makes for an intriguing puzzle!

    Citation

    Yun-Kyeong Sheen et al 2017 ApJL 840 L7. doi:10.3847/2041-8213/aa6d79

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  • richardmitnick 4:41 pm on May 3, 2017 Permalink | Reply
    Tags: AAS NOVA, Anatomy of an Asteroid Breakup, , , ,   

    From AAS NOVA: “Anatomy of an Asteroid Breakup” 

    AASNOVA

    American Astronomical Society

    3 May 2017
    Susanna Kohler

    1
    Keck images of asteroid P/2013 R3 (click for the full view!) taken in October 2013, just as it had started to break up. Four different fragments of the asteroid are labeled. [Jewitt et al. 2017]

    A team of scientists has observed the breakup of an asteroid as it orbits the Sun. In a new study, they reveal what they’ve learned from their ground- and space-based observations of this disintegration.

    Observations of Disintegration

    Active asteroids are objects that move on asteroid-like orbits while displaying comet-like behavior. The cause of their activity can vary — ranging from outgassing as the asteroid heats up in its solar approach, to expelled debris from a collision, to the entire asteroid flying apart because it’s spinning too fast.

    Led by David Jewitt (University of California at Los Angeles), a team of scientists has analyzed observations of the disintegrating asteroid P/2013 R3. The observations span two years and were made by a number of telescopes, including Hubble, Keck (in Hawaii), Magellan (in Chile), and the Very Large Telescope (in Chile).

    NASA/ESA Hubble Telescope

    Keck Observatory, Mauna Kea, Hawaii, USA

    Carnegie 6.5 meter Magellan Baade and Clay Telescopes located at Carnegie’s Las Campanas Observatory, Chile.

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

    Jewitt and collaborators then used these observations — and a bit of modeling — to understand what asteroid R3 was like originally, what its pieces are doing now, and what caused it to break up.

    7
    A schematic diagram of the different fragments of R3 and how they relate to each other. Black numbers estimate the fragment separation velocities; red numbers estimate the separation date. [Jewitt et al. 2017]

    Cause of the Breakup

    The team found that P/2013 R3 broke up into at least 13 pieces, the biggest of which was likely no more than 100-200 meters in size. The original asteroid was probably less than ~400 m in radius.

    By measuring the velocities of the fragments in the various observations, Jewitt and collaborators were able to work backward to determine when each piece broke off. They found that the fragmentation process was spread out over the span of roughly 5 months — suggesting that the asteroid’s breakup wasn’t impact-related (otherwise the fragmentation would likely have been all at once rather than gradual).

    8
    Timeline of the destruction of R3. Calendar dates are in black, day-of-year dates are in red. The letters below the timeline indicate observations. [Jewitt et al. 2017]

    So if it wasn’t an impact, what caused the breakup of R3? Tidal stresses are unlikely; the asteroid wasn’t close enough to the Sun or a planet to experience strong pulls. Gas pressure from sublimating ice also falls short of being strong enough to have caused the disruption, according to the authors’ calculations.

    The authors conclude that the most plausible cause of R3’s breakup was rotational instability. If an asteroid is made up of a collection of rocky material loosely gravitationally bound in what’s known as a “rubble-pile” composition, then it tends to fly apart if the asteroid spins faster than once every ~2.2 hours. The authors show that torques from radiation or anisotropic sublimation could have driven R3 to spin this quickly on a relatively short timescale.

    9
    Zodiacal light, caused by scattering by dust in the Zodiacal Cloud. [ESO]

    A Dusty End

    Lastly, Jewitt and collaborators examine the debris cloud released by the breakup of R3. They use these observations to estimate how much debris disrupted asteroids likely contribute to the Zodiacal Cloud, the cloud of dust found in our solar system, primarily between the Sun and Jupiter.

    The authors estimate that the fractional contribution by asteroids like R3 is roughly 4% — consistent with models that suggest that asteroid dust is a measurable, but not dominant, contributor to the Zodiacal Cloud. Future sky surveys will allow us to better examine this contribution.

    Citation

    David Jewitt et al 2017 AJ 153 223. doi:10.3847/1538-3881/aa6a57

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  • richardmitnick 2:38 pm on April 28, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , Hunting Elusive SPRITEs with Spitzer   

    From AAS NOVA: “Hunting Elusive SPRITEs with Spitzer” 

    AASNOVA

    American Astronomical Society

    28 April 2017
    Susanna Kohler

    1
    Blue dots in this image mark the locations of infared transients — explosive or fleeting events — that were recently discovered by a survey called SPIRITS. [Kasliwal et al. 2017]

    In recent years, astronomers have developed many wide-field imaging surveys in which the same targets are observed again and again. This new form of observing has allowed us to discover optical and radio transients — explosive or irregular events with durations ranging from seconds to years. The dynamic infrared sky, however, has remained largely unexplored … until now.

    Infrared Exploration

    Why hunt for infrared transients? Optical wavelengths don’t allow us to observe events that are obscured, such that their own structure or their surroundings hide them from our view. Both supernovae and luminous red novae (associated with stellar mergers) are discoverable as infrared transients, and there may well be new types of transients in infrared that we haven’t seen before!

    To explore this uncharted territory, a team of scientists developed SPIRITS, the Spitzer Infrared Intensive Transients Survey. Begun in 2014, SPIRITS is a five-year long survey that uses the Spitzer Space Telescope to conduct a systematic search for mid-infrared transients in nearby galaxies.

    NASA/Spitzer Telescope

    NASA Spitzer Infrared Array Camera

    3
    Example of a transient: SPIRITS 14ajc was visible when imaged by SPIRITS in 2014 (left) but it wasn’t there during previous imaging between 2004 and 2008 (right). The bottom frame shows the difference between the two images. [Adapted from Kasliwal et al. 2017]

    In a recent publication led by Mansi Kasliwal (Caltech and the Carnegie Institution for Science), the SPIRITS team has now detailed how their survey works and what they’ve discovered in its first year.

    4
    The light curves of SPRITEs (red stars) lie in the mid-infared luminosity gap between novae (orange) and supernovae (blue). [Kasliwal et al. 2017]

    Mystery Transients

    Kasliwal and collaborators used Spitzer to monitor 190 nearby galaxies. In SPIRITS’ first year, they found over 1958 variable stars and 43 infrared transient sources. Of these 43 transients, 21 were known supernovae, 4 were in the luminosity range of novae, and 4 had optical counterparts. The remaining 14 events were designated “eSPecially Red Intermediate-luminosity Transient Events”, or SPRITEs.

    SPRITEs are unusual infrared transients that lie in the luminosity gap between novae and supernovae, and they have no optical counterparts. They all occur in star-forming galaxies.

    Search for the Cause

    What’s the physical origin of these phenomena? The authors explore a number of possible sources, including obscured supernovae, stellar mergers with dusty winds, collapse of extreme stars, or even weak shocks in failed supernovae.

    5
    Spitzer image of Messier 83, one of the closest barred spiral galaxies in the sky. SPIRITS 14ajc was discovered in one of Messier 83’s spiral arms. [NASA/JPL-Caltech]

    In one case, SPIRITS 14ajc, the SPRITE’s spectrum shows signs of excited molecular hydrogen lines, which are indicative of a shock. Based on the data, Kasliwal and collaborators propose that the shock might have been driven into a molecular cloud after it was triggered by the decay of a system of massive stars that either passed closely or collided and merged.

    The other SPRITEs may all have different origins, however, and in general the infrared photometric data isn’t sufficient to identify which model fits each transient. Future technology, like spectroscopy with the James Webb Space Telescope, may help us to better understand the origins of these elusive transients, though. And future surveying with projects like SPIRITS will help us to discover more SPRITE-like events, expanding our understanding of the dynamic infrared sky.

    Citation

    Mansi M. Kasliwal et al 2017 ApJ 839 88. doi:10.3847/1538-4357/aa6978

    Related Journal Articles

    Spirits 15c and spirits 14buu: two obscured supernovae in the nearby star-forming galaxy ic 2163 doi: 10.3847/1538-4357/aa618f
    Rising from the ashes: mid-infrared re-brightening of the impostor sn 2010da in ngc 300 doi: 10.3847/0004-637X/830/2/142
    A systematic study of mid-infrared emission from core-collapse supernovae with spirits doi: 10.3847/1538-4357/833/2/231
    Common envelope ejection for a luminous red nova in m101 doi: 10.3847/1538-4357/834/2/107
    The ALMA view of the omc1 explosion in orion doi: 10.3847/1538-4357/aa5c8b
    An excess of mid-infrared emission from the type iax sn 2014dt doi: 10.3847/2041-8205/816/1/L13

    See the full article here .

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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.

    See the full article here .

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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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