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  • richardmitnick 5:13 pm on November 20, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , Kepler Planets Tend to Have Siblings of the Same Size   

    From AAS NOVA: “Kepler Planets Tend to Have Siblings of the Same Size” 

    AASNOVA

    AAS NOVA

    20 November 2017
    Susanna Kohler

    1
    Artist’s impression of a Kepler multiplanet system. A new study has found that planets within the same system tend to share similar masses as well as radii. [NASA / JPL-Caltech]

    NASA/Kepler Telescope

    After 8.5 years of observations with the Kepler space observatory, we’ve discovered a large number of close-in, tightly-spaced, multiple-planet systems orbiting distant stars. In the process, we’ve learned a lot about the properties about these systems — and discovered some unexpected behavior. A new study explores one of the properties that has surprised us: planets of the same size tend to live together.

    1
    Orbital architectures for 25 of the authors’ multiplanet systems. The dots are sized according to the planets’ relative radii and colored according to mass. Planets of similar sizes and masses tend to live together in the same system. [Millholland et al. 2017]

    Ordering of Systems

    From Kepler’s observations of extrasolar multiplanet systems, we have seen that the sizes of planets in a given system aren’t completely random. Systems that contain a large planet, for example, are more likely to contain additional large planets rather than additional planets of random size. So though there is a large spread in the radii we’ve observed for transiting exoplanets, the spread within any given multiplanet system tends to be much smaller.

    This odd behavior has led us to ask whether this clustering occurs not just for radius, but also for mass. Since the multiplanet systems discovered by Kepler most often contain super-Earths and mini-Neptunes, which have an extremely large spread in densities, the fact that two such planets have similar radii does not guarantee that they have similar masses.

    If planets don’t cluster in mass within a system, this would raise the question of why planets coordinate only their radii within a given system. If they do cluster in mass, it implies that planets within the same system tend to have similar densities, potentially allowing us to predict the sizes and masses of planets we might find in a given system.

    Insight into Masses

    Led by NSF graduate research fellow Sarah Millholland, a team of scientists at Yale University used recently determined masses for planets in 37 Kepler multiplanet systems to explore this question of whether exoplanets in a multiplanet system are more likely to have similar masses rather than random ones.

    Millholland and collaborators find that the masses do show the same clustering trend as radii in multiplanet systems — i.e., sibling planets in the same system tend to have both masses and radii that are more similar than if the system were randomly assembled from the total population of planets we’ve observed. Furthermore, the masses and radii tend to be ordered within a system when the planets are ranked by their periods.

    4
    The host star’s metallicity is correlated with the median planetary radius for a system. [Adapted from Millholland et al. 2017]

    The authors note two important implications of these results:

    The scatter in the relation between mass and radius of observed exoplanets is primarily due to system-to-system variability, rather than the variability within each system.
    Knowing the properties of a star and its primordial protoplanetary disk might allow us to predict the outcome of the planet formation process for the system.

    Following up on the second point, the authors test whether certain properties of the host star correlate with properties of the planets. They find that the stellar mass and metallicity have a significant effect on the planet properties and the structure of the system.

    Continuing to explore multiplanet systems like these appears to be an excellent path forward for understanding the hidden order in the broad variety of exoplanets we’ve observed.

    Citation

    Sarah Millholland et al 2017 ApJL 849 L33. doi:10.3847/2041-8213/aa9714

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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

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  • richardmitnick 4:06 pm on November 17, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , , , GW170608   

    From AAS NOVA: “LIGO Finds Lightest Black-Hole Binary” 

    AASNOVA

    AAS NOVA

    1
    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Wednesday evening the Laser Interferometer Gravitational-wave Observatory (LIGO) collaboration quietly mentioned that they’d found gravitational waves from yet another black-hole binary back in June. This casual announcement reveals what is so far the lightest pair of black holes we’ve watched merge — opening the door for comparisons to the black holes we’ve detected by electromagnetic means.

    A Routine Detection

    2
    The chirp signal of GW170608 detected by LIGO Hanford and LIGO Livingston. [LIGO collaboration 2017]

    After the fanfare of the previous four black-hole-binary merger announcements over the past year and a half — as well as the announcement of the one neutron-star binary merger in August — GW170608 marks our entry into the era in which gravitational-wave detections are officially “routine”.

    GW170608, a gravitational-wave signal from the merger of two black holes roughly a billion light-years away, was detected in June of this year. This detection occurred after we’d already found gravitational waves from several black-hole binaries with the two LIGO detectors in the U.S., but before the Virgo interferometer came online in Europe and increased the joint ability of the detectors to localize sources.

    3
    Mass estimates for the two components of GW170608 using different models. [LIGO collaboration 2017]

    Overall, GW170608 is fairly unremarkable: it was detected by both LIGO Hanford and LIGO Livingston some 7 ms apart, and the signal looks not unlike those of the previous LIGO detections. But because we’re still in the early days of gravitational-wave astronomy, every discovery is still remarkable in some way! GW170608 stands out as being the lightest pair of black holes we’ve yet to see merge, with component masses before the merger estimated at ~12 and ~7 times the mass of the Sun.

    Why Size Matters

    With the exception of GW151226, the gravitational-wave signal discovered on Boxing Day last year, all of the black holes that have been discovered by LIGO/Virgo have been quite large: the masses of the components have all been estimated at 20 solar masses or more. This has made it difficult to compare these black holes to those detected by electromagnetic means — which are mostly under 10 solar masses in size.

    4
    GW170608 is the lowest-mass of the LIGO/Virgo black-hole mergers shown in blue. The primary mass is comparable to the masses of black holes we have measured by electromagnetic means (purple detections). [LIGO-Virgo/Frank Elavsky/Northwestern]

    One type of electromagnetically detected black hole are those in low-mass X-ray binaries (LMXBs). LMXBs consist of a black hole and a non-compact companion: a low-mass donor star that overflows its Roche lobe, feeding material onto the black hole. It is thought that these black holes form without significant spin, and are later spun up as a result of the mass accretion. Before LIGO, however, we didn’t have any non-accreting black holes of this size to observe for comparison.

    Now, detections like GW170608 and the Boxing Day event (which was also on the low end of the mass scale) are allowing us to start exploring spin distributions of non-accreting black holes to determine if we’re right in our understanding of black-hole spins. We don’t yet have a large enough comparison sample to make a definitive statement, but GW170608 is indicative of a wealth of more discoveries we can hope to find in LIGO’s next observing run, after a series of further design upgrades scheduled to conclude in 2018. The future of gravitational wave astronomy continues to look promising!

    Citation

    LIGO collaboration, submitted to ApJL. https://arxiv.org/abs/1711.05578

    See the full article here .

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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 11:13 am on November 16, 2017 Permalink | Reply
    Tags: A Look at the Milky Way’s Outskirts, AAS NOVA, , , ,   

    From AAS NOVA: “A Look at the Milky Way’s Outskirts” 

    AASNOVA

    AAS NOVA

    15 November 2017
    Susanna Kohler

    1
    Our position inside the Milky Way — which results in views of our galaxy like this 360° panorama — makes it difficult to gain a large-scale picture of what our galaxy’s outskirts look like. [ESO/S. Brunier]

    Milky Way NASA/JPL-Caltech /ESO R. Hurt

    Studying the large-scale structure of the Milky Way is difficult given that we’re stuck in its interior — which means we can’t step back for a broad overview of our home. Instead, a recent study uses distant variable stars to map out a picture of what’s happening in the outskirts of our galaxy.

    Mapping with Tracers

    2
    Phase-folded light curve for two of the RR Lyrae stars in the authors’ sample, each with hundreds of observations over 7 years. [Cohen et al. 2017]

    Since observing the Milky Way from the outside isn’t an option, we have to take creative approaches to mapping its outer regions and measuring its total mass and dark matter content. One tool used by astronomers is tracers: easily identifiable stars that can be treated as massless markers moving only as a result of the galactic potential. Mapping the locations and motions of tracers allows us to measure the larger properties of the galaxy.

    RR Lyrae stars are low-mass, variable stars that make especially good tracers. They pulsate predictably on timescales of less than a day, creating distinctive light curves that can easily be distinguished and tracked in wide-field optical imaging surveys over long periods of time. Their brightness makes them detectable out to large distances, and their blue color helps to separate them from contaminating stars in the foreground.

    Best of all, RR Lyrae stars are very nearly standard candles: their distances can be determined precisely with only knowledge of their measured light curves.

    3
    Locations on the sky of the several hundred outer-halo RR Lyrae stars in the authors’ original sample. The red curve shows the location of the Sagittarius stream, an ordered structure the authors avoided so as to only have unassociated stars in their sample. [Cohen et al. 2017]

    Distant Variables

    In a new study led by Judith Cohen (California Institute of Technology), the signals of hundreds of distant RR Lyrae stars were identified in observations of transient objects made with the Palomar Transient Factory (PTF) survey.

    Caltech Palomar Intermediate Palomar Transient Factory telescope at the Samuel Oschin Telescope at Palomar Observatory,located in San Diego County, California, United States

    Cohen and collaborators then followed up with the Keck II telescope in Hawaii to obtain spectra for a narrower sample of 122 RR Lyrae stars.


    Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft) above sea level

    The stars in the sample lie at whopping distances of ~150,000–350,000 light-years from us. For comparison, we’re about 25,000 light-years from the center of the galaxy, and the stellar disk of the galaxy is only thought to be perhaps 100,000 light-years across — so these variable stars lie firmly in the Milky Way’s outer halo. The spectra of the stars reveal their radial velocity, providing us with precise measurements of how objects in the outer halo move.

    More Space in the Suburbs?

    4
    Histogram with distance for the ~450 RR Lyrae stars in the authors’ broader sample. When the authors include their estimates for the completeness of their sample, the best fit scales with distance as r-4, shown by the red line. [Cohen et al. 2017]

    After reporting the velocity dispersions that they measure — which can be used to make more precise estimates of the Milky Way’s total mass — Cohen and collaborators discuss the stellar density implied by their sample. They find that the density of stars in the outer halo of the Milky Way scales with their distance as r-4. This is similar to the drop-off in density we’ve measured in the inner halo, and it contradicts some studies that have predicted a much sharper drop in stellar density in the Milky Way’s outermost regions.

    The work presented in this study goes a long way toward building our view of the galaxy’s outer halo. Future catalogs like the Pan-STARRS RR Lyrae catalog and upcoming surveys like LSST should also significantly increase the tracer sample size and measurement accuracy, further allowing us to map out the outskirts of the Milky Way.

    Pann-STARS telescope, U Hawaii, Mauna Kea, Hawaii, USA, 4,207 m (13,802 ft) above sea level

    LSST


    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.

    Citation

    Judith G. Cohen et al 2017 ApJ 849 150. doi:10.3847/1538-4357/aa9120

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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 9:12 am on November 9, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , Carrying Energy to the Corona with Waves, , DKIST under construction by the National Solar Observatory atop the Haleakala volcano on the Pacific island of Maui Hawaii USA at an altitude of 3084 m (10118 ft) with a planned completion date of 201, Photosphere- the surface of the Sun,   

    From AAS NOVA: “Carrying Energy to the Corona with Waves” 

    AASNOVA

    AAS NOVA

    8 November 2017
    Susanna Kohler

    1
    How does the solar corona, the Sun’s outer atmosphere visible in this image, get so hot? [Luc Viatour]

    The solar corona has a problem: it’s weirdly hot! A new study explores how magnetic waves might solve the mystery of the unusually hot corona by transporting energy to the outer atmosphere of the Sun.

    The Problem with the Corona

    2
    The temperatures of different layers of the Sun. [ISAS/JAXA]

    The corona, the outer layer of the Sun’s atmosphere, has typical temperatures of 1–3 million K — significantly hotter than the cool 5,800 K of the photosphere, the surface of the Sun far below it. Since temperatures ordinarily drop the further you get from the heat source (in this case, the Sun’s atom-fusing center), this so-called “coronal heating problem” poses a definite puzzle.

    As is the case for many astronomical mysteries, the answer may have something to do with magnetic fields. Alfvén waves, magnetohydrodynamic waves that travel through magnetized plasma, could potentially carry energy from the convective zone beneath the Sun’s photosphere up into the solar atmosphere. There, the Alfvén waves could turn into shock waves that dissipate their energy as heat, causing the increased temperature of the corona.

    Daniel K. Inouye Solar Telescope, DKIST under construction by the National Solar Observatory atop the Haleakala volcano on the Pacific island of Maui, Hawaii, USA, at an altitude of 3,084 m (10,118 ft), with a planned completion date of 2018

    Predicting Observations

    Alfvén waves as a means of delivering heat to the corona makes for a nice picture, but there’s a lot of work to be done before we can be certain that this is the correct model. Observational evidence of Alfvén waves has thus far been limited to specific conditions — and the observations have not yet been enough to convince us that Alfvén waves can deliver enough energy to explain the corona’s temperature.

    Lucas Tarr, a scientist at the Naval Research Laboratory, argues that upcoming solar telescopes may make it easier to detect these waves — but first we need to know what to look for! In a recent study, Tarr uses a simplified analytic model to show which frequencies of waves are likely to carry power when magnetic field lines in the corona are pertubed.

    3
    The power carried by Alfvén waves as a function of frequency, as a result of an initial perturbation, plotted for several different initial conditions (such as the size of the perturbation or the length of the loop on which it is introduced). [Tarr 2017]

    A Promising Future

    Tarr modeled the effects of a minor perturbation — like a local magnetic reconnection event in the corona — on a coronal arcade, a common structure of magnetic field loops found in the corona. Tarr determined that such a disturbance would peak in power at a low frequency (maybe tens of millihertz, or oscillations on scales of minutes), but a substantial portion of the power is carried by waves of higher frequencies (0.5–4 Hz, or oscillations on scales of seconds).

    Tarr’s findings confirm that with the cadence and sensitivity of current instrumentation, we would not expect to be able to detect these Alfvén waves. The results do indicate, however, that high-cadence observations with future telescope technology — like the instrumentation at the upcoming Daniel K. Inouye Solar Telescope, which should be completed in 2018 — may have the ability to reveal the presence of these waves and confirm the model of Alfvén waves as the means by which the Sun achieves its mysteriously hot corona.
    Citation

    Lucas A. Tarr 2017 ApJ 847 1. doi:10.3847/1538-4357/aa880a

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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 2:01 pm on November 7, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , , , Feeding Black Holes Through Galactic Bars   

    From astrobites via AAS NOVA: ” Feeding Black Holes Through Galactic Bars” 


    AAS NOVA

    Astrobites bloc

    astrobites

    1
    Hubble view of NGC 1300, a barred spiral galaxy. [NASA, ESA, and The Hubble Heritage Team (STScI/AURA)].AAS NOVA

    Title: Galaxy-Scale Bars in Late-Type Sloan Digital Sky Survey Galaxies Do Not Influence the Average Accretion Rates of Supermassive Black Holes
    Authors: A.D. Goulding, E. Matthaey, J.E. Greene, et al.
    First Author’s Institution: Princeton University

    Status: Accepted to ApJ, open access

    When it comes to picking their host galaxies, active galactic nuclei (or AGN) are rather promiscuous. They reside in all types of galaxies: ellipticals, irregulars, and spirals. AGN of the same feather tend to flock together — the more luminous and radio-loud ones are found in elliptical galaxies while the lower luminosity ones are more often found in spiral galaxies. This is a manifestation of the black hole mass-host galaxy luminosity correlation, where spiral galaxies like our Milky Way tend to have less massive black holes than elliptical galaxies. Besides spiral arms, spiral galaxies sometimes also boast of having bars, if the right mood strikes. How are bars related to their AGN? Could they trigger the central black holes to light up as AGN?

    Galactic bars are thought to contribute to the dynamical evolution of their host galaxies. Numerical studies show that they can funnel in gas from the outskirts to the central regions of the galaxies, triggering star formation and possibly AGN activity. It is still unclear whether bars actually help trigger AGN, as previous studies have produced conflicting results and tend to suffer from small number statistics and biased AGN diagnostics. In today’s paper, the authors bring better tools to bear on the problem, by utilizing the large wealth of information from the SDSS Galaxy Zoo citizen science project and X-ray stacking analyses.

    2
    Fig. 1: Sample unbarred (blue borders), ambiguously barred (yellow borders), and barred (red borders) spiral galaxies from the Galaxy Zoo project, as determined by fbar, which is the fraction of votes by citizen scientists for the presence of bars. [Goulding et al. 2017]

    See the full article here .

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    What do we do?

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

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

     
  • richardmitnick 5:41 pm on November 6, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , Did Triton Destroy Neptune’s First Moons?   

    From AAS NOVA: “Did Triton Destroy Neptune’s First Moons?” 

    AASNOVA

    AAS NOVA

    6 November 2017
    Susanna Kohler

    1
    This computer-generated image shows Neptune as it would appear from near its largest moon, Triton. [NASA/JPL/USGS]

    Neptune’s moon system is not what we would expect for a gas giant in our solar system. Scientists have now explored the possibility that Neptune started its life with an ordinary system of moons that was later destroyed by the capture of its current giant moon, Triton.

    An Odd System

    Our current understanding of giant-planet formation predicts a period of gas accretion to build up the large size of these planets. According to models, the circumplanetary gas disks that surround the planets during this time then become the birthplaces of the giant planets’ satellite systems, producing systems of co-planar and prograde (i.e., orbiting in the same direction as the planet’s rotation) satellites similar to the many-moon systems of Jupiter or Saturn.

    1
    Triton’s orbit is tilted relative to the inner Neptunian satellite orbits. [NASA, ESA, and A. Feild (STScI)]

    Neptune, however, is quirky. This gas giant has surprisingly few satellites — only 14 compared to, say, the nearly 70 moons of Jupiter — and most of them are extremely small. One of Neptune’s moons is an exception to this, however: Triton, which contains 99.7% of the mass of Neptune’s entire satellite system!

    Triton’s orbit has a number of unusual properties. The orbit is retrograde — Triton orbits in the opposite direction as Neptune’s rotation — which is unique behavior among large moons in our solar system. Triton’s orbit is also highly inclined, and yet the moon’s path is nearly circular and lies very close to Neptune.

    2
    The distribution of impact velocities in the authors’ simulations for primordial satellite interactions with Triton, in three cases of different satellite mass ratios. In the low-mass case — a third of the mass ratio of the Uranian satellite system — 88% of simulations ended with Triton surviving on its high-inclination orbit. The survival rate was only 12% in the high-mass case. [Adapted from Rufu et al. 2017]

    How did this monster of a satellite get its strange properties, and why is Neptune’s system so odd compared to what we would expect for a gas giant’s satellites? Two scientists, Raluca Rufu (Weizmann Institute of Science, Israel) and Robin Canup (Southwest Research Institute), propose an explanation in which Triton long ago wreaked havoc on a former system of satellites around Neptune.

    Destruction After Capture

    Rufu and Canup explore the scenario in which Neptune once had an ordinary, prograde system of moons around it that resembled those of the other gas giants. Triton, the authors suggest, may have been a former Kuiper belt object that was then captured by Neptune. The ensuing interactions between retrograde Triton and Neptune’s original, prograde satellite system may have then resulted in the destruction of this original system, leaving behind only Triton and Neptune’s other current satellites.

    4
    Nereid, a small irregular moon of Neptune, orbits at an average distance of more than 15 times that of Triton. Models of Triton’s orbital evolution must also account for the preservation of satellites like this one. [NASA]

    Using N-body simulations that model a newly captured Triton and a likely primordial prograde system of moons, Rufu and Canup show that if the moons have a mass ratio similar to that of Uranus’s system or smaller, Triton’s interactions with it have a substantial likelihood of reproducing the current Neptunian satellite system. They even demonstrate that the interactions decrease Triton’s initial semimajor axis quickly enough to prevent smaller, outer satellites like Nereid from being kicked out of the system.

    If the authors’ picture is correct, then it neatly explains why Neptune’s satellite system looks so unusual compared to Jupiter’s or Saturn’s — which means that our models of how primordial systems of moons form around gas giants still hold strong.

    Citation

    Raluca Rufu and Robin M. Canup 2017 AJ 154 208. doi:10.3847/1538-3881/aa9184

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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 3:44 pm on November 3, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , Tracing the Fuel for Forming Stars   

    From AAS NOVA: ” Tracing the Fuel for Forming Stars” 

    AASNOVA

    AAS NOVA

    3 November 2017
    Susanna Kohler

    1
    We observe star formation occurring in clouds of molecular gas in the local universe. A new study explores how the reservoirs of cold gas may be different in the more distant universe. [T. A. Rector & B. A. Wolpa, NOAO, AURA]

    Huge reservoirs of cold hydrogen gas — the raw fuel for star formation — lurk in galaxies throughout the universe. A new study examines whether these reservoirs have always been similar, or whether those in distant galaxies are very different from those in local galaxies today.

    2
    Left: Optical SLOAN images of the five HIGHz galaxies in this study. Right: ALMA images of the molecular gas in these galaxies. Both images are 30” wide. [Adapted from Cortese et al. 2017]

    SDSS Telescope at Apache Point Observatory, NM, USA, Altitude2,788 meters (9,147 ft)

    Molecular or Atomic?

    The formation of stars is a crucial process that determines how galaxies are built and evolve over time. We’ve observed that star formation takes place in cold clouds of molecular gas, and that star-formation rates increase in galaxies with a larger surface density of molecular hydrogen — so we know that molecular hydrogen feeds the star-forming process.

    But not all cold gas in the interstellar medium of galaxies exists in molecular form. In the local universe, only around 30% of cold gas is found in molecular form (H2) and able to directly feed star formation; the rest is atomic hydrogen (H I). But is this true of galaxies earlier in the universe as well?

    Studying Distant Galaxies

    Cosmological simulations have predicted that earlier in our universe’s history, the ratio of molecular to atomic hydrogen could be larger — i.e., more cold hydrogen may be in a form ready to fuel star formation — but this prediction is difficult to test observationally. Currently, radio telescopes are not able to measure the atomic hydrogen in very distant galaxies, such as those at the peak of star formation in the universe, 10 billion years ago.

    Recently, however, we have measured atomic hydrogen in closer galaxies: those at a redshift of about z ~ 0.2–0.4, a few billion years ago. One recent study of seven galaxies at this distance, using a sample from a survey known as COOL BUDHIES, showed that the hydrogen reservoirs of these galaxies are dominated by molecular hydrogen, unlike in the local universe. If this is true of most galaxies at this distance, it would suggest that gas reservoirs have drastically changed in the short time between then and now.

    But a team of scientists from the International Centre for Radio Astronomy Research [ICRAR]in Australia, led by Luca Cortese, has now challenged this conclusion.

    3
    Top: molecular vs. atomic hydrogen gas in galaxies between z = 0 and z = 1.5. Bottom: the evolution of the molecular-to-atomic mass ratio with redshift. [Adapted from Cortese et al. 2017]

    Adding to the Sample

    Cortese and collaborators combined observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and Arecibo to estimate the ratio of molecular to atomic hydrogen in five HIGHz-survey massive star-forming galaxies at a redshift of z ~ 0.2.

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

    NAIC/Arecibo Observatory, Puerto Rico, USA, at 497 m (1,631 ft)

    They then combine these results with those of the COOL BUDHIES survey; they argue that, since the two surveys use different selection criteria, the combination of the two samples provides a fairer view of the overall population of star-forming galaxies at z ~ 0.2.

    Intriguingly, the HIGHz galaxies do not show the molecular-gas dominance that the COOL BUDHIES galaxies do. Cortese and collaborators demonstrate that the addition of the HIGHz galaxies to the sample reveals that the gas reservoirs of star-forming disks 3 billion years ago are, in fact, still the same as what we see today, suggesting that star formation in galaxies at z ~ 0.2 is likely fueled in much the same way as it is today.

    As telescope capabilities increase, we may be able to explore whether this continues to hold true for more distant galaxies. In the meantime, increasing our sample size within the range that we can observe will help us to further explore how galaxies have formed stars over time.
    Citation

    Luca Cortese et al 2017 ApJL 848 L7. doi:10.3847/2041-8213/aa8cc3

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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 4:21 pm on November 1, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , RNAAS, Stuff that does not work   

    From AAS NOVA: “RNAAS: A Unique Journal Joins the Family” 

    AASNOVA

    AAS NOVA

    1 November 2017
    Susanna Kohler

    1
    A new publication has joined the AAS journal family: Research Notes of the American Astronomical Society. [AAS Publishing]

    Null results — research outcomes that show what doesn’t work, rather than what does — are a crucial part of science. It’s imperative that these results are shared widely, so that researchers can learn from each others’ experiences instead of unnecessarily repeating work.

    Unfortunately, null results are often difficult to publish in traditional venues, as they represent the steady march of science in the background rather than exciting new discoveries. And null results aren’t alone — there are a number of other types of scientific research that are of interest to the astronomical community, and yet they cannot easily be shared, archived, or cited.

    A Home for Non-Traditional Communications

    2
    An example of a recently submitted Research Note. [AAS Publishing]

    Enter Research Notes of the American Astronomical Society (RNAAS) — a new and unique journal that just joined the AAS journal family this week. RNAAS provides a means of sharing with the astronomical community work that may not fit into traditional publication outlets. There are many types of submissions that could be appropriate for RNAAS, such as:

    Null results
    Timely reports of observations (like the spectrum of a supernova)
    Brief observations (like the discovery of a single exoplanet or contributions to the monitoring of a variable source)
    Work in progress or projects of limited scope (like the results of a summer undergraduate research project)

    Why Publish a Research Note?

    RNAAS is a non-peer-reviewed, non-edited journal that is moderated by one of the AAS journals’ lead editors, Dr. Chris Lintott (University of Oxford). Communications published in RNAAS are brief — they are limited to <1000 words, with space for one table or figure. Research Notes have the benefit of being:

    Searchable and citable
    Since Research Notes are indexed by ADS, this ensures that researchers can easily find work that might otherwise have gone unshared. And since Research Notes are assigned a DOI, this means that information from Research Notes can be referenced in future publications.
    Archived for perpetuity
    Publishing data and results in RNAAS — part of the AAS suite of journals and hosted alongside them by Institute of Physics Publishing — prevents the risk that this less formal information is unintentionally lost to the community as a result of institution changes, outdated websites, etc. (a common problem in academia!).
    Quick to publish
    Need to notify the community of something in a hurry, and don’t have time to wait for a traditional journal’s publication process? Research Notes are typically available online within 72 hours of when they are received.
    Free to access (and to publish!)
    RNAAS is not behind a paywall, so Research Notes can be read by anyone and do not require an institutional or personal subscription to AAS journals to access. What’s more, the AAS is currently suspending charges for all submissions to RNAAS, so publishing a Research Note costs nothing at this time.

    Find Out More and Submit

    Intrigued? You can go see for yourself what people are submitting to RNAAS.
    Convinced? We look forward to receiving your submission to RNAAS!

    Citation

    Ethan T. Vishniac and Chris Lintott 2017 Res. Notes AAS 1 1. doi:10.3847/2515-5172/aa93da

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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 1:05 pm on October 26, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , Little Eyes on Large Solar Motions   

    From AAS NOVA: ” Little Eyes on Large Solar Motions” 

    AASNOVA

    AAS NOVA

    25 October 2017
    Susanna Kohler

    1
    Observations from small telescopes have provided this beautiful view of the solar corona during a solar eclipse in 2013. These data have helped researchers better understand what shapes the large-scale structure in the corona. [Alzate et al. 2017]

    2
    Images taken during the solar eclipse in 2012. The central color composite of the eclipsed solar surface was captured by SDO, the white-light view of the solar corona around it was taken by the authors, and the background, wide-field black-and-white view is from LASCO. The white arrows mark the “atypical” structure. [Alzate et al. 2017]

    It seems like science is increasingly being done with advanced detectors on enormous ground- and space-based telescopes. One might wonder: is there anything left to learn from observations made with digital cameras mounted on ~10-cm telescopes?

    The answer is yes — plenty! Illustrating this point, a new study using such equipment recently reports on the structure and dynamics of the Sun’s corona during two solar eclipses.

    A Full View of the Corona

    The solar corona is the upper part of the Sun’s atmosphere, extending millions of kilometers into space. This plasma is dynamic, with changing structures that arise in response to activity on the Sun’s surface — such as enormous ejections of energy known as coronal mass ejections (CMEs). Studying the corona is therefore important for understanding what drives its structure and how energy is released from the Sun.

    Though there exist a number of space-based telescopes that observe the Sun’s corona, they often have limited fields of view. The Solar Dynamics Observatory AIA, for instance, has spectacular resolution but only images out to 1/3 of a solar radius above the Sun’s limb. The space-based coronagraph LASCO C2, on the other hand, provides a broad view of the outer regions of the corona, but it only images down to 2.2 solar radii above the Sun’s limb. Piecing together observations from these telescopes therefore leaves a gap that prevents a full picture of the large-scale corona and how it connects to activity at the solar surface.

    To provide this broad, continuous picture, a team of scientists used digital cameras mounted on ~10-cm telescopes to capture white-light images from the solar surface out to several solar radii using a natural coronagraph: a solar eclipse. The team made two sets of observations: one during an eclipse in 2012 in Australia, and one during an eclipse in 2013 in Gabon, Africa. In a recent publication led by Nathalia Alzate (Honolulu Community College), the team now reports what they learned from these observations.

    Building Atypical Structures

    The authors’ image processing revealed two “atypical” large-scale structures with sharp edges, somewhat similar in appearance to what is seen near the boundaries of rapidly expanding polar coronal holes. But these structures, visible in the southeast quadrant of the images taken during both eclipses, were not located near the poles.

    By analyzing their images along with space-based images taken at the same time, Alzate and collaborators were able to determine that the shape the structures took was instead a direct consequence of a series of sudden brightenings due to low-level flaring events on the solar surface. These events were followed by small jets, and then very faint, puff-like CMEs that might otherwise have gone unnoticed.

    4
    Impact of the passage of a series of puff-like CMEs (shown in the LASCO time sequence in the bottom panels) on coronal structures. [Alzate et al. 2017]

    Citation

    Nathalia Alzate et al 2017 ApJ 848 84. doi:10.3847/1538-4357/aa8cd2

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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 8:22 am on October 24, 2017 Permalink | Reply
    Tags: AAS NOVA, , , , , How A Black Hole Lights Up Its Surroundings   

    From AAS NOVA: “How A Black Hole Lights Up Its Surroundings” 

    AASNOVA

    AAS NOVA

    23 October 2017
    Susanna Kohler

    1
    This composite image of the galaxy Centaurus A shows an example of how powerful X-ray and radio jets can extend from the supermassive black hole at the center of a galaxy, affecting the black hole’s surroundings. [ESO/WFI (visible); MPIfR/ESO/APEX/A.Weiss et al. (microwave); NASA/CXC/CfA/R.Kraft et al. (X-ray)]

    ESO WFI LaSilla 2.2-m MPG/ESO telescope at La Silla, 600 km north of Santiago de Chile at an altitude of 2400 metres

    ESO/APEX high on the Chajnantor plateau in Chile’s Atacama region, at an altitude of over 4,800 m (15,700 ft)

    NASA/Chandra Telescope

    How do the supermassive black holes that live at the centers of galaxies influence their environments? New observations of a distant active galaxy offer clues about this interaction.

    2
    Plot demonstrating the m-sigma relation, the empirical correlation between the stellar velocity dispersion of a galactic bulge and the mass of the supermassive black hole at its center. [Msigma]

    Signs of Coevolution

    We know that the centers of active galaxies host supermassive black holes with masses of millions to billions of suns. One mystery surrounding these beasts is that they are observed to evolve simultaneously with their host galaxies — for instance, an empirical relationship is seen between the growth of a black hole and the growth of its host galaxy’s bulge. This suggests that there must be a feedback mechanism through which the evolution of a black hole is linked to that of its host galaxy.

    One proposed source of this coupling is the powerful jets emitted from the poles of these supermassive black holes. These jets are thought to be produced as some of the material accreting onto the black hole is flung out, confined by surrounding gas and magnetic fields. Because the jets of hot gas and radiation extend outward through the host galaxy, they provide a means for the black hole to influence the gas and dust of its surroundings.

    3
    In our current model of a radio-loud active galactic nuclei, a region of hot, ionized gas — the narrow-line region — lies beyond the sphere of influence of the supermassive black hole. [C.M. Urry and P. Padovani]

    Clues in the Narrow-Line Region

    The region of gas thought to sit just outside of the black hole’s sphere of influence (at a distance of perhaps a thousand to a few thousand light-years) is known as the narrow line region — so named because we observe narrow emission lines from this gas. Given its hot, ionized state, this gas must somehow be being pummeled with energy. In the canonical picture, radiation from the black hole heats the gas directly in a process called photoionization. But could jets also be involved?

    In a recent study led by Ákos Bogdán, a team of scientists at the Harvard-Smithsonian Center for Astrophysics used X-ray observations of a galaxy’s nucleus to explore the possibility that its narrow-line region is heated and ionized not only by radiation, but also by the shocks produced as radio jets collide with their surrounding environment.

    4
    Chandra X-ray data for Mrk 3, with radio contours overplotted. Both wavelengths show S-shaped morphology of the jets, with the X-ray emission enveloping the radio emission. A strong shock is present in the west and a weaker shock toward the east. [Bogdán et al. 2017]

    Heating from Jets

    Bogdán and collaborators analyzed deep Chandra X-ray observations of the center of Mrk 3, an early-type galaxy located roughly 200 million light-years away. Chandra’s imaging and high-resolution spectroscopy of the galaxy’s narrow-line region allowed the team to build a detailed picture of the hot gas, demonstrating that it shows similar S-shaped morphology to the gas emitting at radio wavelengths, but it’s more broadly distributed.

    The authors demonstrate the presence of shocks in the X-ray gas both toward the west and toward the east of the nucleus. These shocks, combined with the broadening of the X-ray emission and other signs, strongly support the idea that collisions of the jets with the surrounding environment heat the narrow-line-region gas, contributing to its ionization. The authors argue that, given how common small-scale radio jets are in galaxies such as Mrk 3, it’s likely that collisional ionization plays an important role in how the black holes in these galaxies impart energy to their surrounding environments.

    Citation

    Ákos Bogdán et al 2017 ApJ 848 61. doi:10.3847/1538-4357/aa8c76

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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

     
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