SAO: Weekly Science Updates 2015

[bystander]

The Physical Properties of Dense Molecular Clouds
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Jun 12

A near-infrared image of the massive star formation region W51. A new survey of small, dense molecular clouds in the plane of the Milky Way galaxy, including W51, probes the statistical properties of the dense cores that constitute the birth clouds of massive new stars. (Credit: 2MASS/IPAC)

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Small, dense interstellar clouds of gas and dust, containing hundreds to thousands of solar-masses of material, are suspected of being the precursors to stars and stellar clusters. These so-called cores, with gas densities around one thousand molecules per cubic centimeter (a more typical interstellar value is fewer than one per cubic centimeter) have become a primary focus for understanding the process of high-mass star formation. A series of largescale surveys of the Galactic plane have recently detected tens of thousands of them using infrared and submillimeter telescopes that respond to the emission of their dust; the far infrared sensitive Herschel Space Telescope has been particularly important. A detailed census of these dense molecular cloud structures, their temperatures, masses, and environmental conditions, can help constrain the initial conditions of star formation and subsequent galaxy evolution theories. So far, however, a coherent picture has not emerged, in part because the analyses have been based on individual cases which are often influenced by local effects.

CfA astronomer Cara Battersby and her colleagues now report on the properties of a large sample of dense cores - 1710 objects – that were discovered and/or studied in a survey taken at millimeter wavelengths. The survey covered about one-half of the plane of our Milky Way galaxy looking for these dense clumps. In order to characterize accurately the parameters of each core, its distance needs to be known. Even though they all are seen in the direction of the galactic plane, some could be close to the Sun, and others as far away twenty-five thousand light-years (the distance to the galactic center), or even farther. The team estimated the distance to each object using a statistical likelihood method that included data on the sources’ measured velocities. ...

The Bolocam Galactic Plane Survey. XIII. Physical Properties and Mass Functions
of Dense Molecular Cloud Structures - Timothy P. Ellsworth-Bowers et al

• Astrophysical Journal 805(2) 157 (2015 Jun 01) DOI: 10.1088/0004-637X/805/2/157
  arXiv.org > astro-ph > arXiv:1504.01388 > 06 Apr 2015

[bystander]

Star Formation Near Supermassive Black Holes
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Jun 19

The bright radio galaxy 3C219. The blue object at the center is its active nucleus powered by a supermassive black hole; red shows the extent of the radio emission. Infrared observations of a complete set of similar galaxies dating from about seven billion years ago find that although star formation is active in these objects, the nuclear activity dominates the luminosity. (Credit: NRAO and Parijskij et al)

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Most if not all galaxies are thought to host a supermassive black hole in their nuclei, a finding that is both one the most important and amazing in modern astronomy. A supermassive black hole grows by accreting mass, and while growing its feeding frenzy is not hidden from our view -- it generates large amounts of energy. During the evolutionary phase in which it is most active, the object is known as an active galactic nucleus (AGN). Although there is a difference of a factor of about one billion in physical size scales between the black hole’s accreting environment and its host galaxy, the two sizes are found to be closely correlated, suggesting that there is some kind of feedback between the growth of the black hole and that of its host galaxy. Understanding what the feedback mechanisms are, and how they affect the growth of the galaxy (in particular its star formation), are of paramount importance for our understanding galaxy formation and evolution. Both processes are thought to peak in activity when the universe was only a few billion years old. Neither is particularly well understood.

CfA astronomers Belinda Wilkes, Joanna Kuraszkiewicz, Steve Willner, Matt Ashby, and Giovanni Fazio, along with their colleagues, used the Herschel Space Telescope to study the infrared emission from sixty-four bright, radio and X-ray emitting galaxies with AGN nuclei, and which contain more than one hundred billion solar-masses of stars. Their set is a complete sample of objects of a well-defined class dating from about seven billion years ago, and includes some of the most powerful quasars known. All the objects have large bipolar jets that were driven into intergalactic space by the AGN. The scientists set out to determine how much of the luminosity in these powerful galaxies was due to the AGN and how much was due to star formation activity. The infrared is emitted by dust heated by these two processes, and details of the emission (its typical temperature for example) can help sort out the relative contributions of the two processes.

The astronomers conclude that the star formation rates in these monsters run into the hundreds of solar-masses per year, and therefore reject suggestions that the AGN outflows will quench the star formation in such galaxies. Whatever the details of the growth feedback mechanism, therefore, they do not suppress the star formation. Nevertheless, despite the active star formation underway, the majority of the luminosity is due to the AGN, even during periods when the star formation is most active. Their paper is also significant because it can explain the principal observational differences between the galaxies in this set simply by the orientation of their disk to our line-of-sight, with the large, double-lobed jet sources being seen edge on and the quasars being seen more face-on.

Star formation in z>1 3CR host galaxies as seen by Herschel - P. Podigachoski et al

• Astronomy & Astrophysics 575 A80 (Mar 2015) DOI: 10.1051/0004-6361/201425137
  arXiv.org > astro-ph > arXiv:1501.01434 > 07 Jan 2015

[bystander]

The Discovery of the Molecule Si-C-Si in Space
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Jun 26

A deep optical image of the carbon star RW Leo, showing traces of its surrounding envelope. The important dust-forming molecule Si-C-Si (disilicon carbide) has been discovered in space after decades of speculation and searching, found in the envelope of this source. (Izan Leao; ESO/VLT)

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The space between stars is not empty -- it contains a vast reservoir of diffuse material with about 5-10% of the total mass of our Milky Way galaxy. Most of the material is gas, but about 1% of this mass (quite a lot in astronomical terms) takes the form of tiny dust grains made predominantly of silicates (sand is also silicates), although grains can also be composed of carbon and other elements. The dust grains contain a large fraction of many important elements in the universe like silicon, carbon, and iron. They also play several crucially important roles. They are essential to the chemistry that takes place in the interstellar medium by providing gas molecules with a surface on which to react with other molecules. They absorb ultraviolet and optical light, re-emitting the energy as infrared light, and thus they both constrain what astronomers can see and control much of the energy balance in the interstellar medium. Not least, in the early stages of a star's evolution the dust can coagulate into large clumps -- the first step towards forming planets.

Where does all this dust come from? Interstellar grains are synthesized in two main types of sources: the inner winds of a class of evolved stars, and the ejecta of supernovae. The grains form out of molecular seeds. In evolved stars, such seeds might be molecules like TiO, VO, ZrO, C_2, CN, or C_3, species that have been known for a hundred years; supernovae also have numerous possible constituent elements. Because the carbon monoxide (CO) molecule is extremely stable, it uses up nearly all of the carbon or oxygen (whichever happens to be less abundant). Thus, if the oxygen is less abundant than carbon, there some carbon left over and available for the grains. Carbon-rich stars are ones that have this excess carbon. The dust forms from nucleation seeds that grow as molecules condense onto it via numerous steps that are still quite mysterious, not least being the first step of forming the nucleation seeds.

One likely seed is predicted to be the molecule Si-C-Si (disilicon carbide), but it had never been identified in space. Analogous molecules have been found, like Si-C-C (SiC_2), and both SiC molecules and grains are known, and so the search for disilicon carbide has been underway for several decades. ...

Discovery of SiCSi in IRC+10216: A missing link between gas and dust carriers of SiC bonds - J. Cernicharo et al

• Astrophysical Journal Letters 806(1) L3 (10 Jun 2015) DOI: 10.1088/2041-8205/806/1/L3
  arXiv.org > astro-ph > arXiv:1505.01633 > 07 May 2015

[bystander]

Transition Disks
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Jul 03

A false-color image of the circumstellar transition disk around the star LkCa15, taken at submillimeter wavelengths. A new study finds that the most probable explanation for the inner gap in transition disks is the influence of one or more giant planets orbiting nearby. (Credit: Sean Andrews (CfA)

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A star is typically born with a disk of gas and dust encircling it, from which planets develop as dust grains in the disk collide, stick together and grow. These disks, warmed by the star to a range of temperatures above the cold, ambient interstellar material, can be detected at infrared or millimeter wavelengths, and their infrared color used to characterize their properties. Stars older than about five million years lack evidence for these disks, however, suggesting that by this age most of the disk material has either been converted into planets or smaller bodies, accreted onto the star, or dispersed from the system. Transition disks bridge this period in disk evolution: They have not yet been disbursed, but although they are present they emit only slightly in the infrared. Their emission shows characteristically cooler temperatures, and signs that the innermost (hottest) regions have already disappeared and left a gap (or cavity) in the ring.

CfA astronomer Sean Andrews and his colleagues have been studying transition disks in nearby star-forming regions located in constellations of Taurus and Ophiuchus. The astronomers note that the gaps in transition disks they see might have been caused by one or more of three processes: grain growth and planet formation that depleted the material, a giant planet in the vicinity that swept the region clean, or a stellar wind that blew away or evaporated the dust.

The astronomers set out to determine which of these processes was at work by comparing the properties of transition disks and normal disks. In particular, they compared the rates of mass accretion onto the star and the disk masses. They find, first of all, that these two quantities correlate well with the properties of the radiation, enabling them to control for other potential environmental variables. The transition disks tend to be associated with smaller accretion rates and larger masses than disks in the normal comparison stars, suggesting that the wind and grain growth scenarios are secondary. At least in these two star-forming regions, therefore, the team concludes that the most likely explanation for the gap is the presence of one or more giant planets.

Demographics of Transition Discs in Ophiuchus and Taurus - Joan R. Najita, Sean M. Andrews, James Muzerolle

• Monthly Notices of the RAS 450(4) 3559 (2015 Jul 11) DOI: 10.1093/mnras/stv839
  arXiv.org > astro-ph > arXiv:1504.05198 > 20 Apr 2015

[bystander]

An Over-Massive Black Hole in the Young Universe
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Jul 10

An artist's impression of the Chandra X-ray Telescope in Earth orbit. Astronomers have used Chandra to identify an X-ray bright supermassive black hole from an era only two billion years after the big bang. The finding appears to challenge conventional wisdom about how quickly such supermassive black holes can form in galaxies. (Credit: NASA/CXC/D.Berry & M.Weiss)

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Astronomers generally accept the notion that black holes at the centers of galaxies co-evolve with their host galaxies, and that they have done did so during all cosmic epochs, from the early period after the big band until today. This means that as a galaxy grows in mass, which it does by accreting material (and perhaps also consuming other galaxies) from the intergalactic medium, its black hole also accretes matter and grows. Indeed, the black hole is so massive -- perhaps as much as ten percent (!) of the entire stellar mass of the galaxy – that these two growth processes may be related, for example, because the black hole influences accretion onto the galaxy. The black hole accretion process may have other consequences too, like suppressing star formation by heating and/or disrupting nearby molecular clouds.

Testing these ideas is difficult because it requires measuring black hole properties in cosmic epochs when the universe was only a few billion years old and at the correspondingly cosmic distances even ultraluminous galaxies appear faint to us. A few recent studies have indicated that some supermassive black holes actually grew faster than their galaxies in epochs about three billion years after the big bang, but these measurements were made only on exceptionally X-ray luminous objects which are perhaps not representative of most systems and whose galaxy masses are very uncertain. ...

An over-massive black hole in a typical star-forming galaxy, 2 billion years after the Big Bang - Benny Trakhtenbrot et al

• Science 349(6244) 168 (10 July 2015) DOI: 10.1126/science.aaa4506
  arXiv.org > astro-ph > arXiv:1507.02290 > 08 Jul 2015

http://asterisk.apod.com/viewtopic.php?t=34957

[bystander]

The Fastest Unbound Stars in the Universe
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Jul 17

A schematic showing a late stage in the merger of two supermassive black holes and the consequent ejection of some of the orbiting stars. These stars, which could be numerous and detectable, might reach velocities that are a significant fraction of the speed of light, and could traverse large portions of the universe in their lifetimes. Credit: Guillochon and Loeb (CfA)

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Stars do not stand still. They move through the heavens with motions determined by their encounters with other stars and stellar systems. Stellar velocities in the Milky Way are typically a few hundred thousand miles per hour. However, some peculiar stars do move at much greater velocities, in particular the so-called hypervelocity stars, the fastest known case of which moves at about one and one-half million miles per hour. These stars are thought to have been ejected from the vicinity of the supermassive black hole at the center of our galaxy during a close passage. Indeed, the existence of such rapidly moving stars strengthens the evidence for the existence of a massive black hole at the galactic nucleus.

The speeds of hypervelocity stars are fast, but they are thousands of times slower than the speed light, and relativistic effects can be neglected for them. CfA astronomers James Guillochon and Avi Loeb have found a mechanism that can produce even higher stellar velocities. If one of two orbiting stars breaks up or explodes as a supernova, the other star will be ejected with a velocity that is limited to its previous maximum orbital velocity. But if there is a third massive star in the system, the ejected star's velocity can be enhanced. The astronomers show that when a pair of orbiting, supermassive black holes in a galactic center merge, any tightly bound stars orbiting them can be liberated in this enhanced way, and flung out at speeds of as much as thirty percent of the speed of light.

The scientists demonstrate that there could be many of these relativistic, hypervelocity stars in the cosmos. Moreover, they show that their mechanism is the only possible method to accelerate significant numbers of stars to these speeds, so that if any are detected a merger event would be established. Not least, the astronomers note that these semi-relativistic hypervelocity stars can cross large swaths of the observable universe in their lifetime, and hence can serve as new cosmological messengers that should be detectable with a new generation of telescopes.

The Fastest Unbound Stars in the Universe - James Guillochon, Abraham Loeb

• Astrophysical Journal 806(1) 124 (2015 June 10) DOI: 10.1088/0004-637X/806/1/124
  arXiv.org > astro-ph > arXiv:1411.5022 > 18 Nov 2014 (v1), 16 Jun 2015 (v2)

Observational Cosmology With Semi-Relativistic Stars - Abraham Loeb, James Guillochon

• arXiv.org > astro-ph > arXiv:1411.5030 > 18 Nov 2014 (v1), 29 Apr 2015 (v2)

Planets Could Travel Along with Rogue ‘Hypervelocity’ Stars, Spreading Life Throughout the Universe
Universe Today | 2014 Dec 03

[bystander]

Finding O2
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Jul 24

An infrared image of the Orion Nebula; the circled area shows the region where shocks and emission from molecular oxygen are found. (Credit: ESA/NASA/JPL-Caltech)

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Oxygen is the third most abundant element in the universe (after hydrogen and helium) and of course it is important: all known life forms require liquid water and its oxygen content. For over thirty years, astronomers have been searching for molecular oxygen, O­2, as part of an accounting of cosmic oxygen atoms. Despite early predictions that O­2 should be abundant in the molecular clouds that form new stars and planetary systems, it is virtually absent. Only two locations have convincing O­2 detections, a region of shocks near the Orion Nebula, and a cloud in the constellation of Ophiuchus. The theory is clearly wrong. What is not clear is whether O­2 is missing (with dramatic implications for its abundance and the chemistry of molecular clouds), is in some less detectable form (perhaps frozen onto dust grains), or has been taken up to form water, carbon monoxide, or other oxygen-bearing molecules.

The launch of the Herschel Space Observatory enabled much more sensitive searches for O­2, and prompted updated chemical modeling for molecular clouds and their oxygen. CfA astronomer Gary Melnick and his colleague improved the models to take into account the role of ultraviolet radiation in modifying the chemical and physical conditions in shocks. Their goal was to explain the strength and proportions of the line emission in Orion, and to understand what made the Orion environment so special.

The scientists report that when UV illuminates the region of a shock, the width and temperature of the shocked region changes, and enhances the O­2 in two ways: by knocking oxygen off dust grains and breaking apart some molecules, thus increasing the number of free atoms before the shock, and by breaking apart water in the post-shock gas so that more O­2 can form. The new model also successfully resolves the outstanding puzzle about its limited detection, finding that the detectability is very sensitive to the size and geometry of the emitting region. The paper provides the first self-consistent treatment of preshock, shock, and postshock regions under the influence of UV fields, and explains why O2 detections are so rare.

O₂ Emission Toward Orion H₂ Peak 1 and the Role of FUV-Illuminated C-Shocks - Gary J. Melnick, Michael J. Kaufman

• Astrophysical Journal 806(2) 227 (2015 June 20) DOI: 10.1088/0004-637X/806/2/227
  arXiv.org > astro-ph > arXiv:1505.03909 > 14 May 2015

http://asterisk.apod.com/viewtopic.php?t=24749

[bystander]

The Nearest Potentially Habitable Planet
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Jul 31

Most exoplanets have been discovered via the transiting technique used by the Kepler mission, as illustrated here. A new study has examined Kepler observations for exoplanet transits around lower mass, M-dwarf stars. These stars are much more numerous than solar-type stars. The study concludes that the nearest potentially habitable Earth-sized planet is likely to be about nine light-years away. Credit: NASA/Kepler and planethunters.org

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The Kepler mission has so far spotted an astonishing 4696 possible exoplanets; about one third of them, 1030, have been confirmed as exoplanets so far. A small percentage of this set are potentially habitable, by which astronomers generally mean that they have surface temperatures suitable for water to remain liquid, rather than frozen out as ice or suspended as vapor in the atmosphere. Exoplanets can exist around all kinds of stars, not only ones like the Sun. Sun-like stars are comparatively rare - the less massive ("M dwarf") stars are about ten times more common. They are also cooler and dimmer, and so their planetary systems will have habitable zones closer to the star than is the case in our solar system, but exoplanets orbiting in that zone are of great interest, especially rocky, Earth-sized ones large enough to retain an atmosphere.

CfA astronomers Courtney Dressing and David Charbonneau have reanalyzed the full Kepler mission dataset, using their own transiting planet detection software to search for exoplanets around M dwarf stars. They use both archival and followup data to refine the stellar sample and the transit details, explicitly address issues of sample completeness, and incorporate a sophisticated treatment of the habitable zone in these stars. After accounting for sampling effects that influence the transit detection (for example, the line-of-sight geometry to the stellar system), they conclude that Earth-sized planets (rocky planets between 1 to 1.5 times the Earth's size) are common: there is probably one with an orbital period less than fifty days for every 1.8 early M dwarf stars. With a somewhat expanded range of parameters, including sizes up to four Earth-radii and orbiting in less than two hundred days, there are on average an estimated 2.5 Earth-like planets per M dwarf star. Based on these statistics, the astronomers conclude that the nearest potentially habitable Earth-sized planet is likely to be a mere nine light-years away.

The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from
the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity - Courtney D. Dressing, David Charbonneau

• Astrophysical Journal 807(1) 45 (2015 July 01) DOI: 10.1088/0004-637X/807/1/45
  arXiv.org > astro-ph > arXiv:1501.01623 > 07 Jan 2015 (v1), 26 May 2015 (v2)

[bystander]

Direct Imaging of a Young, Extrasolar Kuiper Belt
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Aug 07

An infrared image of a bright, nearly edge-on dusty debris ring around the star HD115600. The ring resembles the Kuiper belt in our own solar system, the region that contains Pluto as well as thousands of remnants of the earliest stages of icy planet formation. The Sun-Pluto distance is projected onto the image for reference. The center of the disk (diamond) appears slightly offset from the location of the star (plus). ((Credit: Thayne Currie et al/NAOJ))

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The Kuiper belt is the region of the solar system situated just beyond Neptune's orbit, about 40 AU from the Sun (one AU - astronomical unit - is the Earth's average distance from the Sun). The Kuiper belt is the location of numerous dwarf planets, such as Pluto, and is also home to thousands of remnants of the earliest stages of icy planet formation which provide keys to understanding the early, unevolved solar system. Beyond the Kuiper belt lies the Oort cloud, a spherical cloud of comets and icy planetesimals that extends out to perhaps one hundred thousand AU. Presumably other stellar systems also contain analogs to the Kuiper belt, and they could help shed light on the Kuiper belt's evolution and composition. Until now, however, the few such rings that have been imaged are unreliable analogs; they are seen around nearby stars whose birth environments are unlike the massive stellar complex where the Sun was probably formed.

The situation has recently changed with the advent of a new generation of astronomical imaging instruments on large telescopes that use adaptive optics to obtain high spatial resolution images. CfA astronomer Scott Kenyon and his colleagues used the new spectrometer on the Gemini telescope to study a star slightly larger than the Sun located about 360 light-years away in a relatively young complex of massive stars, a region roughly analogous to the Sun’s suspected birth environment. The star has a large excess of infrared emission, a clear indication that it hosts a circumstellar dust disk that plausibly is forming a system of planets.

The astronomers imaged the disk, and found that the debris ring is confined to a Kuiper belt–like distance from the star. Moreover, they discovered from the spectrum of its reflected light that its dust has properties consistent with that of the major constituents of Kuiper belt bodies -- including water ice. The result provides a promising reference point for understanding the evolution and composition of the Kuiper belt, and for the early evolution of the whole solar system.

Direct Imaging and Spectroscopy of a Young Extrasolar Kuiper Belt in the Nearest OB Association - Thayne Currie et al

• Astrophysical Journal Letters 807(1):L7 (01 July 2015) DOI: 10.1088/2041-8205/807/1/L7
  arXiv.org > astro-ph > arXiv:1505.06734 > 25 May 2015

http://asterisk.apod.com/viewtopic.php?t=34829

[bystander]

Dust Storms on Mars
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Aug 14

su201533.jpg (25.07 KiB) Viewed 6891 times

An image of a Martian textured dust storm taken with the
Mars Global Surveyor (MGS) mission. The colored contours
indicate the confidence level of the identifications. A new
analysis of textured dust storms examines data over eight
years of MGS observations and finds that such storms
account for about half of all the dust in the Martian
atmosphere. (Credit: NASA/MGS; Guzewich et al. 2015)

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In the 1870's astronomers first noted the presence of yellow clouds on the surface of Mars and suggested they were caused by windblown dust. Today, dust storms on Mars are well known and those that display visible structures are called "textured dust storms." Textured dust storms actively lift dust into the atmosphere, and can result from a range of meteorological effects including strong winds. Besides obscuring views of the Martian surface, dust in these storms affects atmospheric heating, and accumulation of dust on the surface can alter the surface albedo (reflectivity). These dust storms, despite being studied for more than a century, remain rather mysterious. It is not understood, for example, how textured storms are distributed over the surface of the planet, when their frequency peaks, or how much dust is actually swept up.

CfA astronomer Huiqun Wang and his three collaborators used data from the Mars Global Surveyor (MGS) mission and its suite of instruments to study textured dust storms. MGS was launched in 1999 and has been observing Mars almost continuously since then; subsequent missions like the Mars Reconnaissance Orbiter (launched in 2005) added to the database. The scientists examined a complete record of MGS daily global maps to study the climatology of dust storms that exhibit non-smooth textures, and relate these observations to the surface dust distribution and atmospheric effects.

The astronomers found that textured dust storms are seasonally clustered: they occur mostly at the equinoxes (the "spring" and "fall" seasons of Mars) and not the solstices (the "summer" and "winter" seasons). They also occur most frequently in the north-polar region above mid-latitudes. They are able to quantify the relative importance of textured dust storms to dust in the atmosphere, and find that they can account for about half, the rest coming from local “dust devils” or other small-scale disturbances. Not least, they report that the dust in textured dust storms is mixed globally on a timescale of weeks. The new research, which analyzed eight years of spacecraft observations, uses the most complete set of data to date to obtain the most recent understanding of Martian dust storms.

Mars Orbiter Camera Climatology of Textured Dust Storms - Scott D. Guzewich et al

• Icarus 258 1 (15 Sep 2015) DOI: 10.1016/j.icarus.2015.06.023

[bystander]

The Evolving Population of Black Holes
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Aug 21

su201534.jpg (22.07 KiB) Viewed 6876 times

Four galaxies simulated by the Illustris program, which
also tracks the evolution of the supermassive black
holes at their centers. Credit: D. Sijacki, et al.

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Quasars are among the most luminous sources in the universe, shining with a total luminosity of hundreds or thousands of Milky Ways. Their large radiative emission enable us to observe them out to great distances and thus to investigate their evolution over more than ninety per cent of cosmic time. The mechanism most likely to power quasars is accretion onto the supermassive black hole at each galaxy's nucleus.

Although there is a difference of a factor of about one billion in the physical size scales between a black hole's accreting environment and its host galaxy size, the two dimensions are closely correlated in the many observed objects, suggesting that there is some kind of feedback between the growth of the black hole and that of its host galaxy. Understanding what those feedback mechanisms are, and how they affect the growth of the galaxy (in particular its star formation), are of paramount importance for understanding galaxy formation and evolution. Both processes are thought to have peaked in activity when the universe was only a few billion years old, but neither is particularly well understood. Astronomers have a generally good understanding of the range of properties and activity of quasars and their black holes in the local universe, but know much less about quasars dating from the first billion years of the universe.

CfA astronomers Shy Genel, Paul Torrey, Dylan Nelson, and Lars Hernquist and their colleagues have developed a computer simulation that attempts to address these mysteries. "Illustris" is the first simulation capable of modeling galaxy evolution at scales down to relatively small sizes – only a few thousand light-years – a capability that in particular allows the code to track the development and growth of the supermassive nuclear black holes. The code assumes basic cosmological parameters, including dark matter, and by the time the simulation ends in the current epoch it has produced 32,542 black holes, 3,965 of them more massive than ten million suns. ...

The Illustris Simulation: The Evolving Population of Black Holes Across Cosmic Time - Debora Sijacki et al

• Monthly Notices of the RAS 452(1):575 (2015 Sep 01) DOI: 10.1093/mnras/stv1340
  arXiv.org > astro-ph > arXiv:1408.6842 > 28 Aug 2014 (v1), 16 Jun 2015 (v2)

[bystander]

Imaging Lensed, Distant Galaxies with the Large Millimeter Telescope
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Aug 28

The Large Millimeter Telescope, the world's largest single-dish, steerable, millimeter-wavelength telescope designed specifically for astronomical observation. Astronomers have used the LMT in its early stages of operation to study the gas and dust properties of luminous galaxies in the ealey stages of the universe. (Credit: Large Millimeter Telescope Alfonso Serrano)

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In the 1980's, observations of nearby galaxies made with the Infrared Astronomical Satellite, along with observations of the far-infrared/submillimeter background with the Cosmic Background Explorer satellite, showed that the universe emits about as much energy density at infrared and submillimeter wavelengths as it does at optical and ultraviolet wavebands. Where does it all come from? A breakthrough came with the discovery of a large population of sources very bright at submillimeter wavelengths at large cosmic distances. These so-called submillimeter selected galaxies (SMGs) have luminosities hundreds of times larger than that of the Milky Way, powered in part by star formation. Identifying and understanding the nature of these sources has, however, proven to be challenging because they are so distant and hence smaller in angular size than most single telescopes can resolve.

The Large Millimeter Telescope (LMT) is the world's largest single-dish, steerable, millimeter-wavelength telescope designed specifically for astronomical observations. Situated on the summit of Volcán Sierra Negra at an altitude of 4600 meters, it is a binational project between México and the United States, and is the largest and most complex scientific instrument constructed in México. CfA astronomers Mark Gurwell and David Wilner and their colleagues used the newly operational LMT in a set of early science spectroscopic studies of submillimeter galaxies. They selected their objects from the Herschel satellite imaging catalog of luminous far infrared galaxies; the sources are distant luminous SMGs that are being gravitationally lensed by foreground galaxy clusters, making them appear particularly bright.

The team used the carbon monoxide molecule as their spectroscopic tracer. The strength of the line enabled them to estimate the amount of gas, which they could then compare to the amount of dust; they report finding about seventy times more gas, consistent with expectations. The gas also enables them to determine the Doppler shift of the galaxy and hence its distance, something not possible to do accurately from the imaging data alone. The most distant galaxy in their early science study dates from the epoch only one and one-half billion years after the big bang. Remarkably, they were also able to use the spectroscopic measurements to discover that one object is in fact three galaxies, at three different very different cosmic distances, that just happen to all lie along the same line-of-sight. The new results demonstrate the power of this new astronomical telescope, and mark the beginning of a detailed exploration of the cosmic sources responsible for the infrared background.

Early Science with the Large Millimeter Telescope: Observations of Dust Continuum
and CO Emission Lines of Cluster-lensed Submillimetre Galaxies at z = 2.0−4.7 - J. A. Zavala et al

• Monthly Notices of the RAS 452(2):1140 (2015 Sep 11) DOI: 10.1093/mnras/stv1351
  arXiv.org > astro-ph > arXiv:1506.04747 > 15 Jun 2015 (v1), 15 Jul 2015 (v2)

[bystander]

Neutral Hydrogen Gas in Galaxy Clusters
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Sep 04

A composite image of the 'Sausage' merging cluster CIZA J2242.8+5310, made using data from the Subaru and Canada France Hawaii Telescopes. Yellow circles are cluster galaxies, where accelerated star formation is taking place; white circles indicate galaxies outside of the cluster; green marks regions of radio emission that traces shocks; purple marks the hot X-ray emitting gas. The cluster is one of the most massive in the Universe. (Credit: Andra Stroe et al. / MNRAS)

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Most galaxies are members of a cluster, a grouping of several to thousands of galaxies. Our Milky Way, for example, is a member of the "Local Group," a set of about fifty galaxies whose other large member is the Andromeda galaxy about 2.3 million light-years away. The closest large cluster of galaxies to us is the Virgo Cluster, with about 2000 members; its center is about 50 million light-years away. The clustering of galaxies influences how any particular member galaxy will evolve, but what happens and how it happens are not well understood. The cluster's influences on the star-formation activity within its galaxies is a particularly interesting question because the star formation rate helps set the luminosity of a galaxy, its supernovae activity, and the processing of its hydrogen gas into heavier elements.

Astronomers think that when clusters merge, whatever effects they have on star formation should be accentuated. Published results, however, have arrived at divergent conclusions on what these effects are. Most research finds that cluster mergers enhance the star formation activity, but a few studies have concluded that it quenches it, while at least one has argued there is little effect either way.

CfA astronomer Reinout van Weeren and his five colleagues used the Westerbork Synthesis Radio Telescope for very deep studies of the atomic hydrogen gas in the cluster CIZAJ2242.8+5301, nicknamed the "Sausage." Hydrogen gas is the fuel for star formation, and they find, contrary to previous results, that there are comparable amounts of it in star-forming cluster galaxies as in non-cluster galaxies, implying that the cluster merger does not reduce the amount of the gas. Moreover, the scientists find that supernova activity confirms that star formation has been underway in these galaxies for about one hundred million years (not an unusual span for a starburst) and that at the current rate of activity the gas would not be depleted for about a billion years. The new results mark an important milestone in the study of cluster mergers because they show that member galaxies are still gas rich and thus can form stars and stimulate the nuclear engine for a long time. The results also can exclude some previously viable models.

Neutral Hydrogen Gas, Past and Future Star Formation in Galaxies in and around the 'Sausage' Merging Galaxy Cluster - Andra Stroe et al

• Monthly Notices of the RAS 452(3):2731 (2015 Sep 21) DOI: 10.1093/mnras/stv1462
  arXiv.org > astro-ph > arXiv:1506.08822 > 29 Jun 2015

[bystander]

Shocks in a Distant Gamma-Ray Burst
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Sep 11

An optical image of the sky at the location of a faint galaxy (marked with the
cross-hairs) where a gamma-ray burst (GRB) was seen last year. Followup
studies of the burst and its afterglow find that it originated in a supernova -
the death of a massive star - yet also shows signs of shocks more typically
originating in GRBs from merging binary stars. (Credit: Z.Cano et al/MNRAS)

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Gamma ray bursts (GRBs)--flashes of high-energy light occur about once a day, randomly, from around the sky--are the brightest events in the known universe. While a burst is underway, it is many millions of times brighter than an entire galaxy. Astronomers are anxious to decipher their nature not only because of their dramatic energetics, but also because their tremendous brightness enables them to be seen across cosmological distances and times, providing windows into the young universe.

There appear to be two general types of GRBs: those associated with the deaths of massive stars, and ones believed to originate from the coalescence of two extreme objects (neutron stars or black holes) that had been orbiting each other in a binary system. In general the two types can be distinguished by the lengths of their bursts, the former lasting longer than a few seconds, while the latter are briefer. Astronomers think that, despite the differences, both kinds of GRBs have hot discs accreting material leading to the production of bipolar jets of charged particles moving at relativistic speeds. In the standard model, shocks internal to the fireball produce the gamma-rays in the first (longer duration) case, while shocks from the jets' interactions with the external medium produce the initial burst of gamma-rays in the second case. Many details are similar in both scenarios, however, while some others vary according to the type, and astronomers have been trying to constrain these various parameters so that they can trace the origin of each GRB more precisely.

CfA astronomer Raffaella Margutti and her colleagues used several ground-based telescopes to follow-up a GRB event that went off in June of 2014, examining the afterglow from about three days after the detection to about one hundred and twenty days later. They conclude that the burst is associated with a massive star's death (a supernova), but find that some of its emission apparently results from shocks external to the fireball as are seen in the less luminous class of GRBs. The results are consistent with the predictions of supernova modeling, but the fact that this object spans both classes highlights the complexity of the sometimes-overlapping physical processes at work and the importance of observations at multiple wavelengths.

GRB 140606B/iPTF14bfu: Detection of Shock-Breakout Emission from a Cosmological γ-Ray Burst - Zach Cano et al

• Monthly Notices of the RAS 452(2):1535 (2015 Sep 11) DOI: 10.1093/mnras/stv1327
  arXiv.org > astro-ph > arXiv:1505.03522 > 13 May 2015 (v1), 10 Jun 2015 (v2)

[bystander]

The Local Group as a Time Machine
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Sep 18

The Small Magellanic Cloud, a companion to the Milky Way galaxy and a
member of our Local Group of galaxies. Scientists have extrapolated the
character of the current galaxies in the Local Group back to the cosmic
Epoch of Re-ionization, and find that even modest galaxies today were
significant contributors to the reionization of the neutral gas at that time.
The field of view is slightly larger than 3.5 x 3.6 degrees.
(Credit: ESA/Hubble and Digitized Sky Survey 2)

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Most galaxies lie in clusters, groupings of several to many thousands of galaxies. Our Milky Way galaxy itself is a member of the "Local Group," a band of about fifty galaxies whose other large member is the Andromeda Galaxy about 2.3 million light-years away. The closest large cluster of galaxies to us is the Virgo Cluster, about 50 million light-years away, with about 2000 members. Astronomers are peering back into cosmic history, examining distant galaxies and trying to reconstruct the evolution of the universe, but distant galaxies are faint and hard to detect. Analyzing the evolution of clusters of galaxies offers a way to overcome this limitation.

About 380,000 years after the big bang, the nuclei of hydrogen atoms combined with free electrons to form neutral gas, beginning the so-called Dark Ages ("dark" because cold neutral gas does not radiate much). About four hundred million years later this gas began to be re-ionized by ultraviolet radiation from a newly born cosmic denizen: stars. There is a major gap in our understanding of this period, however: There did not seem to have been enough bright galaxies and stars back then to have done the job, at least according to astronomers who model the epoch of re-ionization. They conclude that faint galaxies, those 100-1000 times below the detection limits of current observatories, must have been necessary contributors. NASA's soon-to-be-launched James Webb Space Telescope (JWST) has as one of its primary goals the detection of these hypothesized, faint galaxies.

CfA astronomers Benjamin Johnson and Charlie Conroy and their collaborators argue that even JWST is unlikely to detect the faintest and most numerous of these sources. Instead, they propose an alternative way to solve the problem of the missing ultraviolet: they use the Local Group of galaxies to learn about the faintest galaxies at the epoch of reionization. The scientists applied models of stellar evolution to extrapolate the current galaxies' properties back to the epoch of re-ionization from its end time about one billion years after the big bang to times a few hundred million years earlier. The astronomers were able to show that many of the faintest galaxies in the Local Group today were at one time nearly as bright as the most distant galaxies currently being detected, and that the Milky Way's two major dwarf galaxy companions, the Large and Small Magellanic Clouds, were also substantially more luminous back then. The team reports that even the small irregular galaxies in the Local Group must have been significant contributors to cosmic re-ionization back then, although even so they were so faint that JWST would not be able to see them.

The Local Group as a Time Machine: Studying the High-Redshift Universe with Nearby Galaxies - Michael Boylan-Kolchin et al

• Monthly Notices of the RAS 453(2):1503 (2015 Oct 21) DOI: 10.1093/mnras/stv1736
  arXiv.org > astro-ph > arXiv:1504.06621 > 24 Apr 2015 (v1), 27 Aug 2015 (v2)

[bystander]

Discovery of the Companions of Millisecond Pulsars
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Sep 25

An optical image of the globular cluster, 47 Tucanae. Astronomers have identified the orbiting companions to five millisecond pulsars in this cluster and found them all to be white dwarf stars. (Credit: South African Astronomical Observatory)

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When a star with a mass of roughly ten solar masses finishes its life, it does so in a spectacular explosion known as a supernova, leaving behind as remnant "ash" a neutron star. Neutron stars have masses of one-to-several Suns, but they are tiny in size, only tens of kilometers. Neutron stars spin rapidly, and when they have associated rotating magnetic fields to constrain charged particles, these particles emit electromagnetic radiation in a lighthouse-like beam that can sweep past the Earth with great regularity every few seconds or less. Such neutron stars are known as pulsars. Pulsars are dramatic and powerful probes of supernovae, their progenitor stars, and the properties of nuclear matter under the extreme conditions that exist in these stars.

Some pulsars called millisecond pulsars spin much more quickly, and astronomers have concluded that in order to rotate so rapidly these objects must be regularly accreting material from a nearly companion star which in a binary orbit with it; the new material helps to spin-up the neutron star, which normally would gradually slow down. There are more than 200 known millisecond pulsars. An understanding of these pulsars has been hampered, however, by the fact that only about a dozen of them have had their companion stars directly detected and studied.

CfA astronomers Maureen van den Berg, Josh Grindlay, and Peter Edmonds and their colleagues used ultraviolet images from Hubble to identify the companion stars to two millisecond pulsars located in the globular cluster 47 Tucanae. They were also able to confirm a previous but tentative identification, and to confirm two more. They report that each is of these companions is a white dwarf star – an evolved star that can no longer sustain nuclear burning and which has shrunk to a fraction of its original radius. Each of these pulsars spins more than 120 times per second, and the companions orbit quite closely with periods ranging from only 0.43 days to 1.2 days, close enough to easily satisfy the requirements needed for this kind of cosmic cannibalism as the pulsars gradually feed on material from the white dwarfs. The new work significantly increases the number of identified and characterized millisecond pulsar companions.

Discovery of Near-Ultraviolet Counterparts to Millisecond Pulsars in the Globular Cluster 47 Tucanae - L. E. Rivera-Sandoval et al

• Monthly Notices of the RAS 453(3):2707 (2015 Sep 02) DOI: 10.1093/mnras/stv1810
  arXiv.org > astro-ph > arXiv:1508.05291 > 21 Aug 2015 (v1), 24 Aug 2015 (v2)

[bystander]

The Environments of Radio-Bright Active Galaxies
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Oct 02

A Chandra X-ray Observatory image of the galaxy cluster Abell 2125, showing its complex of galaxies and very hot gas clouds in the process of merging. Some galaxies in clusters host active black-hole nuclei that are ejecting jets of particles and emitting at radio wavelengths. A new study finds evidence that the cluster environment plays an important role in determining the nature of accretion onto the black hole. (Credit: NASA/CXC/UMass/Q.D.Wang et al.)

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The nucleus of an active galaxy contains a massive black hole that is vigorously accreting material. In the process, the nucleus typically ejects jets of rapidly moving charged particles that radiate brightly at many wavelengths, in particular radio wavelengths. Active galaxies display a range of dramatically different properties and one categorization uses the radio emission, finding one class that is bright in the radio and a second group that is comparatively faint. Astronomers suspect that the reason for the difference is a different rate of accretion onto the central black hole, but there are other activities that also seem to correlate with the radio emission including nearby star formation, for example, or the age of the galaxy. Astronomers are therefore trying to identify the ones that might be causal.

Feedback from the intergalactic medium onto a galaxy's nucleus has recently been identified as an important driver of galaxy evolution, and the question naturally arises about the role of such feedback in a galaxy’s radio activity and the accompanying effects. CfA astronomers Ralph Kraft and Dan Evans and their colleagues used the Chandra X-Ray Telescope in the first systematic X-ray study of the cluster environment of radio galaxies all dating from the same epoch. The X-ray emission is the key to understanding how the gas accretes onto the black hole.

The team observed fifty-five radio emitting sources spanning a factor of a thousand in radio luminosity, twenty-five of them classified as bright. They found that the bright radio sources show evidence of high accretion from a circumnuclear disk. The faint sources, on the other hand, have a more uncertain mechanism, perhaps the chaotic accretion of cool gas clouds; significantly, their radio emission strength is strongly correlated with the cluster richness and central density, while no such correlations were found for the bright sources. The scientists conclude that there are strong environmental differences between these two classes consistent with thinking that the cluster environment supports the fueling of emission. This evidence has prompted the team to study next the relationships between the gas in the intracluster medium and the other phenomena associated with the two classes.

The link between accretion mode and environment in radio-loud active galaxies - Judith Ineson et al

• Monthly Notices of the RAS 453(3):2682 (2015 Nov 01) DOI: 10.1093/mnras/stv1807
  arXiv.org > astro-ph > arXiv:1508.01033 > 05 Aug 2015 (v1), 21 Aug 2015 (v2)

[bystander]

Dead Comets and Near-Earth Encounters
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Oct 09

An image of the asteroid Tempel 1 taken during the Deep Impact visit. Tempel 1 is about five kilometers across. CfA astronomers report that there are about one hundred large NEOs that are dormant, short-period comets, and about .3 - 3% of the fainter ones are too. (Credit: NASA/JPL-Caltech/UMd)

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Near Earth Objects (NEOs) are asteroids or comets whose orbits sometimes bring them close to the Earth, thereby posing a potentially threat. The asteroid that struck Chelyabinsk last year was an NEO about 40 meters in diameter. While it is relatively easy to detect an NEO in visible light by watching its movement across the sky from night to night, determining its size and its potential hazard is more difficult because its optical brightness results from both its size and its reflectivity. CfA astronomers have for several years been using the IRAC infrared camera on Spitzer to measure the infrared light emitted by NEOs and, combined with optical measurements, to deduce their probable dimensions.

NEOs are thought to originate from collisional fragments of objects in the asteroid belt beyond the orbit of Mars, with over 10,000 known today. Short–period comets can also be NEOs, but unlike asteroids they most likely originated in the Kuiper belt, a reservoir of icy bodies outside the orbit of Neptune. The orbits of these bodies are disturbed as a result of gravitational perturbations with the giant planets, and some end up as NEOs, developing cometary tails when they approach the Sun and become active. After a while, their volatiles evaporate and these comets become dormant. As a result, it is very likely that the NEO population being studied includes a significant number of extinct comets.

CfA astronomers Joe Hora, Giovanni Fazio and Howard Smith and their colleagues reported two years ago on the discovery that the NEO Don Quixote is actually an extinct comet - they were able to find its faint cometary tail in infrared images. Now they and their colleagues have completed a statistical analysis of the full near infrared catalog of NEOs, searching for possible short-period comets by using a combination of their orbital parameters and their surface albedos as inferred from their near infrared properties. The scientists found that between about 0.3 and 3% of the moderately bright NEOs are actually likely to be dormant, short-term period comets. They identify twenty-three specific ones as dormant comets. They also conclude that about one hundred large NEOs, with diameters larger than a kilometer. are probably also dormant short-period comets.

ExploreNEOs VIII: Dormant Short-Period Comets in the Near-Earth Asteroid Population - M. Mommert et al

• Astronomical Journal 150(4):106 (2015 Oct) DOI: 10.1088/0004-6256/150/4/106
  arXiv.org > astro-ph > arXiv:1508.04116 > 17 Aug 2015

[bystander]

The Minimum Mass of a Proto-Solar System Disk
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Oct 16

An image of the young star forming region IC348 in Perseus (about 2-3 million years old) as seen by the infrared cameras onboard the Spitzer Space Telescope. Astronomers studying the birth of solar systems have found thirteen stars in this complex with detectable disks, none of which is as massive as the early Solar system's disk. (Credit: NASA/JPL-Caltech/SST/L. Cieza (UTx))

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Astronomers estimate that at the time the Solar system formed, its proto-planetary disk contained the equivalent of about twenty Jupiter-masses of gas and dust. This so-called "minimum mass solar nebula (MMSN)" is derived from the current masses of the rocky planets and calculations of how they formed; a minimum mass is used in case the planet formation mechanism is somehow less efficient than expected. (Some earlier estimates had MMSN values up to about 100 Jupiter-masses.) As a nebula ages and its planets develop, its disk mass naturally decreases; current models estimate that a planetary system can form in under five million years.

CfA astronomer Sean Andrews and his colleagues have been studying the early stages of planet-forming nebulae around other stars using the fact that such disks are cool and emit radiation primarily in the infrared and submillimeter regimes. The team used the submillimeter camera on the James Clerk Maxwell Telescope in Hawaii to map the emitting dust in a cluster of young stars known as IC348 located in the Perseus molecular cloud about a thousand light-years away from us. The cluster is estimated to be about two to three million years old, and its planetary systems should therefore be partially developed.

The scientists found thirteen submillimeter point sources in the cloud indicative of disks, in a total population of about three hundred and seventy known objects. From its emitted luminosity the scientists can estimate the mass of a disk, and they find these disks range in size between 1.5 and 16 Jupiter-masses -- smaller than a MMSN. Their results imply that disks as massive as the early solar system's are, at least by this age, very rare. Furthermore, expecting that the undetected sources all have smaller and fainter disks, the team combined the observations of all the sources to estimate what the average disk mass was: one-half a Jupiter-mass. The astronomers conclude that fewer than about 1% of stars have a MMSN disk. If most disks start off with the solar minimum mass value, therefore, they must have evolved very rapidly in order to have depleted most of the mass after a few millions years.

A SCUBA-2 850 micron Survey of Protoplanetary Discs in the IC 348 Cluster - L. Cieza et al

• Monthly Notices of the RAS 454(2):1909 (2015 Dec 01) DOI: 10.1093/mnras/stv2044
  arXiv.org > astro-ph > arXiv:1504.06040 > 23 Apr 2015 (v1), 25 Apr 2015 (v2)

[bystander]

X-Ray Emission from Massive Stars
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Oct 23

A composite image of the massive star forming region Cygnus OB2. Astronomers have determined that the X-ray emission from massive stars arises from shocks. The image shows X-ray emission from Chandra (blue), infrared from Spitzer (red), and optical data from the Isaac Newton Telescope (orange). (Credit: X-ray: NASA/CXC/SAO/J.Drake et al, Optical: Univ. of Hertfordshire/INT/IPHAS, Infrared: NASA/JPL-Caltech)

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Massive young stars are known to emit strong X-rays. Unlike the X-ray emission from lower mass stars, however, which arises in stellar photospheres, the X-rays from massive stars are thought to result from powerful shocks. Several kinds of shocks can be responsible, produced either by very strong winds driven by the star’s radiation, by the head-on collision between winds that have been magnetically channeled by the star’s magnetic field, or by wind collisions in a binary stellar system in which each stars has a wind. Sorting out the mechanisms enables astronomers to identify the most active physical processes at work, and thereby decode additional information about the star’s physical makeup and evolutionary status.

CfA astronomers Nick Wright, Jeremy Drake, and Marcio Guarcello and their colleagues used the Chandra X-ray Observatory to study the emission from 106 massive stars in the relatively nearby Cygnus-OB2 cluster. This relatively large sample enabled the scientist to test their models by examining, for example, whether or not there are clear correlations between a star’s X-ray strength and its luminosity.

The astronomers find for their massive stars that there is a well-defined correlation between the X-ray and total stellar luminosity, with the X-ray strength being about sixteen million times less; indeed, their relation is similar to one previously reported for another massive star-forming region, and favors the first (radiatively driven) kind of shocks. For the most massive stars in the sample, however, the team does find evidence for colliding shocks. The new results help to constrain models of X-ray emission from massive stars. Because the relations are about the same as in other massive star clusters but now extended to different clusters and cluster environments, the new work also shows that the mechanisms are not very sensitive to the local conditions.

X-ray emission from massive stars in Cyg OB2 - G. Rauw et al

• Astrophysical Journal Supplement 221(1):1 (Nov 2015) DOI: 10.1088/0067-0049/221/1/1
  arXiv.org > astro-ph > arXiv:1401.8098 > 31 Jan 2014

http://asterisk.apod.com/viewtopic.php?t=29947
http://asterisk.apod.com/viewtopic.php?t=29752

[bystander]

Unveiling the Active Galactic Nucleus of a Quasar
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Nov 13

An artist's illustration of the NuSTAR X-ray satellite, which studies high energy X-rays. Astronomers have used the satellite to study the quasar 3C273, and found that the emission most likely arises from two components, an accretion disk around the black hole and very rapidly moving charged particles in a jet. (Credit: NASA/JPL-Caltech/NuSTAR)

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3C 273 is the nearest high-luminosity quasar to Earth, about two billion light-years away, and shining with the power of more than about three thousand Milky Way galaxies. Since its discovery in 1963 it has been extensively studied at many wavelengths. It is bright and very variable from the radio wavelengths to the X-ray and gamma-rays, and has rapidly moving jets of charged particles. One of the major theoretical challenges for 3C273 has been explaining the origins of the various kinds of dramatic activities seen. Two generic suggestions have been proposed based on optical variability: emission from an accretion disk around the black hole nucleus, and/or emission from fast-moving charged particles (but unrelated to the radio jet). One of the interesting complications is that the X-rays can be produced not only by very hot gas but also by high velocity particles which add some of their own energy to radiation that they scatter, for example converting radio emission into X-rays. Sorting out the possibilities has been a priority for scientists studying quasars.

The NuSTAR mission (Nuclear Spectroscopic Telescope Array) is a space-based X-ray telescope that can detect X-rays with wavelengths smaller (energies higher) than those detectable by the Chandra X-ray Observatory by about a factor of eight. Launched in 2012, NuSTAR is the first space-based direct-imaging X-ray telescope to be able to observe these energies. CfA astronomer Laura Brenneman joined a team of colleagues to study 3C273 with NuSTAR, combing the results with observations from other space-based X-ray and gamma-ray missions. In 2012 while undertaking a coordinated observing campaign, the team found that the emission was in a relatively weak phase which enabled them to diagnose X-ray emission signatures that are normally overwhelmed by other emission. The scientists conclude that the two-component model is very likely correct, with some emission coming from the accretion disk and higher-energy light coming from a beamed jet of charged particles. With additional, multi-observatory observations of the source, the team hopes to refine their description with more explicit values for the physical conditions present.

3C 273 with NuSTAR: Unveiling the Active Galactic Nucleus - Kristin K. Madsen et al

• Astrophysical Journal 812(1):14 (2015) DOI: 10.1088/0004-637X/812/1/14
• arXiv.org > astro-ph > arXiv:1506.06182 > 19 Jun 2015 (v1), 03 Sep 2015 (v2)

[bystander]

Inferring the Star Formation Rates of Galaxies
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Nov 20

The active star-forming region RCW106 in our galaxy, as seen in the infrared. Astronomers estimate star formation rates in other galaxies by measuring their gas content and infrared emission. In a new study, astronomers counted up the new stars in RCW106 and five other local regions of active star formation to check the reliability of this method. (Credit: NASA/JPL-Caltech/Spitzer

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Our Milky Way galaxy produces on average a few new stars every year across the entire system. Massive young stars emit large amounts of ultraviolet radiation which heats the local dust, and so the star formation process results in infrared emission. The IRAS satellite, launched by NASA in 1983 for a ten-month mission, discovered that some galaxies in the universe are ultra-luminous, radiating a hundred or even a thousand times as much light, mostly in the infrared, as does the Milky Way. Astronomers today attribute the source of that intense luminosity to massive bursts of star formation, simply scaled-up versions (called the Schmidt relation) of the processes in the Milky Way. The colors and other morphological characteristics of ultra-luminous galaxies are generally consistent with this interpretation. If true, these galaxies are forming stars with surprisingly high efficiencies and perhaps in unusual ways. Astronomers refining their models are therefore investigating the extent to which star formation rates can legitimately be derived from a simple scaling relation, as well as the extent to which other processes like black hole accretion at the nucleus might supplement the radiation from star formation.

CfA astronomers Sarah Willis, Andres Guzman, Howard Smith, and Juan Rafael Martinez-Galarza and their colleagues decided to investigate these issues by examining the star formation activity in six regions of current, massive star formation in our Milky Way. These molecular clouds are thought to be small prototypes of the powerful star formation regions active in luminous galaxies, but because the clouds are much closer to us, it is possible to count directly the number of new stars in them, rather than just infer their numbers from a luminosity as with the Schmidt relation extrapolation. Using infrared images from the Spitzer Space Telescope, complemented by ground-based observations, the team identified 2871 newly formed stars in these regions; they then traced the stellar production rates in different zones across the sources, using the visual extinction as a measure of the amount of dust and gas present. Their results were roughly consistent with a conventional Schmidt relation, but the astronomers found significant deviations across the regions, with the most dramatic locations producing stars a thousand times more efficiently than the least active (but still star forming) regions. The scientists conclude that, at least on the local scale, there is no universal relation between the density of molecular gas and the star formation.

The Schmidt Law in Six Galactic Massive Star-forming Regions - S. Willis et al

• Astrophysical Journal 809(1):87 (2015 Aug 10) DOI: 10.1088/0004-637X/809/1/87

[bystander]

Timing a Sextuple Quasar
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Nov 27

The quintuple quasar SDSSJ1029+2623 as seen in the visible. The quasar has been gravitationally imaged by an intervening cluster of galaxies, and the distortions result in six images. Four of these quasar images are labeled A-D; the other two images are obscured in this figure by the foreground galaxies G1-G3 (an arc produced by the lens can also be seen). Astronomers have detected time delays between flaring events in the quasar, the result of the light in different images traveling along different paths in space. (Credits: H. Dahle et al, Nordic Optical Telescope)

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Quasars are galaxies with massive black holes at their cores around which vast amounts of energy are being radiated. Indeed, so much light is emitted that the nucleus of a quasar is much brighter than the rest of the entire host galaxy, and their tremendous luminosities allow quasars to be seen even when they are very far away. The quasar SDSS J1029+2623, for example, is so distant that its light has been traveling towards us for 11.4 billion years, 83% of the age of the universe. This quasar is particularly unusual because it happens to have five quasar neighbors in the sky that look very similar to it and moreover are located at the same cosmological distance.

SDSS J1029+2623 is actually a gravitationally lensed quasar. Its light is being magnified and distorted by the gravity of a cluster of galaxies fortuitously lying between us, in accordance with Einstein's prediction that light can be bent by gravity. Only a few other quasars being gravitationally lensed into multiple images by clusters are known. Over fifty years ago, astronomers predicted that in such cases, because the light from each image travels along a different cosmological path, any time delays between flaring events in the images can be used to probe underlying cosmological parameters such as the age and rate of expansion of the universe. Moreover, these delays can also probe the surface density distribution of the lens. Such delays have now been detected, with the longest delays being of the order of a few years. In the case when individual galaxies (not clusters of galaxies) act as lenses, the time delays are more often weeks or months.

CfA astronomer Matthew Bayliss and four of his colleagues undertook a campaign to monitor the time delays in the images of SDSS J1029+2623 using the 2.56-meter Nordic Optical Telescope in the Canary Islands, Spain. Over three years of systematic observations they found a delay of 722 days between the image whose light is predicted to arrive first ("image C") and the component that is brightest, and a 47.7 day delay between the two brightest components. Fortuitously, during this period image C underwent a sharp flux increase, and models predict that this event should be spotted in the other five images in the next few years. The data are not quite good enough to refine any cosmological parameters, at least not yet, but the team is continuing close monitoring of the quasar and hopes to determine with precision the timing delays in all six components over the next few observing seasons.

Time Delay Measurements for the Cluster-lensed Sextuple Quasar SDSS J2222+2745 - H. Dahle et al

• Astrophysical Journal 813(1):67 (2015 Nov 01) DOI: 10.1088/0004-637X/813/1/67
  arXiv.org > astro-ph > arXiv:1505.06187 > 22 May 2015 (v1), 30 Sep 2015 (v2)

[bystander]

Imaging an Expanding Supernova Shell
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Dec 04

An optical image of the galaxy Messier 51 with the insert showing the location of supernova SN2011dh. Using precise radio imaging techniques, astronomers have determined the size of the shock around this supernovae, and estimated its outward velocity. (Credit: Rafael Ferrando, Observatory Pla D’arguines)

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Supernovae, the explosive deaths of massive stars, are among the most momentous events in the cosmos because they disburse into space all of the chemical elements that were produced inside their progenitor stars, including the elements essential for making planets and life. Their bright emission also enables them to be used as probes of the very distant universe. Not least, supernovae are astrophysical laboratories for the study of very high-velocity shocks and the physics of particles under extreme conditions.

On May 31, 2011, an amateur astronomer spotted a supernova in the relatively nearby Whirlpool Galaxy (Messier 51), about 257 million light-years away. An analysis of the spectrum of SN2011dh showed that the precursor object was a massive supergiant star, about thirteen times bigger than the Sun (there is also some evidence for the presence of a binary companion star). The explosion set off a shock wave whose bright optical emission comes primarily from the inner dense, slow-moving ejecta. In addition, astronomers see a fast-moving component to the shock that is bright at radio wavelengths. The size and expansion velocity of the shock are thought to be basic distinguishing characteristics of different kinds of supernova (for example, having different mass progenitors or different stellar properties). Astronomers have therefore been at work trying to study these shocks. Unfortunately, supernovae are relatively rare, and so far only five supernovae have gone off in galaxies close enough to us, and recently enough, to have had their detailed shock properties studied.

CfA astronomers Atish Kamble and Alicia Soderberg and their colleagues have now measured a sixth. They been following SN2011dh in the radio since the explosion took place using a variety of radio telescope facilities, including very long baseline techniques, to obtain very high spatial resolution images of the shock. Observations they took 453 days after the event have now been combined with more recent measurements, enabling the scientists to determine the basic geometry of the shock: It has swept out a nearly spherical shell of hot material about 120 times larger in radius than the average distance Pluto is from the Sun.

Since the scientists know how long the shock has been propagating, about 453 days, they can estimate its velocity as about 19,000 kilometers per second (over forty-two million miles per hour). Combined with other observations of its radio brightness, the result implies that for nearly all of that time the expansion proceeded without being slowed down significantly by intervening material in space. Only about one-thousandth of a solar-mass of material has been swept up. These measurements are key tests of the robustness of theoretical predictions about supernovae and the underlying assumptions, and the results provide confidence in supernova shock theories. The research is part of an ongoing, cradle-to-grave study of this supernova.

Imaging the Expanding Shell of SN 2011dh - A. de Witt et al

• Monthly Notices of the RAS 455(1):511 (2016 Jan 01) DOI: 10.1093/mnras/stv2306
  arXiv.org > astro-ph > arXiv:1503.00837 > 03 Mar 2015 (v1), 14 Oct 2015 (v2)

http://asterisk.apod.com/viewtopic.php?t=23856
http://asterisk.apod.com/viewtopic.php?t=23871
http://asterisk.apod.com/viewtopic.php?t=23926

[bystander]

Planetary Influences on Young Stellar Disks
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2015 Dec 11

False-color images of the gas (left) and dust (right) disks around the young stellar object DoAr44. The results show that the size of the gas ring is much smaller than the dust ring (dotted line), and support a picture in which orbiting planets have swept out the ring. (Credit: ALMA/N. van der Marel et al (A&A 2015))

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A newborn star typically has a disk of gas and dust from which planets develop as the dust grains collide, stick together and grow. Stars older than about five million years lack evidence for these disks, however, suggesting that by this age most of the disk material has either been converted into planets or smaller bodies, accreted onto the star, or dispersed from the system. Transition disks bridge this period in disk evolution: They have not yet been disbursed, and warmed by the star, can be detected at infrared or millimeter wavelengths. Their infrared colors can be used to characterize their properties. They often show inner dust cavities, which astronomers have sometimes interpreted as evidence of the presence of planets that have cleared out their orbits.

The models of planet-disk interactions, however, indicate that dust cavities are only an indirect consequence of planet clearing. What actually seems to occur is that the planet creates a gap in the gas, and the gas distribution at its outer edges then traps the small dust grains and produces a dust ring that is frequently asymmetric. There is some uncertainty in this picture because other mechanisms could produce a dust cavity or dust ring, including selective evaporation of dust grains by starlight, or instabilities in the dust ring itself. Determining the gas density inside the cavity can help to distinguish between these mechanisms. ...

Resolved gas cavities in transitional disks inferred from CO isotopologues with ALMA - Nienke van der Marel et al

• Astronomy & Astrophysics 585:A58 (Jan 2016) DOI: 10.1051/0004-6361/201526988
  arXiv.org > astro-ph > arXiv:1511.07149 > 23 Nov 2015 (v1), 02 Dec 2015 (v2)