SAO: Weekly Science Updates 2017

Find out the latest thinking about our universe.
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SAO: Weekly Science Updates 2017

Postby bystander » Tue Jan 10, 2017 6:18 pm

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Outflowing Gas in Ultraluminous Galaxies

Postby bystander » Tue Jan 10, 2017 6:27 pm

Outflowing Gas in Ultraluminous Galaxies
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jan 06

Galaxies evolve over billions of years in part through the activity of star formation and their supermassive nuclear black holes, and also by mergers with other galaxies. Some features of galaxies, in particular the strong correlations found between the mass of the central black hole and properties like galaxy velocity structure or luminosity, imply a fundamental connection between the growth of the nuclear black hole and the assembly of stars on a global scale. Feedback of some kind is therefore expected to explain these tight correlations, and astronomers have been working to identify and study it. One prominent suggestion for feedback is the presence of warm outflowing gas, powered by new stars but which would deplete the galaxy of the raw material needed for making new stars, and/or for enhancing the black hole mass.

In the 1990's, the Infrared Space Observatory (ISO) detected evidence for warm gas in luminous galaxies, the molecule OH, and the recent Herschel Space Observatory followed up those detections with velocity-resolved observations of six of the prominent OH far infrared lines. CfA astronomers Eduardo Gonzalez-Alfonso, Matt Ashby, and Howard Smith led a team of scientists reducing and modeling the four strong lines in fourteen ultra-luminous infrared galaxies (ULIRGs). The set of OH lines from ULIRGs is remarkable in that they appear sometimes in absorption, sometimes in emission, and sometimes with a bit of both depending on the particular line and velocity component. Many of these spectral features are characteristic of gas moving in an outflow, and the team has developed a radiative transfer model to deduce the geometry and kinematics of the flowing gas from the complex line shapes.

The scientists report that there are indeed powerful outflows in these ULIRGs, some with more than a thousand solar-masses per year and the power of a hundred billion Suns (a few percent of the total luminous energy of the galaxy). The typical time it would take for this gas to be blown out of the galaxy is only a few hundred million years, and the astronomers conclude that the outflows must occur erratically (not continuously), and are probably tied to the equally random flaring activity of the central black hole, which in turn can be linked to the gas motions induced by galaxy mergers.

Molecular Outflows in Local ULIRGs: Energetics from Multi-Transition OH Analysis - E. González-Alfonso et al
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Re: Outflowing Gas in Ultraluminous Galaxies

Postby geckzilla » Wed Jan 11, 2017 4:23 am

They left all the cosmic rays running through the chip gap... unfortunate...
Just call me "geck" because "zilla" is like a last name.

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A Catalog of Habitable Zone Exoplanets

Postby bystander » Thu Jan 19, 2017 3:44 pm

A Catalog of Habitable Zone Exoplanets
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jan 13

The last two decades have seen an explosion of detections of exoplanets, as the sensitivity to smaller planets has dramatically improved thanks especially to the Kepler mission. These discoveries have found that the frequency of planets increases to smaller sizes: terrestrial planets are more common than gas giants. The significance of a universe rich in terrestrial sized planets naturally leads to the question about the "habitable zone (HZ)" – the region around a star where a suitable planet could sustain the conditions necessary for life. In this zone, the balance between stellar radiation onto the planet and radiative cooling from the planet allows water on the surface to be a liquid. (The definition also includes consideration of the planet’s atmosphere and solid surface.)

In our solar system, the Earth is cozily situated in the middle of the habitable zone which, depending on the model, extends roughly from Venus to Mars. The Kepler mission has as one of its primary goals the determination of the frequency of terrestrial planets in their habitable zones. CfA astronomer Guillermo Torres and his colleagues have now produced a complete catalog of Kepler exoplanet candidates in their habitable zones from the Kepler data releases to date. After reviewing the various criteria for determining the boundaries of the HZ, they report there are 104 candidates within an optimistic (larger) HZ definition, and twenty within a more conservative (smaller) definition of the HZ and which also have radii less than two Earth-radii, making this group in particular potential "Earth-like" candidates. ...

A Catalog of Kepler Habitable Zone Exoplanet Candidates - Stephen R. Kane et al
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The Evolution of Massive Galaxy Clusters

Postby bystander » Sat Jan 21, 2017 4:04 pm

The Evolution of Massive Galaxy Clusters
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jan 20

Galaxy clusters have long been recognized as important laboratories for the study of galaxy formation and evolution. The advent of the new generation of millimeter and submillimeter wave survey telescopes, like the South Pole Telescope (SPT), has made it possible to identify faint galaxy clusters over large fractions of the sky using an effect first recognized by Rashid Sunyaev and Yakov Zel’dovich in 1969: When hot electrons in the cluster gas interact with light from the ubiquitous cosmic microwave background they increase its brightness very slightly.

SAO is a partner institution in the South Pole Telescope, which has been conducting a large survey covering about six percent of the whole sky with a sensitivity and angular resolution suitable for spotting galaxy clusters as far away as those from the epoch about four billion years after the big bang. One advantage of studying this sample of clusters is that because they have been identified from their hot gas signatures (rather than from the starlight of their member galaxies), the evolution of the cluster and its ensemble population is easier to disentangle.

CfA astronomer Brian Stalder and a team of colleagues used the SPT survey data to identify twenty-six of the most massive clusters known, each with a mass of over about a million billion solar-masses. They find that the clusters are broadly in agreement with the current thinking about the evolution of massive clusters and the stars in these galaxies. ...

Galaxy Populations in the 26 Most Massive Galaxy Clusters in the South Pole Telescope SPT-SZ Survey - A. Zenteno et al
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Radio Weak Blazars

Postby bystander » Sat Jan 28, 2017 4:35 pm

Radio Weak Blazars
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jan 27

A blazar is a galaxy whose central nucleus is bright at wavelengths from the low energy radio band to high energy gamma rays (each gamma ray photon is over a hundred million times more energetic than the X-rays seen by the Chandra X-ray Observatory). Astronomers think that the blazar nucleus contains a supermassive black hole, similar to a quasar nucleus. The emission results when matter falls onto the vicinity of the black hole and erupts into powerful, narrow jets of radiating charged particles moving close to the speed of light. Two defining characteristics of blazars, strong radio emission and high variability, are results of the accretion and jets.

Although the nuclei of other galaxies also eject jets of particles, the class of blazars is thought to result from our unique viewing angle: staring directly down the throats of these jets. The orientation makes these objects unique probes of exotic physical activity, with the relative intensities of the radiation providing key diagnostics. In most other galaxies, for example, infrared radiation comes from heated dust, but in blazars the infrared colors indicate that it comes from jet emission. Because the jet emission is so bright the underlying galaxy light can be masked, with the result that in the class of BL Lac blazars emission and absorption lines are not detected, making their distances difficult to determine.

CfA astronomers Raffaele D'Abrusco and Howard Smith and their four colleagues report discovering blazars that challenge this general paradigm. They found two BL Lac blazars with no apparent radio emission: "radio weak" BL Lacs. The astronomers discovered them by using the Fermi catalog of very high energy sources to identify a set of possible new blazars, and the WISE infrared sky catalog to reinforce the categorization and to pinpoint the locations of the sources in the sky. After searching radio catalogs for counterparts to the sources, they discovered two that had no detected radio emission. ...

Radio-Weak BL Lac Objects in the Fermi Era - F. Massaro et al
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A Massive Galaxy Long Ago and Far Away

Postby bystander » Mon Feb 06, 2017 6:08 pm

A Massive Galaxy Long Ago and Far Away
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Feb 03

Galaxies today fall roughly into two categories: elliptically-shaped collections of reddish, old stars that formed predominantly during a period early in the history of the universe, and spiral shaped objects dominated by blue, young stars. The Milky Way is an example of the latter, a spiral galaxy actively making new stars. In order to understand the growth of galaxies over cosmic time and the past star formation history of the universe, astronomers study the population of old stars in distant ellipticals from earlier epochs, stars which in turn formed at an even early time. Star formation produces supernovae which enrich their environments with elements, including the diagnostic element magnesium. Measuring the amount of magnesium (relative to iron) in a galaxy thus helps to fix the strength and duration of prior episodes of star formation.

CfA astronomers Charlie Conroy and Jieun Choi and eight colleagues used the spectrometer on the Keck telescope (along with some secondary datasets) to obtain very sensitive magnesium measurements in one of the most massive and luminous elliptical galaxies known. The galaxy, seen at an epoch only three billion years after the big bang, has a stellar mass of about three hundred billion solar-masses (the Milky Way’s stellar mass is about ten times less) but is currently making stars at a rate only about half that of the Milky Way. However, it's magnesium-to-iron ratio indicates that earlier in its life it was making stars at a phenomenally high rate, perhaps as many as several thousand solar-masses each year, making it one of the most vigorous examples of star-formation known.

The scientists conclude that the bursts of star formation in this galaxy must have been due to mergers with other galaxies. In fact, they estimate that the object probably doubled in sized as a consequence of accreting smaller galaxies. Unfortunately this particular elliptical is so unusual that it cannot be considered a typical progenitor for any local elliptical galaxy. The team argues that additional observations of more, less extreme ellipticals in the early universe are now needed to fill in the rest of the story. The instruments on the James Webb Space Telescope, to be launched next year, should be capable of doing so.

A Massive, Quiescent, Population II Galaxy at a Redshift of 2.1 - Mariska Kriek et al
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The Lifetime of the Solar Nebula

Postby bystander » Fri Mar 03, 2017 5:24 pm

The Lifetime of the Solar Nebula
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Feb 10

Very young stars host gaseous nebulae and protoplanetary disks where planetary systems form. The lifetimes of these disks place important constraints on the timescale of the planet formation, including the final sizes and eccentricities of the rocky terrestrial planets, and so it is a key parameter in the models. Observations of nearby young stellar objects suggest the timescales are typically short, under five million years, but such brief times are surprising because they would require very efficient mechanism(s) to transport material and disperse the disk. How they work is uncertain.

The evolution and dispersal of the gaseous nebula are closely connected to the nebula's magnetic fields. The magnetic fields transport angular momentum, driving disk accretion onto the central star. These processes also determine the structure of the solar nebula, as well as level of disk turbulence, and strongly affect many stages of planet formation. Information about the nebula's magnetic fields may be preserved in solid inclusions, the building blocks of planets and asteroids, and retrieved by analyzing certain types of meteorites, thus placing constraints on the lifetime of the solar nebula.

Angrites are among the oldest and most pristine known samples from meteorites, having cooled rapidly after the solar system was formed. They contain ferromagnetic grains that should have acquired a permanent magnetization if a magnetic field was present when they solidified, and so can be used to obtain direct, precisely dated measurements of the nebular field strength in the terrestrial planet–forming region. ...

Lifetime of the Solar Nebula Constrained by Meteorite Paleomagnetism - Huapei Wang et al

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Dating the Milky Way's Disc

Postby bystander » Fri Mar 03, 2017 5:34 pm

Dating the Milky Way's Disc
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Feb 17

When a star like our sun gets to be very old, after another seven billion years or so, it will no longer be able to sustain burning its nuclear fuel. With only about half of its mass remaining, it will shrink to a fraction of its radius and become a white dwarf star. White dwarfs are common, the most famous one being the companion to the brightest star in the sky, Sirius. As remnants of some of the oldest stars in the galaxy, white dwarfs offer an independent means of dating the lifetimes of different galactic populations.

A globular cluster is a roughly spherical ensemble of stars (as many as several million) that are gravitationally bound together and typically located in the outer regions of galaxies. The white dwarf stars in the Milly Way's globular clusters reveal an age spread of between eleven and thirteen billion years. By contrast, the thick disk of the galaxy is thought to be older than ten billion years but that figure is not very well constrained. White dwarfs in the disc can be used to refine those age estimates and, since they are closer and brighter to us than those in globular clusters, they can provide more detailed information. However, they are not located in well-defined regions like clusters and so they are also harder to spot. ...

New Halo White Dwarf Candidates in the Sloan Digital Sky Survey - Kyra Dame et al
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Constraining the Chemistry of Carbon-Chain Molecules in Space

Postby bystander » Fri Mar 03, 2017 5:43 pm

Constraining the Chemistry of Carbon-Chain Molecules in Space
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Feb 24

The interstellar medium of the Milky Way contains 5-10% of the total mass of the galaxy (excluding its dark matter) and consists primarily of hydrogen gas. There are small but important contributions from other gases as well, including carbon-bearing molecules both simple, like carbon monoxide and carbon dioxide, and complex like ethene, benzene, propynal, methanol and other alcohols, and cyanides. There are even some very large molecules like polycyclic aromatic hydrocarbons and buckyballs with fifty or more carbon atoms. Some species like the cyanides have relative abundances similar to what is seen in comets in our Solar System, suggesting that local carbon chemistry is not unique.

Astronomers think complex interstellar molecules are probably produced on dust grains, although some molecules might be produced in the gas phase. About one percent by mass of the interstellar material, these tiny grains are composed predominantly of silicates and provide the gas molecules with surfaces on which to react with other molecules. Carbon chain molecules are particularly interesting because they are thought to be the starting point for a significant fraction of the known complex chemicals in the interstellar medium. It is even suspected that carbon-chain species are a key stage in the formation of polycyclic aromatic hydrocarbons. Carbon-chain molecular chemistry thus provides insight into a large subset of interstellar chemistry. ...

Non-detection of HC11N towards TMC-1: Constraining the Chemistry of Large Carbon-Chain Molecules - Ryan A. Loomis et al
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Gravity Wave Detection with Atomic Clocks

Postby bystander » Fri Mar 03, 2017 5:55 pm

Gravity Wave Detection with Atomic Clocks
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Mar 03

The recent detection of gravitation waves (GW) from the merger of two black holes of about thirty solar-masses each with the ground-based LIGO facility has generated renewed enthusiasm for developing even more sensitive measurement techniques. Ground-based GW instruments have widely spaced sensors that can detect sub-microscopic changes in their separation -- better than one part in a billion trillion, They suffer, however, from the noise produced by small ground tremors -- vibrations from natural or man-made sources that ripple through the precisely tuned detectors. The vibrations most difficult to compensate for are those that change relatively slowly, at frequencies around once a second or less, yet astronomers predict that GW sources producing these slow variations should be interesting and abundant, from compact stellar-mass binary stars to gravitational events in the early universe.

The CfA has long been renowned for its laboratory work producing some of the best precision devices in the world. In particular are its timekeeping hydrogen-maser clocks, used by NASA to track its satellites as well as by radio astronomers around the world to make precision measurements of cosmic phenomena using Very Long Baseline Interferometry. The CfA maser group has continued to develop advanced clock technologies over the years, and to turn them into new tools to probe the heavens, including recently the so-called "laser-combs" for ultraprecise measurement of stellar velocity shifts induced by extrasolar planets. ...

Gravitational Wave Detection with Optical Lattice Atomic Clocks - Shimon Kolkowitz et al
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Superluminous Supernovae

Postby bystander » Fri Mar 10, 2017 4:10 pm

Superluminous Supernovae
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Mar 10

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 energetic phenomena. One class of supernovae consists of single stars whose mass is at least eight solar masses as they finish their lives.

A typical supernova shines about as brightly as ten billion Suns at its peak. In the last decade, a new type of supernova was discovered that is ten to one hundred times more luminous than a normal massive stellar collapse supernova, and today over a dozen of these superluminous supernovae (SLSN) have been seen. Astronomers are in agreement that these objects come from the collapse of massive stars, but their tremendous luminosities cannot be explained by the usual physical mechanisms invoked. Instead, the debate has centered on whether the excess emission results from an external source, for example the interaction of material ejected from the explosion with a circumstellar shell, or instead by some kind of powerful internal engine such as a highly magnetized, spinning neutron star.

The SLSN "Gaia6apd" was discovered by the European Gaia satellite, and at a distance of about one and one-half billion light-years it is the second-closest SLSN discovered to date. It is also special in another way: it is extraordinarily bright in the ultraviolet, nearly four times brighter than the next nearest known SLSN despite the fact that in the optical both have comparable luminosities. ...

An Ultraviolet Excess in the Superluminous Supernova Gaia16apd Reveals a Powerful Central Engine - M. Nicholl et al
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Textured Dust Storms on Mars

Postby bystander » Wed Mar 22, 2017 4:44 pm

Textured Dust Storms on Mars
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Mar 17
Astronomers studying Mars first noted the presence of yellow clouds on its surface in the 1870's. Today these windblown dust storms on Mars are well known, and can span local, regional or even global in scale. Storms can display visible structures, sometimes periodic with wavelike features, or in other cases streaky or plume-like. Storms with structures are called "textured dust storms" and they result from strong winds or other meteorological effects that lift dust into the Martian atmosphere. In addition to obscuring views of the Martian surface, the dust can affect atmospheric heating and other climatic processes. These dust storms, despite having been 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 two colleagues have been using the Mars Global Surveyor (MGS) images to analyze textured dust storms. Launched in 1999, MGS has provided about four Mars-years of daily global data that are particularly suitable for studying various aspects of clouds and dust storms. The new analysis focuses on the dust storms that occurred between May 1999 and October 2006, including a global dust storm in June 2001. There were 3955 textured dust storms during these periods. The scientists manually mark the position of each textured dust storm in the MGS image, and categorize its texture into one of three new categories they have developed: pebbled textures, characterized by a granular or crinkled appearance suggestive of strong turbulence, puffy textures with a bubbling appearance and cotton-like structures analogous to cumulus clouds indicative of vertical motions, and plume-like textures composed of multiple parallel elongated features suggestive of dust being uplifted and carried downstream by strong winds.

The scientists find that these three texture types have preferred seasons and distinctive locations; puffy storms, for example, tend to appear in the low latitudes and might therefore result from vertical convective winds there. Pebbled storms occur more frequently in the southern mid-latitudes implying that these zones are more turbulent than the northern mid/high-latitudes. The new results suggest links between storm texture type and meteorological conditions that can lead to improved understanding of the Martian climate. ...

The Seasonal and Spatial Distribution of Textured Dust Storms
Observed by Mars Global Surveyor Mars Orbiter Camera
- Laura Kulowski et al
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Re: Superluminous Supernovae

Postby Ann » Thu Mar 23, 2017 1:23 am

bystander wrote:Superluminous Supernovae
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Mar 10

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 energetic phenomena. One class of supernovae consists of single stars whose mass is at least eight solar masses as they finish their lives.

A typical supernova shines about as brightly as ten billion Suns at its peak. In the last decade, a new type of supernova was discovered that is ten to one hundred times more luminous than a normal massive stellar collapse supernova, and today over a dozen of these superluminous supernovae (SLSN) have been seen. Astronomers are in agreement that these objects come from the collapse of massive stars, but their tremendous luminosities cannot be explained by the usual physical mechanisms invoked. Instead, the debate has centered on whether the excess emission results from an external source, for example the interaction of material ejected from the explosion with a circumstellar shell, or instead by some kind of powerful internal engine such as a highly magnetized, spinning neutron star.

The SLSN "Gaia6apd" was discovered by the European Gaia satellite, and at a distance of about one and one-half billion light-years it is the second-closest SLSN discovered to date. It is also special in another way: it is extraordinarily bright in the ultraviolet, nearly four times brighter than the next nearest known SLSN despite the fact that in the optical both have comparable luminosities. ...



Smithsonian Astrophysical Observatory wrote:
The SLSN "Gaia6apd" was discovered by the European Gaia satellite, and at a distance of about one and one-half billion light-years it is the second-closest SLSN discovered to date. It is also special in another way: it is extraordinarily bright in the ultraviolet, nearly four times brighter than the next nearest known SLSN despite the fact that in the optical both have comparable luminosities.
...
The scientists review all the known data and conclude that the most likely source is an internal central engine like a rapidly spinning neutron star. They also emphasize the key role that UV wavelengths played in diagnosing the mechanisms and urge that future studies of SLSN include UV coverage.


I really, really hope that good-quality UV observations will still be made in the coming age of James Webb Infrared Telescope (and planned manned missions to Mars).

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The Fate of Exomoons

Postby bystander » Fri Mar 24, 2017 3:47 pm

The Fate of Exomoons
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Mar 24

When a star like our sun gets to be very old, after another seven billion years or so, it will shrink to a fraction of its radius and become a white dwarf star, no longer able to sustain nuclear burning. Studying the older planetary systems around white dwarfs provides clues to the long-term fate of our Sun and its planetary system. The atmosphere of a white dwarf star is expected to break up any material that accretes onto it into the constituent chemical elements and then to stratify them according to their atomic weights. The result is that the visible, uppermost layers of the atmosphere of a white dwarf should contain only a combination of hydrogen, helium (and some carbon). About one thousand white dwarf stars, however, show evidence in their spectra of pollution by some form of rocky material. This suggests that there is frequent, ongoing accretion onto these white dwarf stars of fragmentary material coming from somewhere - the precise origins are not clear.

CfA astronomers Matt Payne and Matt Holman, with two colleagues, have completed a series of simulations of the late evolution of planetary systems to try to understand where this material might be coming from. It was already known that the moons of planets can be easily knocked out of their orbits during planet-planet interactions in white dwarf systems. The question was whether these freed moons might themselves accrete onto the star to provide the polluting elements, or whether they might act to scatter asteroids towards the star. The difficulty has been the computational limits of simulating a complex evolving system that included the moons around planets. ...

The Fate of Exomoons in White Dwarf Planetary Systems - Matthew J. Payne et al
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The Space Weather Forecast for Proxima Cen b

Postby bystander » Mon Apr 10, 2017 4:40 pm

The Space Weather Forecast for Proxima Cen b
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Mar 31

Proxima Centauri, the closest star to the Earth (only 4.28 light-years away) is getting a lot of attention these days. It hosts a planet, Proxima Cen b, whose mass is about 1.3 Earth-mass (though it could be larger, depending on the angle at which we are viewing it). Moreover, Proxima Cen b orbits the star in its habitable zone. Proxima Cen itself is an M-dwarf star with a mass only about one-tenth the Sun's mass and a luminosity about one-thousandths of the Sun's; because the star is dim, the planet's habitable zone is twenty times closer to the star than the Earth's is to the Sun, and the planet orbits in 11.3 days. M dwarfs are the most abundant type of stars, and their small radii make them easier targets (relatively speaking) to spot transiting exoplanet signatures. Recent statistical estimates have concluded that half of M dwarf stars probably host an exoplanet between about 0.5–1.4 Earth-radii orbiting in or near their "habitable zone." Proxima Cen and its exoplanet, therefore, are important benchmark objects for understanding low-mass stars, their planets, and the planetary environments.

M dwarf stars pose a particular hazard to their planets: A large proportion of their radiation, much more than in Sun-like stars, is in the form of UV, extreme UV, and X-rays. This radiation can evaporate a planet's atmosphere, especially when those planets orbit nearby in the habitable zone. Indeed, the question astronomers ask is whether planets like Proxima Cen b can retain any atmosphere at all, at least over a long enough time for the planet to be "habitable" from any practical point-of-view. An addition danger is posed by the star's magnetic activity, which is not only responsible for the corrosive radiation but which also drives stellar winds and coronal mass ejections that could be even more perilous to atmospheric survival.

The photoevaporation of planetary atmospheres due to stellar radiation has been studied in limited situations, but not much effort has been devoted to the case of active M-dwarf stars and their magnetic activity. CfA astronomers Cecilia Garraffo, Jeremy Drake and Ofer Cohen have begun a program to model the stellar winds and magnetic field for active M-dwarf stars, and to investigate the impact on the atmospheres of planets in habitable zones. Proxima Cen is their first specific example. They found that the pressure of the stellar wind at the exoplanet was a thousand to ten thousand times higher than the solar wind pressure at Earth. Moreover, the pressure is highly nonuniform, and Proxima b will pass through these extreme pressure variations twice each orbit leading to the compression and expansion of its atmosphere by factors of up to 3 every day. The atmosphere of Proxima Cen b is also likely to experience supersonic wind conditions. All these phenomena will have a significant negative effect on any atmosphere that might vweekexist on Proxima b. The extent to which similar hostile conditions prevail on other M-dwarf exoplanets is a subject of further studies.

The Space Weather of Proxima Centauri b - Cecilia Garraffo, Jeremy J. Drake, Ofer Cohen

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The Lifetimes of Massive Star-Forming Regions

Postby bystander » Fri Apr 14, 2017 4:12 pm

The Lifetimes of Massive Star-Forming Regions
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Apr 14

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Astronomers can roughly estimate how long it takes for a new star to form: it is the time it takes for material in a gas cloud to collapse in free-fall, and is set by the mass, the size of the cloud, and gravity. Although an approximation, this scenario of quick, dynamic star formation is consistent with many observations, especially of sources where new material can flow into the cloud, perhaps along filaments, to sustain steady activity. But this simple picture might not apply in the largest systems with star clusters and high-mass stars. Rather than a quick collapse, the process there might be inhibited by pressure, turbulence, or other activities that slow it down.

CfA astronomer Cara Battersby and two colleagues studied the formation, early evolution, and lifetimes of high-mass star-forming regions and their earliest evolutionary phases in dense, molecular regions. These clumps have densities of gas as high as ten million molecules per cubic centimeter (tens of thousands of times higher than typical in gas clouds); the dust associated with this gas blocks the external starlight, leaving the material very cold, only a few tens of degrees above absolute zero. The usual method for identifying these clumps is with submillimeter telescopes, which take images of the sky; automated algorithms can then process the images to identify and characterize cold clumps. The problem is that even a quiescent clump can contain subregions of activity that are not spotted with the relatively poor spatial resolutions of the submillimeter telescopes used to assemble catalogs of these regions.

Rather than rely on the submillimeter images of the entire clumps, the astronomers examined each of the multiple, individual pixels in each clump image and compared the results with data from infrared and far infrared. ...

The Lifetimes of Phases in High-Mass Star-Forming Regions - Cara Battersby, John Bally, Brian Svoboda
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A Black Hole in a Low Mass X-Ray Binary

Postby bystander » Sat Apr 22, 2017 9:07 pm

A Black Hole in a Low Mass X-Ray Binary
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Apr 21

A globular cluster is a roughly spherical ensemble of stars (as many as several million) that are gravitationally bound together, and typically located in the outer regions of galaxies. Low mass X-ray binary stars (LMXBs) are systems in which one star is compact (a neutron star or black hole) and is accreting matter from a companion star. Astronomers have long noticed that there are fractionally many more LMXBs in globular clusters than elsewhere in the galaxy, an overabundance that is usually attributed to the high stellar density in globular clusters. But this calls attention to another unusual feature of the LMXBs in globulars. In the galactic field, most LXMBs are formed from binary stars as they age and evolve, but in globular clusters it has been shown that most LMXBs form when compact objects encounter and then capture another star. There are many neutron stars in clusters, but black holes that form in these dense stellar environments are expected either to sink down to the center of the cluster or else to be gravitationally ejected from the cluster after they are formed. Indeed, all of the LMXBs seen in globular clusters are of the type with a neutron star.

CfA astronomer Javier Garcia was a member of a team studying LMXBs in the 47 Tuc globular cluster. They discovered one, called X9, that appears to contain a black hole, and if the interpretation is correct X9 would be the first such case in our Galaxy. The team used simultaneous observations of the source with Chandra, NuSTAR, and the Australia Compact Array, plus archival datasets, to find a twenty-eight minute modulation of the signal from X9, such as would be produced by a black hole of about one solar mass orbited by a white dwarf star of about 0.02 solar-masses. The source has some other unusual characteristics, including a precession period of 6.8 days, and a relatively high rate of mass transfer for LMXBs, about seven-millionths of an Earth-mass per year. ...

The Ultracompact Nature of the Black Hole Candidate X-ray Binary 47 Tuc X9 - Arash Bahramian et al

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Re: A Black Hole in a Low Mass X-Ray Binary

Postby BDanielMayfield » Sat Apr 22, 2017 10:10 pm

Hey bystander, did you notice this:

There are numerous known low-mass X-ray binary stars (LMXBs) in this and other globular clusters. Astronomers report finding what might be the the first LMXB whose compact object is a black hole rather than a neutron star.

A globular cluster is a roughly spherical ensemble of stars (as many as several million) that are gravitationally bound together, and typically located in the outer regions of galaxies. Low mass X-ray binary stars (LMXBs) are systems in which one star is compact (a neutron star or black hole) and is accreting matter from a companion star. Astronomers have long noticed that there are fractionally many more LMXBs in globular clusters than elsewhere in the galaxy, an overabundance that is usually attributed to the high stellar density in globular clusters. But this calls attention to another unusual feature of the LMXBs in globulars. In the galactic field, most LXMBs are formed from binary stars as they age and evolve, but in globular clusters it has been shown that most LMXBs form when compact objects encounter and then capture another star. There are many neutron stars in clusters, but black holes that form in these dense stellar environments are expected either to sink down to the center of the cluster or else to be gravitationally ejected from the cluster after they are formed. Indeed, all of the LMXBs seen in globular clusters are of the type with a neutron star.

CfA astronomer Javier Garcia was a member of a team studying LMXBs in the 47 Tuc globular cluster. They discovered one, called X9, that appears to contain a black hole, and if the interpretation is correct X9 would be the first such case in our Galaxy. The team used simultaneous observations of the source with Chandra, NuSTAR, and the Australia Compact Array, plus archival datasets, to find a twenty-eight minute modulation of the signal from X9, such as would be produced by a black hole of about one solar mass orbited by a white dwarf star of about 0.02 solar-masses. The source has some other unusual characteristics, including a precession period of 6.8 days, and a relatively high rate of mass transfer for LMXBs, about seven-millionths of an Earth-mass per year. ...


So they are extremely rare, but, apparently the universe can crank out a 1 Sol BH now and then.

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Shocked Gas in Galaxy Collisions

Postby bystander » Fri Apr 28, 2017 4:52 pm

Shocked Gas in Galaxy Collisions
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Apr 28

Collisions between galaxies, especially ones rich in molecular gas, can trigger bursts of star formation that heat the dust and result in their shining brightly in the infrared. Astronomers think that there is also significant gas inflowing to the central regions of galaxies that can stimulate starburst activity. Inflowing gas, as it collides with the gas in the inner regions, should produce powerful shocks that should make the gas itself glow. Some evidence for gas inflows on galactic scales has been discovered, but there have been few observational confirmations of the effects of the inflowing material in the inner region of the galactic nucleus.

CfA astronomers Junko Ueda, David Wilner, and Giovanni Fazio used the ALMA submillimeter array to study the gas in the central regions of the Antennae galaxies, the nearest mid-stage merging system (about seventy-two million light-years away). The star formation rate of the system is estimated to be about ten solar-masses per year, much of it in the off-nuclear region (the so-called “overlap region”) of the two galaxies; the two nuclear regions themselves appear to have lower star formation rates.

The astronomers examined the star formation in one of the two nuclear regions, whose gas abundance is as much as one hundred times more than in the Milky Way’s center. They measured the emission from five organic molecules, CN, HCN, HCO+, CH3OH (methanol), and HNCO (isocyanic acid), looking for evidence of shock activity. And they found it. The methanol and isocyanic acid in particular were detected, for the first time in this object, and show clear evidence ion their intensities, ratios, and velocities for being excited by shocks. The evidence from the geometry of the emission suggests that the shocks are produced by infall, rather than from the collision. However, there is also the possibility that the induced burst of star formation produced local shocks that contributed to the shock activity. Although further work is needed, the results so far indicate that infalling material is likely responsible. ...

ALMA Observations of the Dense and Shocked Gas in the Nuclear
Region of NGC 4038 (Antennae Galaxies)
- Junko Ueda et al
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Obscured Supermassive Black Holes in Galaxies

Postby bystander » Tue May 16, 2017 4:45 pm

Obscured Supermassive Black Holes in Galaxies
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 May 12

Most if not all galaxies are thought to host a supermassive black hole in their nuclei. It grows by accreting mass, and while feeding it is not hidden from our view: it generates X-ray emission and ultraviolet that heats the dust which in turn radiates in the infrared. During the evolutionary phase in which it is most active, the object is known as an active galactic nucleus (AGN). The vast majority of AGN reside in normal galaxies in which the activity of star formation co-evolves with the black hole accretion, but astronomers disagree about the nature of the host galaxies, and in particular whether they resemble normal star forming galaxies in their overall structure.

The main problem lies in the difficulty of distinguishing the contribution the AGN makes to the emission from that of the host galaxy. Even images from the Hubble Space Telescope are unable to distinguish the nuclear component when there is significant dust obscuration in the galaxy. These so-called “obscured AGN” contribute only weakly to the optical emission since it is absorbed by the dust. However, the ones studied to date are extremely luminous overall, with among the largest total luminosities known, equivalent to more than ten billion Suns. ...

Obscured Active Galactic Nuclei Triggered in Compact Star-forming Galaxies - Yu-Yen Chang et al
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Star Forming Filaments

Postby bystander » Sat May 20, 2017 3:19 pm

Star Forming Filaments
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 May 19

Interstellar molecular clouds are often seen to be elongated and "filamentary" in shape, and come in a wide range of sizes. In molecular clouds, where stars form, the filamentary structure is thought to play an important role in star formation as the matter collapses to form protostars. Filamentary clouds are detected because the dust they contain obscures the optical light of background stars while emitting at infrared and submillimeter wavelengths. Observations of some filaments indicate that they are themselves composed of bundles of closely spaced fibers with distinct physical properties. Computer simulations are able to reproduce some of these filamentary structures, and astronomers generally agree that turbulence in the gas combined with gravitational collapse can lead to filaments and protostars within them, but the exact ways in which filaments form, make stars, and finally dissipate are not understood. The number of new stars that develop, for example, varies widely between filaments for reasons that are not known.

The usual model for a star forming filament is a cylinder whose density increases towards the axis according to a specific profile, but which otherwise is uniform along its length. CfA astronomer Phil Myers has developed a variant of this model in which the filament has a star-forming zone along its length where the density and diameter are higher, with three generic profiles to describe their shapes. Besides being a more realistic description of a filament's structure, the different density profiles develop different strength gravitational "wells" naturally leading to different numbers of stars forming within them.

Myers compares the star formation properties of these three kinds of zones with the properties of observed star formation filaments, with excellent results. The filament in the molecular cloud in Musca has relatively little star formation, and can be reasonably well explained with one of the three profiles indicative of an early stage of evolution. A small cluster of young stars in the Corona Australis constellation fits a second model that has evolved for longer, while Ophiuchus hosts a filament that may be near the end of its star forming lifetime and resembles the third type. The three profiles so far seem able to account for the full range of conditions. The new results are an important step in bringing more sophistication and realism to the theory of star forming filaments. Future work will probe the specific processes that fragment the various star-forming zones into their stars.

Star-forming Filament Models - Philip C. Myers
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Understanding Star-Forming Galaxies

Postby bystander » Sun Jun 04, 2017 2:28 pm

Understanding Star-Forming Galaxies
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jun 02

The more stars a typical spiral galaxy contains, the faster it makes new ones. Astronomers call this relatively tight correlation the "galaxy main sequence." The main sequence might be due simply to the fact that galaxies with more stars have more of everything, including material for making new stars. Alternatively, the mechanisms making new stars could be more efficient in some galaxies, or it could be some combination of these and other possibilities.

Star formation in spiral galaxies generates copious amounts of ultraviolet light that is absorbed by dust and re-radiated at infrared wavelengths, and infrared space missions have enabled scientists to measure more precisely the infrared emission from warm dust in galaxies. As astronomers probe very distant galaxies in the early universe and are forced to rely on measured fluxes rather than visual morphologies to interpret what is going on there, the main-sequence relationship has become an important tool for tracing when and how the universe generated its stars. ...

The CfA astronomers find that even across a wide range of stellar masses, at least for local galaxies the correlation between a galaxy's stellar mass and star formation rate is a tight one. They also find that a similarly close correlation holds within small subregions of galaxies, in particular the regions around the supermassive black hole nuclei.

The Sub-Galactic and Nuclear Main Sequences for Local Star-Forming Galaxies - A. Maragkoudakis et al
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Extended Hard X-ray Emission from a Galactic Nucleus

Postby bystander » Tue Jun 13, 2017 6:51 pm

Extended Hard X-ray Emission from a Galactic Nucleus
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jun 09

Supermassive black holes containing millions or even billions of solar-masses of material are found at the nuclei of almost all galaxies. Our Milky Way, for example, has a nucleus with a black hole with about four million solar masses of material. Around the black hole is a torus of dust and gas, and when material falls toward the black hole the inner edge of this disk can be heated to millions of degrees, and emit in X-rays. The accretion heating can also drive dramatic phenomena like bipolar jets of rapidly moving charged particles. The charged particles and intense X-rays found inside this region include the most energetic kind, the so-called “hard X-rays” that are thought to interact with material in the torus to stimulate emission from highly ionized, hot iron atoms. High resolution X-ray images from the Chandra X-ray Observatory appear to confirm that the most extreme conditions that make the hard X-rays are contained in a small region only hundreds of light-years in size around the nuclear black hole.

At a distance of only about seventy million light-years, ESO428-G014 is a comparatively nearby example of a galaxy with nuclear, hard X-ray emission. As it happens, the nucleus is highly obscured in the optical and low-energy X-rays by dust because we view the galaxy edge-wise through its spiral arms; our line-of-sight may also pass through the circumnuclear torus. The galaxy is of particular interest, however, because the nucleus has a radio jet extending from it. CfA astronomers Pepi Fabbiano, Martin Elvis, Alessandro Paggi, Margarita Karovska, Pete Maksym, and John Raymond, with two colleagues, studied this galaxy using Chandra and taking much deeper exposures than ever before attempted. The scientists report discovering that the extreme X-ray emission is extended, not confined to an inner region, and spans a distance of several thousand light-years rather than just a few hundred. While there have been reports of hard emission outside of the nuclear surrounding in two other galaxies, this is the first time such hard X-ray emission has been found on these large extended scales. Without doubt, this discovery challenges the basic presumption that it comes only from the most rapidly moving charged particles inside the torus. The scientists offer a few speculative explanations, including the possible acceleration of particles in the radio jet, and plan to continue their research with additional observations.

Discovery of a Kiloparsec Extended Hard X-Ray Continuum
and Fe–Kα from the Compton Thick AGN ESO 428-G014
- G. Fabbiano et al
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The Anatomy of Orion

Postby bystander » Sat Jun 17, 2017 2:31 pm

The Anatomy of Orion
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jun 16

The Orion molecular cloud is a large complex of hot young stars, nebulae, and dark clouds of gas and dust located in the constellation of Orion. Two particularly famous sights in the night sky, the Orion Nebula and the Horsehead Nebula, are members of this complex, which is relatively nearby, only about 1500 light-years away. Despite its fame, brightness, and relative proximity, however, this complex is not very well understood. Take its star formation, for instance. The relative roles of the local versus galactic-wide conditions are poorly modeled, in particular the contributions of small-scale processes like magnetic fields and turbulence as compared to larger scale activity like gas pressure or the streaming motions of gas within the galaxy’s spiral arms. One reason for this lack of understanding is that the nebula is densely packed with stars and activity while its dust obscures many of the regions from optical view.

CfA astronomers Viviana Guzman and Karin Oberg were part of a team of fourteen astronomers who used the IRAM millimeter telescope to map the Orion-B giant molecular cloud (GMC), located in this complex, over nearly a full degree in the emission from over a dozen molecular lines (for comparison, the angular size of the moon is about one-half a degree). Orion-B is a typical GMC and is useful as a template for other GMCs elsewhere in the Milky Way and in other galaxies. There are a wide range of conditions found in this large region (about 25 light-years in size) and so the scientists are able to obtain a statistically significant breakdown of the region’s activities. One of the key questions the astronomers want to resolve by measuring both small and large-scale gas properties in this example is the linear scale needed to correctly derive star formation characteristics. In extragalactic studies of star formation, small scale measurements are usually not possible: to what extent are the interpretations of emission line ratios, for example, therefore suspect?

The astronomers' study of the molecular anatomy of this complex reveals the detailed relationships between the gas and dust, and quantifies how the spatially varying intensities of the molecular lines reveal the physical conditions. The visual extinction varies with location with values ranging from almost none to nearly opaque even at long infrared wavelengths. The team reports that the amount of molecular gas in any location correlates closely with the extinction, consistent with the picture that more extinction means more dust and thus also more gas. They also find a correlation with the illumination by ultraviolet light from massive young stars at the edges of the map, but no simple correlation between the gas densities and the fraction of radiated light. The paper concludes that the relationships between the line emission and the GMC environment are more complicated than usually assumed, emphasizing (for example) the importance of local chemistry in determining the intensities of the emission here, and in other galaxies.

The anatomy of the Orion B Giant Molecular Cloud:
A local template for studies of nearby galaxies
- Jérôme Pety et al
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