SAO: Weekly Science Updates 2016

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SAO: Weekly Science Updates 2016

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No Fireworks in the Galactic Center
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jan 01

[img3="A false-color infrared image of the region around the nucleus of our Milky Way, the supermassive black hole SagA*. The marker shows the location of the black hole, which glows faintly due to accretion of material; the other objects are stars or dense clouds either orbiting the black hole or in its general vicinity.
Credit: Stefan Gillessen, Max Planck Institute for Extraterrestrial Physics"]https://www.cfa.harvard.edu/sites/www.c ... 201601.jpg[/img3][hr][/hr]

The center of our Milky Way galaxy, about twenty-five thousand light years from Earth, is invisible to us in optical light because of the extensive amounts of absorbing, intervening dust. Radiation at many other wavelengths, however, including the infrared, radio, and energetic X-rays, can penetrate the veiling material. At the heart of the galactic center is a supermassive black hole, Sgr A*, with about four million solar-masses of material. It is a relatively dim object, and shows some slight flickering that is thought to be the result of small blobs of material randomly accreting onto a disk around it. Its general passivity distinguishes Sgr A* from many other supermassive black holes in other galactic centers that actively accrete and heat large amounts of material, and then eject powerful bipolar jets of fast-moving charged particles.

A few years ago astronomers spotted a large cloud of gas (estimated to be three Earth-masses in size) moving relatively quickly towards Sgr A*. Some models projected that the cloud (known as G2) would be disrupted by the black hole during 2015, an event that might be accompanied by detectable radiation that could in turn shed light on a black hole's feeding mechanisms. That did not happen; the year passed without any fireworks, possibly because G2 was too dense to break up.

CfA astronomer Michael McCourt and his colleague have been able to make the most out of this recent non-event. They recognized that the ongoing X-ray emission from Sgr A* implied an inflow rate of about a few Earth-masses per year from ambient material, but that this rate was inconsistent with almost every other measurement, including among other things the total luminosity of Sgr A*. Sorting out the possible solutions required knowing the distribution of the gas very near the black hole - distances less than the distance of the Earth from the Sun. The scientists realized that they could use the changes in the orbit of G2 as it passed through this medium to probe the innermost gas. Even though the object was not devoured as expected, its path would be altered. They also took advantage of a second small cloud in the system, G1, to pin down some parameters as the two traveled during the year along highly eccentric, nearly co-planar orbits around Sgr A*.

The scientists modeled slight changes in the orbital parameters of G1 and G2 as they moved, assuming the changes were due to encounters with the local material. Their analysis provides the first determination of the rotation axis of the accretion flow, and points to the source of the accretion as being from the large torus of molecular gas that is about 4 light-years from the black hole, rather than from the winds of stars that are present in the intervening volume. The result is an important clue to the nature of the black hole’s environment, and moreover it has some observational consequences that might be tested in the next decade, including the future paths of G2 and G2 and the geometry of the emission close to the black hole's boundary.

Going with the flow: using gas clouds to probe the accretion flow feeding Sgr A* - Michael McCourt, Ann-Marie Madigan

• Monthly Notices of the RAS 455(2):2187 (2016 Jan 11) DOI: 10.1093/mnras/stv2232
  arXiv.org > astro-ph > arXiv:1503.04801 > 16 Mar 2015

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The Formation of Carbon-Rich Molecules in Space
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jan 08

[img3="A computer simulation of the formation of complex organic molecules in space. The spherical molecular structures forming on the graphene surface at 3000 K are similar in shape to fullerenes. The red atoms originated in the gas phase and the white atoms are from the surface. Credit: D.W. Marshall, H.R. Sadeghpour"]https://www.cfa.harvard.edu/sites/www.c ... 201602.jpg[/img3][hr][/hr]

The space between stars is not empty, but contains an abundance of diffuse material, about 5-10% of the total mass of our galaxy (excluding dark matter). Most of the material is gas, predominantly hydrogen, but with a small and important component in complex carbon-bearing molecules including ethene, benzene, propynal, methanol and other alcohols, cyanides, simple amino acids, and even larger molecules (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 the local carbon chemistry is not unique.

One important but unsolved issue for astronomers is how these complex organic molecules are made. The answer probably lies in the interstellar dust grains. These tiny grains are about one percent by mass of the interstellar material, and are made predominantly of silicates with some of carbon and/or other elements. These grains seem to be 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.

CfA astrophysicists David Marshall and Hossein Sadeghpour have used a new generation of supercomputers (Harvard's Odyssey research cluster) to simulate how atoms in space might combine to form carbon-rich molecules and clusters, either in the gas phase or on the surface of dust grains. Their simulations used temperatures ranging from 100 to 3000 kelvin (representative of values in stellar neighborhoods) and 4128 atoms (including both grain surface and gas phase atoms) - an unrealistically large number in context compared to the actual density of interstellar matter, but still very indicative of trends and the best computers can currently manage in reasonable calculation times, like days.

The scientists find that at low temperatures the grain surface helps to catalyze faster growth than happens in the gas phase, and that in general the surface temperature is a major factor in determining molecular structures, in particular their geometrical complexity. The research provides important new insights into the formation of large carbon-rich molecules on astrophysical carbonaceous surfaces. THey conclude, for example, that although large chain and branched molecules can form on grain surfaces, they do not have enough energy to stick to the surface and so move around, whereas above 1000 kelvin they tend to stick to the surface and form isolated large clusters that might evolve into complex structures like buckyballs.

Simulating the Formation of Carbon-rich Molecules on an idealised Graphitic Surface - David W. Marshall, H. R. Sadeghpour

• Monthly Notices of the RAS 455(3):2889 (2016 Jan 21) DOI: 10.1093/mnras/stv2524
  arXiv.org > astro-ph > arXiv:1510.05783 > 20 Oct 2015

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The Properties of Pre-Stellar Cores
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jan 15

[img3="A false-color infrared image of a young, star-forming dust cloud with several embedded cores (identified in red). A new infrared study of 3218 cores in various stages of development has enabled astronomers to categorize the temperatures, densities, and evolutionary characters of young stellar nurseries.
(Credit: NASA/Spitzer and P. Myers)"]https://www.cfa.harvard.edu/sites/www.c ... 201603.jpg[/img3][hr][/hr]

Stars like the Sun begin their lives as cold, dense cores of dust and gas that collapse under the influence of gravity until nuclear fusion is ignited. These cores contain hundreds to thousands of solar-masses of material and have gas densities about a thousand times greater than typical interstellar regions (the typical value is about one molecule per cubic centimeter). How the collapse process occurs in these embryos in poorly understood, from the number of stars that form to the factors that determine their ultimate masses, as well as the detailed timescale for stellar birth. Material, for example, might simply fall freely to the center of the core, but in most realistic scenarios the infall is inhibited by pressure from warm gas, turbulent motions, magnetic fields, or some combination of them.

Astronomers are actively studying these issues by observing young stars in the process of being born. The dust in these natal cores (or clumps), however, makes them opaque in the optical, thus requiring observations at other wavelengths, in particular infrared, submillimeter, and radio. In the early stages of star formation, an embryonic star heats the surrounding dust cloud to temperatures between about ten and thirty degrees kelvin before stellar winds and radiation blow the material away and expose the newborn star. CfA astronomers Andres Guzman and Howard Smith, together with their colleagues, have completed an analysis of 3246 star-forming cores, the largest sample ever done. The cold cores themselves were discovered with the APEX submillimeter-wavelength sky survey and then observed in sixteen submillimeter spectral lines; the spectral information enabled the astronomers to determine the distance to each core as well as to probe its chemistry and internal gas motions. The new paper combines these results with far-infrared measurements taken by Herschel Space Observatory surveys. The Herschel data allow the scientists to calculate the dust density, mass, and temperature of each core; the large dataset then permits useful statistical comparisons between cores with varyious parameters.

Sources in the sample fall generically into four categories: quiescent clumps, which have the coldest temperatures (16.8K) and the least infrared emission, protostellar clumps, which are sources with the youngest identifiable stellar objects, ionized hydrogen regions, which are cores within which the stars have ionized some of the surrounding gas, and "photo-dissociation" cores, the warmest of the set, which have dust temperatures around 28K, are slightly more evolved and brighter than the ionized hydrogen cores. Although the groups overlap in their properties, the large sample enables the scientists to conclude that, on average, in the quiescent clumps the dust temperature increases towards the outer regions, whereas the temperatures in protostellar and ionized hydrogen cores increase towards the inner region, consistent with the idea that they are being internally heated. The latter also tend to have dust densities that increase more steeply than the quiescent cores. This study has also identified a population of particularly cold and infrared-dark objects that are probably still in the stages of contraction, or else for some reason have had their star formation aborted. The new paper and its catalog are just the beginning: now that the dust in all these cores has been well characterized, astronomers can associate chemistry with dust temperature, for example, and study subgroups that might represent different stellar masses in gestation.

Far-Infrared Dust Temperatures and Column Densities of the MALT90 Molecular Clump Sample - Andrés E. Guzmán et al

• Astrophysical Journal 815(2):130 (2015 Dec 20) DOI: 10.1088/0004-637X/815/2/130
  arXiv.org > astro-ph > arXiv:1511.00762 > 03 Nov 2015

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Searching for Orbiting Companion Stars
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jan 22

[img3="A Hubble image of the star Gliese 229 together with its brown dwarf companion, Gliese 229B. A new systematic radial velocity search for brown dwarf and stellar-mass companions to stars has discovered one new giant exoplanet and four new companion stars. Credit: . Kulkarni (Caltech), D.Golimowski (JHU) and NASA/ESA/Hubble"]http://cdn.spacetelescope.org/archives/ ... o9548b.jpg[/img3][hr][/hr]

The search for exoplanets via the radial velocity technique has been underway for nearly 30 years. The method searches for wobbles in a star's motion caused by the presence of orbiting bodies. It has been has been very successful, detecting hundreds of exoplanets, but has been overtaken (at least in numbers of detections) by the transit method, which looks for dips in the star's light. The velocity technique also naturally spots orbiting bodies that are larger than planets, which can be either stellar-mass companions or smaller companions that are not quite large enough to become stars, called brown-dwarfs. These larger companions have been largely ignored by surveys dedicated to finding exoplanets, but they are valuable discoveries for astronomers trying to study the smallest classes of stars which are very dim and otherwise difficult to detect. The indications so far are that there are fewer brown dwarf stars than expected in the mass range from about 13 to 80 Jupiter-masses, a phenomenon known as the "brown dwarf desert" that is unexplained. There is another important puzzle: About half of all nearby stars are binary systems yet there are very few known exoplanets around them - only about five percent of all known exoplanets. The dynamics of forming a planetary system around (or within) a multiple-star system are complex and important but poorly understood.

CfA astronomer John Johnson and six colleagues decided to study brown dwarf stars directly with a dedicated, five-year survey that emphasized large companions (stars or brown dwarfs) to mid-sized stars. The scientists selected forty-eight candidate stars for detailed observations from an initial sample of 167 likely candidates based on preliminary observations. They discovered one new giant exoplanet in this set and four stellar-mass companions, one of which may in fact be a brown dwarf. All the objects orbit their stars at distances less than a few astronomical units (one AU is the average distance of the Earth from the Sun). The new results include the orbital parameters of the objects, and the paper considers the possibility of imaging directly these multiple systems with a new generation of optical instruments. The work also marks one of the first efforts to address the nature of the "brown-dwarf desert" by searching for them systematically in order to improve the statistics.

The Pan-Pacific Planet Search III: Five Companions Orbiting Giant Stars - R.A. Wittenmyer et al

• Monthly Notices of the RAS 455(2):1398 (2016 Jan 11) DOI: 10.1093/mnras/stv2396
  arXiv.org > astro-ph > arXiv:1510.04343 > 14 Oct 2015

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The Lives of Massive Compact Galaxies
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jan 29

[img3="An infrared image of a massive, compact galaxy whose
light has been traveling towards us for about 1.5 billion
years. Astronomers have studied how these kinds of
galaxies have evolved over the past 3.5 billion years,
from a time when they were much smaller in size and
mass until today, by using the Illustris simulation.
Credit: Gemini, Trujillo et al."]https://www.cfa.harvard.edu/sites/www.c ... 201605.jpg[/img3][hr][/hr]

Massive quiescent galaxies have masses in stars of over one hundred billion Suns; our Milky Way by comparison has only about half that amount. In the local universe, the stars in these galaxies are mostly old and evolved, and the galaxies themselves are about fifty thousand light-years across and roughly elliptical in shape. The suspected younger counterparts to these objects are massive galaxies from the epoch of peak star formation activity, when the universe was about 3.5 billion years old. They were recently discovered in surveys of the universe and found to be as much as five times smaller in size than their descendants. How they grew so dramatically to become the massive galaxies we see in our cosmic neighborhood is poorly understood.

Two scenarios have been proposed to explain the growth of massive compact galaxies: the merger of galaxies, or their inflation by gas expelled in stellar winds or black hole jets. The problem has been to test these ideas and (since it is expected that at least these two processes are at work) to quantify the relative importance of each. There are just not enough massive compact galaxies known to be able to reach a convincing conclusion from statistical arguments, and so CfA astronomers Sarah Wellons, Vicente Rodriguez-Gomez, Annalisa Pillepich, and Lars Hernquist, together with their colleagues, undertook an analysis of Illustris: a cosmological computer simulation that tracks the evolution of the universe and its contents from a few million years after the big bang to today.

The scientists selected thirty-five massive compact galaxies in the simulation and watched what happened to them as they evolved over the past roughly 3.5 billion years. The galaxies started off with similar masses to within a factor of three, but by the current epoch their masses differed by a factor of twenty – different processes prompted some to grow much faster than others. (At the same time, the difference in their dark matter halo masses, originally a factor of five, grew to a factor of forty.) The team found that about half of the galaxies survived to become the cores of modern massive galaxies, about fifteen percent were destroyed in mergers with much larger galaxies, and about a third – the ones found in the least dense environments – evolved without much change. In particular, the team found that size growth of the galaxies was predominantly driven by the acquisition of new mass through mergers and accretion; winds played only a modest role. The new results are impressive demonstration of the power of Illustris to reveal details of cosmic evolution.

The diverse evolutionary paths of simulated high-z massive, compact galaxies to z=0 - Sarah Wellons et al

• Monthly Notices of the RAS 456(1):1030 (2016 Feb 11) DOI: 10.1093/mnras/stv2738
  arXiv.org > astro-ph > arXiv:1507.02291 > 08 Jul 2015 (v1), 03 Dec 2015 (v2)

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Star Formation in the Outskirts of Galaxies
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Feb 05

[img3="Star formation in the outer spiral regions of the galaxy NGC 4625 is seen in ultraviolet light (blue); these arms are nearly invisible in optical light, but have hot, newborn stars that radiate in the UV. A new study finds that the star formation processes in these outer regions generally resemble the processes at work in more normal, denser regions where molecular gas abounds. The atomic gas is traced in the radio (purple); optical starlight is red. Credit: NASA/JPL-Caltech/Carnegie Observatories/WSRT"]https://www.cfa.harvard.edu/sites/www.c ... 201606.jpg[/img3][hr][/hr]

Star formation environments can be roughly grouped into three types, categorized by the density of their gas (or more precisely, the projected "surface" density of the gas, which is easier to determine than the conventional volume density). In moderately high density regions, where the gas is primarily molecular in form rather than atomic, there is a strong correlation between the amount of star formation taking place and the density. This result is the basis for concluding that stars form from molecular material. In very high density regions like those found in merging and starbursting galaxies, the star-formation rates compared to the total mass of available material are even larger. In low-density regions there is little known about correlations between the total gas and star-formation activity.

Low-density regions, however, are important: they can cover very large spatial extents in the outer domains of galaxies, going well beyond the sizes defined by starlight in the optical. Recently, sensitive searches for molecular gas in these outer regions have been able to map that component, while ultraviolet surveys have spotted UV emission from up to four times farther out in galaxies than the nominal, optical radius. Since the UV is produced by hot young stars, the presumption is that there are new stars forming there. Does star formation in these outer regions correlate with the gas density in the same way as higher density regions, or might perhaps the star formation process proceed differently?

CfA astronomer Linda Watson led a team of five colleagues to address these questions. They analyzed published observations of carbon monoxide gas (a bright tracer of molecular material) in fifteen outer regions of the galaxy NGC 4625 where UV was spotted but which are faint in the optical, and derived the relationship between star formation and gas density. They found that in general the activity is consistent with the same physical processes at work in the brighter, inner regions of the galaxies, a finding that is somewhat reassuring for theorists. But they also spotted a few outer locations where something different was happening: much higher rates of stars were being formed. Molecular gas is a crude tracer for age (because it takes time to convert atomic material into molecular), and most of the regions in this study are estimated to be between about one-to-seven million years old. The scientists raise the possibility that evolutionary effects between these regions could be at work, and urge deeper carbon monoxide observations to facilitate a broader analysis.

Testing the Molecular-Hydrogen Kennicutt-Schmidt Law in the
Low-Density Environments of Extended Ultraviolet Disk Galaxies - Linda C. Watson et al

• Monthly Notices of the RAS 455(2):1807 (2016 Jan 11) DOI: 10.1093/mnras/stv2412
  arXiv.org > astro-ph > arXiv:1510.05658 > 19 Oct 2015

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Star Formation in Distant Galaxy Clusters
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Feb 12

[img3="The galaxy cluster Abell 1689 as seen by Hubble. The mass in the cluster acts as a gravitational lens, distorting the light from background galaxies into blueish arcs of light. Abell 1689 is relatively close by, but astronomers have now spotted clusters in the early universe via their lensing of even more remote, luminous galaxies, and have studied the star formation underway in their outer regions.
(Credit: NASA, N. Benitez (JHU), T. Broadhurst (Racah Institute of Physics/The Hebrew University), H. Ford (JHU), M. Clampin (STScI),G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA)"]https://upload.wikimedia.org/wikipedia/ ... lins-4.jpg[/img3][hr][/hr]

The first stars appeared about one hundred million years after the big bang, and ever since then stars and star formation processes have lit up the cosmos, producing heavy elements, planets, black holes, and arguably all of the other interesting characters in today's universe. When the universe was about three billion years old (currently it is 13.8 billion years old), star formation activity peaked at rates about ten times above current levels. Why this happened, and whether the physical processes back then were different from those today or just more active (and why), are among the most pressing questions in astronomy, and are among the drivers of future facilities from large ground-based telescopes to NASA's James Webb Space Telescope.

The local environment of a galaxy plays a critical role in regulating its star formation. Studies in the local universe find, for example, that in dense galaxy cluster environments (a cluster can contain as many as a thousand galaxies) the star formation is suppressed, consistent with the idea that interactions and other mechanisms are stripping away the raw material for new stars (the neutral gas) and sweeping it into the intergalactic environment. In the distant universe, however, the picture is murkier, and some studies have even found the opposite, perhaps explaining in part the higher star formation rates then. Although studies of individual galaxies in the early universe have made progress, it is usually because these are extremely active and luminous galaxies. A cluster of galaxies, by contrast, might host one or two bright members but most of the membership is ordinary, faint, and hard to study. In fact, clusters are usually even difficult to identify.

CfA astronomers Matt Ashby, Brian Stalder, Tony Stark and their team of colleagues have studied star formation in very dense galaxy clusters in the early universe, dating from about six billion years after the big bang, in an effort to resolve the issue of star formation in cluster environments. They started with a sample of ultraluminous galaxies from the earlier, three billion year-old epoch (or even younger), discovered with the South Pole Telescope. These more distant galaxies were detected in part because their light has been gravitationally lensed by closer clusters; that is how the team was able to locate these clusters in the first place. Knowing where to look, the scientists used infrared data from the Herschel and Planck Space Telescopes (and others) to examine the faint infrared signals from the clusters. That light is presumed to come from star formation, allowing the scientists to determine its level of activity and properties. Their principal finding is that the star formation activity is actually enhanced, not suppressed, in these clusters, up to several thousand new stars are forming per year in these clusters over-and-above the normal levels for these sets of galaxies. They also find that star formation is active out to the edges of clusters, perhaps fifteen million light-years across, and that the effect of this faint infrared emission needs to be taken into consideration in studies of origins of the cosmic background.

Probing star formation in the dense environments of z~1 lensing halos aligned
with dusty star-forming galaxies detected with the South Pole Telescope - N. Welikala et al

• Monthly Notices of the RAS 455(2):1629 (2016 Jan 11) DOI: 10.1093/mnras/stv2302
  arXiv.org > astro-ph > arXiv:1510.01359 > 05 Oct 2015

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Discovering Distant Radio Galaxies via Gravitational Lensing
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Feb 19

[img3="A Hubble Space Telescope image of distant, bright radio galaxies being gravitationally lensed by a very large foreground galaxy cluster. The red contours show the radio emission of these galaxies, which date from an epoch about three billion years after the big bang. A team of X-ray astronomers used these lensed radio galaxies to identify and study distant galaxies with active supermassive black hole nuclei.
(Credit: NASA HST, and van Weeren et al)"]https://www.cfa.harvard.edu/sites/www.c ... 201608.jpg[/img3][hr][/hr]

A lensing cluster is a gravitationally bound collection of galaxies, hundreds or even thousands, whose mass acts as a gravitational lens to collect and reimage the light of more distant objects. These lensing clusters make excellent targets for astronomical research into the early universe because they magnify the faint radiation from more distant galaxies seen behind them, making these remote objects accessible to our telescopes. Most searches in "lensed galaxies" have so far been done at optical, near infrared or submillimeter wavelengths, and the latter have been successful at identifying luminous dusty galaxies from earlier cosmic epochs that are powered by bursts of star formation that were more common back then.

X-ray astronomers study the powerful jets and high energy particles around supermassive black holes at the nuclei of active galaxies (AGN). X-rays are also seen in galaxies dominated by star formation, but they are much dimmer than those seen from AGN and so are difficult to study when these galaxies are at cosmological distances. Even finding distant examples in lensing searches can be challenging, and when the star formation activity is modest they are not even expected to show up in infrared lensing searches. But in galactic nuclei, the same fast-moving particles that emit at X-ray wavelengths also emit at radio wavelengths. A search for lensed radio emission, therefore, is a way to study distant, faint galaxies and their black hole nuclei.

CfA astronomers Reinout van Weeren, G. Ogrean, Christine Jones, Bill Forman, Felipe Andrade-Santos, E. Bulbul, Lawrence David, Ralph Kraft, Steve Murray (deceased), Paul Nulsen, Scott Randall, and Alexey Vikhlinin and their colleagues have completed a radio survey of the large cluster known as MACS J0717.5+3745. This group of galaxies, one of the largest and most complex known with the equivalent of over ten thousand Milky Way-sized galaxies, is located about five billion light-years away.

The astronomers used the Jansky Very Large Array to hunt for lensed radio sources in this cluster, and detected fifty-one compact galaxies -- seven whose light seems to be magnified by the cluster by more than factor of two and as much as a factor of nine. The scientists infer from the radio fluxes that most of these seven are forming new stars at a modest rate, ten to fifty per year, and date from an epoch about three billion years after the big bang. Two are also detected in X-rays by the Chandra X-ray Observatory, and so host AGN, each one radiating about as much light in X-rays as a billion Suns. The two AGN are interesting in themselves, but finding them both in this one region suggests that, like bright star forming galaxies, these AGN were more common back then too.

The Discovery of Lensed Radio and X-Ray Sources behind the Frontier Fields
Cluster MACS J0717.5+3745 with the JVLA and Chandra - R. J. van Weeren et al

• Astrophysical Journal 817(2):98 (2016 Feb 01) DOI: 10.3847/0004-637X/817/2/98
  arXiv.org > astro-ph > arXiv:1512.04527 > 14 Dec 2015

[bystander]

The Milky Way's Central Molecular Zone
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Feb 26

[img3="An infrared and multi-wavelength image of the Central Molecular Zone in the Milky Way. Dense gas is shown in red, and warm and cold dust in green and blue respectively. Several key objects in the region are labeled, along with a set of embedded young stellar clusters seen at 24 microns. (Credit: Cara Battersby, CfA)"]https://www.cfa.harvard.edu/sites/www.c ... 201609.jpg[/img3][hr][/hr]

The center of our Milky Way galaxy lies about 27,000 light-years away in the direction of the constellation of Sagittarius. At its core is a black hole about four million solar masses in size. Around the black hole is a donut-shaped structure about eight light-years across that rings the inner volume of neutral gas and thousands of individual stars. Around that, stretching out to about 700 light-years, is a dense zone of activity called the Central Molecular Zone (CMZ). It contains almost eighty percent of all the dense gas in the galaxy - a reservoir of tens of millions of solar masses of material - and hosts giant molecular clouds and massive star forming clusters of luminous stars, among other regions many of which are poorly understood. For example, the CMZ contains many dense molecular clouds that would normally be expected to produce new stars, but which are instead eerily desolate. It also contains gas moving at highly supersonic velocities - hundreds of kilometers per second (hundreds of thousands of miles per hours).

Where did the CMZ come from? No place else in the Milky Way is remotely like it (although there may be analogues in other galaxies). How does it retain its structure as its molecular gas moves, and how do those rapid motions determine its evolution? One difficulty facing astronomers is that there is so much obscuring dust between us and the CMZ that visible light is extinguished by factors of over a trillion. Infrared, radio, and some X-ray radiation can penetrate the veil, however, and they have allowed astronomers to develop the picture just outlined.

CfA astronomers Cara Battersby, Dan Walker, and Qizhou Zhang, with their team of colleagues, used the Australian Mopra radio telescope to study the three molecules HNCO, N2H+, and HNC in the CMZ. These particular molecules were selected because they do a good job of tracing the wide range of conditions thought to be present in the CMZ, from shocked gas to quiescent material, and also because they suffer only minimally from cluttering and extinction effects that complicate more abundant species like carbon monoxide. The scientists developed a new computer code to analyze efficiently the large amounts of data they had.

The astronomers find, consistent with previous results, that the CMZ is not centered on the black hole, but (for reasons that are not understood) is offset; they also confirm that the gas motions throughout are supersonic. They identify two large-scale flows across the region, and suggest they represent one coherent (or at most two independent) streams of material, perhaps even spiral-like arms. They also analyze the gas in several previously identified zones of the CMZ, finding that one shell-like region thought to be the result of supernova explosions may instead be several regions that are physically unrelated, and that a giant cloud thought to be independent is actually an extension of the large-scale flows. The scientists note that this work is a first step in unraveling an intrinsically complex galactic environment, and that pending research will trace the gas motions to larger distances and try to model the CMZ gas motions with computer simulations.

Molecular gas kinematics within the central 250 pc of the Milky Way - J. D. Henshaw et al

• Monthly Notices of the RAS 457(3):2675 (2016 Apr 11) DOI: 10.1093/mnras/stw121
  arXiv.org > astro-ph > arXiv:1601.03732 > 14 Jan 2016 (v1), 10 Feb 2016 (v2)

[bystander]

A New Record for the Farthest Galaxy Known
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Mar 04

A near-infrared image of the sky showing the source GN-z11 (in the circle), the farthest known galaxy with a spectroscopically confirmed distance; it dates from a time a mere 400 million years after the big bang. (The lines show details of the spectrometer orientation.) (Credit: NASA, P. A. Oesch et al)

---

The epoch when the very first stars appeared in the universe is unknown but is of tremendous interest. These stars began the manufacturing of the chemical elements (heavier than hydrogen and helium), the reionization of the neutral cosmic gas, and mark the dawn of the universe as we know it today. Astronomers estimate that they appeared roughly a few hundred million years after the big bang and soon, as their host galaxies matured, their activity dominated the complex evolution of the cosmos. Thousands of candidates for these early galaxies have been spotted so far despite their being distant and faint. Many were discovered at far infrared or submillimeter wavelengths because they are making stars at fantastic rates (over a thousand per year) and are correspondingly luminous in infrared bands.

Identifying a faint galaxy as being from this early epoch requires measuring its distance, that is its redshift, the amount its light has shifted to the red due to the expansion of the universe (Hubble discovered that the redshift was proportional to the distance). The most reliable redshifts are determined from spectroscopic measurements of known atomic lines, but because distant galaxies are so faint the more common method is to estimate the redshift from their overall color. There are currently about 800 known galaxies whose redshifts using this latter method place them in the epoch around six-to-eight hundred million years after the big bang. The most distant known galaxy with a firm spectroscopic redshift is also from this period, about 580 million years after the big bang.

CfA astronomers Matt Ashby, Giovanni Fazio, and Steve Willner and their colleagues used the Hubble Space Telescope and the IRAC camera on Spitzer to make a spectroscopic discovery of the most distant galaxy known, dating from an epoch around 400 million years after the big bang – about 150 million years earlier than the previous record-holder. If the first stars did indeed appear a few hundred millions years earlier than this time, then this new galaxy only took as long to descend from those first galaxies as we took to succeed the dinosaurs. The galaxy is known only as GN-z11, from the GOODS-North project (a sky survey which took very sensitive infrared images), and the numerical value of its redshift (z=11.1). The success of the measurement was possible because of the considerable luminosity of this galaxy, the result of star formation at a rate of about twenty stars per year. As for the bulk of the galaxy's stars, their aggregate color suggests that most of them are only about forty million years old. The implications of this dramatic new discovery is that the models are in reasonable shape, that massive galaxies formed quite early in cosmic times, and that upcoming, sensitive space missions should easily be able to discover many more of them.

A Remarkably Luminous Galaxy at z=11.1 Measured with
Hubble Space Telescope Grism Spectroscopy - P. A. Oesch et al

• arXiv.org > astro-ph > arXiv:1603.00461 > 01 Mar 2016

http://asterisk.apod.com/viewtopic.php?t=35702

[bystander]

The Distribution of Globular Clusters
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Mar 11

[img3="An optical image of the Fornax Cluster of galaxies. Astronomers have analyzed the distribution of over eighty thousand globular clusters in the region to infer an active history of galaxy-galaxy interactions. (Credit: ESO and Digitized Sky Survey 2)"]https://www.cfa.harvard.edu/sites/www.c ... 201611.jpg[/img3][hr][/hr]

Globular clusters are gravitationally bound, roughly spherical ensembles of stars. Some contain as many as a million stars, and their sizes are as small as only tens of light-years in diameter. Globular clusters are typically located in the outer regions (the halos) of galaxies, and our Milky Way galaxy has about two hundred of them. Astronomers are interested in globular clusters in part because they are home to many of the oldest known stars, but also because of their locations in the halos.

Collisions between galaxies are commonplace, and the locations of globular clusters can offer evidence of these encounters because they are strongly affected by such interactions. During a collision, a galaxy can grow by absorbing or merging with its neighbor, and some models even predict that clusters form during these interactions. Moreover, it is possible that in a merger large numbers of globular clusters originally belonging to a smaller galaxy may be captured by a larger one. The distribution of globular clusters around a galaxy holds therefore clues to their origins and to the history of the host galaxy. There is an additional benefit: Standard models of cosmology make predictions for how galaxies form and evolve that depend on fundamental properties of the universe like the amounts of dark matter and dark energy. The properties of globular cluster, mostly determined by the accretion histories and mergers of their host galaxies, also reflect the values of these fundamental cosmic quantities. ...

The Extended Spatial Distribution of Globular Clusters in the Core of the Fornax Cluster - R. D'Abrusco et al

• Astrophysical Journal Letters 819(2):L31 (2016 Mar 10) DOI: 10.3847/2041-8205/819/2/L31
  arXiv.org > astro-ph > arXiv:1602.06076 > 19 Feb 2016

[starsurfer]

This is a great thread! You do an excellent job bystander!
I do wish there were some planetary nebulae related thingies but then I would say that.

[Ann]

This is a great thread! You do an excellent job bystander!
I do wish there were some planetary nebulae related thingies but then I would say that.

I agree. This is a great thread indeed. I, too, want to thank bystander for making new fascinating posts here every week.

Ann

[bystander]

The Signature of Dark Matter Annihilation, Detected?
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Mar 25

[img3="A false-color image of the anomalous gamma-ray emission from the central region of the Milky Way galaxy; this emission is suspected of coming from dark matter annihilation. In this image, the emission from conventional sources has been subtracted from the total. The region covers roughly five degrees; the brightest emission is colored red and faintest blue. (Credit: Daylan et al)"]https://www.cfa.harvard.edu/sites/www.c ... 201612.jpg[/img3][hr][/hr]

We live in a dramatic epoch of astrophysics. Breakthrough discoveries like exoplanets, gravity waves from merging black holes, or cosmic acceleration seem to arrive every decade, or even more often. But perhaps no discovery was more unexpected, mysterious, and challenging to our grasp of the "known universe" than the recognition that the vast majority of matter in the universe cannot be directly seen. This matter is dubbed "dark matter," and its nature is unknown. According to the latest results from the Planck satellite, a mere 4.9% of the universe is made of ordinary matter (that is, matter composed of atoms or their constituents). The rest is dark matter, and it has been firmly detected via its gravitational influence on stars and other normal matter. Dark energy is a separate constituent.

Understanding this ubiquitous yet mysterious substance is a prime goal of modern astrophysics. Some astronomers have speculated that dark matter might have another property besides gravity in common with ordinary matter: It might come in two flavors, matter and anti-matter, that annihilate and emit high energy radiation when coming into contact. The leading class of particles in this category are called weakly interacting massive particles (WIMPS). If dark matter annihilation does occur, the range of options for the theoretical nature of dark matter would be considerably narrowed. ...

The Characterization of the Gamma-Ray Signal from the Central Milky Way:
A Compelling Case for Annihilating Dark Matter - Tansu Daylan et al

• arXiv.org > astro-ph > arXiv:1402.6703 > 26 Feb 2014 (v1), 17 Mar 2015 (v2)

[bystander]

Challenging the Brightness Limits of Quasars
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Apr 01

Ground-based radio observations of the quasar 3C 273, (the label explains that these are VLBA observations taken at a radio frequency of 1.7GHz), showing the powerful jet and bright core. New observations in concert with the ten meter RadioAstron Space Radio Telescope discovered that the bright core is even brighter than previously thought, by a factor of about one hundred, challenging current theories. (Credit: YY Kovalev et al)

---

Quasars are galaxies with massive black holes at their cores from which vast amounts of energy are being radiated. So much light is emitted that the nucleus of a quasar is much brighter than the rest of the entire galaxy, and these tremendous luminosities allow quasars to be seen even when they are very far away. Much of the radiation is at radio wavelengths, and is produced by electrons in powerful jets ejected from the core at speeds very close to that of light. Such fast-moving charged particles can scatter photons of light, kicking them up in energy while losing a corresponding amount of their own energy. In extreme situations, as is found in quasars, the moving particles will scatter the photons they emit, with the result that the velocities they can attain are self-limited. Astronomers quantify these conditions with a brightness temperature, and conventional estimates conclude that the maximum such temperature is about a trillion (!) degrees.

Brightness is a quantity that depends on size: A small source that emits as much energy as a large source appears to be brighter. In an effort to constrain the size of the emitting region in quasars, the Russian Federal Space Agency launched the ten meter RadioAstron Space Radio Telescope in 2011 with a goal of conducting unprecedented, high spatial resolution radio "very long baseline interferometry" measurements when combined with ground-based radio dishes. The instrument was used to study the very luminous quasar 3C 273, located about two billion light-years away. Previous observations of 3C 273 suggested that its emission region was less than about a light-month in size (the distance light can travel in one month, one twelfth of a light-year), in part because the light varies on a timescale of months. The corresponding brightness temperature is close to the nominal limit.

CfA astronomers Michael Johnson and Ramesh Narayan, together with their RadioAstron teammates, measured the brightness temperature of 3C 273 and discovered that it appeared to exceed the so-called limit by nearly a factor of one hundred. The scientists considered a number of possible effects that might ameliorate the dramatic conclusion, for example, the angle at which we view the beam of fast-moving particles is not well known but enters into the calculation. Their conclusion is that the discrepancy appears to be real. In a series of two papers, the scientists describe the results and offer some possible explanations. One is that turbulent gas in our galaxy is distorting the light from the quasar, an effect that had not been seen until these higher resolution measurements. Another, more dramatic option, is that some unknown physical processes are at work in the vicinity of the supermassive black hole.

RadioAstron Observations of the Quasar 3C273: a Challenge to the Brightness Temperature Limit - Y. Y. Kovalev et al

• Astrophysical Journal Letters 820(1):L9 (2016 Mar 20) DOI: 10.3847/2041-8205/820/1/L9
• arXiv.org > astro-ph > arXiv:1601.05806 > 21 Jan 2016 (v1), 02 Mar 2016 (v2)

Extreme Brightness Temperatures and Refractive Substructure in 3C273 with RadioAstron - Michael D. Johnson et al

• Astrophysical Journal Letters 820(1):L10 (2016 Mar 20) DOI: 10.3847/2041-8205/820/1/L10
• arXiv.org > astro-ph > arXiv:1601.05810 > 21 Jan 2016 (v1), 19 Feb 2016 (v2)

viewtopic.php?t=35779

[bystander]

Pulsation-Driven Winds in Giant Stars
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Apr 08

[img3="The APEX (Atacama Pathfinder Experiment) telescope in Chile. This submillimeter telescope was used to study the velocity of the CO gas in the wind of the giant star EU Del to help constrain the mechanism responsible for driving its wind.
(Credit: ESO,APEX)"]https://www.cfa.harvard.edu/sites/www.c ... 201615.jpg[/img3][hr][/hr]

Nearly all stars have winds. The Sun's wind, which originates from its hot outer layer (corona), contains charged particles emitted at a rate equivalent to about one-millionth of the moon's mass each year. Some of these particles bombard the Earth, producing radio static, auroral glows, and (in extreme cases) disrupted global communications. The winds of stars more evolved than the Sun (like the so-called giant stars that are cooler and larger in diameter than the Sun) often contain dust particles which enrich the interstellar medium with heavy elements. These winds also contain small grains on whose surfaces chemical reactions produce complex molecules. The dust also absorbs radiation and obscures visible light. Understanding the mechanism(s) that produce these winds in evolved stars is important both for modeling the wind and the character of the stellar environment, and for predicting the future evolution of the star.

The mechanism that drives the winds of giant stars is poorly determined. Astronomers think there are three possibilities: radiative, in which the pressure of the light pushes out the grains, magnetically driven, in which the stellar magnetic field plays a role in powering the flow, and pulsation driven, in which a periodic build-up of radiative energy in the stellar interior is suddenly released. Over the years scientific opinion has varied among these alternatives, depending on each particular stellar example. CfA astronomer Chris Johnson and his colleagues explored the problem of wind-driving mechanism in giant stars by measuring the motion of the outflowing CO gas around one the nearest and brightest giant stars, EU Del, which is only about 380 light-years away and shines with 1600 solar-luminosities. Its radius, if the star were placed at the position of the Sun, would extend past the orbit of Venus. EU Del is known to be a semi-regular variable star which pulses every sixty days or so (but with some secondary periods as well), and infrared observations suggest it has a circumstellar dust shell.

The astronomers used the submillimeter APEX (Atacama Pathfinder Experiment) telescope to look at warm CO gas in the wind, making EU Del one of the first stars of its class to be studied with this relatively new tool. The team reports finding the CO moving at about ten kilometers per second (twenty two thousand miles per hour) with a total mass-loss rate equal to about the mass of the Moon each year. Analyzing this and related behavior, they conclude that although a number of uncertainties remain, the most likely mechanism to power the wind is stellar pulsations. They strengthen this conclusion by comparing the EU Del wind results to winds in other giant stars which have different pulsation and wind properties.

EU Del: Exploring the Onset of Pulsation-Driven Winds in Giant Stars - I. McDonald et al

• Monthly Notices of the RAS 456(4):4542 (2016 Mar 11) DOI: 10.1093/mnras/stv2942
  arXiv.org > astro-ph > arXiv:1512.04695 > 15 Dec 2015

[bystander]

Like Galaxies, Like Cities
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Apr 15

[img3="A schematic illustration of Zip's Law for galaxies or cities. On the left is a simulated population distribution with darker spots indicating higher densities; on the right, in pink, are the locations where the population density exceeds some critical value. The distribution of pink clusters agrees with Zipf's Law. (Credit: HW Lin and A Loeb, Phys. Rev. E, 2016)"]https://www.cfa.harvard.edu/sites/www.c ... 201616.jpg[/img3][hr][/hr]

In the last century, the linguist George Zipf noticed that the second most common word in English ("of") was used about half as often as the most common word ("the"), the third most common word ("and") occurred about one-third as often, and so on. This curious behavior, that the frequency of any word is inversely proportional to its ranking in the list of words, became known as Zipf's Law. Others had noticed the same behavior for the populations of cities, namely, that the second most populous city had roughly half the population of the most populous city, the third most populous city had one-third the population, and so on. Scientists studying the detection of faint signals in a background of noise also began to notice a similar effect, with most systems having a component of noise whose intensity varied inversely with the frequency, so-called "one-over-f" noise. Theoretical statistical analyses have found many other cases in which Zipf's Law, or close approximations to it, could result from quasi-random distributions of the element being considered, whether words or cities. There are many slight deviations, however, and no consensus exists on the origin of Zipf's Law.

Galaxies form when the density of matter exceeds some critical value. CfA astronomers Henry Lin, a Harvard undergraduate, and Avi Loeb noted that, like galaxies, cities also might be thought of as forming once their populations exceeded some critical value, with the larger the population, the larger the city. Since Zipf's Law applied to cities, they investigated whether it might also apply to galaxies, and why this might be the case. Rather than focusing on how the law emerges from specific situations, they argue that it occurs naturally in all statistical systems with two key properties: a two-dimensional geometry (galaxies are seen projected onto the two-dimensional plane of the sky) and a clustering behavior that is independent of size ("scale-invariance") so that a small region looks the same as a large region. The scientists show mathematically that with these two characteristics, a Zipf-Law behavior naturally emerges. (Of course for some systems, like words, different reasons may be responsible for producing a Zipf's Law character.) The new theory can derive Zipf's Law and successfully predict population density fluctuations.

Zipf's Law from Scale-Free Geometry - Henry W. Lin, Abraham Loeb

• Physical Review E 93(3):032306 (Mar 2016) DOI: 10.1103/PhysRevE.93.032306
  arXiv.org > physics > arXiv:1501.00738 > 05 Jan 2015 (v1), 15 Feb 2016 (v3)

[rstevenson]

Nobody, not even Zipf, would have deduced any such law if the given data had been the population of the 20 largest cities in Canada. Here's the actual ratios, based on 2011 census data...

1 1:1
2 1:1.5
3 1:2.4
4 1:4.5
5 1:4.6
6 1:4.8
7 1:7.3
8 1:7.6
9 1:7.7
10 1:11.7
11 1:11.7
12 1:14.2
13 1:14.3
14 1:15.6
15 1:16.2
16 1:17.4
17 1:21.3
18 1:26.4
19 1:27.5
20 1:28.2

The chart of these ratios would make for an interstingly wavy line, which with very large error bars might be forced into the expected straight line. Despite this, I find the coincidence of pattern described in the above paper interesting, if not revealing as to cause.

Rob

[bystander]

Understanding Spica
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Apr 22

[img3="A schematic of the binary stars in Spica, showing four stages of an orbital period. Massive binary stars often have a "mass discrepancy problem," meaning that the masses derived from orbital and evolutionary methods disagree. (Credit: A. Tkachenko et al.)"]https://www.cfa.harvard.edu/sites/www.c ... 201617.jpg[/img3][hr][/hr]

The familiar star Spica (Alpha Virginis) is the fifteenth brightest star in the night sky, in part because it is relatively nearby, only about 250 light-years away. It is easy to find by following the arc of the Big Dipper's handle and using the mnemonic, "Arc to Arcturus (Alpha Bootes) and then spike to Spica." In fact Spica is a "spectroscopic" binary, two stars orbiting each other and too close together to separate visually. They were revealed as being a binary pair in 1890 when spectroscopic observations discovered that the stellar lines were doubled due to each star having a slightly different velocity and corresponding Doppler shift. The stars in Spica are, moreover, an unusual pair: They are very close, separated by about twenty-eight solar-radii, and orbit each other in only 4.01 days. This puts them so close together that their mutual gravity tidally distorts their atmospheres, with the result that the stars are not spherical. Oh, and the more massive star pulses in size and luminosity.

Binary stars play a critical role for astronomers studying stars. Because mass and gravity determine the dynamics of orbital behavior, in a binary system it is possible to get at the stars' masses by studying the orbital motions, something that nominally can be done with great accuracy. In contrast, for a single star the mass must be inferred from a much more complicated set of stellar properties and evolutionary models, although even so these models are thought to be excellent and reliable. Sometimes, however, the mass determined from spectroscopy (kinematics) differs from that determined from evolutionary modeling. In the case of massive binary stars (and Spica's two stars are both massive, at 11.4 and 7.2 solar-masses, respectively) this is known as the "mass discrepancy problem."

Enter CfA astronomer Dimitar Sasselov who is part of a team trying to resolve the mass discrepancy problem. In previous work on massive binaries, the team found that the single-star evolutionary models were slightly in error, in particular for the smaller partner. For their analysis of Spica, the astronomers obtained 1731 high-quality spectra and broadband measurements over the course of nearly twenty-three days. They were able to refine all of the system parameters, and realized that the pulsations in the larger star are actually tidally induced, the first such case found for a massive binary. They also found, somewhat surprisingly, that there was no mass discrepancy problem for Spica - the stellar masses derived in both ways are actually consistent, although there are large uncertainties introduced by the complex nature of the Spica system. The research program continues, and the astronomers plan to observe and analyze another few dozen systems in a consistent way, to improve their insights into the nature of the mass discrepancy problem for massive stars.

Stellar modelling of Spica, a high-mass spectroscopic binary with a β Cep variable primary component - A. Tkachenko et al

• Monthly Notices of the RAS 458(2):1964 (2016 May 11) DOI: 10.1093/mnras/stw255
  arXiv.org > astro-ph > arXiv:1601.08069 > 29 Jan 2016 (v1), 08 Feb 2016 (v2)

[Ann]

Spica. Artist's impression by
Manuel Perez de Lema Lopez.

Since it pains me to see Spica depicted as yellow and black, even if I understand that the components have been differently colored for clarity, and even if I understand that just one of the components can't be shown as blue since both are blue, I nevertheless have to post a more realistically colored depiction of Spica.

Ann

[rstevenson]

Not to mention that the schematic is about as confusing as it can be, for us amateurs, at least. In the four charts, they've used three different sets of scales, making chart to chart visual comparisons effectively impossible. I'm going to try to redraw them (as best I can without original data) to see what the orbit actually looks like. ... ...

Rob

[rstevenson]

Okay, here's a reasonable rendition, I think, of the orbital positions, all on the same scale. I used dark blue circles for the positions of the 'black' star, and cyan circles for the positions of the 'yellow' star. I joined the four positions of each with a suitably coloured oval. (My software will only draw horizontal and vertical ovals; I think the yellow one should be tilted up a bit on the left.) I used the Phase=0.75 image (the bottom-right one in the article above) as the base. The ovals have little arrowheads on them indicating the positions of the stars in numerical order by phase. I've no idea if that indicates actual orbital direction from our POV. I drew the cyan stars all at the same size, as best I could by eye. The dark blue ones were drawn at different sizes as shown in the original image, again as best I could by eye. (My image is a little fuzzy because I doubled its size to make it easier to work on.)

Rob

[bystander]

Oxygen in Stars
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Apr 29

[img3="An optical image of the brightest globular cluster, Omega Centauri, a group of over ten million stars older than the Sun. Astronomers have developed a new computational method to determine the abundance of oxygen in these and similar stars, and in particular in giant stars. The code finds values that are more self-consistent than previous estimates. (Credit: Joaquin Polleri & Ezequiel Etcheverry (Observatorio Panameño en San Pedro de Atacama))"]https://www.cfa.harvard.edu/sites/www.c ... 201618.jpg[/img3][hr][/hr]

Oxygen is the third most abundant element in the universe, after hydrogen and helium. It is an important constituent of the clouds of gas and dust in space, especially when combined in molecules with other atoms like carbon, and it is from this interstellar material that new stars and planets develop. Oxygen is, of course, also essential for life as we know it, and all known life forms require liquid water and its oxygen content. Oxygen in molecular form, especially as water, was supposed to be relatively abundant, but over the past decade considerable attention has been paid to observations suggesting that at least in molecular form oxygen is scarcer than expected, a deficit that has not yet been entirely resolved.

Atomic oxygen by contrast, seen most prominently in the light of stars, was thought to be in good agreement with expectations. The neutral oxygen atom produces strong lines that are frequently used to calculate its abundance. Models fit the line strengths by taking into account the radiation field, the star's hot gas motions, and the internal structure of the star (for example, the way the temperature and pressure change with radius). It turns out, however, that varying assumptions in these calculations can result in oxygen abundance predictions that differ significantly, and in the case of giant stars, which are larger and cooler and often have hot outer chromospheres, those abundance results can disagree with one another by as much as a factor of 15. This discrepancy has often been discounted by scientists arguing that some of the proposed stellar models are themselves unrealistic.

CfA astronomers Andrea Dupree, Eugene Avrett, and Bob Kurucz have tacked this fundamental problem with Avrett's PANDORA code for stellar atmospheres. In particular, they include the effects of a hot outer atmosphere in giant stars, something that was typically ignored. Moreover, they do not tie the excitation of oxygen atoms (and the corresponding line strengths) to the local temperature. That constraint, imposed by most previous methods in order to simplify the calculations, does not take more complex situations (like the hot atmosphere) adequately into account. The astronomers find that their new computations can resolve several outstanding issues. The lines themselves are actually as much as three times stronger than previously thought, reducing the implied oxygen abundances, and thereby also affecting details of the stellar interior models, especially for giants seen in globular clusters of stars. Similar improvements are seen in the results for stars known to be lacking other heavier elements, and even some normal, Sun-like stars. The possible implications extend to estimating more accurately the amount of oxygen present in a solar nebula when exoplanets form.

Chromospheric Models and the Oxygen Abundance in Giant Stars - A. K. Dupree, E. H. Avrett, R. L. Kurucz

• Astrophysical Journal Letters 821(1):L7 (2016 Apr 10) DOI: 10.3847/2041-8205/821/1/L7
  arXiv.org > astro-ph > arXiv:1603.07381 > 23 Mar 2016

[bystander]

The Turbulent Interstellar Medium
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 May 06

[img3="The galaxy M101 as seen in the optical and in the light of atomic hydrogen gas (red). The hydrogen lines reveal that the gas is rapidly moving and turbulent, and a new study of turbulence in galaxies concludes that in many cases it is generated not by star formation but by gravitational effects alone. Image Credit: Terry Hancock"]https://www.cfa.harvard.edu/sites/www.c ... 201619.jpg[/img3][hr][/hr]

The gas in galaxies is typically seen to be moving at very rapid, even supersonic velocities, providing clear evidence that the medium is highly turbulent. Looking more closely at gas clouds in our own Milky Way, astronomers have similarly demonstrated using a variety of different observations that the interstellar medium is turbulent. Turbulence is a key physical parameter in the star formation process because, like the thermal pressure of warm gas, it counters the collapse of clouds into stars from gravitational contraction. Despite its importance and ubiquity, however, turbulence is poorly understood. Even its origin is far from clear. Some scientists argue that turbulence results from star formation itself, as new stars and their associated supernovae drive winds that stir up the interstellar medium. Other astronomers counter that the influence of gravity alone is enough to induce supersonic motions in gas as it moves through and across a rotating galaxy.

CfA astronomer Blakesley Burkhart and her colleague examine in theoretical detail the physical processes involved in generating turbulence, and compare their conclusions with observations of galaxies. It has long been noted that the star formation rate in galaxies appears to correlate approximately with the spread of gas velocities seen in that galaxy. Indeed, that result was one of the reasons that a causal link between star formation and turbulence was proposed. The scientists point out, however, that if star formation were responsible for that spread then the correlation would be much tighter than is observed. Actually, the gravity-driven model of turbulence shows much better agreement with the data. For example, the latter has no trouble reproducing galaxies with very high velocities yet having very low star formation rates; in these situations there is just not much gas to make new stars, but gravity nevertheless drives the fast motions. The scientists' results are highly suggestive though not definitive, and very likely there are cases where both star formation and gravity play comparable roles. The authors conclude by discussing the limitations of the current observational dataset, and they suggest future measures to refine their conclusions, but in the meantime they have demonstrated that turbulence has a more complicated origin than was typically thought.

Is Turbulence in the Interstellar Medium Driven by Feedback or Gravity? An Observational Test - Mark R. Krumholz, Blakesley Burkhart

• Monthly Notices of the RAS 458(2):1671 (2016 May 11) DOI: 10.1093/mnras/stw434
  arXiv.org > astro-ph > arXiv:1512.03439 > 10 Dec 2015 (v1), 22 Feb 2016 (v2)