SAO: Weekly Science Updates 2016

[bystander]

Mysterious Happenings Around the Star KIC 8462852
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 May 13

A portion of a curve plotting light flux versus time (in days) for the strange star KIC 8462852. The dips are irregular and therefore not due to orbiting planetary transits; the actual cause is not well understood. A new measurement of the dust in this system concludes there is too little to satisfy most suggested explanations, but is possibly consistent with the breakup of a cluster of Halley-like comets. Credit: Boyajian et al.

---

The Kepler satellite was designed to search for Earth-sized planets in the habitable zone of stars by measuring dips in a star's brightness as orbiting planets move across the stellar disc (transits). Its sensitive camera stares at more than 150,000 stars towards the constellations of Cygnus and Lyrae, and so far has found over 5000 exoplanet candidates. But Kepler also monitors the light fluctuations in all the other stars, even dips not caused by transits, and has found some bizarre situations. Perhaps the strangest is the case of KIC 8462852, an otherwise normal star slightly larger than the Sun that has exhibited significant, irregular dips in the flux that last as short as a few days or as long as eighty days, and are as deep as 20%. The source is so far unique in the Kepler database. The irregular and extreme nature of the episodes excludes planetary transits, and other suggestions have ranged from a catastrophic collision between planets that released a cloud of obscuring debris, to the presence of a huge alien artifact like a so-called "Dyson sphere!"

CfA astronomers Mike Dunham, Glen Petitpas, and Lars Kristensen, and their colleagues, realized that if a cloud of dust particles were present in the stellar system, it should be detectable at submillimeter and millimeter wavelengths because of its warm temperature. They used the Submillimeter Array and the James Clerk Maxwell Telescope to search for any such dust. They found no signs of it. They can therefore limit the amount of material to less than about one tenth of the Moon's mass (at least in the regions mostly likely to host dust) and fewer than about eight Earth-masses throughout the entire stellar system. According to the scientists, such small amounts of dust make the catastrophic planetary collision scenario very unlikely, but might be consistent with the picture of the complete breakup of a cluster of about thirty Halley-like comets. The cause of such a dramatic event, however, is not understood, and meanwhile other imaginable scenarios are still allowable, but the new results put a firm limit on the amount of dusty material around this strange and unique star.

Constraints on the Circumstellar Dust around KIC 8462852 - M.A. Thompson et al

• Monthly Notices of the RAS: Letters 458(1):L39 (2016 May 01) DOI: 10.1093/mnrasl/slw008
  arXiv.org > astro-ph > arXiv:1512.03693 > 11 Dec 2015

http://asterisk.apod.com/viewtopic.php?t=35401

[bystander]

Water on the Moon?
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 May 27

[img3="This illustration shows a promising new technique for locating water on the Moon. Galactic cosmic rays (GCR) that penetrate the lunar surface, when encountering a layer with material containing hydrogen atoms (like water), trigger the ejection of protons (red spheres) that can be detected by the appropriate instrumentation in an orbiting satellite. (Credit: Schwadron et al. 2016)"]https://www.cfa.harvard.edu/sites/www.c ... 201621.jpg[/img3][hr][/hr]

Prior to the Apollo missions to the Moon, scientists speculated that volatiles - including water - may have accumulated in permanently shaded regions at the poles. Then the Apollo era brought the return of lunar samples, enabling real measurements: They found none of the water-bearing minerals common on Earth. Over the past ten years, however, several developments have reinvigorated the discussion. In particular, new analyses of volcanic glasses in the sample returns have inferred the presence of water in the Moon’s interior. Meanwhile, several new lunar missions have been launched. The ones using neutron spectroscopy to search for water have come up with mixed conclusions, but those using infrared spectroscopy seemed to reach unambiguous identification of water on the lunar surface, although in disagreement with the neutron experiments.

CfA astronomers Anthony Case and Justin Kasper were members of a team of astronomers who propose a new method to detect hydrated material on the Moon - like water - by measuring the strength of protons coming from the lunar surface with the CRaTer instrument (Cosmic Ray Telescope for the Effects of Radiation) on the Lunar Reconnaissance Orbiter, a NASA robotic orbiter launched in 2009. Cosmic rays from the galaxy, when striking the lunar surface, will knock protons out of material on the surface which can be detected by the CRaTER instrument. The team completed a set of laboratory tests using high energy particle accelerators to simulate the effects of cosmic rays on materials containing hydrogen, and found that the presence of hydrogen - in water for example – actually suppresses the overall proton emission. The implication is that if water is present near the poles of the Moon, a scan across the lunar surface should show a clear reduction in numbers of protons as it crosses the poles.

Actually, the CRaTER scans found an increase in the proton emission at the poles. The scientists soon realized that there were some effects, originally thought to be negligible, that were responsible. Protons and neutrons, released from material below the surface down to about ten centimeters, will collide with other atoms and produce the emission of secondary particles. The enhancement of these secondary protons is indeed consistent with the presence of hydrogen. But it turns out that there are other possible solutions as well, and the team is continuing to investigate them. Meanwhile their current paper shows that the technique of using CRaTER measurements to search for water is at least in principle possible, and when the remaining issues are resolved, the techniques could be used in other missions to probe other solar system bodies.

Signatures of Volatiles in the Lunar Proton Albedo - N.A. Schwadron et al

• Icarus 273:25 (15 July 2016) DOI: 10.1016/j.icarus.2015.12.003

[bystander]

A Blazing Gamma-Ray Source
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jun 03

[img3="An artist's conception of a blazar, whose powerful jet of high velocity particles is generated around a supermassive black hole and aimed almost directly at Earth. Astronomers have measured and successfully modeled the very high energy gamma ray emission from a blazar using VERITAS. (Credit: Marscher et al., Wolfgang Steffen, Cosmovision, NRAO/AUI/NSF)"]https://www.cfa.harvard.edu/sites/www.c ... 201622.jpg[/img3][hr][/hr]

Blazars are galaxies whose central, supermassive black holes are accreting material from surrounding regions. Although black hole accretion happens in many galaxies and situations, in the case of a blazar the infalling material erupts into a powerful, narrow beam of high velocity charged particles that, fortuitously, is pointed in our direction. The charged particles produce gamma ray photons, each photon packing over a hundred million times the energy of the highest energy X-ray photon seen by the Chandra X-ray Observatory. The electron beam produces many other effects, and in blazars these include rapid, strong, and incessant variability. They sometimes also include the ability to generate high-energy gamma rays.

The blazar 1ES1741+196 was first spotted in 1996 by the Einstein X-ray satellite. Followup observations determined that it is a triplet system: an elliptical galaxy with two companion galaxies close enough nearby to be an interacting triplet; a tidal tail is observed, for example, presumably the result of mutual gravitation influences. The interactions might play a role in stirring up material for black hole accretion. In 2011, astronomers discovered that the object also emitted gamma rays, but at an intensity that made it one of the faintest such sources known.

VERITAS, the Very Energetic Radiation Imaging Telescope Array System, is an observatory designed to study gamma rays. It consists of four 12-m telescopes located at the Fred L. Whipple Observatory at Mt. Hopkins, Arizona. CfA astronomers Wystan Benbow, M. Cerruti, Pascal Fortin, V. Pelassa, and Thomas Roche were members of a team of eighty-eight astronomers who used VERITAS to study 1ES1741+196 in an effort to model this weak gamma ray blazar. They observed it successfully in several energy ranges for thirty hours over a period of several years and were able to obtain, and model, the first very high energy spectrum for the source. In general, high energy photons are produced in two processes: direct radiation from the charged particles, and scattering by the fast-moving particles of lower energy photons to much higher energies. The team successfully modeled this source including only these two, energetic processes. The result suggests that the scientists have accurately characterized this blazar - despite its faintness - as being among those that produce the most energetic gamma rays. They also found, curiously, that there is no evidence for any significant flaring in this source.

VERITAS and Multiwavelength Observations of the BL Lacertae Object 1ES 1741+196 - VERITAS Collaboration

• Monthly Notices of the RAS 459(3):2550 (2016 Jul 01) DOI: 10.1093/mnras/stw664
  arXiv.org > astro-ph > arXiv:1603.07286 > 23 Mar 2016

[bystander]

T-Tauri Stars
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jun 10

[c][attachment=0]eso9921c.jpg[/attachment][/c][hr][/hr]

A newborn star typically goes through four stages of adolescence. It begins life as a protostar still enshrouded in its natal molecular cloud, accreting new material and developing a proto-planetary disc. Slowly, stellar winds and radiation blow away the surrounding shell of gas and dust, and the third stage, when the surrounding envelope has cleared, is called the T-Tauri phase. T-Tauri stars (the class is named after the first star of this type that was so identified) are less than about ten million years old, and provide astronomers with promising candidates in which to study the early lives of stars and planets. They were among the first young stars to be identified because the earlier stages, still embedded in their birth clouds, were blocked from optical observations by the dust. In the fourth stage, the disk stops accreting and the source’s radiation comes from the star’s photosphere. T-Tauri stars produce strong X-rays, primarily by what is thought to be coronal activity much like the coronal activity in our own Sun, although in some cases a component might be coming from hot material in the dusty disk.

Measurements of T-Tauri circumstellar disks provide important tests for theories of planet formation and migration. Near-infrared results, for example, sample the hotter temperature dust grains, and can reveal the presence of gaps in the disk (perhaps cleared by massive planets) when an expected ring of warm dust around the star is not detected. Astronomers during the past few decades have been able to use infrared space telescopes like Spitzer to probe T-Tauri disks, but there are still many puzzles, in particular about the mechanisms responsible for the accretion, the subsequent dissipation of material, and the evolutionary ages when these processes occur. ...

Fundamental Stellar Parameters for Selected T-Tauri Stars
in the Chamaeleon and Rho Ophiuchus Star-Forming Regions - D.J. James et al

• Monthly Notices of the RAS 459(2):1363 (2016 Jun 21) DOI: 10.1093/mnras/stw481
  arXiv.org > astro-ph > arXiv:1603.00820 > 02 Mar 2016

[bystander]

The Stability of the Solar Wind
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jun 17

[img3="Astronomers have used data from Wind to make the first complete analysis of two key instabilities in the plasma flow of the solar wind. (Credit: NASA/Wind)"]https://www.cfa.harvard.edu/sites/www.c ... 201624.jpg[/img3][hr][/hr]

NASA's Wind spacecraft observes the solar wind before it impacts the magnetosphere of Earth. Launched in 1994 into an orbit more than two hundred Earth-radii away, one of Wind's prime objectives is to investigate the basic physical processes occurring in the ionized gas in the near-Earth solar wind. One of the open issues is the stability of the solar wind plasma.

The three main particle species in the solar wind are protons, electrons, and helium nuclei (two protons and two neutrons). Unlike particles in a conventional dense gas, the charged particles in the wind can have motions and collective behaviors that are not characterized simply by their temperature. For example, the wind can host regions of different temperatures and densities that propagate along as waves, or that dissipate, and the energy in the wind can be converted between different modes. Moreover the wind can host two unmixed gases with different temperatures and flows in the same vicinity, and these regions can react differently to the magnetic fields that are present.

CfA astronomer Mike Stevens and his colleagues used the Wind spacecraft to analyze two key instabilities in the wind, the first such analysis that includes all three species of particles. One of the instabilities results when the forward pressure is large enough to generate ripples in the plasma, and the other results when pressure builds up within dense clumps in the wind, causing them to break up. The scientists concluded from the Wind data that protons were the species that dominated for both of these instabilities, but that the presence of the other two species was significant, contributing about one third of the observed effects for each instability. They also concluded that for the majority of the time, the solar wind is stable. The new results will be useful for other situations involving astrophysical plasmas.

Multi-Species Measurements of the Firehose and Mirror Instability Thresholds in the Solar Wind - C. H. K. Chen et al

• arXiv.org > physics > arXiv:1606.02624 > 08 Jun 2016

[bystander]

Measuring the Shape of the Milky Way's Black Hole
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jun 24

[img3="This figure shows the locations of the radio telescopes linked together to observe the supermassive black hole at the center of our Milky Way. (Credit: Ortiz-LeOn et al)"]https://www.cfa.harvard.edu/sites/www.c ... 201625.jpg[/img3][hr][/hr]

At the heart of our galaxy's center is SagA*, a supermassive black hole containing about four million solar-masses of material. SgrA* is relatively faint, unlike the supermassive black holes in some other galaxies. This is probably because, unlike its active cousins, it is not aggressively accreting material and so is neither heating up its environment nor ejecting particularly intense jets of fast-moving charged particles. Of course, it is also faint because it is located about twenty-five thousand light years from Earth and because it is shrouded in absorbing, intervening dust. Nevertheless, radiation at radio, submillimeter, infrared and X-rays can penetrate the veiling material. As the closest super massive black hole to Earth, SgA* is a template for astronomers actively studying black holes, offering the best views of the physical properties and environments. The radio emission in particular is thought to come from material falling onto a disk around the black hole and heating up electrons, and from ejected material both within the jet itself and its nozzle.

One of the most exciting new projects studying SgrA* uses Very Long Baseline Interferometry (VLBI) techniques, which links an array of widely-spaced radio telescopes to obtain very high spatial resolutions. CfA astronomers Michael Johnson, Shep Doeleman, Lindy Blackburn, Mark Reid, Andrew Chael, Katherine Rosenfeld, Hotaka Shiokawa, and Laura Vertatschitsch and their colleagues used a VLBI network to detect SgrA* in millimeter wavelengths. They were successfully able to model its size, thanks to the inclusion in the array for the first time of the Large Millimeter Telescope Alfonso Serrano in Mexico.

The scientists conclude that the radio emission comes from a region only 1.2 astronomical units in diameter (one AU is approximatley the average distance of the Earth from the Sun). The black hole's radius of no return itself (its Schwarzchild radius) is only about ten times smaller. They estimate that the emission they see comes from hot electrons in the inner parts of the accretion flow, but there are many details to sort out and additional observations are needed to eliminate other possibilities. Nevertheless, this first result is a remarkable achievement in probing the nature of supermassive black holes, their environments, and the processes taking place around them.

The Intrinsic Shape of Sagittarius A* at 3.5-mm Wavelength - Gisela N. Ortiz-León et al

• Astrophysical Journal 824(1):40 (10 June 2016) DOI: 10.3847/0004-637X/824/1/40
  arXiv.org > astro-ph > arXiv:1601.06571 > 25 Jan 2016 (v1), 14 Mar 2016 (v2)

[bystander]

The Quiet Intracluster Medium in the Core of the Perseus Cluster
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jul 15

[img3="An X-ray image of the Perseus cluster of galaxies taken by Chandra. The central bubbles (dark regions) and ripples are associated with outflows from material that surrounds the cluster's central supermassive black hole. Astronomers using the (now lost) Hitomi satellite find that turbulence in the central region of the cluster is low, implying that errors in previous measurements of the masses of galaxy clusters using X-ray observations are small. (Credit: NASA/Chandra, Nature, and the Hitomi Collaboration"]https://www.cfa.harvard.edu/sites/www.c ... 201626.jpg[/img3][hr][/hr]

Most galaxies lie in clusters, groupings of a few to many thousands of galaxies. Our Milky Way galaxy itself is a member of the Local Group, a band of about fifty galaxies whose other large member is the Andromeda Galaxy about 2.3 million light-years away. The closest large cluster of galaxies to us is the Virgo Cluster, about 50 million light-years away, with about 2000 members. Clusters are the most massive gravitationally bound objects in the universe and consequently are key probes of cosmological parameters. Currently, astronomers think that galaxies form in a "bottoms-up" fashion, in which smaller structures develop first and large clusters only assemble later in cosmic history. How and when this proceeds, however, is poorly understood and depends on several competing physical processes, including the behavior of the intracluster gas. As clusters grow, they incorporate large quantities of very hot, inter-galaxy gas whose temperature is ten million kelvin or even much higher. There is more mass in the gas than there is in all the stars of a cluster's galaxies, and so the gas plays an important role in the cluster's evolution.

Last February, the Japanese Space Agency JAXA launched Hitomi, an international X-ray satellite designed among other things to study the hot intracluster gas. Unfortunately, Hitomi was lost shortly after launch, but before it failed it was able to study the hot gas in the one large cluster of galaxies, the Perseus Cluster. A large team of astronomers including CfA scientists Laura Brenneman, Adam Foster, and Randall Smith have just published their conclusions from this key observation in Nature.

The scientists studied the motions of the hot gas using Hitomi's uniquely high velocity resolution capability. If the gas were moving rapidly, then this energy might help support the material from gravitationally collapsing. Before Hitomi, astronomers assumed that such rapid kinematics were modest when compared with the thermal motions of the hot gas. The Hitomi results confirmed that this is indeed the case, and in fact finding that the kinematic motion was even smaller than previously expected. These results relied in part on a model of the X-ray emission provided by the CfA's AtomDB team.

The new work excludes the high velocity scenario, at least for Perseus, and since this cluster is thought to be representative the result means that related conclusions about cluster behavior and evolution are reliable.

The Quiescent Intracluster Medium in the Core of the Perseus Cluster - Hitomi Collaboration

• Nature 535(7610):117 (07 July 2016) DOI: 10.1038/nature18627
  arXiv.org > astro-ph > arXiv:1607.04487 > 15 Jul 2016

http://asterisk.apod.com/viewtopic.php?t=36129

[bystander]

Orphaned Protostars
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jul 22

[img3="A far-infrared dust map of the star formation region in Orion showing protostellar candidates (of various types) with very low extinction using blue and green markers. The background color corresponds to the infrared brightness; the black line represents one survey region. Astronomers have confirmed that ten of these candidates objects are true protostars even though, contrary to expectations, they are not deeply buried within their clouds. (Credit: Lewis & Lada, ApJ, 2016)"]https://www.cfa.harvard.edu/sites/www.c ... 201627.jpg[/img3][hr][/hr]

Stars form as gravity contracts the gas and dust in an interstellar cloud until clumps develop that are dense enough to coalesce into stars. Precisely how this happens, however, is very uncertain, and the processes are hard to study because they occur inside the opaque interiors of these clouds. The numbers of stars that form from a single clump is not well understood, for example, nor are the stellar masses. In the earliest stages of formation, the so-called protostellar stage, the newborn star is still accreting material from an envelope and is ringed by a circumstellar disk (which can develop into a system of planets). This earliest stage, which occurs when the star is the most heavily obscured, is therefore also the most mysterious.

Infrared observations can penetrate the dust in a stellar womb, at least partially, with the reddest wavelengths being least obscured. Astronomers examining Spitzer Space Telescope infrared images of the Orion star-forming complex discovered 329 protostars in the region, and forty-four of them did not appear to be very red -- and hence not deeply embedded, contrary to expectations. CfA astronomers John Lewis and Charlie Lada wondered how that could be. After re-examining the dataset with more stringent criteria, they concluded that only ten were true protostars, with the others being extragalactic objects and nebulous regions with a few unclassified sources. But these ten were still ten more than would be predicted.

The astronomers modeled these ten protostars (including for their analysis a variety of other data) and conclude that indeed they are protostars … and they are not presently embedded in a cloud. The scientists dub them "orphan protostars," and speculate that they have left their nursery at a young age, perhaps ejected in a gravitational interaction with a more-massive sibling. Alternative possibilities include that they just migrated away, or that somehow they were able to dissipate their immediate surroundings through winds. Additional research will narrow these choices. On the one hand these new results make the class of protostars even more varied, but on the other hand they confirm that the general theory is steadily improving even though the range of conditions has increased.

Protostars at Low Extinction in Orion A - John Arban Lewis, Charles J Lada

• Astrophysical Journal 825(2):91 (10 July 2016) DOI: 10.3847/0004-637X/825/2/91
  arXiv.org > astro-ph > arXiv:1605.02732 > 09 May 2016 (v1), 11 May 2016 (v2)

http://asterisk.apod.com/viewtopic.php?t=36147

[bystander]

The Aligned Spin of a Black Hole
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Jul 29

[img3="An artist's conception of an X-ray emitting black hole binary system. A new study has measured the spin of one notable example and confirmed, contrary to some earlier claims, that the spin is aligned with the spin of the accretion disk.
(Credit: ESA, NASA, and Felix Mirabel)"]https://www.cfa.harvard.edu/sites/www.c ... 201629.jpg[/img3][hr][/hr]

A black hole in traditional theory is characterized by having "no hair," that is, it is so simple that it can be completely described by just three parameters, its mass, its spin, and its electric charge. Even though it may have formed out of a complex mix of matter and energy, all the specific details are lost when it collapses to a singular point. This is surrounded by a "horizon," and once anything – matter or light (energy) – falls within that horizon, it cannot escape. Hence, the singularity appears black. Outside this horizon a rotating, accreting disk can radiate freely.

Astronomers are able to measure the spins of black holes by closely modeling the X-ray radiation from the environment in one of two ways: fitting the continuum emission spectrum, or modeling the shape of an emission iron line from very highly ionized iron. So far the spins of ten stellar-mass black holes have been determined and the robustness of the continuum-fitting method has been well demonstrated. Recently one bright black hole, "Nova Muscae 1991," was found to be rotating in a sense opposite to the spin of its disk, a very unusual and curious result since both might be expected to develop somewhat in concert. The spin of this black hole had previously determined to be small, about ten percent of the limit allowed by relativity.

CfA astronomers Jeff McClintock, James Steiner and Jainfeng Wu and their colleagues have re-reduced archival data for this source, and obtained much improved measurements for the three key parameters needed in the continuum-fitting method: mass (11.0 solar-masses), disk inclination (43.2 degrees), and distance (16,300 light-years), each with a corresponding (and modest) uncertainty. Using the new numbers to reevaluate the model of the Nova Muscae 1991 spin, the scientists report that the spin is actually about five times larger than previously estimated. More significantly, that the spin is definitely prograde (aligned with the direction of the disk spin), and not retrograde. The new results resolve a potential mystery, and offer a confirmation of the general methods for modeling black holes.

The Spin of The Black Hole in the X-ray Binary Nova Muscae 1991 - Zihan Chen et al

• Astrophysical Journal 825/1/45 (01 July 2016) DOI: 10.3847/0004-637X/825/1/45
  arXiv.org > astro-ph > arXiv:1601.00615 > 27 Dec 2015

[bystander]

Chiral Molecules in Space
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Aug 05

[img3="The molecular structure of S-propylene oxide (A) and R-propylene oxide (B).
Carbon, hydrogen, and oxygen atoms are indicated by gray, white, and red spheres, respectively. Astronomers have detected this chiral molecule, the first one found in interstellar space. Life on Earth is based on chiral molecules.
(Credit: McGuire et al, Science, 2016)"]https://www.cfa.harvard.edu/sites/www.c ... 201631.jpg[/img3][hr][/hr]

Chiral molecules are those that have a "handedness," that is, they take one of two forms, identical in chemical composition but with mirror image shapes - like our right and left hands: no rotation can convert one into the other. For reasons that are not known, essentially all biologically molecules are homochiral (having only one shape). On Earth this shape is left-handed; our bodies cannot digest right-handed sugars.

The origin of homochirality is a key mystery in the study of our cosmic origins. Although some scientists have argued it has some evolutionary advantages, the mechanisms for the selection of one shape over another are uncertain. Disentangling them requires that we understand the potential origins of the phenomenon. The oldest materials studied in the laboratory so far are meteoritic samples, but astronomers have been searching for even more ancient examples in the rich chemistry of the interstellar medium using techniques of radio astronomy to look for spectral signatures of gaseous chiral molecules. ...

Both chiralities have identical spectra, and so although the new results may have detected ancient, chiral molecules it is not known if they are homochiral. Nor do these results identify any mechanism that might eventually preferentially select one handedness over the other. However the team can calculate the likely environment of these molecules, which is in a cold, shocked shell surrounding a dense core with many other organic species. They speculate briefly on the role of one particularly interesting possibility: circularly polarized light in the cloud, which also has a handedness. More research is clearly needed; meanwhile the new results demonstrate that chiral molecules exist in space and are ready for study.

Discovery of the Interstellar Chiral Molecule Propylene Oxide (CH₃CHCH₂O) - Brett A. McGuire et al

• Science 352(6292):1449 (17 June 2016) DOI: 10.1126/science.aae0328
  arXiv.org > astro-ph > arXiv:1606.07483 > 23 Jun 2016

http://asterisk.apod.com/viewtopic.php?t=36050

[bystander]

Resolving the Planetesimal Belt around HR 8799
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Aug 12

[img3="A submillimeter image of the planetesimal disk around the star HR8799, the first directly imaged system of four exoplanets and their dust disk. The insert shows the innermost region of the system and the location of the four exoplanets.
(Credit: ALMA; Booth et al)"]https://www.cfa.harvard.edu/sites/www.c ... 201632.jpg[/img3][hr][/hr]

Planets develop from the dusty placental disk of material that surrounds a star after it begins to shine. The dust in that disk, according to most models, starts to stick to itself until clumps develop large enough to attract other clumps gravitationally. Astronomers believe the process of building planets and dissipating the disk takes about ten million years. Many mysteries remain, however, including the tendency of dust not to stick together, and the likelihood that colliding clumps could break apart rather than agglomerate. Recent discoveries of exoplanets have begun to overlap with studies of planetesimal disks, and enable astronomers to probe the development and evolution of a star's system of planets and their interactions with the disk.

Direct imaging of dust disks has been very limited, and so far has principally probed regions in disks at the outer zones of planetary systems – analogous to the Kuiper Belt in our own solar system. At the same time, the vast majority of exoplanets discovered and studied so far have been very close to the star, even within a distance that in the solar system would be within the orbit of Mercury. The star HR8799 is so far the only star around which direct imaging has found multiple planets. Its circumstellar disk has been known to exist for several decades, and has been modeled as having three zones: an inner asteroid belt analogue, a planetesimal belt from about one hunded astronomical units (au) to about 430 au, and a halo region extending out to over 1500 au. ...

Resolving the Planetesimal Belt of HR 8799 with ALMA - Mark Booth et al

• Monthly Notices of the RAS: Letters 460(1):L10 (2016 July 21) DOI: 10.1093/mnrasl/slw040
  arXiv.org > astro-ph > arXiv:1603.04853 > 15 Mar 2016

[bystander]

Statistical Properties of Star Formation in Molecular Clouds
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Sep 02

[img3="An image of the giant molecular cloud complex, Mon R2. A far-infrared study of the numbers of dense clumps across this cloud has found statistical correlations that depend on the relative dominance of turbulence versus gravity.
(Credit: Adam Block, Mt. Lemmon SkyCenter, U. Arizona)"]https://www.cfa.harvard.edu/sites/www.c ... 201633.jpg[/img3][hr][/hr]

Stars form within the dense regions of diffuse molecular clouds, but the physical processes that determine the locations, rate, and efficiency of star formation are poorly understood. Recent thinking envisions an approximately two-step process: first, a network of dense filaments form due to large-scale turbulence and then fragmentation into cores occurs as gravity starts to dominate. In the dense gas the structure formation is affected by motions induced primarily by three processes: supersonic turbulence, self-gravity, and magnetic fields, although the role of each process is still debated.

Recent research suggests that the statistical properties of the column density (for example, the numbers of cores denser than a fixed value) offer a key to unraveling structure formation mechanisms. Computer simulations of star formation show that if the number of dense cores having any particular density value is random, then turbulence is probably dominant, but if denser cores tend to cluster non-randomly, then gravity is probably dominant.

CfA astronomers Scott Wolk and Phil Myers and their colleagues have analyzed the Herschel Space Observatory's five wavelength far infrared images of the giant molecular cloud Mon R2, looking for evidence of non-randomness in core formation. The far infrared images can be combined to map the dust densities (or more precisely, the dust column densities). The astronomers found that in lower density regions -- in this Mon R2 characterized by a clear density cut-off -- the distribution was indeed random, signaling the dominance of turbulence over most of the giant cloud. However, in the denser regions, self-gravity appears to be predominant. Moreover, the deviation from randomness can be quantified, and the scientists found that the measure of the variation correlates with the numbers of young stars seen in the vicinity. They even discovered some instances when a second value of this measure was needed for the densest regions. Although relating these findings to more fundamental issues like the star formation efficiency will require additional research, the new paper demonstrates that far infrared imaging techniques can provide critical insights into early stages of star formation.

A Herschel-SPIRE Survey of the Mon R2 Giant Molecular Cloud:
Analysis of the Gas Column Density Probability Density Function - R. Pokhrel et al

• Monthly Notices of the RAS 461(1):22 (2016 Sep 01) DOI: 10.1093/mnras/stw1303
  arXiv.org > astro-ph > arXiv:1606.01752 > 06 Jun 2016

[bystander]

A Riddle for Our Time
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Sep 09

[img3="A schematic of cosmic history. Astronomers have called attention to a neglected riddle of the expansion: although the outward flaring seen in the graphic (representing cosmic acceleration) might have started at any time, curiously it has happened in our own time. (Credit: NASA/WMAP)"]https://www.cfa.harvard.edu/sites/www.c ... 201634.jpg[/img3][hr][/hr]

One of the most remarkable successes of astrophysics in the last century was its discovery that the age of the universe as measured by its oldest stars was about the same as the age estimated in an entirely different way, from the recession of galaxies. Both came up with surprisingly long times - billions of years - providing reassuring confirmation that both were probably on the right track. But the two values were not identical and scientists very quickly realized a major discrepancy: the oldest stars were older than the universe itself. Refinements to the measurements and the models to resolve this contradiction were underway until 1998 when cosmic acceleration was discovered. It proved, in a single sweep, that the universe was actually much older than had been thought, and in particular was older than the oldest stars.

But there was a riddle in the discovery: The motion of the universe is governed by matter, whose gravity tends to slow the expansion down, and by acceleration which speeds it up. Since the average density of matter in the universe steadily drops as the universe swells, in time it has a smaller and smaller value. Curiously, today it just happens to have almost exactly the same value (when expressed in the same units) as the acceleration parameter. Why? There was a second riddle too: The theoretical size of the acceleration parameter could be almost anything; indeed, basic quantum mechanical calculations suggest it should be vastly larger than it is. Why it is as small as we measure is a mystery.

CfA astronomers Arturo Avelino and Bob Kirshner have just published a paper calling attention to yet another riddle. The universe did not expand at a constant rate that was just the blend of these two factors. For the first nine billion years of cosmic evolution, contraction dominated and the universe gradually slowed its expansion. Since the relative importance of cosmic acceleration grows with time, however, for the past five billion years acceleration has dominated and the universe has sped up its expansion. Curiously, though, today the universe looks the same way it would have if it had always been expanding steadily at a constant rate (the rate required to prevent ultimate re-collapse). Although it sounds slightly similar to the original riddle, the authors describe why this new puzzle is actually different: We are living (apparently) in a privileged epoch; the other puzzles do not have this implication. The explanation(s) for these riddles is not yet known. If some specific new kinds of elementary particles exist, the scientists suggest, they could provide the answer, but for now the only thing that is certain is that more observational research is needed.

The Dimensionless Age of the Universe: A Riddle for Our Time - Arturo Avelino, Robert P. Kirshner

• Astrophysical Journal 828(1):35 (2016 Sep 01) DOI: 10.3847/0004-637X/828/1/35
  arXiv.org > astro-ph > arXiv:1607.00002 > 30 Jun 2016

[Ann]

Harvard-Smithsonian Center for Astrophysics wrote:
We are living (apparently) in a privileged epoch

Click to view full size image

Keukenhof Gardens, Netherlands.
Source: http://www.wovow.org/4-beautiful-places-earth-spring/

Indeed, we are living in a life-friendly, and possibly brief, epoch of the universe.

Similarly, we are living in an era of the history of the Earth that is good for us humans.

It is easy to think that the world around us and the sky above us is eternal and unchanging, but it is not so. Our time is now, both in the universe and on the Earth. It is up to us not to make our species' lease on life shorter than it has to be.

Ann

[bystander]

Migrating Exoplanets
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Sep 16

[img3="A photo of an array of exoplanet-hunting telescopes in the MEarth Project,
which emphasizes M-dwarf stars. Credit: The MEarth Project"]https://www.cfa.harvard.edu/sites/www.c ... 2/base.jpg[/img3][hr][/hr]

Many known exoplanets orbit close to their host star, within one-tenth of an astronomical unit (one AU is the average distance of the Earth from the Sun). Since their orbital periods are therefore very short and their gravitational influences on the host star's wobble comparatively large, they can be readily detected by the transit and velocity methods. What astronomers do not yet know is whether these planets formed near their present positions from natal circumstellar material close to their star, or if instead they formed at distances larger than an AU and subsequently migrated inwards.

There is solid evidence for the effectiveness of migration from computer simulations and observed planetary characteristics. But there are three possible physical mechanisms for migration and (if the planets did migrate) it is not known which mechanism was responsible. The mechanisms all involve planetary interactions: with the protoplanetary disk, with a stellar companion (like a binary), or among multiple planets. Each one imposes its own set of conditions, with different timescales that offer a possible way to discriminate between them. Simulations show, for example, that migrations due to interactions with another body typically take much longer than disk migration. Therefore, close-in super-Earths or Jupiter-size planets around stars younger than one hundred million years would not have had enough time to migrate inward by a slow process such as planet–planet or planet–star interaction.

CfA astronomers Elisabeth Newton, Jonathan Irwin, David Charbonneau, and Andrew Vanderburg and their team of colleagues have studied the close-in exoplanet K2-33b, a super-Neptune-sized object with a radius of 5.04 Earth-radii orbiting a young (only eleven million years old) star every 5.425 days. The team considered many alternative explanations to a transiting exoplanet for the light variations (like star spots) before concluding that the planet is real. Because the star is so young, they also conclude that close-in planets can form in-situ, or else they must be able to migrate in short times - a possibility with planet-disk interactions but not with the other two mechanisms. A precise interpretation of the transiting data also required knowing the star's mass and radius, which the astronomers carefully modeled, and the new result is also significant for being one of the most precise characterizations of a young star's radius (to within 7%) and mass (to within 16%).

Zodiacal Exoplanets in Time (ZEIT). III. A Short-Period Planet Orbiting
a Pre-Main-Sequence Star in the Upper Scorpius OB Association - Andrew W. Mann et al

• Astronomical Journal 152(3):61 (2016 Sep) DOI: 10.3847/0004-6256/152/3/61
  arXiv.org > astro-ph > arXiv:1604.06165 > 21 Apr 2016 (v1), 17 Jun 2016 (v2)

[bystander]

Precision Measurements of Exoplanet Velocities
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Sep 23

[img3="An artist's conception of Kepler 62-e, a super-Earth exoplanet. Astronomers working to detect super-Earths around the most common kind of stars, M dwarfs, have successfully tested infrared techniques that overcome some of the limitations of optical measurements. Credit: NASA/Ames/JPL-Caltech/Kepler mission"]http://www.jpl.nasa.gov/spaceimages/ima ... _hires.jpg[/img3][hr][/hr]

The search for exoplanets via the radial velocity technique has been underway for nearly thirty years, measuring the wobbles in a star's motion caused by the presence of orbiting bodies. The method has been very successful and has detected 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 radial velocity method has some powerful advantages, however, most notably that it can spot planets that do not pass across the face of the star ("transit"). The majority of radial velocity targets (so far) have been stars similar roughly to our Sun, but this neglects the majority of stars, the less massive class M dwarfs, which make up 75% of the stars in the solar neighborhood. Surveys of some nearby M dwarfs have been able to reach astonishing velocity precisions - as tiny as a few meters per second (4.5 miles per hour) -- adequate to detect a super-Earth orbiting in the star’s habitable zone (where surface water remains liquid). In order to detect an Earth-mass planet around a solar-type star, however, precisions twenty times better are needed.

One of the technical challenges in measuring radial velocities for M-dwarfs is that they are relatively faint in the optical. Near infrared techniques can ameliorate this issue because the stars are brighter in the infrared, but naturally face some other problems. CfA astronomers John Johnson and Dave Latham were part of a team of scientists working to advance infrared techniques for radial velocity studies of M-dwarfs. Using the current infrared instruments on NASA’s Infrared Telescope Facility in Hawaii, the astronomers were able to achieve about three meters per second precision on some test M stars, demonstrating that the technique and the methods used to process and analyze the data are reliable. There are next generation infrared instruments are in the pipeline, and the new paper demonstrates that they should be able to spot super-Earths and mini-Neptunes in the habitable zones of M dwarfs.

Retrieval of Precise Radial Velocities from Near-infrared High-resolution Spectra of Low-mass Stars - Peter Gao et al

• Publications of the Astronomical Society of Pacific 128(968):104501 (2016 Oct) DOI: 10.1088/1538-3873/128/968/104501
  arXiv.org > astro-ph > arXiv:1603.05997 > 18 Mar 2016

[bystander]

Millisecond Pulsars
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Sep 30

[img3="An artist's impression of a millisecond pulsar and its companion. The pulsar (seen in blue with two radiation beams) is accreting material from its bloated red companion star and increasing its rotation rate. Astronomers have measured the orbital parameters of four millisecond pulsars in the globular cluster 47 Tuc and modeled their possible formation and evolution paths. Credit: ESA/Francesco Ferraro (Bologna Astronomical Observatory)"]https://www.cfa.harvard.edu/sites/www.c ... 201638.jpg[/img3][hr][/hr]

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

Millisecond pulsars are ones that spin particularly rapidly, hundreds of times per second. Astronomers have concluded that these objects must be increasing their rotation rates through the accretion of material from a nearby companion star. There are nearly 3000 known millisecond pulsars. About five percent of them are found in globular clusters -- gravitationally bound, roughly spherical ensembles of stars containing as many as a million stars, with sizes as small as only tens of light-years in diameter. Their crowded environments provide ideal conditions for forming binary stars, and nearly eighty percent of the pulsars in globular clusters are millisecond pulsars. The globular cluster 47 Tucanae (47 Tuc) has twenty-five of them. ...

Long-term observations of the pulsars in 47 Tucanae –
I. A study of four elusive binary systems - A. Ridolfi et al

• Monthly Notices of the RAS 462(3):2918 (2016 Nov 01) DOI: 10.1093/mnras/stw1850
  arXiv.org > astro-ph > arXiv:1607.07248 > 25 Jul 2016

[bystander]

Preparing to Study the Epoch of Reionization
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Oct 07

[img3="Murchison Widefield Array"]https://upload.wikimedia.org/wikipedia/ ... T_Tile.jpg[/img3][hr][/hr]

The epoch when the very first stars appeared is a key period of cosmic history. These stars began the manufacture of the chemical elements (those heavier than hydrogen and helium) and their light began the reionization of the neutral cosmic gas. These stars thus mark the dawn of the universe as we know it today and the start of the so-called Epoch of Reionization. The term “reionization” refers to the process whereby these atoms are prompted (by the ultraviolet light from new stars) to shed some of their electrons. Astronomers estimate that this period occurred a few hundred million years or so after the big bang.

Neutral hydrogen atoms were the dominant element in the universe from the time they first arose, about 380,000 years after the big bang, until the Epoch of Reionization. Astronomers are now constructing facilities like the radio telescope Murchison Wide-field Array (MWA) to search for light from the hydrogen atoms at the dawn of this Epoch, a daunting task not only because the sources are so distant and faint, but also because there are so many other galaxies from much later cosmic times lying in the way and contaminating our lines-of-sight, as well as more local sources of contamination. ...

A High Reliability Survey of Discrete Epoch of Reionization Foreground Sources in the MWA EoR0 Field - P. A. Carroll et al

• Monthly Notices of the RAS 461(4):4151 (2016 Oct 01) DOI: 10.1093/mnras/stw1599
  arXiv.org > astro-ph > arXiv:1607.03861 > 13 Jul 2016

[bystander]

A Novel Approach to Studying a Star's Environment from Light Curves
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Oct 14

[img3="A plot of the light intensity versus time (light curve) for the star EPIC 204278916. The star was observed for over 78.8 days with a 29.4 minute cadence. The first 15 days are plotted here, and show the light changing in intensity by over a factor of two. Astronomers used the ALMA telescopes to discover a circumstellar disk around the star, and offer several explanations for the dips including that they are caused by clumps of material in the warped inner portion of the disk.
(Credit: S. Scaringi et al, 2016, MNRAS)"]https://www.cfa.harvard.edu/sites/www.c ... 201640.jpg[/img3][hr][/hr]

There are now about 2700 confirmed exoplanets discovered by transiting techniques including those used with the Kepler satellite. The transit method observes the stellar light curve (the flux versus time), and spots the slight dimming of starlight when an exoplanet crosses the face of the star as seen from Earth. The orbital period of the exoplanet can be determined from multiple transits, with the duration and details of the transit dip providing a measure of the planet's size. Light curves actually monitor a number physical processes including some at the stellar surface (like sunspots) as well as in their surroundings. Periodic variability of the observed stellar flux has long been used to measure stellar rotation periods, for example; transiting exoplanets produce a characteristic shape in the light curve.

Variability in young stellar objects (YSOs) is also related to the presence of a protoplanetary disc. Material in the disc can intermittently obscure the central star, while the highly variable accretion on to the central star can also modify the emission from the system. The variability of YSOs is known to show different kinds of periodicities and symmetries that are possibly related to different environmental processes, from comets orbiting around the stars to variations in circumstellar material with occulting material located at the inner edges of a disc viewed edge-on. ...

The Peculiar Dipping Events in the Disk-Bearing Young-Stellar Object EPIC 204278916 - S. Scaringi et al

• Monthly Notices of the RAS 463(2):2265 (2016 Aug 26) DOI: 10.1093/mnras/stw2155
  arXiv.org > astro-ph > arXiv:1608.07291 > 25 Aug 2016

[bystander]

Hotspots in an Active Galactic Nucleus
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Oct 21

[img3="An image taken at radio wavelengths of the dramatic jets of charged particles being ejected from the nucleus of the galaxy Cygnus-A. Newly obtained radio images were able to resolve hotspots in the jets at the places where they impact the surrounding medium. The conventional thinking is that the bulk of the radiation in such hotspots is produced by shocks, but the new results found that some other processes, perhaps absorption, must be involved. (Credit: NRAO/AUI)"]https://www.cfa.harvard.edu/sites/www.c ... 201641.jpg[/img3][hr][/hr]

The nucleus of a so-called "active" galaxy contains a massive black hole that is vigorously accreting material. As a result, the nucleus often ejects bipolar jets of rapidly moving charged particles that radiate brightly at many wavelengths, in particular radio wavelengths. Active galaxies display a range of dramatically different properties, and the ones that are bright in the radio can beam as much as one trillion solar luminosities of radiation into space at those wavelengths.

The intense emission arises from the hot environment of the black hole because electrons, moving at close to the speed of light in an environment of strong magnetic fields, radiate in the radio. The directed particle jets eventually collide with the ambient medium and convert much of their bulk energy of motion into shocks. The points of termination in the jet flow are seen as very hot spots, bright and compact structures. The hotspots can reverse the flow the jets back towards the black hole, and thereby generate additional turbulence and random motions. The characteristic temperature of a hot spot (or more accurately, the spectral dependence of the brightness versus wavelength) reveals the nature of the physical processes at work. Most known active radio galaxies have hotspots whose spectral dependence conforms well with the idea of termination shocks and reverse flows, but some very luminous radio galaxies do not conform. ...

LOFAR Imaging of Cygnus A – Direct Detection of a Turnover in the Hotspot Radio Spectra - J. P. McKean et al

• Monthly Notices of the RAS 463(3):3143 (2016 Dec 11) DOI: 10.1093/mnras/stw2105

[bystander]

Hypervariable Galactic Nuclei
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Oct 28

[img3="A photo of the PanSTARRS (Panoramic Survey Telescope & Rapid Response System) telescope in Hawaii. Astronomers have used a sky survey from this facility to identify a class of blue, hypervariable galaxies; the origin of the variability is uncertain but might in some cases be due to microlensing. (Credit: PanSTARRS)"]https://www.cfa.harvard.edu/sites/www.c ... 201642.jpg[/img3][hr][/hr]

Extreme variability in the intensity of the optical light of galaxies, by factors of two or more, is of great interest to astronomers. It can flag the presence of rare types of supernovae, for example, or spot sudden accretion activity around quiescent black holes or around the supermassive black hole at the galaxy’s nucleus. In recent years systematic searches for such variability have been made using instruments that can survey wide swaths of the sky. One, the Panoramic Survey Telescope & Rapid Response System (PanSTARRS), is a facility capable very wide-field imaging using a combination of relatively small mirrors coupled with very large digital cameras, and it can observe the entire sky accessible to it several times a month.

CfA astronomer Martin Elvis was part of a team of scientists that looked for variability in galaxies by comparing PanSTARRS images of the sky with images taken by an earlier survey, the Sloan Digital Sky Survey, about ten years before; the results were followed up with several other telescopes. Their comparison spanned nearly one-third of the whole sky. After sifting through thousands of apparent transients per month to check, among other things, for accurate spatial coincidences, that the candidates were galaxies, and that multiple observations confirmed the variability, the team reports finding seventy-six reliable objects. Spectroscopic followups and other observations were then able to classify these into nine categories, including supernovae and radio-emitting galaxies. In the end, the team found fifteen hypervariable sources that have brightened by almost a factor of ten in the past decade; light from the most distant one of these has been traveling for about nine billion years. The galaxies' light is blue in color and has been steadily changing, usually getting weaker. ...

Slow-Blue Nuclear Hypervariables in PanSTARRS-1 - A. Lawrence et al

• Monthly Notices of the RAS 463(1):296 (2016) DOI: 10.1093/mnras/stw1963
  arXiv.org > astro-ph > arXiv:1605.07842 > 25 May 2016 (v1), 05 Aug 2016 (v2)

[bystander]

Pulsar Wind Nebulae
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Nov 04

[img3="The Crab Nebula seen in the optical by Hubble. The Crab is an example of a pulsar wind nebula. Astronomers have modeled the detailed shape of another pulsar wind nebula to conclude, among other things, that the pulsar’s spin axis is pointed almost directly towards us. Credit: NASA, ESA, J. Hester and A. Loll (ASU)"]http://imgsrc.hubblesite.org/hu/db/imag ... ge_web.jpg[/img3][hr][/hr]

Neutron stars are the detritus of supernova explosions, with masses between one and several suns and diameters only tens of kilometers across. A pulsar is a spinning neutron star with a strong magnetic field; charged particles in the field radiate in a lighthouse-like beam that can sweep past the Earth with extreme regularity every few seconds or less. A pulsar also has a wind, and charged particles, sometimes accelerated to near the speed of light, form a nebula around the pulsar: a pulsar wind nebula. The particles’ high energies make them strong X-ray emitters, and the nebulae can be seen and studied with X-ray observatories. The most famous example of a pulsar wind nebula is the beautiful and dramatic Crab Nebula.

When a pulsar moves through the interstellar medium, the nebula can develop a bow-shaped shock. Most of the wind particles are confined to a direction opposite to that of the pulsar's motion and form a tail of nebulosity. Recent X-ray and radio observations of fast-moving pulsars confirm the existence of the bright, extended tails as well as compact nebulosity near the pulsars. The length of an X-ray tail can significantly exceed the size of the compact nebula, extending several light-years or more behind the pulsar. ...

The scientists used deep Chandra observations to examine the nebula’s faint emission structures, and found that the shape of the nebula, when compared to the direction of the pulsar’s motion through the medium, suggests that the spin axis of the pulsar is pointed nearly directly towards us. They also estimate many of the basic parameters of the nebula including the strength of its magnetic field, which is lower than expected (or else turbulence is re-accelerating the particles and modifying the field). Other conclusions include properties of the compact core and details of the physical mechanisms powering the X-ray and radio radiation.

Deep Chandra Observations of the Pulsar Wind Nebula Created by PSR B0355+54 - Noel Klingler et al

• arXiv.org > astro-ph > arXiv:1610.06167 > 19 Oct 2016

[bystander]

A Hydrogen Rich, Passive Galaxy
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Nov 11

[img3="A deep optical image of the gas-rich galaxy GASS 3505 which in at radio wavelengths shows a ring of neutral hydrogen gas, probably a result of accretion (there is a faint streamer seen to the left in this image). Astronomers conclude that the star formation in this object is very weak, less than about 0.1 solar-masses per year.
Credit: K.Gereb et al. 2016 MNRAS"]https://www.cfa.harvard.edu/sites/www.c ... 201644.jpg[/img3][hr][/hr]

Cold gas in the form of neutral hydrogen atoms provides the reservoir for star formation in galaxies from the distant to the nearby Universe. Understanding how it accretes onto galaxies is of crucial importance because fresh supplies of gas fuel the ongoing star-forming. In the most popular version, accretion onto the galaxy occurs along cosmic filaments, and at least in more massive galaxies is heated by shocks in the process; in smaller galaxies the infalling material stays relatively cool. Since galaxies in the early universe are smaller, it is thought that this cold process of growth is more typical for them as well.

Astronomers studying accretion need to look at nearby galaxies both because they are brighter and because they have distinguishable spatial features such as tails, bridges, ringlike structures, warped discs, or lopsidedness that could result from accumulating gas. The GALEX Arecibo SDSS Survey (GASS) is a multi-wavelength, deep survey designed specifically to search for galaxies rich in atomic hydrogen. CfA astronomer Sean Moran and five colleagues searched GASS to select one object, GASS 3505, that has nearly ten billion solar-masses of atomic hydrogen and a round, relatively unstructured appearance in the optical. The team followed up with deep radio maps of the hydrogen emission using the Jansky Very Large Array.

The astronomers found that the cold gas is distributed in a ring around the galaxy about one hundred and sixty thousand light-years in diameter, within which extremely inefficient star formation is happening (about ten times less than the Milky Way’s value). The ring, it turns out, is connected to a complex stream of material that is a signature for infall and accretion; the stream is a reminder of how important faint morphological features are in understanding a galaxy’s evolution. ...

GASS 3505: The Prototype of HI-Excess, Passive Galaxies - K. Gereb et al

• Monthly Notices of the RAS 462(1):382 (2016 Oct 11) DOI: 10.1093/mnras/stw1675
  arXiv.org > astro-ph > arXiv:1607.01446 > 06 Jul 2016

[bystander]

Forming Stars in the Early Universe
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Nov 18

[img3="A Hubble image of a field of distant galaxies. A new study of the gas content in galaxies so distant their light has been traveling for about ten billion years suggests that the processes converting gas into stars is about the same back then as in the local universe. Credit: NASA, ESA, G. Illingworth (UCO/Lick, UCSC), R. Bouwens (UCO/Lick, Leiden U.), and the HUDF09 Team"]https://www.cfa.harvard.edu/sites/www.c ... 201645.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. When the universe was about three 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. Since stars are made from gas, the gas content of galaxies is a measure of their star formation potential and (at least in the local universe) the fraction of matter in form of gas, the "gas fraction", is a measure of the star formation capability.

Gas in galaxies is depleted as new stars are formed and as some of it is blown out of the system by supernovae or by winds; gas can also be added by infall from the intergalactic medium. These processes are roughly understood in the local universe, mostly because the galaxies are bright and close enough to be studied in detail. For galaxies in the peak epoch of star formation, the evolution of the gas fraction is much less well constrained. Measuring the gas content is often done with observations of carbon monoxide, an abundant gas molecule, but in the early universe it is difficult to do because the distances make the lines faint, while the cosmic redshift pushes the usual diagnostic transitions to wavelengths that are beyond the capability of current facilities. ...

Gas Fraction and Depletion Time of Massive Star Forming Galaxies at z~3.2:
No Change in Global Star Formation Process out to z>3 - E. Schinnerer et al

• arXiv.org > astro-ph > arXiv:1610.03656 > 12 Oct 2016

[bystander]

Near Earth Objects
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
Weekly Science Update | 2016 Nov 25

[img3="An artist's conception of an asteroid recovery mechanism. Astronomers have been characterizing potential NASA asteroid targets using the IRAC camera on the Spitzer Space Telescope. (Credit: NASA)"]https://www.cfa.harvard.edu/sites/www.c ... 201646.jpg[/img3][hr][/hr]

Near Earth Objects (NEOs) are small solar system bodies whose orbits sometimes bring them close to the Earth, thereby posing a potential threat. Because NEOs are constantly being replenished from the solar system, they are tracers of the composition, dynamics and environmental conditions throughout the solar system, and of the history of our planetary system. NEOs are the parent bodies of meteorites, one of our key sources of detailed knowledge about the solar system’s development. NEOs are also potential targets for NASA missions. They are easier to reach with spacecraft than the moon, and NEOs offer a large number of targets with a wide range of physical properties and histories. NASA’s exploration plans for the next decade and beyond include an Asteroid Retrieval Mission.

While it is relatively easy to detect an NEO in visible light by watching its movement across the sky from night to night, determining its size and its potential hazard is more difficult because its optical brightness results from both its size and its reflectivity (albedo). The Spitzer Space Telescope’s infrared camera, IRAC, is a powerful NEO characterization system because NEOs typically have daytime temperatures around room temperature and their emitted radiation in the infrared is almost always significantly larger than their reflected radiation. Thermal models of the radiation can then be used to derive NEO properties, in particular the sizes and albedos. ...

NEOSurvey 1: Initial Results from the Warm Spitzer Exploration
Science Survey of Near Earth Object Properties - David E. Trilling et al

• Astronomical Journal 152(6):172 (2016 Dec) DOI: 10.3847/0004-6256/152/6/172
  arXiv.org > astro-ph > arXiv:1608.03673 > 12 Aug 2016