SAO: Science Updates 2019

Find out the latest thinking about our universe.
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The Low Density of Some Exoplanets is Confirmed

Post by bystander » Mon Jun 24, 2019 7:05 pm

The Low Density of Some Exoplanets is Confirmed
SAO Science Updates | 2019 Jun 21
The Kepler mission and its extension, called K2, discovered thousands of exoplanets. It detected them using the transit technique, measuring the dip in light intensity whenever an orbiting planet moved across the face of its host star as viewed from Earth. Transits can not only measure the orbital period, they often can determine the size of the exoplanet from the detailed depth and shape of its transit curve and the host star’s properties. The transit method, however, does not measure the mass of the planet. The radial velocity method, by contrast, which measures the wobble of a host star under the gravitational pull of an orbiting exoplanet, allows for the measurement of its mass. Knowing a planet's radius and mass allows for the determination of its average density, and hence clues to its composition.

About fifteen years ago, CfA astronomers and others realized that in planetary systems with multiple planets, the periodic gravitational tug of one planet on another will alter their orbital parameters. Although the transit method cannot directly measure exoplanet masses, it can detect these orbital variations and these can be modeled to infer masses. Kepler has identified hundreds of exoplanet systems with transit-timing variations, and dozens have been successfully modeled. Surprisingly, this procedure seemed to find a prevalence of exoplanets with very low densities. The Kepler-9 system, for example, appears to have two planets with densities respectively of 0.42 and 0.31 grams per cubic centimeter. (For comparison, the rocky Earth’s average density is 5.51 grams per cubic centimeter, water is, by definition, 1.0 grams per cubic centimeter, and the gas giant Saturn is 0.69 grams per cubic centimeter.) The striking results cast some doubt on one or more parts of the transit timing variation methodology and created a long-standing concern.

CfA astronomers David Charbonneau, David Latham, Mercedes Lopez-Morales, and David Phillips, and their colleagues tested the reliability of the method by measuring the densities of the Kepler-9 planets using the radial velocity method, its two Saturn-like planets being among a small group of exoplanets whose masses can be measured (if just barely) with either technique. They used the HARPS-N spectrometer on the Telescopio Nazionale Galileo in La Palma in sixteen observing epochs; HARPS-N can typically measure velocity variations with an error as tiny as about twenty miles an hour. Their results confirm the very low densities obtained by the transit-timing method, and verify the power of the transit-variation method.

HARPS-N Radial Velocities Confirm the Low Densities of the Kepler-9 Planets ~ L. Borsato et al
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How Shiny are Near Earth Objects?

Post by bystander » Sun Jul 14, 2019 9:31 pm

How Shiny are Near Earth Objects?
SAO Science Updates | 2019 Jun 28
Near Earth Objects (NEOs) are small solar system bodies whose orbits sometimes bring them close to the Earth. NEOs are consequently potential collision threats, but scientists are also interested in them because they offer keys to the composition, dynamics and environmental conditions of solar system and its evolution. Most meteorites for example, one of the key sources of knowledge about the early solar system, come from NEOs. The large majority NEOs were discovered in optical searches, and today the total number of known NEOs exceeds 20,000. The crucial NEO parameter of interest for most problems, including the possible dangers from an impact, is the size, but unfortunately optical detections usually cannot determine size. This is because an NEO's optical light is reflected sunlight, and the object could be bright either because it is large or because it has a high reflectivity (albedo).

CfA astronomers Joe Hora, Howard Smith, and Giovanni Fazio helped lead the team that was the first to undertake the systematic measurement of NEO sizes using their infrared brightnesses. An NEO's infrared signal is the result of its thermal emission, and that provides an independent measure of its size. The team used Spitzer IRAC infrared observations of NEOs together with optical data and their sophisticated thermal model to break the size/albedo degeneracy and determine the sizes of NEOs. (The NASA WISE mission and its NEOWISE team subsequently also undertook infrared size determinations.) So far, infrared measurements have been made on over 3000 NEOs, the vast majority of them using IRAC. The smallest NEO characterized this way, so far, is only about twelve meters in diameter (with about a 20% uncertainty). But strangely, the results also suggest an abundance of high-albedo objects, nearly eight times more than had been expected based on the current thinking about the population distribution.

The scientists had previously analyzed and published the variations of NEO brightness that resulted as their non-spherical bodies rotated in space (their light-curves). They wondered whether the large apparent excess of high-albedo objects was the result of an inadequate correction for light-curve variations. They performed a statistical analysis using Monte-Carlo simulations to estimate what might be expected of a population of rotating, non-spherical NEOs. They conclude that while light-curve variations could indeed be the cause of the large high-albedo excess, the excess is also consistent with a real – and still unexplained – overabundance of shiny objects. They also concluded that whatever the explanation, it is unlikely that NEOs have albedos exceeding 50%. Additional observations of full NEO light-curves are need to resolve the uncertainties.

Spitzer Albedos of Near-Earth Objects ~ Annika Gustafsson et al
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Powering the Extreme Jets of Active Galaxies

Post by bystander » Sun Jul 14, 2019 9:38 pm

Powering the Extreme Jets of Active Galaxies
SAO Science Updates | 2019 Jul 05
An active galaxy nucleus (AGN) contains a supermassive black hole that is vigorously accreting material. It typically ejects jets of particles that move at close to the speed of light, radiating across many wavelengths, in particular the X-ray, in processes are among the most energetic phenomena in the universe. The jets are often also highly collimated and extend far beyond their host galaxy, and if they happen to be pointed along our line of sight they are the most spectacular class of this phenomenon: blazars.

A few years ago astronomers noticed that some types of blazars have jet powers that appear to exceed the power provided by the accretion. Two ideas were put forward to explain the difference: the jets are also extracting power from the spin of the black hole or from the magnetic flux around the object. How either process happens – if indeed they do happen - is hotly debated, but one popular line of argument asserts that the processes are somehow related to the mass of the supermassive black hole, with the most massive cases (more than a hundred million solar-masses) being the most anomalous. Recently the Fermi Gamma-Ray Space Telescope detected gamma-rays (even more energetic photons than X-rays) coming from jets in a class of galaxies called Seyferts, spiral galaxies with relatively small supermassive black hole masses, typically about ten million solar-masses. Astronomers speculated that these relatively low-mass yet powerful emission engines might provide keys to sorting out the various sources of jet power.

CfA astronomer Mislav Balokovic and his colleagues completed a multi-wavelength study of the bright blazar-like Seyfert galaxy PKSJ1222+0413 and included data from the gamma-ray to the radio, both archival and new observations, including new results from the NuSTAR space observatory They then undertook a complete modeling of this source, the most distant one of its type known - its light has been traveling towards us for about eight billion years. They detected the pronounced signature of an accretion disk, and estimated the mass of the supermassive black hole from the widths and strengths of the emission lines to be about two hundred million solar-masses, about ten times higher than most other Seyferts of its type. The jet luminosity is only about half the accretion luminosity, unlike cases like galaxies whose jet power exceeds the accretion. But the object nonetheless clearly falls into a transition regime for jet strengths, enabling future studies to study in more detail the origins of jet power both Seyfert galaxies and in blazars.

The Relativistic Jet of the γ-ray Emitting Narrow-Line Seyfert 1 Galaxy PKS J1222+0413 ~ Daniel Kynoch et al
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Re: The Low Density of Some Exoplanets is Confirmed

Post by BDanielMayfield » Mon Jul 15, 2019 6:40 pm

bystander wrote:
Mon Jun 24, 2019 7:05 pm
The Low Density of Some Exoplanets is Confirmed
SAO Science Updates | 2019 Jun 21
The Kepler mission and its extension, called K2, discovered thousands of exoplanets. It detected them using the transit technique, measuring the dip in light intensity whenever an orbiting planet moved across the face of its host star as viewed from Earth. Transits can not only measure the orbital period, they often can determine the size of the exoplanet from the detailed depth and shape of its transit curve and the host star’s properties. The transit method, however, does not measure the mass of the planet. The radial velocity method, by contrast, which measures the wobble of a host star under the gravitational pull of an orbiting exoplanet, allows for the measurement of its mass. Knowing a planet's radius and mass allows for the determination of its average density, and hence clues to its composition.

About fifteen years ago, CfA astronomers and others realized that in planetary systems with multiple planets, the periodic gravitational tug of one planet on another will alter their orbital parameters. Although the transit method cannot directly measure exoplanet masses, it can detect these orbital variations and these can be modeled to infer masses. Kepler has identified hundreds of exoplanet systems with transit-timing variations, and dozens have been successfully modeled. Surprisingly, this procedure seemed to find a prevalence of exoplanets with very low densities. The Kepler-9 system, for example, appears to have two planets with densities respectively of 0.42 and 0.31 grams per cubic centimeter. (For comparison, the rocky Earth’s average density is 5.51 grams per cubic centimeter, water is, by definition, 1.0 grams per cubic centimeter, and the gas giant Saturn is 0.69 grams per cubic centimeter.) The striking results cast some doubt on one or more parts of the transit timing variation methodology and created a long-standing concern.

CfA astronomers David Charbonneau, David Latham, Mercedes Lopez-Morales, and David Phillips, and their colleagues tested the reliability of the method by measuring the densities of the Kepler-9 planets using the radial velocity method, its two Saturn-like planets being among a small group of exoplanets whose masses can be measured (if just barely) with either technique. They used the HARPS-N spectrometer on the Telescopio Nazionale Galileo in La Palma in sixteen observing epochs; HARPS-N can typically measure velocity variations with an error as tiny as about twenty miles an hour. Their results confirm the very low densities obtained by the transit-timing method, and verify the power of the transit-variation method.

HARPS-N Radial Velocities Confirm the Low Densities of the Kepler-9 Planets ~ L. Borsato et al
Nice confirmation of transit timing variation as a planetary density estimation tool.

As so many of the Kepler planets are in star hugging orbits, it shouldn't be all that surprising that many of them have low density. They are inflated by heat from the star they orbit.

Bruce
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Re: How Shiny are Near Earth Objects?

Post by BDanielMayfield » Mon Jul 15, 2019 6:59 pm

bystander wrote:
Sun Jul 14, 2019 9:31 pm
How Shiny are Near Earth Objects?
SAO Science Updates | 2019 Jun 28
Near Earth Objects (NEOs) are small solar system bodies whose orbits sometimes bring them close to the Earth. NEOs are consequently potential collision threats, but scientists are also interested in them because they offer keys to the composition, dynamics and environmental conditions of solar system and its evolution. Most meteorites for example, one of the key sources of knowledge about the early solar system, come from NEOs. The large majority NEOs were discovered in optical searches, and today the total number of known NEOs exceeds 20,000. The crucial NEO parameter of interest for most problems, including the possible dangers from an impact, is the size, but unfortunately optical detections usually cannot determine size. This is because an NEO's optical light is reflected sunlight, and the object could be bright either because it is large or because it has a high reflectivity (albedo).

CfA astronomers Joe Hora, Howard Smith, and Giovanni Fazio helped lead the team that was the first to undertake the systematic measurement of NEO sizes using their infrared brightnesses. An NEO's infrared signal is the result of its thermal emission, and that provides an independent measure of its size. The team used Spitzer IRAC infrared observations of NEOs together with optical data and their sophisticated thermal model to break the size/albedo degeneracy and determine the sizes of NEOs. (The NASA WISE mission and its NEOWISE team subsequently also undertook infrared size determinations.) So far, infrared measurements have been made on over 3000 NEOs, the vast majority of them using IRAC. The smallest NEO characterized this way, so far, is only about twelve meters in diameter (with about a 20% uncertainty). But strangely, the results also suggest an abundance of high-albedo objects, nearly eight times more than had been expected based on the current thinking about the population distribution.

The scientists had previously analyzed and published the variations of NEO brightness that resulted as their non-spherical bodies rotated in space (their light-curves). They wondered whether the large apparent excess of high-albedo objects was the result of an inadequate correction for light-curve variations. They performed a statistical analysis using Monte-Carlo simulations to estimate what might be expected of a population of rotating, non-spherical NEOs. They conclude that while light-curve variations could indeed be the cause of the large high-albedo excess, the excess is also consistent with a real – and still unexplained – overabundance of shiny objects. They also concluded that whatever the explanation, it is unlikely that NEOs have albedos exceeding 50%. Additional observations of full NEO light-curves are need to resolve the uncertainties.

Spitzer Albedos of Near-Earth Objects ~ Annika Gustafsson et al
Don't worry, be happy. (Really, no sarcasm intended.) This study shows that the threat from a NEO asteroid is considerably less than what has been advertised. Since they are on average brighter than expected they would also be less massive than expected as well.

I still hope we develop impact warning and avoidance methods of course, but this isn't the dire threat it is played up to be in the popular press.

Bruce
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Tracer Galaxies Probe the Cosmic Background

Post by bystander » Fri Aug 16, 2019 4:16 pm

Tracer Galaxies Probe the Cosmic Background
SAO Science Updates | 2019 Jul 12
The universe, perhaps surprisingly, is not comprised of galaxies randomly distributed throughout space; that is, it is not very homogeneous. Instead, its galaxies are clustered into distinct structures, typically gigantic filaments separated by vast voids -- the "large-scale structure (LSS)," an architecture whose discovery and mappings were pioneered by CfA astronomers about thirty years ago. Astronomers since have combined LSS maps with results from the cosmic microwave background radiation (CMBR) and ideas about the inflationary big bang to assemble a remarkably consistent picture of the universe, its origins and its evolution.

Mysteries remain, for example dark matter, which is also expected to gather in large-scale structures. CfA astronomers David James and Tony Stark were members of a large international team that used photons from galaxies in the early universe ("tracer galaxies") to probe the LSS in more detail. As these photons traverse the universe on their way to us, their paths are perturbed by the gravitational influences of the LSS, including in particular the effects of gravitational lensing. The apparent placements of young galaxies as projected on the sky and their statistical distributions are sensitive both to the current and the evolving geometry and structure of matter in the universe.

The astronomers recognized that although the details of the projected mass distribution are extremely complex, using the ratios of some parameters could obviate some uncertainties, enabling them to obtain important constraints on the current models of cosmic evolution. The team combined observations from the Dark Energy Survey (an optical survey that has mapped millions of galaxies), the South Pole Telescope (a submillimeter-wave facility studying the CMBR and early galaxies), and the Planck mission (a far infrared and millimeter survey spacecraft). One particularly valuable advantage of this approach is that it does not require knowing the distances to the tracer galaxies (distances would require their being able to measure the faint spectroscopic redshifts). The scientists were able to obtain constraints with a precision of about ten percent on some of the detailed parameters of current cosmological models, and they forecast that with further research these techniques will even enable them to constrain some of the essential features of dark matter, like its equatrion of state, and properties that have so-far remained elusive.

Cosmological Lensing Ratios with DES Y1, SPT, and Planck ~ DES Collaboration, SPT Collaboration, J. Prat et al
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Modeling Exoplanet Atmospheres

Post by bystander » Fri Aug 16, 2019 4:22 pm

Modeling Exoplanet Atmospheres
SAO Science Updates | 2019 Jul 19
All atoms and molecules emit distinctive spectral lines across the spectrum, the details of which depend on the internal structures of the species (for example, the vibration and rotation properties of molecules) and how they are excited by their environments. Measurements of the features' brightnesses, relative intensities, and shapes enable astronomers, at least in principle, to reconstruct most of the essential properties of these environments, including species abundances, temperatures, densities, and motions. But in order to be successful, scientists need to know quantitatively exactly how the temperature, density, and so forth, affect the excitation of each atom or molecule, and then how each species emits light in response. A collision between oxygen and nitrogen molecules, for example, will affect an oxygen molecule differently than its collision with hydrogen.

CfA astronomers develop and maintain the HITRAN (High Resolution Transmission) database, a compilation of diagnostic spectroscopic parameters that is the worldwide standard for calculating atmospheric molecular radiation from the microwave through the ultraviolet region of the spectrum. HITRAN has acquired particular new importance in recent years with the discovery of thousands of exoplanets and the steadily improving technology to detect their atmospheres and measure their compositions. HITRAN is commonly used to model these exotic atmospheres. Molecular oxygen absorption stimulated by collisions between oxygen molecules is thought, for example, to be an important biomarker on potentially habitable exoplanets, but the detection of this absorption feature is not enough: it needs an interpretation.

CfA astrophysicists Tijs Karman, Iouli Gordon, Bob Kurucz, Larry Rothman, and Kang Sun led a team of colleagues in updating HITRAN with many of the essential collision-induced absorption properties of the molecules needed for modeling exoplanet atmospheres. Key molecular species include nitrogen, oxygen, methane, carbon dioxide, and hydrogen. The numerical parameters were gleaned from a wide collection of recent laboratory and theoretical papers and incorporated into the HITRAN database after being validated. The updated compilation goes a long way towards addressing the current needs, but the authors note that additional laboratory and theoretical work is needed to include other effects, water for example, as well as the isotopic variations of the currently included species.

Update of the HITRAN Collision-Induced Absorption Section ~ Tijs Karman et al
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Imaging a Young, Planet-Forming Disk

Post by bystander » Fri Aug 16, 2019 4:43 pm

Imaging a Young, Planet-Forming Disk
SAO Science Updates | 2019 Jul 26
Protoplanetary disks are a natural product of the star formation process. As material in a pre-stellar condensation collapses to form the central star, angular momentum conservation prompts it to develop a circumstellar disk. Planets form out of the material in these disks, whose structure and evolution are therefore keys to understanding the planet building process. Two main scenarios dominate in current theories. In the core accretion model, planets assemble through the aggregation of the dust grains, eventually forming planetesimals followed by a balance between their growth and fragmentation as they collide. In the alternative scenario, gravitational instabilities develop during the initial stages of the disk evolution and the associated density perturbations grow until they form into planetesimals.

In either of these two scenarios, young massive planets are expected to imprint their signatures on the structure of their parent disks, carving out cavities, gaps or other asymmetries which should be detectable. Infrared observations of young stars have uncovered a set whose emission seems to lack a contribution from warm dust. The implication is that the hottest dust closest to the star is absent, perhaps because a cavity has been carved out by an unseen, orbiting planet. Checking these ideas, and identifying examples of young stars in the midst of their planet birthing process, are key goals of modern exoplanetary research.

CfA astronomers Sean Andrews and David Wilner were members of a team that used the ALMA millimeter array to image the disk around CQ Tau, a young star about 530 light-years away that is known from infrared observations to have a cavity in its massive circumstellar disk, estimated to contain about .03 solar-masses of material. The dramatic new images have a spatial resolution of about twenty-four astronomical units (AU), more than enough to resolve the roughly face-on disk which is over 175 AU across (in our solar system, Pluto’s farthest passage is about fifty AU from the Sun). The ALMA images clearly reveal some details of a cavity in both the gas and dust components which the astronomers find to be between about 25 and 40 AU in radius. The team concludes that a massive planet roughly eight Jupiter-masses in size located at twenty AU can produce some (though not all) of the cavity's dimensions, and additional observations and modeling are needed to refine the picture. The new results, coupled with other ALMA observations by the team, offer ground-breaking details about the early stages of planet formation.

A Dust and Gas Cavity in the Disc around CQ Tau Revealed by ALMA ~ M. Giulia Ubeira Gabellini et al
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Looking for Warm Dark Matter

Post by bystander » Fri Aug 16, 2019 5:50 pm

Looking for Warm Dark Matter
SAO Science Updates | 2019 Aug 02
su201930-ls.jpg
Two simulations of galaxy formation at the epoch when the universe was only about
one billion years old. The left (CDM) shows clumps and filaments of young galaxies
using a conventional treatment of non-interacting dark matter, while the right (sDAO)
shows the slightly different - but measurable - differences that occur if dark matter
instead could interact with some particles. Astronomers show that future precise
measurements of large-scale galaxy structures could help constrain the nature of
the mysterious dark matter in the universe. Credit: Bose et al. 2019 MNRAS

In the last century, astronomers studying the motions of galaxies and the character of the cosmic microwave background radiation came to realize that most of the matter in the universe was not visible. About 84% of the matter in the cosmos is dark, emitting neither light nor any other known kind of radiation. Hence it is called dark matter. One of its other primary qualities is that it only interacts with other matter via gravity: it carries no electromagnetic charge, for example. Dark matter is also "dark" because it is mysterious: it is not composed of atoms or their usual constituents like electrons and protons. Particle physicists have imagined new kinds of matter, consistent with the known laws of the universe, but so far none has been detected or its existence confirmed. The Large Hadron Collider's discovery of the Higgs boson in 2012 prompted a burst of optimism that dark matter particles would soon be discovered, but so far none has been seen and previously promising classes of particles now seem to be long-shots.

Astronomers realize that dark matter is the dominant component of matter in the universe. Whatever its nature, it profoundly influenced the evolution of galactic structures and the distribution of the cosmic microwave background radiation (CMBR). The remarkable agreement between the values of key cosmic parameters (like the rate of expansion) derived from observations of two completely different kinds of large-scale cosmic structures, galaxies and the CMBR. lend credence to inflationary big bang models that include the role dark matter.

Current models of dark matter presume it is "cold," that is, that it does not interact with any other kinds of matter or radiation - or even with itself – beyond the influences of gravity. This version of cosmology is therefore called the cold dark matter scenario. But cosmologists wonder whether more precise observations might be able to exclude even small levels of interactions. CfA astronomer Sownak Bose led a team of colleagues in a study of one very popular (if speculative) "dark matter" particle, one that has some ability to interact with very light particles that move close to the speed of light. This version forms one of several possible warm dark matter (perhaps more accurately called interacting dark matter) scenarios. In particular, the hypothetical particles are allowed to interact with neutrinos (neutrinos are expected to be extremely abundant in the hot early universe).

The scientists used state-of-the-art cosmological simulations of galaxy formation to a model universe with this kind of warm dark matter. They find that for many observations the effects are too small to be noticeable. However, the signature of this warm dark matter is present in some distinct ways, and in particular in the way distant galaxies are distributed in space, something that can be tested by mapping galaxies by looking at their hydrogen gas. The authors conclude that future, highly sensitive observations should be able to make these tests. Detailed new maps of the distribution of hydrogen gas absorption could be used to support -- or exclude -- this warm dark matter possibility (see the figure), and shed light on this mysterious cosmic component.

ETHOS – an Effective Theory of Structure Formation: Detecting
Dark Matter Interactions through the Lyman-α Forest
~ Sownak Bose et al
  • Monthly Notices of the RAS 487(1):522 (July 2019) DOI: 10.1093/mnras/stz1276
  • arXiv.org > astro-ph > arXiv:1811.10630 > 26 Nov 2018 (v1), 30 May 2019 (v2)
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Modeling a Core Collapse Supernova

Post by bystander » Fri Aug 16, 2019 6:16 pm

Modeling a Core Collapse Supernova
SAO Science Updates | 2019 Aug 09
su201931ls.jpg
Before and after – optical images of the field of stars around the core collapse
supernova ASASSN-15oz in the relatively nearby galaxy HIPASSJ1919-33.
Astronomers studying the photometric and spectral lightcurves have concluded
that the progenitor star was a red supergiant star with a substantial shell of
previously ejected material. Credit: Bostroem et al. 2019 MNRAS

Stars greater than eight solar-masses end their lives spectacularly -- as supernovae. These single-star supernovae are called core collapse supernovae because when their dense cores (at this stage composed primarily of iron) are no longer able to withstand the inward pressure of gravity they collapse inward before exploding. Core collapse supernovae with strong hydrogen emission lines are thought to result from the explosions of red supergiant stars, massive stars that have evolved beyond their principle hydrogen burning stage and grown in radius. Until recently, astronomers thought these stars were relatively quiescent until their final demise, but evidence has been accumulating that they actually experience strong mass loss before exploding. In some models, emission resulting when ejecta from the supernovae encounter these envelopes produces the observed variations in core collapse supernova.

CfA astronomer Griffin Hosseinzadeh was a member of a team of astronomers testing these ideas by studying the core collapse supernova ASASSN-15oz. He assisted in the multiband observations, which included X-ray, UV, optical, infrared, and radio measurements. ASASSN-15oz exploded almost exactly four years ago, around 31 August 2015, and is located in the relatively nearby galaxy HIPASSJ1919-33, about one hundred million light-years away. The astronomers were able to obtain spectra and photometric lightcurves of the object over a period of about 750 days. They successfully modeled the event as the explosion of a red supergiant star that had ejected material in a wind for most of its later evolution and underwent an extreme eruption just prior to its demise. They estimate that about 1.5 solar-masses of material was ejected in total. The new analysis is consistent with the idea that this class of core collapse supernova is indeed surrounded by a substantial circumstellar shell that was the result of eruptive mass loss from the red supergiant progenitor.

Signatures of Circumstellar Interaction in the Type IIL Supernova ASASSN-15oz ~ K. Azalee Bostroem et al
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Spotting Merging Galaxies

Post by bystander » Fri Aug 16, 2019 6:35 pm

Spotting Merging Galaxies
SAO Science Updates | 2019 Aug 16
su201932.jpg
A Hubble image of a suspected galaxy merger about seven hundred million
light-years away. Might it actually be a single spiral galaxy? A new paper
proposes an algorithm to decide. The method was developed with computer
training techniques applied to a million simulated merging galaxy images.
Credit: NASA/Hubble; Kim et al. 2013

Over thirty years ago, the Infrared Astronomy Satellite discovered that the universe contained many extremely luminous galaxies, some more than a thousand times brighter than our own Milky Way, but which are practically invisible at optical wavelengths. These galaxies are powered by bursts of star formation buried deep within clouds of dust and gas. The dust absorbs the ultraviolet light while radiating at infrared wavelengths. In many cases the hyperactivity was triggered by a collisional encounter between galaxies that facilitated the collapse of interstellar gas into new stars.

Collisions between galaxies are common. Indeed, most galaxies have probably been involved in one or more encounters during their lifetimes, making these interactions an important phase in galaxy evolution and the formation of stars in the universe. The Milky Way, for example, is bound by gravity to the Andromeda galaxy and is approaching it at a speed of about 50 kilometers per second; we are expected to meet in another billion years or so. In the local universe about five percent of galaxies are currently in a merger, and mergers usually can be easily identified by the visible morphological distortions they produce such as tidal tails sweeping out from the galactic discs.

Not all infrared luminous galaxies show such distortions, however, and the issue of identifying (and classifying) mergers becomes especially problematic for studies of earlier cosmic epochs when the star formation rates were much higher than today, and when the merger rate of galaxies was also higher. (Moreover, such systems are preferentially discovered in deep galaxy surveys precisely because they are so luminous.) But galaxies in the distant cosmos are too remote to detect spatial signatures like tidal arms (at least with current telescopes). It is possible that other processes besides merger-induced star formation are lighting up some of these bright galaxies, for example accreting supermassive black holes can emit copious amounts of ultraviolet radiation. Because of such cases, estimates of star formation in the early universe based on luminosity measurements alone could be incorrect.

CfA astronomer Lars Hernquist is a pioneer in the development of computer simulations of merging galaxies. Several years ago he and a team of colleagues produced a massive new simulation of the formation and evolution of galaxies in the universe, called Illustris. In a new paper based on Ilustris simulated images of merger galaxies, the astronomers present a way to help identify when imaged systems are mergers. They created about one million synthetic Hubble and James Webb Space Telescope images from their simulated mergers, and then looked for common morphological indicators of merging. They developed an algorithm that successfully identified mergers at roughly a seventy percent level of completeness out to distances of as much as eighty-five billion light-years (the current distance value), corresponding to light dating from the epoch about 2 billion years after the big bang. Results from the algorithm indicated that spatial features associated with strong central concentrations (or bulges) were most important for selecting past mergers, while double nuclei and asymmetries were most important for selecting future mergers (that is, sometime in the next 250 million years). The new algorithm will be particularly valuable when applied to future Webb images of very distant mergers.

Automated Distant Galaxy Merger Classifications from
Space Telescope Images using the Illustris Simulation
~ Gregory F. Snyder et al
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Optical Identification of Gamma-Ray Blazars

Post by bystander » Wed Sep 04, 2019 4:18 pm

Optical Identification of Gamma-Ray Blazars
SAO Science Updates | 2019 Aug 23
Blazars are galaxies whose central, supermassive black holes are accreting material from surrounding regions and emitting powerful beams of high velocity charged particles that coincidentally are pointed in our direction. The charged particles include electron that can produce gamma ray photons, with each photon packing over a hundred million times the energy of the highest energy X-ray photons seen by the Chandra X-ray Observatory. The electron jets in blazars also exhibit rapid and strong variability.

One of the main challenges in modern gamma-ray astronomy is the identification of the exact sources of strong gamma-ray emission seen by the Fermi Large Area Telescope. Blazars fall primarily into two classes distinguished on the basis of their optical spectra: those lacking prominent emission lines, and those with strong, broad lines. Pinpointing Fermi sources would enable followup observations to determine if the galaxy is a blazar or something else, and if it is a blazar, what type it is. Optical spectra could also measure the source’s redshift and thereby establish the absolute luminosity of the galaxy. The problem is that the location of a Fermi source in the sky is very uncertain, typically by about four arcminutes (about one-tenth the size of the full moon), and dozens of possible galaxies or more can lie in a region of this size. Of the over five thousand gamma-ray sources discovered by Fermi so far, nearly 30% have no obvious optical counterpart.

CfA astronomers Raffaele D'Abrusco and Howard Smith are members of a blazar team that realized that the infrared color of blazars as seen with the WISE (Wide-field Infrared Survey Explorer) filters are distinctive, and that WISE images of the Fermi field can therefore often spot candidate blazars for subsequent optical confirmation. The team recently released two catalogs of WISE-identified blazar candidates, and decided to search the Sloan Digital Sky Survey archive of spectra for matches. The astronomers examined 1830 Sloan galaxy spectra for blazar signatures, and found that about 30% of them are indeed blazars, with many other candidates being quasars. The team further analyzed the results to categorize the blazar types and evaluate the impact of selection effects. In the coming era of the James Webb Space telescope and very large ground-based telescope it should be possible to obtain diagnostic spectra of even fainter candidate galaxies.

Optical Characterization of WISE Selected Blazar Candidates ~ Raniere de Menezes et al
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Stellar Heartbeats

Post by bystander » Wed Sep 04, 2019 4:39 pm

Stellar Heartbeats
SAO Science Updates | 2019 Aug 30
The Kepler satellite is famous for its discovery of thousands of exoplanets by continuously and meticulously measuring the brightnesses of over half-a-million stars for the signatures of transiting exoplanets. Less well known are the revolutionary consequences of its monitoring program for stellar astrophysics, in particular for the study of stellar oscillations. Our own star, the Sun, has been known since the 1960’s to exhibit oscillations, which are analogous to a bell's ringing, as pressure waves generated by its rotation and internal structure circulate around its surface. The oscillations can be analyzed to reveal details of a star's internal structures. Red giant stars, which are in a phase of stellar evolution after normal hydrogen burning is completed and have swelled in diameter, have been of particular interest because the oscillations in their surfaces are slower and of larger amplitude than in smaller stars, and hence easier to measure. Before the advent of space telescopes, however, even such measurements on red giants succeeded on only a few objects. The Kepler and Corot missions have since measured oscillations in thousands of red giant stars.

The Kepler mission discovered a new class of binary star that showed evidence for tidal distortions in its surface (and its light curve) as the companion passed close by, and these systems were dubbed “heartbeat” stars. The morphology of the distortions can be used to help refine details of the binary orbit even when the companion star does not pass through our line-of-sight to eclipse the primary. Some red giant stars with solar-like oscillations are also heartbeat stars, and the combination, together with velocity measurements, allows these systems to be characterized in great detail.

The star KIC-3890 is such a binary system. It contains a red giant and an M-dwarf star in a highly eccentric, 153-day orbit, and Kepler observed it nearly continuously for nearly four years. CfA astronomers Allyson Bieryla and Dave Latham were members of a team that analyzed its heartbeat and oscillations with a new set of synthetic models. They determined that the mass of the red giant is 1.04 solar-masses to within about 6%, and its radius is 5.8 solar-radii to within about 3.4%, and that the companion is an M-dwarf with a mass of 0.23 solar-masses and a radius of 0.256 solar-radii with even smaller uncertainties. Since stellar oscillations evolve as a star ages, the oscillations allowed the astronomers to constrain the age of the red giant – and hence of the system itself – to about 9 billion years, with an uncertainty of about 25%.

KOI-3890: A high mass-ratio asteroseismic red-giant+M-dwarf
eclipsing binary undergoing heartbeat tidal interactions
~ James S. Kuszlewicz et al
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Photosynthesis on Habitable Planets around Low-Mass Stars

Post by bystander » Tue Sep 10, 2019 3:28 pm

Photosynthesis on Habitable Planets around Low-Mass Stars
SAO Science Updates | 2019 Sep 06
Life on Earth is dominated by photosynthesis, the process by which green plants and some organisms use sunlight at visible wavelengths to synthesize carbon-based nutrients from carbon dioxide gas and water. Photosynthesis arose relatively early in Earth's evolutionary history, perhaps as long ago as three billion years. One of the important products of the associated chemical reaction is the release of oxygen molecules to the air, with dramatic consequences for the major evolutionary developments that were thereby enabled. It is estimated that after photosynthesis took hold, roughly six hundred million years were needed to oxygenate the Earth's atmosphere. So far about 4100 exoplanets are known, with about fifty that lie within their host star’s habitable zone where temperatures could permit surface liquid water to exist. Might photosynthesis be possible on some of these planets - and could we detect atmospheric oxygen or other signs of its activity?

CfA astronomers Manasvi Lingham and Avi Loeb analyzed the properties of one class of stars, M-dwarfs smaller than about 0.2 solar-masses. Although these kinds of stars are not the most similar ones to the Sun they are abundant, as are their planets. The closest star to the Sun, for example, Proxima Centauri, has a mass of only 0.12 solar-masses and it has an orbiting planet, Proxima Centauri b, that lies in its habitable zone. The problem for life is that these low mass stars are much cooler and dimmer than the Sun. Proxima Cen has a surface temperature of only about 3000K as compared with Sun’s surface temperature of 5780K. The cooler and dimmer surface results in much less visible light being available for photosynthetic reactions. The astronomers analyzed these stars (they also considered stars up to 2.5 times the Sun’s mass) and calculated whether they might be able to sustain Earth-like biospheres or host oxygen in their atmospheres. They also considered whether additional effects in low mass stars like flaring might contribute usefully to the visible light budget.

The scientists conclude, in agreement with earlier studies, that planets in the habitable zones around low-mass stars may not receive enough visible light to support Earth-like biospheres. They find, in particular, that although stars more massive than 0.21 solar-masses do provide sufficient light, less massive stars like Proxima Centauri are unlikely to be able to sustain photosynthesis or to build up atmospheric oxygen. Finally, they conclude that radiation from flares is not likely to be of much help. They also caution that a null detection of atmospheric oxygen in an exoplanet does not by itself exclude the possibility of photosynthetic life being present either because (at least in the case of a low mass star) not enough time has passed for the oxygen to accumulate or because the gas has dissipated.

Photosynthesis on Habitable Planets around Low-Mass Stars ~ Manasvi Lingam, Abraham Loeb
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The Galaxy Cluster Abell 959

Post by bystander » Tue Sep 17, 2019 11:29 pm

The Galaxy Cluster Abell 959
SAO Science Updates | 2019 Sep 13
Most galaxies lie in clusters containing from a few to thousands of objects. Our Milky Way, for example, belongs to the Local Group, a cluster of about fifty galaxies whose other large member is the Andromeda galaxy about 2.3 million light-years away. Clusters are the most massive gravitationally bound objects in the universe and form (according to current ideas) in a "bottoms-up" fashion with smaller structures developing first and larger groupings assembling later in cosmic history.

Astronomers have detected massive clusters of galaxies, some with more mass than a hundred Milky Way galaxies, dating from as early as only about three billion years after the big bang, and their stars had to form at even earlier times. In the present-day Universe, clusters are still forming through hierarchical processes like major mergers with neighboring clusters. Astronomers are working to better understand cluster formation and evolution in part because the details will also help to constrain cosmological parameters and the properties of dark matter.

CfA astronomer Felipe Andrade-Santos was a member of a team that studied Abell 959, a galaxy cluster whose mass is that of about 3000 Milky-Way galaxies and which lies about three billion light-years away. All of the processes important to the formation of clusters like Abell 959 dissipate energy through shocks. Processes include, for example, mergerd, mass accretion, and phenomena related to their supermassive black hole nuclei. Those shocks in turn produce large-scale diffuse emission features as electrons in the hot gas are accelerated and radiate, and these structures (called radio relics) can be studied with radio telescopes. Gas turbulence in the post-merger cluster also produces radio features - these are called giant radio halos. Abell 959 hosts one radio relic over twelve hundred light-years in length and five hundred in width, and also a giant radio halo.

The scientists analyzed the Abell 959 structures and compared them with an analysis of about eighty other known radio halo systems to test and refine competing theories of cluster evolution. They find that the current model of turbulent re-acceleration of electrons is consistent with their results, and moreover that new simulations of cluster formation are in good agreement with their observations. Their results overall strengthen our confidence in models of how massive galaxy clusters form.

A Massive Cluster at z = 0.288 Caught in the
Process of Formation: The Case of Abell 959
~ L. Bîrzan et al
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Bright Galaxy Clusters in the Era of Peak Star Formation

Post by bystander » Fri Sep 20, 2019 8:27 pm

Bright Galaxy Clusters in the Era of Peak Star Formation
SAO Science Updates | 2019 Sep 20
Galaxies actively engaged in making stars produce many hot massive stars that emit copious amounts of uv radiation. The neutral hydrogen gas in these galaxies (or in the intervening intergalactic medium) absorbs nearly all the uv light that is shorter than 912 Angstroms, the characteristic wavelength of the hydrogen transition. Observers of these starburst galaxies thus see a sudden drop in their spectrum at this wavelength, called the Lyman-break. (For reference, visible blue light lies at a much longer wavelength range, around 4000 Angstroms.) Since galaxies in the distant universe are expanding away from us, as their apparent spectrum is shifted to the red, their Lyman break is shifted to visible wavelengths where optical instruments can detect it.

Massive galaxy clusters in the local universe, and their massive members, must have begun forming stars in the early universe to have grown so large today. Astronomers do in fact see significantly enhanced star-formation activity in distant proto-galaxy clusters, identified in part with searches for their Lyman break signatures. CfA astronomer Mark Gurwell was a member of an international team that used the Submillimeter Array (SMA) and the SCUBA-2 submillimeter camera on the James Clerk Maxwell Telescope to study two of the largest known dense galaxy clusters from an epoch about two and one-half billion years after the big bang. The SMA was able to resolve the spatially blended galaxies. All together the team found fifty-six individual bright galaxies in very densely packed regions only about five million light-years in diameter (for comparison, the closest galaxy to our Milky Way, Andromeda, is two and one-half million light-years away).

The scientists used the infrared and submillimeter brightness of these clusters to estimate their star formation activity and find fantastic rates -- about ten thousand and three thousand solar-masses per year of new stars, respectively (the Milky Way is making about one new star per year). The results are consistent with the idea that during this epoch the universe was making stars at a considerable higher rate than today. The work also helps to refine our understanding of the distribution of dark matter (these galaxies form in dark matter halos) by refining simulations of galaxy structure formation.

Two Sub-Millimetre Bright Protoclusters Bounding the Epoch of Peak Star-Formation Activity ~ Kevin M Lacaille et al
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Heating the Solar Corona

Post by bystander » Tue Oct 01, 2019 2:56 pm

Heating the Solar Corona
SAO Science Updates | 2019 Sep 27
The hot outer layer of the sun, the corona, has a temperature of over a million degrees Kelvin, much more than the surface temperature of the Sun which is only about 5500 degrees Kelvin. Moreover, the corona is very active and ejects a wind of charged particles at a rate equivalent to about one-millionth of the moon's mass each year. Some of these particles bombard the Earth, producing auroral glows and occasionally disrupting global communications. There are two important, longstanding, and related questions about the corona that astronomers are working to answer: how is it heated to temperatures that are so much hotter than the surface? And how does the corona produce the wind?

The role of impulsive events is thought to be key to unraveling this problem. Flares are the most prominent such events, but it is believed that flaring also scales down to much smaller levels of activity – so-called nanoflares. The origins and properties of the energy release mechanisms in flares are often obscured by local heating effects, and instruments need to have good sensitivity, rapid response time, and some luck to retrieve useful data on flares amidst the complex seething cauldron of activity, while nanoflares are faint and elusive. Intermediate scale events are therefore thought to offer important ways to probe the energy release processes.

CfA astronomer Paola Testa is a member of a team of astronomers studying flares using IRIS (the Interface Region Imaging Spectrograph), an instrument on the Solar Dynamics Observatory, a NASA small explorer spacecraft that was launched in 2013 (the telescope for IRIS was provided by SAO). Recently, IRIS observed intermediate scale flaring events that were detected through brightenings at the footpoints of coronal loops and characterized by having high velocity, upward motions caused by impulsive heating. IRIS measured the ultraviolet line of highly ionized silicon to reveal highly variable activity over timescales of twenty to sixty seconds, implying the presence of magnetic loops of activity.

The clear correspondence between the brightening seen by IRIS and these coronal loops prompted the scientists to undertake a systematic study of the events. The scientists report that the localized brightenings found at the base of very hot coronal loops can indeed be treated as systems of interacting loops, and argue that the loop interactions determine the characteristic high temperatures and other behaviors that flag the production of intermediate-sized flares.

Impulsive Coronal Heating from Large-scale Magnetic
Rearrangements: From IRIS to SDO/AIA
~ Fabio Reale et al
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The Hot X-Ray Gas in Giant Elliptical Galaxies

Post by bystander » Wed Oct 09, 2019 4:50 pm

The Hot X-Ray Gas in Giant Elliptical Galaxies
SAO Science Updates | 2019 Oct 04
The oldest known large galactic structures in the universe are giant elliptical galaxies. Unlike our Milky Way and other spiral galaxies, elliptical galaxies have no spiral arms and little or no current star formation activity. Astronomers think that these giants formed in the early universe, less than a billion years after the big bang, after a phase of rapid star-formation. (In contrast, spiral galaxies continue to form stars.) Giant ellipticals evolved to become even larger after this initial phase, but they did so through galaxy mergers and accretion. Those interactions, astronomers think, were dominated by the gravitation of the dark matter that accompanies galaxies.

Giant ellipticals at some point became so large (more than a trillion solar-masses) that the inflowing gas was shock-heated to temperatures above several million Kelvin. The resultant X-ray emitting gas further suppressed the star formation. The same accretion, however, continues to feed their central supermassive black holes, driving the ejection of powerful jets of rapidly moving charged particles that shine brightly at radio frequencies, making these objects early targets of radio telescopes. One of the outstanding puzzlers about these ellipticals is how the hot X-ray emitting gas can stay so hot for so long without cooling down and falling back onto the galaxy to produce new stars. A related question is how the supermassive black holes can continue to grow if material is inhibited from flowing into the center. One hypothesis is that the outflow itself helps to reheat the galaxy and power some circulation.

The giant elliptical galaxy IC4296 is the most massive and centrally located member of a cluster of galaxies called Abell 3565 located about one hundred and fifty million light-years from us. Its radio jets have been known for over thirty years, and extend almost half-a-million light-years into the intergalactic medium. CfA astronomer Paul Nulsen was a member of an international team that combined new radio images from the Karl G. Jansky Very Large Array with archival data from Chandra, XMM-Newton, Hubble, and the Southern Astrophysical Research optical telescope. Their multi-wavelength analysis concludes that this giant elliptical does have dramatic, supersonically expanding jets whose power is comparable to about a hundred billion solar luminosities. They find that the jets appear to not deposit all that energy in the innermost gas of the galaxy, but instead to penetrate into the intercluster medium and heat that gas. Thus the inner material can continue to accrete onto the black hole, thereby resolving the problem of how the supermassive black hole is powered.

Powerful AGN Jets and Unbalanced Cooling in the Hot Atmosphere of IC 4296 ~ R. Grossová et al
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The Nature of Obscured Active Galactic Nuclei

Post by bystander » Sun Oct 13, 2019 2:52 pm

The Nature of Obscured Active Galactic Nuclei
SAO Science Updates | 2019 Oct 11
Most galaxies host a supermassive black hole (SMBH) at their nucleus, one whose mass exceeds a million solar-masses. When material actively accretes onto the SMBH, associated processes can produce an active galactic nucleus (AGN) with a hot torus and dramatic bipolar jets of rapidly moving charged particles. The most luminous known AGN emit over ten trillion solar-luminosities. Astronomers are trying to understand what powers AGN, how they evolve, and how their jets and radiation affect their environments, and these extreme cases are expected to provide key insights.

Quasars are perhaps the best-known luminous AGN, and their nuclei are visible and relatively unobscured by dust. But there are cases where the torus of material around the nuclei happens to block our line-of-sight. These obscured AGN have no visible emission lines and so are often omitted from studies, but they are needed to provide a more complete view of the population. A foremost question is whether these very luminous AGN are powered by moderate accretion onto very massive black holes, or instead by extreme accretion rates onto moderate-mass black holes, or perhaps something in between.

CfA astronomer Fabio Pacucci was a member of a large international team of scientists that combined observations from the Swift satellite's X-Ray Burst Alert Telescope AGN Spectroscopic Survey (BASS) with radio, optical, and infrared datasets. They studied twenty-eight of the most luminous, relatively nearby AGN, most of which are located in elliptical galaxies. Apart from their dramatic nuclear activity, the set have no other distinctive properties; their radio emission, for example, spans a factor of ten thousand, and the inferred masses of their supermassive black holes also cover a large range. Some of these results are surprising: since the jets are responsible for radio emission, it was thought that radio strength would correlate more closely with the X-ray emission or the black hole masses. The authors conclude by speculating that, if the results hold up in larger studies, the intense growth of supermassive black holes and correspondingly intense emission is not a property of any particular type of galaxy but rather is linked to the transformation of galaxies from star-forming spirals to quiescent ellipticals during episodes of major merging. ...

BAT AGN Spectroscopic Survey – XIII. The nature of the most luminous
obscured AGN in the low-redshift universe
~ Rudolf E. Bär et al
BAT AGN Spectroscopic Survey
Monthly Notices of the RAS
arxiv.org > Astrophysics
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The Rotation of Venus

Post by bystander » Thu Oct 24, 2019 3:01 pm

The Rotation of Venus
SAO Science Updates | 2019 Oct 18
Venus is covered in a thick layer of clouds, one reason that it appears so bright in the sky. Ancient astronomers had a good idea of what (since Copernicus) we know as its orbital period; the modern measurement is that Venus takes 224.65 days to complete one revolution around the Sun, a Venusian year. Because of the clouds, however, it has been difficult to measure the length of the Venusian day since the nominal method of watching a visible surface feature rotate around 360 degrees is not possible. In 1963, Earth-based radar observations penetrated the cloud cover and were able to measure a rotation rate of 243 days; more surprising is that Venus rotates on its axis in the opposite direction from that of most planets, so-called retrograde rotation. Subsequent ground-based radar studies came up with inconsistent values for the length, differing by about 6 minutes. The Magellan spacecraft completed its 487 day orbital mapping program in 1991 and concluded the correct number was slightly different still: 243.0185 days with an uncertainty of about 9 seconds. But subsequent missions and ground-based observations found that the rate of rotation was actually not constant but seemed to vary, with models arguing that solar tidal torques and atmosphere drag on the surface could account for at least some of the variation.

Knowing the rotation rate of Venus is critically important for future lander missions to Venus, like ones being considered for the next decade. The current rotation uncertainties correspond to a distance on the surface of about thirteen miles, more than enough to miss a landing site. To try to reduce the uncertainties, CfA astronomer John Chandler and the team of scientists to which he belonged undertook a new analysis of twenty-nine years of Earth-based radar observation taken between 1988 to 2017. They conclude that the mean Venusian day length is 243.0212 +- .00006 days. In particular, their formal uncertainty is the smallest yet obtained, although they note that their result is an average since it does not reflect short-term oscillations. In the next decade, the authors anticipate that further improvements will insure that a Venus lander mission in the late 2020's will be able to succeed.

The Mean Rotation Rate of Venus from 29 years of Earth-based Radar Observations ~ Bruce A.Campbell et al
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Feeding the Supermassive Black Holes in Galaxies

Post by bystander » Sat Oct 26, 2019 5:45 am

Feeding the Supermassive Black Holes in Galaxies
SAO Science Updates | 2019 Oct 25
When cold gas and dust accretes onto the supermassive black hole at a galaxy's core, the galactic nucleus can become activated and eject jets of rapidly moving charged particles. These particles in turn radiate strongly across the electromagnetic spectrum. What prompts the material to accrete in the first place, and the paths it follows as it descends into the accretion maelstrom, are poorly understood physical processes.

Theory, and some simulations, have attempted to model the accretion processes in clusters of galaxies based on limited numbers of emission line studies. They predict, for example, that there are large reservoirs of cold gas in small clouds orbiting a few hundreds of light-years from the black hole, with collisions prompting them to release material for accretion. CfA astronomer Grant Tremblay and a team of colleagues have undertaken a study of the absorption lines in these clouds in submillimeter and radio molecular lines. The advantages of absorption versus emission lines are that absorption line studies probe only the narrow line-of-sight to bright nuclear background whereas emission usually comes from a much larger region because the telescope beam sizes are relatively large; moreover, absorption lines provide key kinematic information about the gas motions along this constrained line-of-sight.

The scientists used the ALMA facility to study absorption in fifteen molecular transitions from carbon monoxide and two other simple molecules (and also atomic hydrogen gas) in what are thought to be cold gas clouds near the nuclei of eighteen of the brightest galaxies in clusters. They find that the gas temperatures vary between about twenty and eighty degrees Kelvin, and that the gas velocities imply the material is falling towards the respective nuclear black holes. The new results are consistent with models of chaotic, cold accretion. The sources are not all identical in behavior, however, but as a group they support the model of molecular clouds drifting and infalling onto the black hole's environment, thereby triggering nuclear activity.

Constraining Cold Accretion onto Supermassive Black Holes: Molecular Gas in the Cores of
Eight Brightest Cluster Galaxies Revealed by Joint CO and CN Absorption
~ Tom Rose et al
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Jupiter-Sized Exoplanet Discovered through Microlensing

Post by bystander » Fri Nov 01, 2019 4:21 pm

Jupiter-Sized Exoplanet Discovered through Microlensing
SAO Science Updates | 2019 Nov 01
The path of a light beam is bent by the presence of mass, and a massive body can therefore act like a lens (a "gravitational lens") to distort the image of an object seen behind it. Scientists first confirmed Einstein's prediction quantitatively during the now famous total eclipse of 29 May 1919 by observing starlight bent by the mass of the Sun. Microlensing is the name given to a related phenomenon: the brightening of light from a star as a cosmic body, acting as a gravitational lens, passing fortuitously in front of it, the light then dimming to normal as the body moves beyond the line-of sight. About one hundred exoplanets have been discovered to date by the microlensing technique, ranging in masses from about fifty Jupiter-masses to less than a few Earth-masses.

The Korea Microlensing Telescope Network (KMTNet) was inaugurated over four years ago with three 1.6-meter telescopes located in Chile, South Africa, and Australia. Its goal is to discover exoplanets through microlensing brightening events using constant monitoring of selected regions of the sky. Depending on the timing cadence used for the observations, from four per hour to once every five hours, KMTNet should be able to detect and characterize planets whose masses are respectively from about one Earth-mass to one Jupiter-mass.

CfA astronomers In-Gu Shin and Jennifer Yee were members of a KMTNet microlensing team that used microlensing techniques to discover a Jupiter-sized exoplanet (its mass is about 0.57 Jupiter-masses) orbiting a small M-dwarf star (mass of about 0.14 solar- masses) located about four thousand light-years away. In the past, most exoplanets dicoveries by microlensing were made by intensive follow-up observations of microlensing events first spotted in large sky surveys as a variation in a star’s light. The constant monitoring of a field means that both the discovery and followups are done with the same telescopes. This is the thirty-third exoplanet discovered by KMTNet, and in addition to confirming the utility of modest-sized telescopes and the team's cadence strategy for observations, its discovery shows that the statistics on this population of exoplanets are rapidly improving, and they are expected to lead to a better understanding of how gas giants form and evolve.

KMT-2018-BLG-1990Lb: A Nearby Jovian Planet from a Low-Cadence Microlensing Field ~ Yoon-Hyun Ryu et al
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Near-Earth Asteroid Pairs

Post by bystander » Fri Nov 08, 2019 6:27 pm

Near-Earth Asteroid Pairs
SAO Science Updates | 2019 Nov 08
Near Earth Objects (NEOs) are small solar system bodies whose orbits sometimes bring them close to the Earth, making them potential collision threats. NEOs also offer clues to the composition, dynamics and environmental conditions of the early solar system and its evolution, and because they are relatively close to the Earth they lend themselves to astronomical measurements. Most NEOs are discovered in optical searches, but one crucial NEO parameter, its size, usually cannot be determined from optical detections alone. This is because an NEO's optical light is reflected sunlight, and an object can be bright either because it is large or because it has a high reflectivity. A CfA team has been using the IRAC infrared camera on Spitzer to measure NEO infrared emission signals which provide an independent measure of its size.

CfA astronomer Joe Hora is a member of a team of scientists who realized that four recently studied NEOs are actually two pairs of objects, each pair consisting of two "genetically related" asteroids of similar composition and size. The pairing was confirmed when optical and near-infrared spectroscopy found that each of the two NEOs were so similar, composed of dehydrated and volatile-poor rocks, suggesting a common origin. The question then became: What was their origin? Thermal fragmentation is unlikely for objects of this composition, while a tidal disruption or collisional origin are seen as unlikely given their modeled orbital paths through the solar system.

The scientists argue that the most likely cause for breakup is the YORP mechanism, an acronym derived from the last names of the four scientists who proposed it in the 19th century. The mechanism refers to asteroid spin-up as the result of its non-uniform reflection and emission of photons. Photons carry momentum, and when an asteroid is non-uniform in its surface properties, the departing photons can gradually spin it up until eventually it fragments. The team estimates that probably both pairs of NEOs fragmented less than about ten thousand years ago, making them among the youngest known multiple asteroid systems. The astronomers expect that more NEO pairs will be found, thus providing an ideal laboratory for studying time dependent evolutionary processes that are relevant to asteroids throughout the solar system.

A Common Origin for Dynamically Associated Near-Earth Asteroid Pairs ~ Nicholas Moskovitz et al
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Discovering an Unseen Black Hole

Post by bystander » Sat Nov 16, 2019 3:14 pm

Discovering an Unseen Black Hole
SAO Science Updates | 2019 Nov 15
Paradoxically black holes, which emit no light because of their strong gravitational fields, are regularly discovered via the bright X-ray emission that arises when material accretes onto a torus around a black hole, heating the torus to high temperatures. Thousands of X-ray emitting black holes are known, ranging in size from supermassive monsters at the hearts of galaxies (like the recently imaged shadow of the giant in M87 with nearly a billion solar-masses) to smaller, stellar-mass-size black holes. The remarkable measurements of gravitational wave emission from merging dense objects, including black holes, illustrate how non-electromagnetic radiation can also be used to spot black holes. Astronomers, however, think that many and perhaps even most black holes are currently neither accreting material nor in the final stages of merging. How, then, to spot them since they do not radiate?

The first stellar-mass black hole that is not emitting any detectable radiation has now very likely been discovered by a team of scientists including CfA astronomers Dave Latham, Allyson Bieryla, Gilbert Esquerdo, Perry Berlind, and Michale Calkins. Their method was to hunt for a star whose wobble implied that it was orbiting with a massive, unseen binary companion star, probably a black hole. This velocity wobble method is the same as is used in the search for planets around other stars, a technique that CfA astronomers helped to pioneer.

The star 2MASSJ05215658+4359220 (its long name incorporates its coordinates in the sky) is a giant star located in the direction of the constellation Auriga and which is about twelve thousand light-years away. The team discovered that the star was wobbling with a period of about eighty-three days with a velocity variation of about eighty km/sec. The peculiar behavior of the star was first noticed using infrared spectroscopy from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), and that was followed up by searching for light (versus velocity) variations using the All-Sky Automated Survey for Supernovae (ASAS-SN) instrument and then with more precise spectroscopy using the Tillinghast Reflector Echelle Spectrograph (TRES) at SAO's Fred L. Whipple Observatory.

The combination of datasets from these campaigns enabled the scientists to determine the mass of the star as being about 3.2 solar-masses; the mass of its non-accreting, unseen companion has a lower mass limit (because the inclination of the orbit is not known) that is about the same, about 3.3 solar-masses . This size black hole is itself an important discovery since the size falls in the so-called compact object “mass gap” between more commonly known neutron stars and more typical sized stellar-mass black holes. The unseen object might in principle be a neutron star, buts since its mass is larger than the highest known neutron star mass (2.0 solar-masses), and since the size is compatible with some models of black hole mass sizes, the team argues that the unseen object is probably a black hole, the first one to be detected without its being directly seen.

A Noninteracting Low-Mass Black Hole – Giant Star Binary System ~ Todd A. Thompson et al Discovery of a Candidate Black Hole - Giant Star Binary System in the Galactic Field ~ Todd A. Thompson et al
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Chemistry in the Turbulent Interstellar Medium

Post by bystander » Sat Nov 23, 2019 4:05 pm

Chemistry in the Turbulent Interstellar Medium
SAO Science Updates | 2019 Nov 22
Over two hundred molecules have been discovered in space, some (like Buckminsterfullerene) very complex with carbon atoms. Besides being intrinsically interesting, these molecules radiate away heat, helping giant clouds of interstellar material cool and contract to form new stars. Moreover, astronomers use the radiation from these molecules to study the local conditions, for example, as planets form in disks around young stars.

The relative abundance of these molecular species is an important but longstanding puzzle, dependent on many factors from the abundances of the basic elements and the strength of the ultraviolet radiation field to a cloud’s density, temperature, and age. The abundances of the small molecules (those with two or three atoms) are particularly important since they form stepping stones to larger species, and among these the ones that carry a net charge are even more important since they undergo chemical reactions more readily. Current models of the diffuse interstellar medium assume uniform layers of ultraviolet illuminated gas with either a constant density or a density that varies smoothly with depth into the cloud. The problem is that the models' predictions often disagree with observations.

Decades of observations have also shown, however, that the interstellar medium is not uniform but rather turbulent, with large variations in density and temperature over small distances. CfA astronomer Shmuel Bialy led a team of scientists investigating the abundances of four key molecules -- H2, OH+, H2O+, and ArH+ -- in a supersonic (with motions exceeding the speed of sound) and turbulent medium. These particular molecules are both useful astronomical probes and highly sensitive to the density fluctuations that naturally arise in turbulent media. Building on their previous studies of the behavior of molecular hydrogen (H2) in turbulent media, the scientists performed detailed computer simulations that incorporate a wide range of chemical pathways together with models of supersonic turbulent motions under a variety of excitation scenarios driven by ultraviolet radiation and cosmic rays. Their results, when compared to extensive observations of molecules, show good agreement. The range of turbulent conditions is wide and the predictions correspondingly wide, however, so that while the new models do a better job of explaining the observed ranges, they can be ambiguous and explain a particular situation with several different combinations of parameters. The authors make a case for additional observations and a next-generation of models to constrain the conclusions more tightly.

Chemical Abundances in a Turbulent Medium — H2, OH+, H2O+, ArH+ ~ Shmuel Bialy et al
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor