SAO: Science Updates 2020

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The Megamaser Cosmology Project Measures the Age of the Universe

Post by bystander » Sun Jun 21, 2020 4:30 pm

The Megamaser Cosmology Project Measures the Age of the Universe
SAO Weekly Science Update | 2020 Jun 19
A maser, like a laser, is a source of bright, monochromatic electromagnetic radiation, with the difference being that maser radiation is not optical light but rather longer wavelength microwave radiation. Dense molecular clouds in interstellar space sometimes produce natural masers when specific molecules (water and OH are two examples) or atoms are stimulated by the local conditions to emit very narrow-line radiation.

Such astronomical masers were first identified in space over fifty years ago, and have since been found in many locations in our Milky Way as well as in other galaxies, with the most spectacular examples found in regions of active star formation. In some cases the energy emitted in a single maser line exceeds the emission from the Sun over its entire visible spectrum making masers valuable diagnostic probes of their local conditions. These "megamasers" can be found in the nuclear regions of galaxies with active supermassive black holes and their brightness makes them potentially useful tools for cosmological studies. ...

The Megamaser Cosmology Project is a multi-year campaign to find, monitor, and map systems with the goal of constraining Ho to a precision of several percent with precise geometric distance measurements to water megamaser galaxies whose known recession velocities were also remeasured precisely. CfA astronomers Dom Pesce and Mark Reid are lead members of the team, which has just published its improved value for Ho of 73.9 +-3.0 (in usual units) corresponding to an age of the universe (with some assumptions) of 12.9 +-0.5 billion years. The team used their analyses of megamasers in six galaxies for this result. For comparison, other projects using measurements from galaxies have reported a consistent value, about 74.0, however the CMBR results from the Planck satellite give a value of value for Ho of about 67.4 and a corresponding age that is significantly older: 14.2 billion years. The team notes that their future megamaser observations will improve on this precision and help astronomers address this critical discrepancy.

The Megamaser Cosmology Project. XIII. Combined Hubble Constant Constraints ~ D. W. Pesce et al
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Inferring Temperature Structure of Circumstellar Disks

Post by bystander » Sat Jun 27, 2020 4:24 pm

Inferring the Temperature Structure of Circumstellar Disks from Polarized Emission
SAO Weekly Science Update | 2020 Jun 26
Image
An ALMA submiilimeter image of the dusty circumstellar disc around a young
star. Astronomers are using ALMA polarization maps of the radiation from discs
similar to this one to infer the presence of a temperature gradient, and infer
possible accretion onto the disc. © Lee, Chin-Fei et al., 2017 SciAdv

Polarized light is a familiar phenomenon because the scattering or reflection of light results in one of its two components being preferentially absorbed. The majority of sunlight on Earth, for example, is preferentially polarized due to scattering in the atmosphere (this helps make polarized sunglasses effective). Electromagnetic radiation from astrophysical sources can also be polarized, typically because of scattering from elongated dust grains that are aligned with each other by the local magnetic fields. These fields are thought to play a major, perhaps even a dominant role in controlling the shapes and motions of interstellar gas clouds and are extremely difficult to measure directly. Observations of polarization by dust grains offer a unique way to probe the magnetic fields.

The polarized emission from aligned grains in discs around young stellar objects is of particular interest to astronomers studying how planets develop and evolve in these discs. The polarized emission can reveal not only the details of the magnetic fields present but also (depending on the grain shapes and properties) other structural features of the disk environment, for example the presence of anisotropic stellar radiation. ...

Probing the Temperature Structure of Optically Thick Disks
Using Polarized Emission of Aligned Grains
~ Zhe-Yu Daniel Lin et al
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Measuring the Structure of a Giant Solar Flare

Post by bystander » Sat Jul 04, 2020 2:34 pm

Measuring the Structure of a Giant Solar Flare
SAO Weekly Science Update | 2020 Jul 03
The sun's corona, its hot outermost layer, has a temperature of over a million degrees Kelvin, and produces a wind of charged particles, about one-millionth of the moon's mass is ejected each year. Transient events have been known to cause large eruptions of high-energy charged particles into space, some of which bombard the Earth, producing auroral glows and occasionally veven disrupting global communications. One issue that has long puzzled astronomers is how the sun produces these high-energy particles.

Flares or other kinds of impulsive events are thought to be key mechanisms. The hot gas is ionized and produces an underlying sheet of circulating current that generates powerful magnetic field loops. When these loops twist and break they can abruptly eject pulses of charged particles. In the standard picture of solar flares, large-scale motions drive this activity, but where and how the energy is released locally, and how the particles are accelerated, have remained uncertain because the magnetic properties of the large-scale current sheet have not been measured at sizes small enough to correspond to the domains of flaring activity.

CfA astronomers Chengcai Shen, Katharine Reeves and a team of their collaborators report spatially resolved observations of the regions of magnetic field and flare-ejected electron activity. The team used the thirteen antenna array at the Expanded Owens Valley Solar Array (EOVSA) and its microwave imaging techniques to observe the giant solar flare on 2017 September 10. As the event progressed they saw a rapidly ascending, balloon-shaped dark cavity, corresponding to twisted magnetic field lines rising, breaking, and ejecting electrons as viewed roughly along the axis of the field lines. The scientists were able to model the details of the configuration, and by estimating the strength of the magnetic field and the speed of the plasma flow, they determined that this one large flare alone released during its peak few minutes about .02% of the energy of the entire sun. Their results suggest that these kinds of spatial structures in the field are the primary locations for accelerating and channeling the fast-moving electrons into interplanetary space, and demonstrate the power of these new, spatially resolved imaging techniques.

Measurement of Magnetic Field and Relativistic Electrons
along a Solar Flare Current Sheet
~ Bin Chen et al
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Modeling the Sun's Heliosphere

Post by bystander » Mon Jul 27, 2020 5:15 pm

Modeling the Sun's Heliosphere
SAO Weekly Science Update | 2020 Jul 24
The heliosphere is the bubble-like cavity around the Sun, created by the solar wind, that extends well beyond Pluto until the pressure of the interstellar medium ultimately terminates it. For decades astronomers thought that the heliosphere had a roughly cometary shape, spherical with a long tail, shaped by a combination of the solar wind, the interstellar medium, and the Sun's motion through the galaxy. The Voyager 1 and 2 missions, both launched in 1977, have since traversed the heliosphere and crossed its edge (the heliopause) into the interstellar medium; Voyager 1 did so in 2012 and Voyager 2 in 2018. Along the way they measured many properties of the heliosphere, and forced astronomers to conclude that the solar magnetic field, previously neglected, plays a crucial role in shaping the heliosphere and that its shape is not really cometary. The Cassini mission, launched in 1997 to explore Saturn, also found evidence that there is no tail to the heliosphere. The question has now become what the shape of the heliosphere really is and how it is determined.

CfA astronomers Merav Opher and Avi Loeb and two of their colleagues tackled these issues with detailed modeling of the heliosphere that is unique in that included as a separate component the pick-up ions in the wind. These ions are originally neutral atoms in the solar wind that become ionized when they become excited by ultraviolet radiation or by collisions and then can respond to magnetic effects. The New Horizons mission to Pluto recently made the first direct detection of pick-up ions in the solar wind; it measured them as far away from the Sun as thirty AU (one AU is the average distance of the Earth from the Sun, and Pluto’s average distance from the Sun is about forty AU). Voyager found that near the shocked edge of the heliopause, the heliosheath, the pressure of these ions actually dominated the pressure.

The astronomers ran several sets of calculations of the heliosphere in order to account for the relatively unconstrained parameters of the magnetic field’s strength and direction, and took into account the production and depletion of pick-up ions. They find that when the ions encounter neutral hydrogen they become neutral, cool, and deflate, making the heliosheath smaller and rounder, in agreement with the Cassini observations. The new model reproduces the behaviors of both the pick-up ions and the solar wind ions and matches the observations made by all the spacecraft. The model does not include effects of the solar cycle variations, however, but future model improvements will do so, and together with new measurements should be able to refine our understanding of the Sun’s heliosphere.

A Small and Round Heliosphere Suggested by Magnetohydrodynamic Modelling of Pick-up Ions ~ Merav Opher et al
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Where Might Very Unequal Mass Black Hole Binaries Come From?

Post by bystander » Wed Aug 05, 2020 8:03 pm

Where Might Very Unequal Mass Black Hole Binaries Come From?
SAO Weekly Science Update | 2020 Jul 31
The direct detection of gravitational waves from at least eleven sources during the past five years has offered spectacular confirmation of Einstein's model of gravity and space-time, while the modeling of these events has provided information on star formation, gamma-ray bursts,neutron stars, the age of the universe, and even verification of ideas about how very heavy elements are produced. The majority of these gravitational wave events arose from the merger of two black holes of comparable masses in an orbiting pair. Near-equal mass pairs are strongly preferred in models of binary black hole formation, whether they result from the evolution of isolated binary stars or from the dynamical pairing of two black holes. This year, however, the LIGO and Virgo gravitational wave observatories reported the first detection of a very unequal mass pair of black holes, GW190412, whose estimated masses are about 30 and 8 solar-masses. The question, then, is how were they formed?

CfA astronomer Carl Rodriguez led a team of colleagues in a theoretical investigation of how such an unequal mass binary might form. The most obvious solution is look in a dense star cluster, where low-spin, comparable mass black hole pairs can naturally form, in part because massive black holes and stars tend to sink toward the center of the cluster and can more readily encounter each other. But even there those encounters are unlikely to produce an unequal mass pair. The spin of each black hole adds a further complicating factor. The spin is quantified by a number between zero and one. If each of the black holes in a merger has a low value of spin, as is expected, then their merger will normally produce a more massive black hole whose spin is large, perhaps around 0.7, but the inferred spin of the massive black hole in GW190412 is well determined to be about 0.43, suggesting that it did not arise from such a simple merger. ...

GW190412 as a Third-Generation Black Hole Merger from a Super Star Cluster ~ Carl L. Rodriguez et al
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Finding the Optical Counterparts of High-Energy Blazars

Post by bystander » Fri Aug 28, 2020 4:37 pm

Finding the Optical Counterparts of High-Energy Blazars
SAO Weekly Science Update | 2020 Jul 10
A blazar is a galaxy whose central nucleus is bright across the spectrum, from low energy radio wavelengths to the high energy gamma rays like those observed by the Fermi Gamma Ray Space Telescope. Astronomers think that a blazar nucleus contains a supermassive black hole that powers jets of charged particles as matter falls into its vicinity. Although the nuclei of other galaxies also eject jets of particles, the class of blazars is thought to result from our unique viewing angle: staring almost directly down the throats of the jets. Two defining characteristics of blazars,strong radio emission and high variability, are the results of the accretion and jets.

As Fermi surveys the sky it spots many powerful gamma-ray sources, but their locations are very uncertain, as much as twice the angular size of the moon, making it difficult to use the spatial location to identify any individual galaxy (or other obejct) as being responsible for the emission. Instead, indirect arguments are made from similarities of brightness, variability, and/or other inferences. Astronomers think that many of these sources should be blazars, and the most recent catalog of about 5100 sources has about 3000 blazars, but one quarter of the sources still lack any clear counterpart.

CfA astronomer Raffaele D'Abrusco was a member of a team that developed a new statistical method to associate Fermi gamma-ray sources with optical, infrared or radio counterparts. The Likelihood Ratio (LR) method, often used to identify counterparts of high energy sources, has typically relied on angular separation in a geometrical approach to calculate associating probabilities of the counterparts. The new version of this method builds instead on fluxes and colors from infrared and radio sky surveys. Infrared surveys have been instrumental in the discovery (made by the same team of researchers) that gamma-ray detected blazars have unique infrared colors that clearly distinguish them from other extragalactic sources. The new procedure takes advantage of these strong correlations between infrared colors and gamma-ray spectral properties of known blazars. ...

On the Physical Association of Fermi-LAT Blazars with Their Low-Energy Counterparts ~ Raniere de Menezes et al
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Recurrent Cometary Activity in Don Quixote

Post by bystander » Fri Aug 28, 2020 4:50 pm

Recurrent Cometary Activity in Don Quixote
SAO Weekly Science Update | 2020 Jul 17
Don Quixote (alias Near-Earth Object NEO 3552) was discovered in 1983. Subsequent infrared and optical observations measured its diameter as about 18.7 kilometers, making it the third largest known NEO. Its reflectivity and its orbit suggested to astronomers that it was actually an extinct comet, but it had no apparent comet-like tail and was instead classified as an asteroid. But physical characterization over the following years revealed further hints at a potentially cometary origin of this body, including infrared observations made with the IRAC camera on Spitzer.

CfA astronomers Joe Hora and Howard Smith, together with a team of colleagues, used Spitzer to study Don Quixote, and in 2014 they reported evidence for extended emission in one of the two IRAC channels. Because the activity was limited and had never been seen at optical wavelengths, the team attributed the activity to a short outburst, perhaps caused by a recent impact. The group continued to monitor the object when it was visible, including another set of observations with IRAC in 2017 and ground-based observations six months later, in 2018 when Don Quixote was close to perihelion, using optical and submillimeter telescopes. The new IRAC observations found extended emission around the nucleus which the astronomers argue is due to either carbon dioxide or carbon monoxide gas. The slightly later optical observations detected activity for the first time which the astronomers attribute to reflected sunlight from dust particles, probably centimeter-sized, in the object's coma and narrow tail. The submillimeter results also detected emission, but no obvious carbon monoxide line, tentatively ruling out this molecule. The combined observations support the original conjecture that Don Quixote is a weakly active comet, with recurrent activity, and confirms it as an important member of an unusual class of NEOs.

Recurrent Cometary Activity in Near-Earth Object (3552) Don Quixote ~ Michael Mommert et al
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Very High Energy Gamma-Ray Emission from a Radio Galaxy

Post by bystander » Fri Aug 28, 2020 5:14 pm

Very High Energy Gamma-Ray Emission from a Radio Galaxy
SAO Weekly Science Update | 2020 Aug 21
Image
A Hubble image of the radio galaxy 3C264 and its jet (the light
from the galaxy itself and its inner disk have been subtracted to
make it easier to see the jet). The red cross marks the location
of the supermassive black hole AGN (the green cross marks the
location of a jet feature). The image is about one thousand light
years across. Astronomers have detected variable, very high
energy gamma-ray emission from the supermassicve black hole
nucleus of this galaxy. Credit: NASA/Archer et al., 2020 ApJ

Giant elliptical galaxies, the oldest known large galactic structures in the universe, have no spiral arms and little or no current star formation activity, but their central supermassive black holes are often active galactic nuclei (AGN). While nearly all galaxies host a supermassive black hole in their nuclei, most nuclei are not AGN. Astronomers think that giant ellipticals formed in the early universe, less than a billion years after the big bang, after a phase of rapid star-formation, and then evolved to become even larger through galaxy mergers and accretion of gas from the intergalactic medium. The same accretion helps feed the AGN that drive the ejection of powerful jets of rapidly moving charged particles. The particles emit strongly at radio frequencies, making these objects bright targets for radio telescopes, and many of these galaxies were first discovered in radio surveys.

VERITAS, the Very Energetic Radiation Imaging Telescope Array System, is a CfA observatory consisting of four 12-m telescopes located at the Fred L. Whipple Observatory at Mt. Hopkins, Arizona. VERITAS is designed to study gamma ray photons, each one packing approximately hundred million times the energy of the highest energy X-ray photon seen by the Chandra X-ray Observatory. CfA astronomers Wystan Benbow, Michael Daniel, Pascal Fortin, Gareth Hughes, and Emmet Roache, together with a large team of colleagues, used VERITAS to search for gamma-ray photons from the AGN in radio-bright, old elliptical galaxies. They and other astronomers realized that the same AGN-produced jets of charged particles that radiate at radio wavelengths can, because they are moving at speeds close to that of light, produce gamma-ray emission when its particles interact with low energy photons. This emission is particularly bright if those jets are being observed nearly face-on.

The astronomers used VERITAS to study the AGN in the elliptical galaxy 3C264 during 2017 - 2019. They discovered very high energy gamma-ray emission in early 2018 and realized this emission must be variable. The emission made this AGN, located about three hundred million light-years from Earth, the most distant very high energy gamma-ray emitting AGN of only four known with jets that are not observed face-on. They followed up this discovery with a large campaign of observations by a variety of multi-wavelength telescopes: Swift, Fermi-LAT, Chandra X-ray Observatory, and Hubble in space, and optical and radio observations on the ground using the Kitt Peak robotically controlled telescope, the Very Long Baseline Array, and the Very Large Array. The team’s complex multi-wavelength data and analysis program allowed them to determine that 3C264 is probably similar to the famous (and very much closer) galaxy M87 and its jet; M87 contains the supermassive black hole that was imaged last year. Only about two hundred very high energy gamma-ray sources have been discovered so far, including both AGN and non-AGN. The new results on 3C264, as one of only four known non face-on AGN in elliptical galaxies, expand our knowledge of AGN jets and their underlying physics. The team is continuing to monitor the source: four bright knots are seen in the radio jet and two are expected to collide in the next few years, with some fireworks expected when this happens.

VERITAS Discovery of VHE Emission from the Radio Galaxy 3C 264: A Multi-Wavelength Study ~ A. Archer et al
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Microlensing Measurement of a Quasar’s Accretion Disk

Post by bystander » Fri Aug 28, 2020 5:41 pm

Microlensing Measurement of a Quasar’s Accretion Disk
SAO Weekly Science Update | 2020 Aug 28
wrote: An active galactic nucleus (AGN) is a supermassive black hole residing at the core of a galaxy that is accreting material. The accretion occurs in the vicinity of the hot torus around the nucleus, and it can generate rapidly moving jets of charged particles that emit bright, variable radiation as material ccelertes as it falls inward. Quasars are perhaps the best-known luminous AGN, and their nuclei are relatively unobscured by dust. Quasar nuclear regions and disks are too far away and much too small to be resolved with telescopes and astronomers trying to understand the behavior of quasars, AGN, and accretion disks are forced to infer the physics from indirect measurements. Flux variability measurements offer one such avenue.

Microlensing refers to the short flashes of light produced when moving cosmic bodies, acting as gravitational lenses, modulate the intensity of light from background sources. Because the path of light is bent by the presence of a mass, material bodies can act like gravitational lenses to distort the images of objects seen behind them. Microlensing offers an opportunity to measure the sizes of quasar AGN. Lensed quasar images are occasionally found that have been magnified and distorted into multiple images by a foreground galaxy and the stellar objects within it. As the quasar moves relative to our line of sight, this magnification changes, generating significant uncorrelated variability between the images over months or years. If the time delays between the multiple images of the quasar are monitored closely enough during multiple epochs it is possible to unravel the intrinsic quasar variability from the microlensing variability. Only fourteen multi-epoch size measurements of quasars have been made until now.

CfA astronomer Emilio Falco was a member of a team that used these variability techniques to estimate the size and mass of the accretion disk and black hole in the quasar WFI2026-4536, a quasar so distant that its light has been traveling towards us for nearly eleven billion years; the age of the universe is only 13.7 billion years. The scientists analyzed optical light variability data over thirteen years, from 2004 to 2017, and developed lensing models that were able to constrain the size of the quasar’s accretion disk to about three hundred and sixty astronomical units and the mass of its supermassive black hole to about one and one-half billion solar-masses. The mass is in rough agreement with other expectations and with the range of masses in the fourteen other similarly measured quasars, but about twice as large as expected from methods based on the luminosity. They also report the first mass measurements of the central black hole using spectroscopic data, with results consistent with the variability method. The impressive results further refine our understadning of these distant monsters and refine the models of AGN.

A Microlensing Accretion Disk Size Measurement in the Lensed Quasar WFI 2026–4536 ~ Matthew A. Cornachione et al
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Magnetic Fields in the Early Stages of Massive Star Formation

Post by bystander » Fri Sep 11, 2020 8:20 pm

Magnetic Fields in the Early Stages of Massive Star Formation
SAO Weekly Science Update | 2020 Sep 04
Star formation in molecular clouds usually occurs in a two-step process. Supersonic flows first compress the clouds into dense filaments light-years long, after which gravity collapses the densest material in the filaments into cores. Massive cores, each more than about twenty solar–masses, preferentially form at intersections where filaments cross, producing sites of clustered star formation. The process is expected to be efficient yet the observed rate of star formation in dense gas is only a few percent of the rate expected if the material really were freely collapsing. To solve the problem, astronomers have proposed that turbulence and/or magnetic fields support the cores against gravitational collapse.

Magnetic fields are difficult to measure. One common approach is to measure the polarized light, because magnetic fields can align elongated dust grains in the interstellar medium which then scatter light with a preferred polarization direction enabling the field strengths to be estimated. CfA astronomers Junhao Liu and Qizhou Zhang led a team that used the ALMA submillimeter facility to study the polarized emission in three massive cores in a dark cloud with a spatial resolution of about 0.7 light-years, small enough to probe the spatial structures of the cores. The region is in our galaxy, about fifteen thousand light-years away, and is known to have more than ten potentially star forming cores with masses between one hundred and one thousand solar-masses. Three of them show signs that star formation is underway, and the scientists observed these three in their submillimeter continuum emission and the molecular emission from their carbon monoxide gas and several other species.

Each of the three cores is slightly different in mass, temperature, gas motions, and substructure, perhaps in part because they are in different stages in their star formation activity. The astronomers find magnetic fields in all three of the clumps, but the strengths also differ slightly from between 1.6 and 0.32 milliGauss (for comparison, the strength of the magnetic field at the Earth’s surface is on average about 500 milliGauss). Their analysis of the energetics shows that turbulence in the gas motions dominates (or compares to) the effects of magnetic fields and that the magnetic force alone cannot prevent gravitational collapse. However the fields may play a key role in another way: There are twelve outflows from the young stars in these cores, and half of them are roughly aligned with the magnetic field directions. Since outflows are related to the disk structures around young stars, it suggests that the fields play a key role in shaping the disks as they develop in the early stages of star formation.

Magnetic Fields in the Early Stages of Massive Star Formation as Revealed by ALMA ~ Junhao Liu et al
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The Evolving Chemistry of Protoplanetary Disks

Post by bystander » Fri Sep 11, 2020 8:28 pm

The Evolving Chemistry of Protoplanetary Disks
SAO Weekly Science Update | 2020 Sep 11
Planets form from the gas and dust in disks that surround young stars. Chemicals in the disk that evaporate easily, called volatiles, include important molecules like water, carbon monoxide, nitrogen, as well as other simple organic molecules. The amount of volatile material that accumulates in a planet as it forms is a key factor in determining the planet’s atmosphere and suitability for life, and depends on the details of the gas and ice reservoirs in the disk at the time of planet formation. Since disk compositions evolve over disk lifetimes, astronomers interested in planet composition are working hard to understand the evolution of disk chemistry. They have already determined that water and carbon monoxide gas are depleted in young systems as compared with their abundances in the normal interstellar medium, sometimes by as much as a factor of one hundred. Current thinking argues that this is because the volatiles have frozen onto the surfaces of dust grains that then accumulate toward the cold midplane of the disk where they remain frozen out. Since each volatile has different properties, however, each one is depleted to a different extent; oxygen is the most depleted element, followed by carbon and then nitrogen. This general framework explains the observations of the few individual sources studied, but astronomers still lack a systematic view of how volatile chemistry evolves with time.

CfA astronomers Karin Oberg, Sean Andrews, Jane Huang, Chunhua Qi, and David Wilner were members of a team that used the ALMA facility to study volatiles in five young disk candidates. They combined the results with data from an early study of fourteen more evolved disks and modeled them to develop an evolutionary view of volatile chemistry over the disks' lifetimes. They conclude that carbon monoxide depletes quickly -- in the first 0.5 - 1 million years of a disk’s lifetime. They also find that youngest objects, those still deeply embedded in their envelope of natal material, have distinct chemical signatures probably because molecules in the disk are shielded from the ultraviolet radiation that can disrupt the chemical bonds. The scientists also consider whether evaporation of the ice mantles could add ingredients back into the gas but conclude that too many uncertainties still remain to reach a definitive answer and they argue for the need for a larger sample of young disks. The new study is a significant advance in understanding the evolution of the chemistry of young, planet-forming disks.

An Evolutionary Study of Volatile Chemistry in Protoplanetary Disks ~ Jennifer B. Bergner et al
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How Planetary Nebulae Get Their Shapes

Post by bystander » Tue Sep 29, 2020 7:10 pm

How Planetary Nebulae Get Their Shapes
SAO Weekly Science Update | 2020 Sep 18
About seven and one-half billion years from now our sun will have converted most of its hydrogen fuel into helium through fusion, and then burned most of that helium into carbon and oxygen. It will have swollen to a size large enough to fill the solar system nearly to the current orbit of Mars, and lost almost half of its mass in winds. At this stage the very hot remnant star will ionize the ejected material, lighting it up and causing it to glow as a planetary nebula (so-called not because it is a planet but because it surrounds its star). All low-to-intermediate mass stars (stars with between about 0.8 to 8 solar masses) will eventually mature into stars hosting planetary nebulae. This simple description suggests that planetary nebulae should all be spherically symmetric shells, but in fact they come in a wide range of shapes from butterfly or bipolar to eye-like or spiral- shapes. Astronomers think that the stellar wind is somehow responsible for these asymmetries, or perhaps the rapid spinning of the host star plays a role, but so far most of the proposed processes are not efficient enough.

A team of scientists including CfA astronomer Carl Gottlieb used the ALMA facility to study the wind morphology of fourteen planetary nebulae at millimeter wavelengths in an effort to understand the origin of their widely varying structures. Previous observations had found that the winds take complex shapes including arcs, shells, clumps, and bipolar structures, shifting some of the puzzle to how winds acquire their varied structures. The astronomers used high spatial resolution imaging in the emission lines of carbon monoxide and silicon monoxide to map the winds. Comparing the results with other datasets, they conclude that a binary star origin can explain both wind and nebular shapes.

Stars in this mass range, on average, have one companion object orbiting that is more massive than about five Jupiter-masses. Interactions between binary stars are known to dominate the evolution of more massive stars, and the scientists speculate that in these lower mass stars the role of the binary companion can similarly affect the evolution. They estimate the binary’s changing influence on the wind and nebula as the primary star evolves, its wind increases, and the separation grows, and report that they can successfully explain the various nebular morphologies in this evolutionary framework. The new model also solves other related puzzles, such as why certain nebular structures (like disks) tend to be preferentially found around stars with specific chemical enrichments (oxygen or carbon), by tracing them as well to evolutionary stages.

Complex Stellar Winds from Evolved Stars ~ Keith T. Smith
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Nanojets in the Solar Corona

Post by bystander » Tue Sep 29, 2020 7:21 pm

Nanojets in the Solar Corona
SAO Weekly Science Update | 2020 Sep 25
The solar corona, the hot outer layer of the Sun, has a temperature of over a million degrees Kelvin and powers a wind of charged particles, some of which bombard the Earth producing auroral glows and often disrupting global communications. These properties of the corona are poorly understood, but astronomers have hypothesized that the heating is due to a myriad of tiny magnetic energy outbursts, called nanoflares, that result when the strong magnetic field lines in the corona (primarily the result of the motions of charged particles below the solar surface) break and reconnect. However, no direct observations of nanoflares exist and this lack has cast doubt on the proffered solutions to the coronal heating problem.

The technical issues are three-fold. First, although some ultraviolet satellites have seen some small intensity bursts in the nanoflare range, and X-ray observations of coronal magnetic loops have detected heating activity, none of these has been unequivocally linked to magnetic reconnection, the originally proposed mechanism for nanoflares. In fact many scientists thought any such linkage would be difficult to prove because the sizes of nanoflares are so small. Finally, numerical modeling of coronal heating has found that some nanoflare-like intensity bursts could be produced in non-reconnection events, like wave heating, forcing any observational diagnostics to discriminate between these alternative mechanisms at very small scales. Instead, astronomers have focused on large-scale changes in the magnetic field or other processes to study magnetic reconnection. ...

Reconnection Nanojets in the Solar Corona ~ Patrick Antolin et al
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Dust in the Galaxy

Post by bystander » Fri Oct 16, 2020 3:52 pm

Dust in the Galaxy
SAO Weekly Science Update | 2020 Oct 02
Dust is found scattered throughout space. It is composed of fine particles made of silicates, like sand on Earth, or of carbon, and these are often blended with other elements. Dust strongly absorbs light at ultraviolet and visible wavelengths, and in our galaxy large clouds of dust are sometimes seen as dark shapes in front of bright nebula as the dust dims our view of the light behind it (the famous Horsehead Nebula is an example of such optical extinction effects). Dust particles are produced and ejected during the final stages of a star life, or in stellar winds, and are the most important component of the interstellar medium after gas, playing a crucial role in the formation of new stars, in facilitating interstellar chemistry, and in producing circumstellar disks of material from which planets form.

Astronomers have been working for decades to model the physical properties of dust grains, not only to better understand all of the processes just mentioned, but also because knowing the size distribution and composition of the dust allows them to correct for the effects of interstellar extinction and to determine accurately the properties of stars whose light has been partially obscured, their intrinsic brightness, for example. The strength of the interstellar dust absorption, moreover, depends on the wavelength such that it tends to make transmitted starlight appear red (described by a "reddening curve") and an ability to recover intrinsic stellar colors is yet another critical goal of dust models. Not least, the radiation that the dust absorbs heats the grains, causing them to re-radiate the energy in the infrared, with this ubiquitous though uneven emission being a major contributor to the foreground contamination encountered in cosmology experiments trying to measure the cosmic microwave background radiation and its subtly varying distribution on the sky. ...

Several major new conclusions emerged from this study. The common assumption has been that dust's infrared emission and uv/optical absorption are perfectly correlated, but the scientists find that this relation is not fixed, but instead depends on the shape of the reddening curve which in turn depends in part on the composition. They also find, in agreement with expectations, that a larger grain size usually leads to a larger value of extinction, as does a higher proportion of carbonaceous to silicate material; they also describe the temperature dependence of the slope of the infrared emission. The new statistical analysis offers possible solutions to a number of outstanding puzzles, but several issues remain (for example, the assumed spherical shape of all grains), and in ongoing work the scientists will include a wider range of dust models for comparison.

Implications of Grain Size Distribution and Composition for the Correlation
Between Dust Extinction and Emissivity
~ Ioana A. Zelko, Douglas P. Finkbeiner
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The Spin of the Supermassive Black Hole in our Galaxy

Post by bystander » Fri Oct 16, 2020 4:09 pm

The Spin of the Supermassive Black Hole in our Galaxy
SAO Weekly Science Update | 2020 Oct 09
Once a black hole forms, its intense gravitational field produces a surface beyond which even light cannot escape, and it appears black to outsiders. All the details of the complex mix of matter and energy in its past are lost, leaving it so simple that it can be completely described by just three parameters: mass, spin, and electric charge. Astronomers can measure the masses of black holes in a relatively straightforward way by watching how matter moves in their vicinity (including other black holes) under the influence of their gravitational fields. The charges of black holes are thought to be insignificant when positive and negative infalling charges are balanced in number. The spins of black holes are difficult to determine; typically they are determined by interpreting the X-ray emission from the hot inner edge of the accretion disk around the black hole. The spin is quantified by a number between zero and one, and black hole spins have been measured with results ranging from a few tenths to close to the one.

Our Milky Way galaxy hosts a supermassive black hole (SMBH) at its center, Sagittarius A*, with about four million solar-masses. At a distance of about twenty-seven thousand light-years, it is by far the closest such object to us, and even though it is not nearly as active or luminous as other supermassive galactic nuclei, its relative proximity provides astronomers with a unique opportunity to probe what happens close to the "edge" of a massive black hole. The Galactic Center SMBH is surrounded by a cluster of stars and clumps of faintly glowing material, and in recent years astronomers have been able to push tests of General Relativity to new limits by measuring and modeling the motions of these clumps as they swing around the SMBH. The spin of the black hole, however, has not been determined in any consistent fashion, but its value would help constrain models of possible jet activity. ...

An Upper Limit on the Spin of Sgr A* Based on Stellar Orbits in Its Vicinity ~ Giacomo Fragione, Abraham Loeb
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Planet Formation in Stellar Infancy

Post by bystander » Fri Oct 16, 2020 4:23 pm

Planet Formation in Stellar Infancy
SAO Weekly Science Update | 2020 Oct 16
As the slightly spinning material in a pre-stellar condensation collapses to form a star, angular momentum conservation shapes it into a circumstellar disk. Planets form from the gas and dust in these disks, whose structure and evolution are thus keys to unraveling the planet-building process. There are currently two favored scenarios, a core accretion model in which planets assemble through the aggregation of dust grains and a gravitational instability model in which clumps develop during the initial stages of the disk evolution and grow into planetesimals.

Astronomers have been able to image disks in over one hundred young stars so far and infer their presence in many more. Thirty-five of the imaged disks are only about one million years old and have recently begun to disperse the natal cloud of material from which they were born. Imaged at submillimeter wavelengths where the cool dust radiates efficiently, these disks reveal bright rings and empty gaps which are thought to be due to the presence of young planets that shepherd the dust into rings and clear out the gaps. These discoveries imply, however, that planet formation must have begun at even earlier stages -- but no younger stars with disk have been discovered, until now.

CfA astronomer Ian Stephens was a member of a team that used the ALMA millimeter facility to image the object IRS63 whose disk had been identified by the Submillimeter Array but with insufficient resolution to see rings. The infrared emission from this system indicates it is younger than about five hundred thousand years. The star is relatively nearby, only about five hundred light-years distant; ALMA images can resolve structures as small as five astronomical units (one AU is the average Earth-Sun distance), and they reveal two rings and two gaps, the first detection of such protoplanetary disks in such a young star. Notably, the rings and gaps appear to be much less mature than those around older stars in the sense that the rings are much less well-defined and the gaps are not yet cleared of dust. The inner ring is situated about twenty-seven AU from the star, and the outer ring is about fifty-one AU away. ...

Four Annular Structures in a Protostellar Disk Less Than 500,000 Years Old ~ D. M. Segura-Cox et al
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The First Habitable-Zone, Earth-sized Planet Discovered with TESS

Post by bystander » Sat Oct 24, 2020 4:48 pm

The First Habitable-Zone, Earth-sized Planet Discovered with TESS
SAO Weekly Science Update | 2020 Oct 23
TESS, the Transiting Exoplanet Survey Satellite, was launched in 2018 with the goal of discovering small planets around the Sun's nearest neighbors, stars bright enough to allow for follow-up characterizations of their planets' masses and atmospheres. TESS has so far discovered seventeen small planets around eleven nearby stars that are M dwarfs -- stars that are smaller than the Sun (less than about 60% of the Sun's mass) and cooler (surface temperatures less than about 3900 kelvin). In a series of three papers that appeared together this month, astronomers report that one of these planets, TOI-700d, is Earth-sized and also located in its star’s habitable zone; they also discuss its possible climate.

CfA astronomers Joseph Rodriguez, Laura Kreidberg, Karen Collins, Samuel Quinn, Dave Latham, Ryan Cloutier, Jennifer Winters, Jason Eastman, and David Charbonneau were on the teams that studied TOI-700d, one of three small planets orbiting one M dwarf star (its mass is 0.415 solar masses) located one hundred and two light-years from Earth. The TESS data analysis found the tentative sizes of the planets as being approximately Earth-sized, 1.04, 2.65 and 1.14 Earth-radii, respectively, and their orbital periods as 9.98, 16.05, and 37.42 days, respectively. In our solar system, Mercury orbits the Sun in about 88 days; it is so close to the Sun that its temperature can reach over 400 Celsius. But because this M-dwarf star is comparatively cool the orbit of its third planet, even though much closer to the star than Mercury is to the Sun, places it in the habitable zone – the region within which the temperatures allow surface water (if any) to remain liquid when there is also an atmosphere. That makes this Earth-sized planet TOI-700d particularly interesting as a potential host for life. ...

The First Habitable Zone Earth-sized Planet from TESS.
I: Validation of the TOI-700 System
~ Emily A. Gilbert et al The First Habitable Zone Earth-sized Planet from TESS.
II. Spitzer Confirms TOI-700 d
~ Joseph E. Rodriguez et al The First Habitable Zone Earth-sized Planet from TESS.
III. Climate States and Characterization Prospects for TOI-700 d
~ Gabrielle Suissa et al
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