SAO: Weekly Science Updates 2019

Find out the latest thinking about our universe.
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SAO: Weekly Science Updates 2019

Post by bystander » Fri Jan 04, 2019 5:20 pm

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Chandra Detection of a Circumnuclear Torus

Post by bystander » Fri Jan 04, 2019 5:30 pm

Chandra Detection of a Circumnuclear Torus
SAO Weekly Science Update | 2019 Jan 04
Most galaxies host supermassive black holes at their nuclei, each with millions or billions of solar-masses of material. There is thought to be a torus of dust and gas around the black holes, and an accreting disk that becomes very hot as material falls onto it, in turn heating the torus and circumnuclear gas and dust. Such an active galactic nucleus (AGN) radiates across the spectrum while the dust often blocks the innermost regions from view. Powerful bipolar jets of charged particles are often ejected as well. Radiation from the torus can be seen directly at infrared wavelengths and, when it scatters off the fast moving particles, at X-ray energies.

Active galactic nuclei (AGN) are among the most dramatic and interesting phenomena in extragalactic astronomy. All of the standard AGN models predict the presence of a torus and accretion disk but the details of the region have been difficult to study directly because the torus is thought to be relatively small, only hundreds of light-years in size. The ALMA millimeter array, however, has recently enabled detection of nearby AGN structures in both continuum and molecular line emission. NGC5643 is a face-on spiral galaxy that hosts an AGN and bipolar jets. Last year ALMA spotted an elongated structure in its nucleus about eighty light-years across (about 200 light-years across in emission from the cooler molecular gas component). Scientists had proposed that the structure was the expected AGN torus and the related molecular material responsible for the obscuration of the AGN and the collimation of the jets. ...

Chandra Detection of the Circumnuclear Molecular Torus of the
Compton-thick Active Galactic Nucleus in NGC 5643
~ G. Fabbiano et al
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The Disintegrating Exoplanet K2-22b

Post by bystander » Fri Jan 18, 2019 8:35 pm

The Disintegrating Exoplanet K2-22b
SAO Weekly Science Update | 2019 Jan 11
Image
An artist's conception of K2-22b, an exoplanet slightly
smaller in size than Neptune. Observations suggest
that this exoplanet is in disintegrating, and has debris
in a trailing and leading dust tails. (Credit: NASA)

Exoplanet surveys have yielded many surprises over the years, and the discovery of "disintegrating" exoplanets was one of them. These are planets that produce asymmetric shapes in the dips of the light curves seen as they transit across the faces of their stars. The asymmetry is hypothesized to be due to tails of dusty material from the planets' disintegration. At present, only three such planets known around main sequence stars, one being K2-22b. There are currently over 3800 confirmed exoplanets, suggesting either that such objects are intrinsically rare or that they have very short lifetimes, in which case it is lucky to catch any in the act of disintegration. These systems have been under intense study to better understand their formation and evolution and to constrain the properties of the grains in the dust tails.

CfA astronomers George Zhou, Karen Collins, Allyson Bieryla, and Dave Latham were members of a team that obtained forty-five ground-based observations of the K2-22 system in their study of the evolution of its transit. K2-22b is a Neptune-sized exoplanet that orbits its star in only about nine hours; it is unusual in that it appears to have not only a trailing dust tail but a leading trail as well. The team’s observations of the dust tails included observing the transits at multiple wavelengths to try to use color to characterize the dust grain size or composition, but except in one transit event no differences were seen. The color information is, however, consistent with the previous model of dust grains as being small - comparable to or smaller than optical light wavelengths. The astronomers also confirmed the variability of the transits, thought to be evidence of the continuing rapid evolution of the dust tails. The scientists point out that this variability appears in all three disintegrating planets, and the shape variability occurs on all the timescales observed, from transit to transit and over several years. They conclude that a continuous observing campaign would be a valuable tool in unraveling the mystery of these dusty trails. ...

A Large Ground-based Observing Campaign of the Disintegrating Planet K2-22b ~ Knicole D. Colón et al
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Making Stars When the Universe was Half Its Age

Post by bystander » Fri Jan 18, 2019 8:48 pm

Making Stars When the Universe was Half Its Age
SAO Weekly Science Update | 2019 Jan 18
The universe is about 13.8 billion years old, and its stars are arguably its most momentous handiwork. Astronomers studying the intricacies of star formation across cosmic time are trying to understand whether stars and the processes that produce them were the same when the universe was younger, about half its current age. They already know that from three to six billion years after the big bang stars were being made at a rate roughly ten times faster than they are today. How this happened, and why, are some of the key questions being posed for the next decade of research.

Star formation in a galaxy is thought to be triggered by the accretion of gas from the intergalactic medium (gas accretion via mergers between galaxies is thought to play a relatively minor role in the total numbers of stars produced). In galaxies that are actively making stars there is a tight relationship between their mass in stars and their rate of forming new stars, and this relationship approximately holds not only locally but even back when the universe was billions of years younger. In contrast, galaxies that are undergoing an active starburst - or the opposite, the quenching of star formation - fall above and below that relation respectively. The relationship supports the general picture of galaxy growth by gas accretion, except that for some reason smaller galaxies – those with fewer than about ten billion stars – seem to make slighter fewer stars than expected for their masses (the Milky Way is right at the turnover, with about ten billion stars and a rate of roughly one new star per year). A particularly significant consequence of this paucity, if real, is that simulations of galaxy growth do not show it, implying that the simulations are incorrect for smaller galaxies and that some physics is missing. ...

The MUSE Hubble Ultra Deep Field Survey XI. Constraining the low-mass end of
the stellar mass - star formation rate relation at z < 1
~ Leindert A. Boogaard et al
viewtopic.php?t=37800
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Re: Making Stars When the Universe was Half Its Age

Post by Ann » Sat Jan 19, 2019 6:07 am

Smithsonian Astrophysical Observatory wrote:

The relationship supports the general picture of galaxy growth by gas accretion, except that for some reason smaller galaxies – those with fewer than about ten billion stars – seem to make slighter fewer stars than expected for their masses (the Milky Way is right at the turnover, with about ten billion stars and a rate of roughly one new star per year).
What does it mean that the Milky Way is right at the turnover? Does it mean that if the Milky Way had made just slightly fewer stars than it does, then the Milky Way, too, would have made fewer stars than expected for its mass?

I find it interesting, by the way, that low-mass galaxies make fewer stars than expected for their masses. That would explain why small galaxies in the nearby universe still undergo bright starbursts. These galaxies have a lot of gas "to spare", and if they can get a starburst going, they have a lot of fuel available.

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Re: Making Stars When the Universe was Half Its Age

Post by BDanielMayfield » Sun Jan 20, 2019 6:22 am

Ann wrote:
Sat Jan 19, 2019 6:07 am
Smithsonian Astrophysical Observatory wrote:

The relationship supports the general picture of galaxy growth by gas accretion, except that for some reason smaller galaxies – those with fewer than about ten billion stars – seem to make slighter fewer stars than expected for their masses (the Milky Way is right at the turnover, with about ten billion stars and a rate of roughly one new star per year).
What does it mean that the Milky Way is right at the turnover? Does it mean that if the Milky Way had made just slightly fewer stars than it does, then the Milky Way, too, would have made fewer stars than expected for its mass?

I find it interesting, by the way, that low-mass galaxies make fewer stars than expected for their masses. That would explain why small galaxies in the nearby universe still undergo bright starbursts. These galaxies have a lot of gas "to spare", and if they can get a starburst going, they have a lot of fuel available.

Ann
That didn't make sense to me either Ann. The MW now has many times more than ten billion stars, and I'll expect that it even had many times more than that 6 BYA too.

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A Primordial Star Forming Galaxy

Post by bystander » Mon Jan 28, 2019 6:04 pm

A Primordial Star Forming Galaxy
SAO Weekly Science Update | 2019 Jan 25
Galaxies with extremely high rates of star formation (from hundreds to thousands of solar-masses worth of stars per year) are rare. Our Milky Way, for example, makes only about one star a year. The process of star formation heats up dust to emit in the infrared, and extreme starburst galaxies that make this many per year shine so brightly they can be spotted at cosmological distances. When gravitational lensing by a fortuitously intervening galaxy or cluster of galaxies magnifies the signal, even farther away and cosmically earlier galaxies can be detected. To date only a handful of these extreme starburst galaxies have been confirmed from the universe’s first billion years of existence. Although still a small sample, they offer important insights into how stars were made at primordial times when most chemical elements were less abundant. They also help astronomers understand star formation in cases where the physical processes are so dramatic when compared to the process in our galaxy.

Far infrared and submillimeter sky surveys identified the first extreme galaxies from the emission of dust heated by their star formation activity. The rate of star formation is inferred from the luminosity of the galaxy, and this is calculated from the observed brightness and distance. As usual in astronomy, the distance parameter is key but difficult to measure. For these remote monsters it is generally obtained from the redshift of some strong lines emitted by the galaxy in the far infrared or submillimeter, typically from carbon monoxide (an abundant molecule) and/or from singly ionized atomic carbon. ...

A dusty star-forming galaxy at z = 6 revealed by strong gravitational lensing - Jorge A. Zavala et al
viewtopic.php?t=37735
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Energetic Particles Can Bombard Exoplanets

Post by bystander » Mon Feb 18, 2019 6:42 pm

Energetic Particles Can Bombard Exoplanets
SAO Weekly Science Update | 2019 Feb 15
TRAPPIST-1 is a system of seven Earth-sized worlds orbiting an ultra-cool dwarf star about 120 light-years away. The star, and hence its system of planets, is thought to be between five-to-ten billion years old, up to twice as old as our own solar system. For scientists seeking evidence for life elsewhere, the advanced age provides more time for chemistry and evolution to operate than the Earth had. On the other hand, the planets are all close to the star (in fact they are probably tidally locked to the star with one side always facing it), and consequently would have soaked up billions more year's-worth of high energy radiation from the star’s winds, adversely affecting any atmospheres they host.

In a new paper in The Astrophysical Journal, CfA astronomers Federico Fraschetti, Jeremy Drake, Julian Alvardo-Gomez, Sofia Moschou, and Cecilia Garraffo and a colleague carry out theoretical simulations of the effects of high-energy protons from a stellar wind on nearby exoplanets. These particles are produced by stellar flares or by shock waves driven by magnetic events in the stellar corona. Measurements of solar eruptive events provide the scientists with a basis for their simulations.

The astronomers calculate the first realistic simulation of the propagation of energetic particles through the turbulent magnetic field environment of an M dwarf star and its wind, and they tailored the details to the TRAPPIST-1 system. They find that particles are trapped within the star's magnetic field and are directed into two polar streams focused onto the planets’ orbital plane - independent of many of the details. The scientists conclude that the innermost putative habitable planet in the system, TRAPPIST-1e, is bombarded by a proton flux up to a million times larger than that experienced by the present-day Earth. Nevertheless, there are many variables at play, for example the angle between the magnetic field and the rotation axis of the star, and consequently a large uncertainty remains in how these effects actually are manifest in individual situations.

Stellar energetic particles in the magnetically turbulent
habitable zones of TRAPPIST-1-like planetary systems
~ F. Fraschetti et al
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An Exoplanet With an 11-Hour Orbit

Post by bystander » Thu Feb 21, 2019 7:01 pm

An Exoplanet With an 11-Hour Orbit
SAO Weekly Science Update | 2019 Feb 01
The Transiting Exoplanet Survey Satellite (TESS) was launched on April 18 of last year with the primary objective of discovering transiting planets smaller than Neptune around stars bright enough for spectroscopic investigations of their masses and atmospheres. Before TESS there were roughly 385 exoplanets known with masses smaller than Neptune, with orbital periods ranging from less than half-a-day to about two Earth-years.

CfA astronomers Dave Latham, Samuel Quinn, Dave Charbonneau, Jonathan Irwin, Kristo Ment, Jennifer Winters, Martin Paegert, Dimitar Sasselov, and Willie Torres and a large team of TESS collaborators report that TESS has found a "hot Earth" exoplanet, rocky in composition, only about fifty light-years away and orbiting its dwarf star in a mere eleven hours. The planet has a radius of about 1.3 Earth-radii, enough to host an atmosphere, but its short orbital period means it lies very close to its star – only about seven stellar radii. The inferred surface temperature is about 800 kelvin, rather hot to be able to retain an atmosphere but possible. The scientists note, however, that if the planet had formed in roughly this close-in location, its atmosphere would likely have been stripped away in the star’s youth when it was more luminous and had more intense chromospheric activity. In any case, the planet's proximity to us offers the opportunity of characterizing any atmosphere it might have using transit and occultation spectra of the source and the result, interesting in its own right, would also shed light on the planet's formation.

TESS Discovery of an Ultra-Short-Period Planet around the Nearby M Dwarf LHS 3844 ~ Roland Vanderspek et al
viewtopic.php?t=31141#p288754
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Simultaneous X-Ray and Infrared Observations of the Galactic Center

Post by bystander » Wed Feb 27, 2019 7:08 pm

Simultaneous X-Ray and Infrared Observations of the Galactic Center
SAO Weekly Science Update | 2019 Feb 22
The supermassive black hole (SMBH) at the center of our Milky Way galaxy, Sagittarius A*, is by far the closest such object to us, only about 25 thousand light-years away. Although not nearly as active or luminous as other SMBHs, its relative proximity provides astronomers with a unique opportunity to probe what happens close to the "edge" of a black hole. Monitored in the radio since its discovery and more recently in the infrared and the X-ray, Sgr A* appears to be accreting material at a very low rate, only a few hundredths of an Earth-mass per year. Its X-ray emission is persistent, probably resulting from the rapid motions of electrons in the hot accretion flow associated with the black hole. Once a day there are also flares of emission that are highly variable; they appear more often in the infrared than in X-rays. Some submillimeter wavelength flares have also been tentatively linked to IR flares, although their timing seems to be delayed with respect to infrared events. Despite these intensive observational efforts, the physical mechanisms producing flaring around this SMBH are still unknown and are the topic of intense theoretical modeling.

CfA astronomers Steve Willner, Joe Hora, Giovanni Fazio, and Howard Smith joined their colleagues in undertaking a systematic campaign of simultaneous multiwavelength observations of flaring in SagA* using the Spitzer and Chandra observatories (the Submillimeter Array was also used in some of the series). In over one hundred hours of data taken over four years (the longest such dataset ever obtained), the team observed four flare events in both X-ray and infrared in which the X-ray event appears to lead the infrared by ten to twenty minutes. The correlation between the observed peaks implies there is some physical connection between them, and the slight timing difference is in agreement with models that describe the flares as coming from magnetically driven particle acceleration and shocks. Exactly simultaneous events can’t be completely ruled out, however, but the results are nevertheless inconsistent with some of the more exotic models that involve the relativistic motion of electrons. If future simultaneous observations planned for the summer of 2019 also see flaring, they can provide new constraints on the time lag and on associated physical models.

Simultaneous X-Ray and Infrared Observations of Sagittarius A*'s Variability ~ H. Boyce et al
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Nearby Clustered Star Formation

Post by bystander » Sat Mar 02, 2019 7:51 pm

Nearby Clustered Star Formation
SAO Weekly Science Update | 2019 Feb 08
Most stars form in clusters of hundreds of stars. In contrast to isolated stars, whose formation is increasingly well understood by astronomers, the hows and whys of cluster formation are much less well understood. Issues include the structure of the cluster in its early stages and the physical processes that determine how it fragments into many stars. To address these questions, astronomers focus on observations of clusters of young protostellar objects, those early in their evolutionary lives. The molecular gas associated with these regions is used to reveal the densities and kinematics of the regions.

CfA astronomers Phil Myers, Mike Dunham, and Riwaj Pokhrel joined with colleagues to analyze twenty-four clusters, all closer than about three thousand light-years. The median diameter of these clusters is only about one light-year, and in an area of this radius they host on average about four protostars and one newly formed main-sequence star (for comparison, the nearest star to the Sun, Proxima Centauri, is 4.2 light-years away). Comparisons between the clusters reveal that continuous low-mass star formation often occurs over periods of several millions of years, much longer than the time it typically takes for a single star to become active. This result implies that many generations of stars are present in the cluster. The scientists also found that for the more compact clusters, characterized by sizes of about one light-year, the stars are separated by distances that agree well with simple theoretical expectations about fragmentation. For more diffuse groups, however, the protostellar separations are always larger than expected with the implication that other processes are at work.

Catalog of High Protostellar Surface Density Regions in Nearby Embedded Clusters ~ Juan Li et al
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Discovering a Brown Dwarf Binary Star with Microlensing

Post by bystander » Sat Mar 02, 2019 8:20 pm

Discovering a Brown Dwarf Binary Star with Microlensing
SAO Weekly Science Update | 2019 Mar 01
Brown dwarfs are stars less massive than the sun and unable to burn hydrogen. They comprise (at least in mass) a bridge between planets and stars, and astronomers think that they form and evolve in ways different from either planets or stars. Gravitational microlensing is an excellent method for detecting them because it does not depend on their light, which is dim, but rather their mass. When the path of light from a star passes by a brown dwarf acting as a lens, it is magnified into a distorted image, like an object seen through the stem of a wineglass, allowing the detection and characterization of the lensing object. Thirty-two brown dwarfs have been detected by microlensing so far. Five are in isolation, but most are in binary systems, companions to faint M-dwarf stars. They provide important constraints on brown dwarf formation scenarios.

The critical parameter of a brown dwarf is its mass, but it is difficult to measure the mass of a lens using microlensing. Using this method, one measures the magnified and distorted stellar image as it changed in time (it varies as the Earth's vantage point moves), but the technique offers no handle on the distance, and the larger the distance, the larger is the mass needed to generate the same-sized distortion. Recognizing this problem, scientists had predicted that if it ever became possible to observe a microlensing flash from two well-separated vantage points, a parallax measurement (the apparent angular difference between the positions of the star as seen from the two separated sites) would determine the distance of the dark object. The Spitzer Space Telescope circles the Sun in an Earth-trailing orbit, and is currently 1.66 astronomical units away from Earth (one AU is the average distance of the Earth from the Sun). Spitzer is unique in this capability, and it has in fact been used successfully to measure the parallax distance for hundreds of microlensing events, thereby helping to determine the masses of the lenses.

CfA astronomers Jennifer Yee and In-Gu Shin were members of a large team of microlensing astronomers who used Spitzer together with ground-based telescopes to study an unusual microlensing event. The object, MOA-2016-BLG-231, is located 9400 light-years away in the disk of our galaxy. The shape of its distorted light curve reveals it probably to be a pair of brown dwarfs of masses approximately twenty-one and nine Jupiter-masses, respectively (the smaller one is right at the lower mass limit for being a brown dwarf rather than a giant planet). This is only the fifth brown dwarf binary system discovered in which both objects are brown dwarfs; improved statistics enable astronomers to address the formation mechanisms needed.

Spitzer Microlensing of MOA-2016-BLG-231L: A Counter-Rotating
Brown Dwarf Binary in the Galactic Disk
~ Sun-Ju Chung et al
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First Detection of the Pre-Biotic Molecule Glycolonitrile in Space

Post by bystander » Fri Mar 08, 2019 7:57 pm

First Detection of the Pre-Biotic Molecule Glycolonitrile in Space
SAO Weekly Science Update | 2019 Mar 08
Heterocyclic molecules are those containing atoms of at least two different elements (plus hydrogen) arranged in a ring structure. Nitrogen heterocycles are key components in biological nucleic acids, and in theories of the origins of biogenic molecules they were synthesized from abundant, simpler nitrogen molecules like hydrogen cyanide, HCN. Adenine, one of the four constituent bases of nucleic acids, is thought to have formed from one of the two known two-ring nitrogen heterocycles, glycolonitrile (HOCH2CN). In the cold interstellar medium of space, glycolonitrile could assemble on the surfaces of icy grain surfaces via reactions between formaldehyde (H2CO) and hydrogen cyanide. Astronomers have calculated that glycolonitrile could then be broken apart by ultraviolet light, leaving a variety of simpler nitrogen-bearing molecules, some of which have been detected in molecular clouds in space. Glycolonitrile itself, however, has not been reported leaving a step in the theory of the formation of nucleic acids unconfirmed.

CfA astronomer Rafael Martin-Domenech and his colleagues used the ALMA telescope facility to search for glycolonitrile in the young, solar-type protostar IRAS16293-2422B. This well-studied object lies about five hundred light-years in the constellation of Ophiuchus. It has a cold outer envelope of gas and dust and a hotter inner region heated by the star extending out to about a hundred astronomical units. Numerous, simpler organic molecules had already been seen in this warm zone. The team searched for the characteristic spectral signature of glycolonitrile in three frequency bands of ALMA, and found thirty-five of its transitions that were unambiguous. They modeled the data to reveal two components at two temperatures, about 24K and 158K, coming correspondingly from material in both the cold outer envelope of the star and its hotter inner zone. Their chemical analysis predicts a smaller abundance of the species than is actually seen, for both the cold and warm components, including under a variety of likely conditions including the cosmic ray ionization rate. The team concludes that some other chemical pathways must be operative, but that this critical chemical has now been measured and the theory is in general on the right track.

First Detection of the Pre-Biotic Molecule Glycolonitrile (HOCH2CN) in the Interstellar Medium ~ S. Zeng et al
viewtopic.php?t=30931
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Bright X-Ray Galactic Nuclei

Post by bystander » Sat Mar 16, 2019 4:34 pm

Bright X-Ray Galactic Nuclei
SAO Weekly Science Update | 2019 Mar 15
All massive galaxies are believed to host supermassive black holes (SMBH) at their centers that grow by accreting mass from their environment. The current picture also imagines that the black holes grow in size as their host galaxy evolves, perhaps because galaxy evolution includes accretion triggered, for example, by galaxy mergers. This general picture has been substantiated by two lines of data. The peak epoch of black hole accretion can be measured by observations of nuclear activity, and coincides with the peak epoch of star formation in the universe about ten billion years after the big bang. Star formation is associated with disruptions that stir up the gas and induce accretion. Moreover, the local universe shows a tight correlation between SMBH mass, host galaxy bulge mass, and the spread of stellar velocities. These methods (but with weaker confirmation) can similarly estimate the sizes of SMBH in galaxies in the earlier universe, and find that SMBH growth and galaxy growth are co-evolutionary processes. Indeed, it seems the processes may regulate each other over time to produce the galaxy and SMBH sizes we observe today.

Both central black hole growth and star formation are fed by the abundance of molecular gas and dust that can be traced by the infrared emitted by the dust. Dust grains, heated by the radiation from young stars and AGN accretion, emit strongly in the infrared. Since AGN activity also produces X-rays, the expectation is that AGN should track strong dust emission and that X-ray and infrared emission should be correlated. CfA astronomer Mojegan Azadi was a member of a team that examined 703 galaxies with active SMBH nuclei using both X-ray data from Chandra and infrared from Spitzer and Herschel, the largest sample to date making this comparison. Although the team did find a trend consistent with the infrared correlating with AGN X-ray activity over a wide range of cases, they did not find one when compared with the AGN's infrared (not- X-ray) contributions. Since the AGN infrared comes largely from a dusty emitting torus around the SMBH, the difference could point to the role of the angle with which we view the torus. These results help to refine the current models of AGN activity, but the authors note that more sensitive, deeper observations should be able to sort out more clearly the physical processes associated with the AGN.

Infrared Contributions of X-Ray Selected Active Galactic Nuclei
in Dusty Star-forming Galaxies
~ Arianna Brown et al
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What Ionized the Universe?

Post by bystander » Fri Mar 22, 2019 5:27 pm

What Ionized the Universe?
SAO Weekly Science Update | 2019 Mar 22
The sparsely distributed hot gas that exists in the space between galaxies, the intergalactic medium, is ionized. The question is, how? Astronomers know that once the early universe expanded and cooled enough, hydrogen (its main constituent) recombined into neutral atoms. Then, once newly formed massive stars began to shine in the so-called “era of reionization,” their extreme ultraviolet radiation presumably ionized the gas in processes that continue today. One of the key steps, however, is not well understood, namely the extent to which the stellar ionizing radiation escapes from the galaxies into the IGM. Only if the fraction escaping was high enough during the era of reionization could starlight have done the job, otherwise some other significant source of ionizing radiation is required. That might imply the existence of an important population of more exotic objects like faint quasars, X-ray binary stars, or perhaps even decaying/annihilating particles.

Direct studies of extreme ultraviolet light are difficult because the neutral gas absorbs it very strongly. Because the universe is expanding, the spectrum absorbed covers more and more of the optical range with distance until optical observations of cosmologically remote galaxies are essentially impossible. CfA astronomer Edo Berger joined a large team of colleagues to estimate the amount of absorbing gas by looking at the spectra of gamma-ray burst (GRB) afterglows. GRBs are very bright bursts of radiation produced when the core of a massive star collapses. They are bright enough that when their radiation is absorbed in narrow spectral features by gas along the line-of sight, those features can be measured and used to calculate the amount of absorbing atomic hydrogen. That number can then be directly converted into an escape fraction for the ultraviolet light of the associated galaxy. Although a single observation of a GRB in one galaxy does not provide a robust measure, a sample of GRBs is thought to be able to provide a representative measure across all sightlines to massive stars.

The astronomers carefully measured the spectra of 140 GRB afterglows in galaxies ranging as far away as epochs slightly less than one billion years after the big bang. They find a remarkably small escape fraction – less than about 1% of the ionizing photons make it out into the intergalactic medium. The dramatic result finds that stars provide only a small contribution to the ionizing radiation budget in the universe from that early period until today, not even in galaxies actively making new stars. The authors discuss possible reasons why GRBs might not provide an accurate measure of the absorption, although none is particularly convincing. The result needs confirmation and additional measurements, but suggests that a serious reconsideration of the ionizing budget of the intergalactic medium of the universe is needed.

The Fraction of Ionizing Radiation from Massive Stars
That Escapes to the Intergalactic Medium
~ N.R. Tanvir et al
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Star Formation in Galactic Centers

Post by bystander » Tue Apr 02, 2019 3:58 pm

Star Formation in Galactic Centers
SAO Weekly Science Update | 2019 Mar 29
Stars form from the gas and dust in molecular clouds via a series of complex processes that are currently only partly understood, and the evolution of these clouds drives the evolution of the stellar populations in the universe. Astronomers studying the formation of stars have, over the past decades, concentrated on a few select regions of active star formation: the solar neighborhood, the disc of the Milky Way, and the neighboring Magellanic Cloud galaxies. This range of environments is limited, however, and not representative of the conditions under which most stars in the Universe formed. For instance, the densities, pressures, and motions of the gas in these local environments are considerably lower than those thought to be present during the time of peak cosmic star formation about ten billion years ago. Moreover the disparate conditions make it difficult to untangle evolutionary effects.

Recent Galactic plane surveys at a wide range of wavelengths using facilities like the Submillimeter Array and ALMA telescopes have made it possible to study cloud evolution and star formation in the Central Molecular Zone (CMZ), the central 1500 light-years of the Milky Way, whose extreme physical conditions more nearly resemble those at the peak of cosmic star formation. CfA astronomers Eric Keto and Qizhou Zhang and their colleagues carried out a series of computer simulations of massive molecular clouds in a CMZ environment with the goal of characterizing their morphological and kinematic evolution as they orbit the galactic center in this dense, complex region. These computations are the first specifically aimed at modeling the clouds in the CMZ ridge and were designed to compare against recent observations.

The team finds that the CMZ environment causes the clouds to be compressed, with stress and shear forces fragmenting them and developing features such as filaments and spinning, pancake-like structures. The simulations are able to reproduce key observed features like the “Brick,” a very dense, flattened molecular cloud that, despite its dense gas, lacks star formation activity; the simulations can mimic its general morphology, inclination, and velocity gradients. The results reveal that the evolution of molecular clouds near galactic centers is closely coupled to their orbital dynamics. When accompanied by accretion of gas, these clouds can evolve to produce the starbursts observed in many galactic nuclei.

The Dynamical Evolution of Molecular Clouds Near the Galactic Centre –
II. Spatial Structure and Kinematics of Simulated Clouds
~ J.M.D. Kruijssen et al
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Gamma-Ray Blazars in the Sky

Post by bystander » Sun Apr 07, 2019 4:31 pm

Gamma-Ray Blazars in the Sky
SAO Weekly Science Update | 2019 Apr 05
When the supermassive black holes at the center of galaxies accrete material, they can eject powerful jets of charged particles at speeds approaching that of light. These particles in turn emit radiation across the electromagnetic spectrum, from radio to gamma-rays. When the jets happen to be aligned toward the Earth, these objects are called blazars, and in a flare they can emit as much radiation as a million billion suns.

NASA's Fermi Gamma Ray Astronomy satellite, launched in 2008, has detected many bright gamma-ray sources, but determining what they are and if any are blazars is difficult because Fermi’s resolution on the sky is only about the size of a quarter full moon, and that large area of sky typically contains many sources that could be emitting in the gamma-rays. Moreover, blazars are notoriously variable at high energies and their irregular flickering can make them difficult to pinpoint exactly. Extensive observations of possible candidates with optical or other facilities can be successful but are very time consuming. A team of astronomers including CfA astronomers Raffaele D’Abrusco and Howard Smith found that the infrared colors of blazars are generally unique because the emission, rather than from hot dust as is typical, is instead coming from processes associated with the blazar jets. They used color data from the Wide-field Infrared Survey Explorer (WISE) in conjunction with catalogs of radio source to identify the most promising gamma-ray candidate objects in the Fermi survey and for the past several years have followed them up, with a success rate confirming blazars of about 90%.

The same team of astronomers has now extended their original 2014 work using all the most recent data collected by Fermi and a new algorithm for analyzing the WISE data, and have produced two new catalogs of WISE blazar candidates with a total of 15,120 candidate sources. The new work will enable many more detailed followup analyses of gamma-ray blazars, as well as other kinds of blazars. The gamma-ray sky is known to reflect, in its dramatic appearance, many other kinds of extreme physical processes, for example possible dark matter annihilation activity in some scenarios, and the new catalogs will enable a much more complete investigation of the dominant contributor to the gamma-ray universe: blazars.

Two New Catalogs of Blazar Candidates in the WISE Infrared Sky ~ Raffaele D'Abrusco et al
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Breezing through the Space Environment of Barnard's Star b

Post by bystander » Sat Apr 13, 2019 5:17 pm

Breezing through the Space Environment of Barnard's Star b
SAO Weekly Science Update | 2019 Apr 12
The closest exoplanet to us, if we include only single stars like the Sun, is the planet around Barnard's Star, Barnard's Star-b ("BSb"). (The planet Promixa Centauri-b is closer, but Proxima Cen is part of a triple-star system with Alpha and Beta Centauri, and understanding the evolutionary development of the planet is more complicated.) BSb orbits at a distance similar to that of Mercury around the Sun, but Barnard's Star is a cool M-dwarf star and so despite the planet being close to the star it probably resides near the snow line – the distance where stellar irradiation is weak enough to allow volatile elements to condense onto the planet’s surface. This makes BSb an especially interesting planet and possibly a keystone for future progress understanding planet formation and atmospheric evolution.

Extreme stellar activity and winds, especially in M dwarf stars, play an important role in the development of a planet and its atmosphere. These kinds of activity are linked to a star's magnetic activity, but unfortunately models are still unable to predict how atmospheric initial conditions evolve under intense radiation environments. Nevertheless, progress has been made using simple models. In the case of Proxima Centauri b, scientists have found that it is probably subject to wind pressures ten thousand times larger than occur at the Earth. Might stellar wind effects also be disrupting any atmosphere on Barnard’s Star b?

CfA astronomers Julian Alvarado-Gomez, Cecilia Garraffo, Jeremy Drake, and Sofia Moschou and their colleagues conclude otherwise. The scientists note that BSb is much farther away from its star than is Promixa Cen b, well outside of the domain of the star's corona. Moreover, an analysis of Barnard Star’s rotation and other properties implies that it is much older, between about seven and ten billion years, and any magnetic field processes should be considerably smaller. The astronomers conclude that although today the planet Barnard’s Star b may have a relatively mild space climate (comparable, nevertheless, to bad space weather conditions for Earth), in its early years it probably did undergo significant disruption. Today, however, BSb might retain an atmosphere that could studied.

Breezing through the Space Environment of Barnard's Star b ~ Julián D. Alvarado-Gómez 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.
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