SAO: Science Updates 2019

Find out the latest thinking about our universe.
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The Principles of Star Formation

Post by bystander » Tue Dec 03, 2019 12:45 am

The Principles of Star Formation
SAO Science Updates | 2019 Nov 29
Nearly forty years ago, as he thought about the dramatic new millimeter wavelength observations of gas in star-forming molecular clouds and their newly derived masses and motions, Gary Larson realized a few basic principles seemed to be at work. Larger clouds showed a wider range of motions, the motions implied that the kinetic energies were roughly balanced (in equilibrium) with the gravitational energies so that the clouds held together, and smaller clouds hosted denser gas. Combined, these observations could be explained by the straightforward roles of gravity and gravity driven turbulence. These principles have guided studies of star formation in the decades since, but recent measurements have begun to highlight situations in which they do not quite apply.

CfA astronomer Maria Jimenez-Donaire and her colleagues reconsider the data on carbon monoxide (CO) emission from molecular clouds in the Milky Way, CO being the most abundant and frequently used species, and test them using models with some slightly alternative relationships. They conclude that the differences between the ideal Larson behaviors of molecular clouds and the observed ones can often be attributed to the presence of CO emission along the line-of-sight from clouds that are not related to the primary source. In the case of dense cores, they argue that an additional complication could come from selection effects that preferentially pick out conventional cores. Among their most significant conclusions is that clouds satisfying the Larson conditions are not necessarily in equilibrium. As a result, an accurate analysis of a cloud requires a detailed understanding of its structure.

What Is the Physics behind the Larson Mass–Size Relation? ~ Javier Ballesteros-Paredes et al
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The Discovery of a Black Hole Binary Companion

Post by bystander » Tue Dec 10, 2019 9:12 pm

The Discovery of a Black Hole Binary Companion
SAO Science Updates | 2019 Dec 06
Most galaxies are thought to host a supermassive black hole at their nucleus, and dozens have been indirectly measured using techniques ranging from imaging (the most famous recent example using the Event Horizon Telescope) to modeling the motion of matter that orbits them. Our Milky Way, for example, has a four million solar-mass black hole at its center; the most extreme examples are estimated to have as much as ten billion solar-masses. At the extreme other end of the mass scale, X-ray observations of matter accreting onto stellar-mass black holes from orbiting companion stars have measured about sixty cases whose masses are a few to ten solar-masses. Gravitational wave detections have so far made roughly a dozen convincing detections of black holes with masses from a few to dozens of solar-masses. The wide gap between these stellar mass sized objects and supermassive ones in galaxy cores is striking and thought to be due to their different origins: stellar-mass black holes are the result of stars going supernova (stars are smaller than about sixty solar-masses) while supermassive black holes, though more mysterious, are thought to grow from mergers and massive accretion.

CfA astronomer Rosanne Di Stefano was a member of a team of astronomers that announced in the latest issue of Nature the discovery of a stellar-mass black hole in a binary star system with a very hot star with a period of 78.9 days and a separation of about one astronomical unit, the widest separation known. The scientists used an optical spectrometer to search for periodic velocity changes in the spectra of each of three thousand target stars known to be members of binaries and thought likely to host an extreme companion. Each target has been measured twenty-six times since the project's start in 2016 using LAMOST (Large Aperture Multi-Object Spectroscopic Telescope). One star in particular, designated LB-1, was found to have a large, unseen companion. Analysis of followup observations with much higher spectral resolution instruments determined its mass to be about sixty-eight solar-masses, with an uncertainly of about ten solar-masses due in large part to the uncertainty in the viewing angle of the orbit.

This massive object is almost surely a black hole because there are no stars this massive (any that might form would have exploded as supernova) but that leaves the origin of this object as something of a puzzle. The team considers several possibilities by varying the mass of the progenitor star, its winds (winds make it lose mass), details of the supernova process, and more complicated initial binary star configurations. In the end, they conclude that some of the more unusual but possible alternatives offer a consistent scenario, thereby shedding new light not only on the population of black holes in this mass range, but also on extreme situations of stellar collapse and binary star evolution. The discovery also signals the likelihood that other black holes will be discovered by this technique, with corresponding insights into how they form.

A wide star–black-hole binary system from radial-velocity measurements ~ Jifeng Liu et al
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Re: The Discovery of a Black Hole Binary Companion

Post by BDanielMayfield » Thu Dec 12, 2019 6:34 pm

Here's the abstract of the above paper:
All stellar mass black holes have hitherto been identified by X-rays emitted by gas that is accreting onto the black hole from a companion star. These systems are all binaries with black holes below 30 M⊙1−4. Theory predicts, however, that X-ray emitting systems form a minority of the total population of star-black hole binaries 5,6. When the black hole is not accreting gas, it can be found through radial velocity measurements of the motion of the companion star. Here we report radial velocity measurements of a Galactic star, LB-1, which is a B-type star, taken over two years. We find that the motion of the B-star and an accompanying Hα emission line require the presence of a dark companion with a mass of 68+11−13 M⊙, which can only be a black hole. The long orbital period of 78.9 days shows that this is a wide binary system. The gravitational wave experiments have detected similarly massive black holes 7,8, but forming such massive ones in a high-metallicity environment would be extremely challenging to current stellar evolution theories 9−11.
So we have a very massive stellar mass BH who's binary companion orbits too far away for mass exchange. How are such high stellar mass BHs forming? New theories are needed to explain such systems.

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Probing the Solar Wind Up Close

Post by bystander » Tue Dec 17, 2019 7:19 pm

Probing the Solar Wind Up Close
SAO Science Updates | 2019 Dec 13
su201949[1].jpg
Artist's concept of the Parker Solar Probe spacecraft approaching the Sun. Launched
in 2018, its first pass in this past year has provided new data on solar wind activity
and made critical contributions to our ability to forecast major space-weather events
on Earth. Credit: NASA/GSFC/Parker Solar Probe

The sun glows with a surface temperature of about 5500 degrees Celsius but its hot outer layer, the corona, has a temperature of over a million degrees. The corona ejects a wind of charged particles, and in 1957 Eugene Parker realized that this wind, already known to be responsible for the direction of comets' tails, could move faster than the speed of sound and could easily bombard the Earth. He developed a theory for the solar wind, and today that wind is known for producing auroral glows and even disrupting global communications.

There are two important, longstanding, and related questions about the wind that astronomers have been working to answer: How does the corona become heated to temperatures so much hotter than the surface, and how does the corona generate and then shape the wind as it expels particles into space? The approximate answer to the first question involves the ionized material in the hot corona. The moving gas generates powerful magnetic field loops that, when they twist and break, can accelerate charged particles. The answer to the second question has been even more difficult to ascertain because the solar wind has only been sampled so far by spacecraft whose closest approach to the sun has been about thirty million miles, about the same distance from the sun as the orbit of Mercury. By this distance, however, scientists think that the wind has already undergone changes that obscure key details of its driving sources in the corona. ...

Probing the energetic particle environment near the Sun ~ D. J. McComas et al Alfvénic velocity spikes and rotational flows in the near-Sun solar wind ~ J. C. Kasper et al Near-Sun observations of an F-corona decrease and K-corona fine structure ~ R. A. Howard et al Highly structured slow solar wind emerging from an equatorial coronal hole ~ S. D. Bale et al
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Simulating Galactic Outflows

Post by bystander » Fri Dec 20, 2019 8:01 pm

Simulating Galactic Outflows
SAO Science Updates | 2019 Dec 20
Astronomers have known for decades that massive outflows of gas are being ejected from galaxies. These fast-moving, bipolar streams act to slow down the rate of star formation and inhibit the gravitational collapse of the galaxy, and they help to counterbalance the inflow of material from the intergalactic medium. Two physical mechanisms power these outflows, supernovae explosions in star-forming regions and winds produced in the vicinities of the central supermassive blackholes as they accrete material. Understanding these processes is essential to understanding how galaxies develop, but attempts using numerical simulations have been stymied for decades because both star formation and black hole accretion operate at small scales,roughly ten billion times smaller than the scale of the whole galaxy and its host environment. It is computationally very challenging to model both large-scale and small-scale processes with the same code. As a result, cosmological simulations of galaxy evolution developed over the years have not been able to be compared directly to observations of outflows.

The Illustris project is an international collaboration that has been producing simulated galaxy evolution scenarios for over five years. The smallest sizes in its simulations are about 2300 light-years and, to describe processes occurring in volumes smaller than that,the code invokes a generic algorithm rather than perform detailed calculations.The project has been extremely successful in being able to reproduce the vast cosmological web of galaxies that developed after the big bang. IllustrisTNG ("the next generation") is a new version of the Illustris simulation project that partially addresses the scale problem by focusing on detailed consideration of selected small volumes while still capturing the essential large-scale processes. The IllustrisTNG50 simulation, the third and final version in this series, simulates activity in dimensions as small as hundreds of light-years in an overall volume fifty million parsecs (163 million light-years) on a side, offering a unique combination of both large volume and fine resolution. ...

First Results from the TNG50 Simulation: Galactic Outflows Driven
by Supernovae and Black Hole Feedback
~ Dylan Nelson et al First Results from the TNG50 Simulation: The Evolution of Stellar
and Gaseous Disks Across Cosmic Time
~ Annalisa Pillepich et al
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Blazar Variability

Post by bystander » Tue Dec 31, 2019 8:45 pm

Blazar Variability
SAO Science Updates | 2019 Dec 27
Active galactic nuclei (AGN) are supermassive black holes at the centers of galaxies that are accreting material. These AGN emit jets of charged particles that move at speeds close to that of light, transporting huge amounts of energy away from the central black hole region and radiating across the electromagnetic spectrum. Blazars are extreme examples of AGN in which the collimated jets are coincidentally aligned towards us. Blazar jets have two peak emission wavelengths, one that spans the range from the radio to the X-ray, the result of charged particle acceleration, and one at extremely short wavelength, high energy gamma ray bands usually (and somewhat controversially) attributed to the charged particles scattering infrared "seed" photons from a variety of other sources. All these bands manifest strong and unpredictable variability. Simultaneous, long-term observations across multiple bands therefore, by modeling the relative timing of flares and other variable emission, offer a valuable way to investigate the numerous possible physical mechanisms at work.

CfA astronomer Mark Gurwell was a member of a large team of astronomers that monitored variability of the blazar CTA102 from 2013-2017 spanning the electromagnetic spectrum from radio to gamma rays, in particular using the Submillimeter Array to measure crucial short (mm/submm) wavelength radio emission. Although this bright blazar had been under surveillance since 1978, it was only since the launch of the Compton Gamma Ray Observatory in 1992 that its gamma-ray variability was discovered, and the launch of the Fermi Gamma-Ray Space Telescope mission 2008 enabled continued observations.

In 2016, CTA102 entered a new phase of very high gamma-ray activity, flaring for a few weeks with corresponding emission changes at all wavelengths. In December of that year a flare was spotted that was more than 250 times brighter than its usual faint state. Several detailed physical scenarios were proposed for that event, one of them based on changes in the geometrical orientation of the jets. In the new paper, the team notes that because the two emission peaks arise from two different processes with different geometrical characteristics, the geometrical scenario can be tested. The gamma-ray and optical fluxes arise from the same particle motions in the jets, for example, and should be strongly correlated. The astronomers undertook an analysis of all the available variability data from 2013-2017. They conclude that an inhomogeneous, curved jet modulated by changes in orientation can explain the long-term flux and spectral evolution of CTA102 in a straightforward way.

Investigating the Multiwavelength Behaviour of the Flat Spectrum
Radio Quasar CTA 102 During 2013–2017
~ F D’Ammando et al
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