SAO: Weekly Science Updates 2017

[bystander]

The Shapes of Galaxies
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jun 23

[img3="An image of the colliding galaxies NGC4676, "The Mice," as seen by Hubble. An analysis of eighteen thousand colliding galaxies in the computer simulation Illustris has found that mergers like this one are the dominant mechanism determining the shape of galaxies more massive than the Milky Way, while for lower mass galaxies mergers do not play a significant role. (Credits: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA)"]http://imgsrc.hubblesite.org/hvi/upload ... lpaper.jpg[/img3][hr][/hr]

Since Edwin Hubble proposed his galaxy classification scheme in 1926, numerous studies have investigated the physical mechanisms responsible for the shapes of spiral and elliptical galaxies. Because the processes are complex, however, studies frequently rely on computer simulations as their main tool. The discs of galaxies are believed to form through the collapse of gas which acquires its initial spin in the early Universe. During their subsequent evolution, galaxies undergo a wide range of phenomena, from the accretion of matter -- or its outflow -- to mergers with other galaxies, all of which modify the disk’s spin and angular momentum.

Astronomers think that spiral galaxies with the largest galactic discs formed preferentially in protogalaxies with the highest angular momentum, although early attempts to verify this prediction using computer simulations failed. (More recently, simulations have been able to verify this trend.) Elliptical galaxies, on the other hand, are believed to be the remnants of repeated galaxy mergers, but their shapes depend on many details like the galaxies' masses, gas content, and the collision parameters. As a result, these mergers need to be considered over a cumulative, cosmological context with large numbers of examples to evaluate their development from a statistical perspective.

CfA astronomers Vicente Rodriguez-Gomez, Annalisa Pillepich and Lars Hernquist led a team that analyzed the morphologies of about eighteen thousand galaxies in the Illustris computer simulation. Both disc and spheroidal galaxies arise naturally in this simulation. They find that massive merging galaxies develop into spirals or spheroidal shapes depending on their gas content (as expected, since the star formation activity depends crucially on the gas). Unexpectedly, they find that for lower mass galaxies -- roughly the mass of the Milky Way or smaller -- mergers do not seem to play a significant role in determining the morphology. The reason appears to be that in higher mass mergers a galaxy accretes many more stars from the partner, and this plays the a critical role. Their significant conclusion is that only in massive galaxies are mergers the dominant factor in shaping the system.

The Role of Mergers and Halo Spin in Shaping Galaxy Morphology - Vicente Rodriguez-Gomez et al

• Monthly Notices of the RAS 467(3):3083 (June 2017) DOI: 10.1093/mnras/stx305
  arXiv.org > astro-ph > arXiv:1609.09498 > 29 Sep 2016 (v1), 01 Feb 2017 (v2)

[bystander]

The Puzzling Detection of X-Rays from Pluto
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jun 30

[c][attachment=0]Fig.1-EMLisse-etal-Icarus-May2017..png[/attachment][/c][hr][/hr]

Pluto is the largest known body belonging to the Kuiper Belt, an orbiting disk of small objects that extends roughly from the orbit of Neptune to fifty AU from the Sun (one AU is the average distance of the Earth from the Sun). Pluto is known to have an atmosphere which changes size and density with its seasons, and preliminary results from the New Horizons flyby revealed that the atmosphere is primarily composed of nitrogen. Pluto, like all solar system objects, is immersed in the interplanetary solar wind, and the way it interacts with the wind depends on the properties of its atmosphere. Most models of Pluto’s atmosphere before the flyby expected it to be quite extended. When the solar wind interacts with neutral gas like nitrogen it is expected to induce X-ray emission; such emission is seen from other solar system bodies, like comets, Venus and Mars. Astronomers therefore decided to look for analogous emission from Pluto’s atmosphere using the Chandra X-ray Observatory.

CfA astronomer Scott Wolk was a member of a team that undertook the Chandra measurements. From its close flyby, New Horizons found that Pluto's atmosphere was not as extended as had been expected with an escape rate of the gas into space that is hundreds of times smaller than expected. But, to the surprise of the team, the X-ray emission was strong anyway, noticeably stronger than would have been expected for the smaller atmosphere. X-rays from other solar system objects arise from strong aurorae, for example, or the scattering of solar x-rays from small dust grains composed of carbon, nitrogen, and oxygen. Pluto's X-rays, although relatively strong, are unlike these in their energy distribution. The cause of the X-ray emission remains mysterious, but the astronomers speculate that it could be due to some process(es) that focus the solar wind near Pluto to enhance the effect of its modest atmosphere.

The Puzzling Detection of X-rays from Pluto by Chandra - C. M. Lisse et al

• Icarus 287:103 (May 2017) DOI: 10.1016/j.icarus.2016.07.008
  arXiv.org > astro-ph > arXiv:1610.07963 > 25 Oct 2017

http://asterisk.apod.com/viewtopic.php?p=262250#p262250
http://asterisk.apod.com/viewtopic.php?t=36388

[bystander]

Y-Type Stars
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jul 07

[img3="An artist's conception of a brown dwarf star, an object that is more massive and hotter than a planet but not massive enough to become a normal star. Y-type brown dwarfs are the coolest subset with surface temperatures between about 200-500 degrees kelvin. A new study of the twenty-four known Y-dwarfs finds that the models for the coolest of them fail to explain the observed properties.
Credit: NASA/JPL-Caltech/T. Pyle (IPAC)"]https://www.cfa.harvard.edu/sites/www.c ... 201725.jpg[/img3][hr][/hr]

Brown dwarf stars are failed stars. Their masses are so small, less than about eighty Jupiter-masses, that they lack the ability to heat up their interiors to the roughly ten million kelvin temperatures required for normal hydrogen burning (hydrogen burning fuels the Sun, whose surface temperature is about 5700 kelvin). The surface temperatures and properties of brown dwarfs depend on their precise masses and ages, and range from a few thousand degrees down to a mere 200 kelvin (comparable to the Earth’s surface temperature) with the warmest group being designated as L Dwarfs, the next warmest group as T Dwarfs, and the coolest objects as Y Dwarfs. Not surprisingly, because they are so cool, brown dwarfs are faint and hard to detect, and so although theorists predict that there could be as many brown dwarf stars as there are normal stars our understanding of their evolution and interior properties is quite incomplete.

NASA's Wide-field Infrared Survey Explorer (WISE), which was sensitive to the emission from cool objects, discovered the Y class of brown dwarfs in 2011, and today there are twenty-four of them known. CfA astronomer Caroline Morley and her colleagues used the Spitzer Space Telescope and the Gemini observatory, as well as some other facilities, to refine the distances, luminosities, colors, and spectral characteristics of these objects and compared the results to current models. The scientists determined the masses and ages for twenty-two of them, and confirmed that, at least for the slightly warmer Y-dwarfs (whose temperatures are around 450 kelvin) the cloud-free surface models agree with observations. All of them have elemental abundances comparable to those found in the Sun, and all appear to have turbulent atmospheres. However for the coolest few objects, whose temperatures are more like 250 kelvin, the models do not agree. A larger sample of objects for study would help to constrain the parameters, but the authors note that it is unlikely more will be found until a more sensitive infrared mission is flown.

The Y-type Brown Dwarfs: Estimates of Mass and Age from New Astrometry,
Homogenized Photometry, and Near-infrared Spectroscopy - S. K. Leggett et al

• Astrophysical Journal 842(2):118 (2017 Jun 20) DOI: 10.3847/1538-4357/aa6fb5
  arXiv.org > astro-ph > arXiv:1704.03573 > 12 Apr 2017

[BDanielMayfield]

Y, as in whY call it a star? Many of these now colder than room temperature Brown Dwarfs may have never been able to ignite fusion at all.

Bruce

[Ann]

Y, as in whY call it a star? Many of these now colder than room temperature Brown Dwarfs may have never been able to ignite fusion at all.

Bruce

Click to play embedded YouTube video.

Good question, Bruce. I think - and make that think - that these little Y thingies are called stars because they form like stars. That is, they form at the center of a cool rotating dusty gas cloud, not from the accretion disk surrounding the young star that has already formed.

So they form like this, apart from the fusion part of the process:

They don't form like this!

Or so I think anyway!

Ann

[BDanielMayfield]

But since all stars are thought to form in pairs that formation distinction may not really exist, since most stars would have formed as part of an orbiting clump of cloud too.

The common definition of 'star' is broad, simply meaning point of light in the sky, without regard to what powers the illumination. The heat and light is first produced by gravitational compression. That would be common to all "stars", even the ones that fail to ignite any fusion at the core.

Bruce

[neufer]

Y, as in whY call it a star?

Many of these now colder than room temperature Brown Dwarfs may have never been able to ignite fusion at all.

Brown Dwarfs are all assumed to fuse deuterium (²H) and/or lithium (⁷Li).

(We certainly think that we understand that much better than how anything was once formed.)

If it fuses it is a star; if it doesn't fuse it (probably) isn't; Y R U confused

[BDanielMayfield]

Y, as in whY call it a star?

Many of these now colder than room temperature Brown Dwarfs may have never been able to ignite fusion at all.

Brown Dwarfs are all assumed to fuse deuterium (²H) and/or lithium (⁷Li).

(We certainly think that we understand that much better than how anything was once formed.)

If it fuses it is a star; if it doesn't fuse it (probably) isn't; Y R U confused

There are things I never knew (vast set), things I think I know (some of which are true, some just possible, and some false), and some things I used to know but have forgotten. Plenty of room for confusion.

Yeah, I should have remembered that by definition all BDs begin fusion but cannot fuse H via the proton-proton chain. Doh!

Bruce

[bystander]

The Formation of Giant Planets
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jul 14

[img3="A Hubble image of the protoplanetary disk around TW Hydra. Submillimeter wavelength observations have been able to measure the distribution of the material in the disk and its implication for the growth of gas giant planets. (Credit: NASA, ESA, and Z. Levay (STScI/AURA)"]https://www.cfa.harvard.edu/sites/www.c ... 201726.jpg[/img3][hr][/hr]

The planets in our Solar System formed in orbits that depended on the initial distribution of the matter in solar nebula. In particular, the most popular theory for the formation of giant gas planets argues that their rock-and-ice cores formed gradually through coagulation of smaller planetesimals until they were massive enough to accrete gaseous envelopes. The spatial distribution of gas in a primitive nebula is therefore critical not only to the accretion of the atmosphere of its giant planets but also to the formation of these early planetesimals. Many young stars are ringed by disks of dust from which new planets will form. Since that dust emits in the infrared, astronomers have been studying the rings in the infrared to constrain the models of solar system evolution. Only about one percent of the matter is in the form of dust however; the bulk is in gas, which is much harder to detect. Astronomers have tried, but so far have primarily only been able to detect the surface layer of the gas lying above the bulk mass reservoir.

CfA astronomer Ilse Cleeves and her colleagues used the ALMA facility to obtain the first spatially resolved observations of gas emission in a protoplanetary disk, the closest one to us around the star TW Hydrae. They observed it in a relatively rare isotopic species of CO that enabled them to probe the full thickness of the disk. By combining their results with other datasets, they were able to constrain the temperature, gas and dust abundances throughout the disk. They were also able to measure how these quantities vary with distance from the star, in particular in the key zone from about five to twenty astronomical units where giant planets are expected to form (one AU is the average distance of the Earth from the Sun). ...

Mass Inventory of the Giant-Planet Formation Zone in a Solar Nebula Analogue - Ke Zhang et al

• Nature Astronomy 1:0130 (15 May 2017) DOI: 10.1038/s41550-017-0130
  arXiv.org > astro-ph > arXiv:1705.04746 > 12 May 2017

[bystander]

Mapping Dark Matter
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jul 21

[img3="Abell 2744, a cluster of galaxies whose dark matter halo has imaged more distant galaxies as seen in this Hubble Space Telescope image. Astronomers have compared the image to simulations of dark matter lensing and found excellent agreement, indicating that that current models of dark matter behavior on the large scale are quite good. (Credit: NASA, ESA, HFF Team (STScI)"]https://www.cfa.harvard.edu/sites/www.c ... 201727.jpg[/img3][hr][/hr]

About eighty-five percent of the matter in the universe is in the form of dark matter, whose nature remains a mystery. The rest of the matter in the universe is of the kind found in atoms. Astronomers studying the evolution of galaxies in the universe find that dark matter exhibits gravity and, because it is so abundant, it dominates the formation of large-scale structures in the universe like clusters of galaxies. Dark matter is hard to observe directly, needless to say, and it shows no evidence of interacting with itself or other matter other than via gravity, but fortunately it can be traced by modeling sensitive observations of the distributions of galaxies across a range of scales.

Galaxies generally reside at the centers of vast clumps of dark matter called haloes because they surround the clusters of galaxies. Gravitational lensing of more distant galaxies by dark matter haloes offers a particularly unique and powerful probe of the detailed distribution of dark matter. So-called strong gravitational lensing creates highly distorted, magnified and occasionally multiple images of a single source; so-called weak lensing results in modestly yet systematically deformed shapes of background galaxies that can also provide robust constraints on the distribution of dark matter within the clusters.

CfA astronomers Annalisa Pillepich and Lars Hernquist and their colleagues compared gravitationally distorted Hubble images of the galaxy cluster Abell 2744 and two other clusters with the results of computer simulations of dark matter haloes. They found, in agreement with key predictions in the conventional dark matter picture, that the detailed galaxy substructures depend on the dark matter halo distribution, and that the total mass and the light trace each other. They also found a few discrepancies: the radial distribution of the dark matter is different from that predicted by the simulations, and the effects of tidal stripping and friction in galaxies are smaller than expected, but they suggest these issues might be resolved with more precise simulations. Overall, however, the standard model of dark matter does an excellent and reassuring job of describing galaxy clustering.

Mapping substructure in the HST Frontier Fields cluster lenses and in cosmological simulations - Priyamvada Natarajan et al

• Monthly Notices of the RAS 468(2):1962 (June 2017) DOI: 10.1093/mnras/stw3385
  arXiv.org > astro-ph > arXiv:1702.04348 > 14 Feb 2017 (v1), 26 Feb 2017 (v2)

http://asterisk.apod.com/viewtopic.php?t=36910

[bystander]

The Outer Galaxy
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Jul 28

[img3="An artist's reconstruction of the Milky Way galaxy showing the locations of the
various spiral arms. Astronomers have detected massive young stars forming in
the outer part of the Scutum-Centarus Arm, the outermost portions of the galaxy.
Credit: NASA/JPL-Caltech"]https://www.cfa.harvard.edu/sites/www.c ... 01728.jpeg[/img3][hr][/hr]

The Sun is located inside one of the spiral arms of the Milky Way galaxy, roughly two-thirds of the way from the galactic center to the outer regions. Because we are inside the galaxy, obscuration by dust and the confusion of sources along our lines-of-sight make mapping the galaxy a difficult task. Astronomers think that the galaxy is a symmetric spiral, and about ten years ago CfA astronomers Tom Dame and Pat Thaddeus using millimeter observations of the gas carbon monoxide discovered symmetric components to the spiral arms deep in the inner galaxy that lent support to this model.

The galaxy is not perfectly flat. It has a slight warp that allows some distant structures, at least in the direction of the constellations of Scutum and Centaurus, to be seen more distinctly above much of the foreground confusion. In 2011 the same CfA astronomers were the first to discover a large-scale spiral feature within this distant warp which they called the “Outer Scutum–Centaurus Arm (OSC).” Subsequent studies placed the OSC at a distance from the galactic center of over forty thousand light-years; it appears to be a symmetric counterpart to a spiral arm on the opposite side, in the direction of Perseus. ...

High-Mass Star Formation in the Outer Scutum-Centaurus Arm - W. P. Armentrout et al

• Astrophysical Journal 841(2):121 (2017 Jun 01) DOI: 10.3847/1538-4357/aa71a1
  arXiv.org > astro-ph > arXiv:1705.02795 > 08 May 2017

[Ann]

Smithsonian Astrophysical Observatory wrote:

CfA astronomer Tom Dame has joined with a set of collaborators to probe the extent of massive star formation in the OSC (Outer Scutum-Centaurus Arm); sadly, his colleague Pat Thaddeus passed away earlier this year.

Using radio measurements of ionized gas, which traces the hot ultraviolet from massive young stars, as well as bright emission from masers associated with massive star formation, the scientists observed 140 candidate locations and discovered evidence for massive young stars in about sixty percent of them.

Sorry for growing unnecessarily poetic, but I find this really moving. An astronomer, Pat Thaddeus, is diligently searching an awkwardly placed part of the sky for signs of star formation in a previously undiscovered outer spiral arm of our galaxy, and he finds enough evidence to make it very likely that this outer arm exists. Bur before Pat Thaddeus could see the his work come to an end, he died.

This is how astronomy is being done by humans. Individual tiny, terribly brief carbon-based life forms search the incredibly huge universe to tease out evidence of what this amazing vastness is, and what components it contains.

The tiny little carbon-based life forms work hard and make large and small discoveries. Some time afterwards, the little life forms grow feeble and die in less than a cosmic blink of an eye. Other, equally brief life forms preserve their deceased colleagues' work and keep on adding to it, themselves ceasing to be soon afterwards but handing over their knowledge to yet another generation of astronomers. Thus the scientists of humanity use their brief little lives to build an ever more complete (but never truly complete) picture of the universe.

Seen this way, humanity's curiosity and courage is quite moving.

Click to play embedded YouTube video.

Ann

[bystander]

Magnetic Fields in Massive, Star Formation Cores
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Aug 04

[img3="A far-infrared image of the long filament of star formation activity known as DR21, seen here in emission by the Herschel Space Telescope. A study of the magnetic field along the filament and around six star-forming cores within it finds that magnetic effects are primarily important during the early stages of star formation.
Credit: ESA/Herschel"]https://www.cfa.harvard.edu/sites/www.c ... 201729.jpg[/img3][hr][/hr]

Studies of molecular clouds have revealed that star formation usually occurs in a two step process. First, supersonic flows compress the clouds into dense filaments light-years long, after which gravity collapses the densest material in the filament into cores. In this scenario, massive cores (each more than about twenty solar–masses) preferentially form at intersections where filaments cross, producing sites of clustered star formation. The process sounds reasonable and is expected to be efficient, but 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 magnetic fields support the cores against the collapse induced by self-gravity.

Magnetic fields are difficult to measure and difficult to interpret. CfA astronomers Tao-Chung Ching, Qizhou Zhang, and Josep Girat led a team that used the Submillimeter Array to study six dense cores in a nearby star formation region in Cygnus. They measured the field strengths from the polarization of the millimeter radiation; elongated dust grains are known to be aligned by magnetic fields and to scatter light with a preferred polarization direction. The scientists then correlated the field direction in these cores with the field direction along the filament out of which the cores developed.

The astronomers find that the magnetic field along the filament is well-ordered and parallel to the structure, but at the cores themselves the field direction is much more complex, sometimes parallel and sometimes perpendicular. They conclude that during the formation of the cores the magnetic fields, at least at small scales, become unimportant compared to turbulence and infall. Although the field may play an important role as the filament initially collapses, once the dense cores develop the local kinematics from infall and gravitational effects become more important.

Magnetic Fields in the Massive Dense Cores of the DR21 Filament:
Weakly Magnetized Cores in a Strongly Magnetized Filament - Tao-Chung Ching et al

• Astrophysical Journal 838(2):121 (2017 Apr 01) DOI: 10.3847/1538-4357/aa65cc
  arXiv.org > astro-ph > arXiv:1703.02566 > 07 Mar 2017

[bystander]

Properties of a Massive Galaxy 800 Million Years after the Big Bang
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Aug 11

[img3="A Hubble image of the galaxy cluster Abell 1689, which acts as a lens to focus the light from much more distant galaxies, including some very dusty star-forming galaxies in the early universe (seen as the nearly point-like blue smudges in this image). A submillimeter study of a different massive dusty galaxy in the early universe uses carbon monoxide gas to characterize the interstellar medium and determine the mass and star-formation rate. Credit: NASA, ESA, Hubble Heritage Team (STScI/AURA)"]https://cdn.spacetelescope.org/archives ... c1317a.jpg[/img3][hr][/hr]

Searches for the most distant galaxies have now probed earlier than the first billion years in the history of the universe, early enough to start seeing the primary effects of the first stars: the reionization of neutral atoms. Astronomers want to understand how galaxies formed and evolved in this period, the timescale over which this reionization took place, the nature of the objects that provided the ionizing photons, and the scenarios in which galaxies and their interstellar medium (ISM) become enriched with atoms made in stellar furnaces. Although galaxies from this era are currently being discovered in deep optical and near-infrared surveys, most of them are low-mass galaxies, very faint, and the enrichment process is difficult to study. More luminous, massive star-forming galaxies are thought to be present and to play a major role in reionization, but because these large objects are difficult to assemble so early in cosmic time there are not many of them.

Massive star-forming galaxies that contain dust emit strongly radiation at submillimeter wavelengths and these objects can be find using submillimeter telescopes. They therefore offer the opportunity to study extreme cases of metal/dust enrichment of the ISM early in the era of reionization. CfA astronomers Matt Ashby and Chris Hayward were members of a large team using the South Pole Telescope to detect a set of these dusty galaxies. They determined their distances using the ALMA telescopes by looking at the redshifted wavelength of carbon monoxide molecule in their ISM. The farthest known dusty galaxy was detected in this way, and subsequent observations of it with other facilities confirmed its cosmological distance. The scientists constrained the properties of the object by modeling the observed continuum and spectral lines, and found that the object has a mass in gas of about 330 billion solar-masses; for comparison, the estimated gas mass of the Milky Way is about five billion solar-masses (most of its mass is in stars). The dusty galaxy is forming new stars at an estimated rate of several thousand per year - although with the assumption that the process is similar to what is seen in nearby galaxies. This rare and distant object offers one of the best probes so far into the activity in galaxies when the universe was very young.

ISM Properties of a Massive Dusty Star-Forming Galaxy Discovered at z ~ 7 - M. L. Strandet et al

• Astrophysical Journal Letters 842(2):L15 (2017 Jun 20) DOI: 10.3847/2041-8213/aa74b0
  arXiv.org > astro-ph > arXiv:1705.07912 > 22 May 2017 (v1), 25 May 2017 (v2)

[bystander]

The Origin of Binary Stars
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Aug 18

[c][attachment=0]53a4cf9444c742510cbaa3c550ef4f74[1].jpg[/attachment][/c][hr][/hr]

The origin of binary stars has long been one of the central problems of astronomy. One of the main questions is how stellar mass affects the tendency to be multiple. There have been numerous studies of young stars in molecular clouds to look for variations in binary frequency with stellar mass, but so many other effects can influence the result that the results have been inconclusive. These complicating factors include dynamical interactions between stars that can eject one member of a multiple system, or on the other hand might capture a passing star under the right circumstances. Some studies, for example, found that younger stars are more likely to be found in binary pairs. One issue with much of the previous observational work, however, has been the small sample sizes.

CfA astronomer Sarah Sadavoy and her colleague used combined observations from a large radio wavelength survey of young stars in the Perseus cloud with submillimeter observations of the natal dense core material around these stars to identify twenty-four multiple systems. The scientists then used a submillimeter study to identify and characterize the dust cores in which the stars are buried. They found that most of the embedded binaries are located near the centers of their dust cores, indicative of their still being young enough to have not drifted away. About half of the binaries are in elongated core structures, and they conclude that the initial cores were also elongated structures. After modeling their findings, they argue that the most likely scenarios are the ones predicting that all stars, both single and binaries, form in widely separated binary pair systems, but that most of these break apart either due to ejection or to the core itself breaking apart. A few systems become more tightly bound. Although other studies have suggested this idea as well, this is the first study to do so based on observations of very young, still embedded stars. One of their most significant major conclusions is that each dusty core of material is likely to be the birthplace of two stars, not the single star usually modeled. This means that there are probably twice as many stars being formed per core than is generally believed.

Embedded Binaries and Their Dense Cores - Sarah I Sadavoy, Steven W. Stahler

• Monthly Notices of the RAS 469(4):21 (Aug 2017) DOI: 10.1093/mnras/stx1061
  arXiv.org > astro-ph > arXiv:1705.00049 > 28 Apr 2017

[bystander]

Extreme Jets
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Sep 08

[img3="V404 Cygni is a binary star system in which a sun-like star is orbited by a black hole. X-rays are produced when the black hole's gravity pulls matter in from the normal star. In this X-ray image, V404 Cyg is the bright point source at the center and the bright rings are X-ray "echoes" produced by walls of dust. Astronomers have modeled the recent flare as a combination of eight discrete ejections.
Credit: Andrew Beardmore (Univ. of Leicester) and NASA/Swift"]https://www.cfa.harvard.edu/sites/www.c ... 201732.jpg[/img3][hr][/hr]

A black hole X-ray binary (BHXB) is a black hole orbiting a normal star. When matter from the normal star accretes onto the black hole, a jet of charged particles is ejected at relativistic (near-light) speeds, and these particles emit strong X-ray radiation. The processes involved are thought to be similar to ones active under the more dramatic conditions in active galactic nuclei. Most known BHXBs are located in our galaxy, and being much closer to us they can be studied in more detail than their more distant cousins.

Black hole X-ray binaries occasionally flare in outbursts that can last days to weeks, offering an opportunity to probe how their jets evolve. Two different types of relativistic jets are known, depending on the accretion rate of mass in the system. At low mass accretion rates, the magnetic fields bend the compact jet, prompting it to emit radiation. At high accretion rates, discrete jet ejecta are launched that can interfere with this process in several ways, resulting in more complex emission characteristics. (A very rare third type of emission displays quasi-periodic oscillations.) There are usually a few bright BHXB events each year, but the more powerful kind occurs only about once a decade.

On June 15, 2015, the BHXB V404 Cygni underwent just such a rare, active outburst, and CfA astronomers Glen Petitpas and Mark Gurwell were members of a team that obtained simultaneous radio through submillimeter observations of the emission using the Submillimeter Array along with the Very Large Array and the James Clerk Maxwell Telescope (SCUBA-2). They tracked the activity over four hours, during which time they saw multiple, rapidly changing flares that were bright at all the frequencies they observed. The scientists best-fitting model worked well with eight discrete, bipolar, jet ejection events. The model also estimated the speed, structural properties, geometry, and energetics of the jets. These unprecedented coordinated observations of a BHXB highlight the importance of multi-band observations in studying BHXB jet emission. ...

Extreme Jet Ejections from the Black Hole X-ray Binary V404 Cygni - A.J. Tetarenko et al

• Monthly Notices of the RAS 469(3):3141 (Aug 2017) DOI: 10.1093/mnras/stx1048
  arXiv.org > astro-ph > arXiv:1704.08726 > 27 Apr 2017

http://asterisk.apod.com/viewtopic.php?t=34912
http://asterisk.apod.com/viewtopic.php?t=34968
http://asterisk.apod.com/viewtopic.php?t=36682

[geckzilla]

Oh, x-ray echoes! A form of light echo I have not seen until now.

[MargaritaMc]

A quick note to point out that http://asterisk.apod.com/viewtopic.php?t=34912 has the info that the star is about half the mass of the Sun and the black hole is about 12 Earth masses. Therefore the BH will not be orbiting the star (as is said in the SAO image text), but both will be orbiting the common centre of mass.

@geck - light echoes are splendid aren't they?! Nadia Drake's article was what got me excited about them.
http://phenomena.nationalgeographic.com ... e-capsule/

[bystander]

The Stratosphere of a Hot Exoplanet
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Sep 15

[img3="An artist's conception of the hot-Jupiter exoplanet WASP-121b with its star. Astronomers have discovered in this object the first unambiguous signs of a hot stratosphere, heated to a blazing 2400 Celsius. Credit: NASA, ESA, G. Bacon (STScI)"]https://www.cfa.harvard.edu/sites/www.c ... 201734.jpg[/img3][hr][/hr]

The stratosphere of a planetary atmosphere is the layer in which the temperatures rise with altitude, in contrast to the tropospheric layer (near the ground) in which the temperature falls with altitude. On the Earth the stratosphere begins at thirty to sixty thousand feet, depending on the latitude; it terminates at about 160,000 feet. The temperature inversion of the Earth's stratosphere is caused principally by ozone, which exists at these higher altitudes and absorbs sunlight to heat the layer. A planet's stratosphere thus contains information about its chemical composition, and astronomers probing the atmospheres of exoplanets have wondered whether any of them show signs of having a stratosphere. The molecules in this warm gas layer should emit diagnostic spectral lines that could be spotted from Earth.

CfA astronomer Mercedes Lopez-Morales was the Co-Principal Investigator in a large team of international exoplanet hunters who discovered a stratosphere on the exoplanet WASP-121b, the first unambiguous such discovery. The exoplanet itself, discovered in 2015, is a so-called hot-Jupiter, and orbits its star every 1.27 days. Its mass is slightly larger than Jupiter’s, and it orbits so close to its star that its atmospheric temperature is thought to be heated to about 2200 Celsius (hence the name, "hot Jupiter").

The astronomers used the Hubble Space Telescope and the IRAC camera on the Spitzer Space Telescope to study in the planet during its secondary eclipses: as it passed behind the star, its illuminated Earth-facing side can be seen. The reflected light from the atmosphere enabled scientists to spot faint emission lines of the molecules present, including in this case water molecules at a temperature of about 2400 Celsius, as well as some other species that are still uncertain. The hot gas is consistent with the presence of a stratospheric layer on the planet. Water had been previously spotted on WASP-121b during a normal transit, but this new result shows that there is water in the stratosphere. The models were able to infer even more detailed information about the atmosphere, for example, that the molecules in that layer absorb about twenty percent of the incident stellar radiation, contributing to the stratosphere being hotter than the general atmosphere.

An ultrahot gas-giant exoplanet with a stratosphere - Thomas M. Evans et al

• Nature 548(7665):58 (03 Aug 2017) DOI: 10.1038/nature23266
  arXiv.org > astro-ph > arXiv:1708.01076 > 03 Aug 2017

http://asterisk.apod.com/viewtopic.php?t=37447
http://asterisk.apod.com/viewtopic.php?p=273742#p273742

[bystander]

The Nature of Galaxy Cluster Mini-Halos
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Sep 22

[img3="A galaxy cluster mini-halo as seen around the galaxy NGC 1275 in the radio, with its main structures labeled: the northern extension, the two eastern spurs, the concave edge to the south, the south-western edge and a plume of emission to the south-south-west. Astronomers used radio and X-ray data to conclude that mini-halos, rather than being simple structures resulting from turbulence, are actually the result of multiple processes. Credit: Gendron-Marsolais et al."]https://www.cfa.harvard.edu/sites/www.c ... 201735.jpg[/img3][hr][/hr]

A mini-halo is a faint, diffuse region of radio emission that surrounds a cluster of galaxies. So far about thirty of these cluster mini-halos have been detected via their X-ray and radio emission, the result of radiation from electrons in the ionized gas, including one mini-halo in the nearby Perseus cluster of galaxies. These electrons are thought to arise from activity around a supermassive black hole at a galactic nucleus, which injects steams of particles into the intracluster medium and which also produces turbulence and shocks. One issue puzzling astronomers is that such electrons should rapidly lose their energy, faster than the time it takes for them to reach the mini-halo regions. Suggested solutions include processes in which turbulence reaccelerates the electrons, and in which cosmic rays generate new ones.

CfA astronomer Reinout van Weeren and his colleagues used the radio Karl G. Jansky Very Large Array (JVLA) to obtain the first detailed study of the structure of the mini-halo in Perseus, and to compare it with Chandra X-Ray images. They find that the radio emission comes primarily from gas behind a cold front as would be expected if the gas is sloshing around within the cluster as particles are re-accelerated. They also detect unexpected, filamentary structures that seem to be associated with edges of X-ray features. The scientists conclude that mini-halos are not simply diffuse structures produced by a single process, but reflect a variety of structures and processes including turbulent re-acceleration of electrons, relativistic activity from the black hole jets, and also some magnetic field effects. Not least, the results demonstrate the sensitivity of the new JVLA and the need to obtain such sensitive images to understand the mini-halo phenomenon.

Deep 230-470 MHz VLA Observations of the Mini-Halo in the Perseus Cluster - M. Gendron-Marsolais et al

• Monthly Notices of the RAS 469(4):3872 (Aug 2017) DOI: 10.1093/mnras/stx1042
  arXiv.org > astro-ph > arXiv:1701.03791 > 13 Jan 2017 (v1), 13 Jun 2017 (v2)

http://asterisk.apod.com/viewtopic.php?t=37279

[bystander]

New Insights on Dark Energy
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Sep 29

[img3="A representation of the evolution of the universe over 13.8 billion years. Different methods of studying cosmic expansion yield slightly different results, including for the age of the universe. Astronomers have calculated that these discrepancies could be reconciled if the dark energy that drives cosmic acceleration were not constant in time. Credit: NASA and the WMAP Consortium"]https://www.cfa.harvard.edu/sites/www.c ... 201737.jpg[/img3][hr][/hr]

The universe is not only expanding - it is accelerating outward, driven by what is commonly referred to as "dark energy." The term is a poetic analogy to label for dark matter, the mysterious material that dominates the matter in the universe and that really is dark because it does not radiate light (it reveals itself via its gravitational influence on galaxies). Two explanations are commonly advanced to explain dark energy. The first, as Einstein once speculated, is that gravity itself causes objects to repel one another when they are far enough apart (he added this "cosmological constant" term to his equations). The second explanation hypothesizes (based on our current understanding of elementary particle physics) that the vacuum has properties that provide energy to the cosmos for expansion.

For several decades cosmologies have successfully used a relativistic equation with dark matter and dark energy to explain increasingly precise observations about the cosmic microwave background, the cosmological distribution of galaxies, and other large-scale cosmic features. But as the observations have improved, some apparent discrepancies have emerged. One of the most notable is the age of the universe: there is an almost 10% difference between measurements inferred from the Planck satellite data and those from so-called Baryon Acoustic Oscillation experiments. The former relies on far-infrared and submillimeter measurements of the cosmic microwave background and the latter on spatial distribution of visible galaxies.

CfA astronomer Daniel Eisenstein was a member of a large consortium of scientists who suggest that most of the difference between these two methods, which sample different components of the cosmic fabric, could be reconciled if the dark energy were not constant in time. The scientists apply sophisticated statistical techniques to the relevant cosmological datasets and conclude that if the dark energy term varied slightly as the universe expanded (though still subject to other constraints), it could explain the discrepancy. Direct evidence for such a variation would be a dramatic breakthrough, but so far has not been obtained. One of the team's major new experiments, the Dark Energy Spectroscopic Instrument (DESI) Survey, could settle the matter. It will map over twenty-five million galaxies in the universe, reaching back to objects only a few billion years after the big bang, and should be completed sometime in the mid 2020's.

Dynamical Dark Energy in Light of the Latest Observations - Gong-Bo Zhao et al

• Nature Astronomy 1:627 (28 Aug 2017) DOI: 10.1038/s41550-017-0216-z
  arXiv.org > astro-ph > arXiv:1701.08165 > 27 Jan 2017 (v1), 13 Jul 2017 (v2)

http://asterisk.apod.com/viewtopic.php?t=35198

[MargaritaMc]

New Insights on Dark Energy
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Sep 29

Dynamical Dark Energy in Light of the Latest Observations - Gong-Bo Zhao et al

• Nature Astronomy 1:627 (28 Aug 2017) DOI: 10.1038/s41550-017-0216-z
  arXiv.org > astro-ph > arXiv:1701.08165 > 27 Jan 2017 (v1), 13 Jul 2017 (v2)

A somewhat related topic: http://asterisk.apod.com/viewtopic.php?f=31&t=37567

[bystander]

Astronomers Reveal Evidence of Dynamical Dark Energy
University of Portsmouth, UK | 2017 Oct 03

[img3="The cosmological ‘constant’ (illustrated by the straight yellow line) is introduced to explain the accelerated expansion of the Universe (shown as the expanding blue cone) due to the presence of dark energy. The study instead suggests that the contribution of dark energy to this expansion is time-dependent (grey curve). The uncertainty of this time dependency is also shown (blue shaded area)."]http://uopnews.port.ac.uk/wp-content/up ... %80%99.png[/img3][hr][/hr]

An international research team, including astronomers from the University of Portsmouth, has revealed evidence of dynamical dark energy.

The discovery, recently published in the journal Nature Astronomy, found that the nature of dark energy may not be the cosmological constant introduced by Albert Einstein 100 years ago, which is crucial for the study of dark energy. ...

Revealing the nature of dark energy is one of key goals of modern sciences. The physical property of dark energy is represented by its Equation of State (EoS), which is the ratio of pressure and energy density of dark energy.

In the traditional Lambda-Cold Dark Matter (LCDM) model, dark energy is essentially the cosmological constant, i.e., the vacuum energy, with a constant EoS of -1. In this model, dark energy has no dynamical features. ...

The dynamics of dark energy needs to be confirmed by next-generation astronomical surveys. The team points to the upcoming Dark Energy Spectroscopic Instrument (DESI) survey, which aims to begin creating a 3D cosmic map in 2018.

In the next five to ten years, the world largest galaxy surveys will provide observables which may be key to unveil the mystery of dark energy. ...

[BDanielMayfield]

[img3="The cosmological ‘constant’ (illustrated by the straight yellow line) is introduced to explain the accelerated expansion of the Universe (shown as the expanding blue cone) due to the presence of dark energy. The study instead suggests that the contribution of dark energy to this expansion is time-dependent (grey curve). The uncertainty of this time dependency is also shown (blue shaded area)."]http://uopnews.port.ac.uk/wp-content/up ... %80%99.png[/img3][hr][/hr]

An international research team, including astronomers from the University of Portsmouth, has revealed evidence of dynamical dark energy.

I like this possibility. It deflates the proposed Big Rip. The graph appears to have the Universe oscillating between periods of inflation and deflation, with the amplitude dampening out over time. Must read more about this ...

Bruce

[bystander]

A Young Protostellar Dust Disk
Smithsonian Astrophysical Observatory
Weekly Science Update | 2017 Oct 06

[img3="An artificial color submillimeter image of the disk around the young protostar HH-212. This is the first submillimeter image of the disk around a very early-stage star, and shows a dark lane of cold dense material implying that the small dust grains in the disk have already begun to coalesce. Credit: ALMA (ESO/NAOJ/NRAO)/CF Lee et al"]https://www.cfa.harvard.edu/sites/www.c ... 201738.jpg[/img3][hr][/hr]

Stars form as gravity contracts the gas and dust in an interstellar cloud until cores develop that become dense enough to coalesce into stars. The simple-sounding process is made much more complex by the presence of magnetic fields and rotation, which produce circumstellar disks around the developing star that in turn play a role in controlling the material accreting onto the protostar. Disks as large as 500 astronomical units in radius (one AU is the average distance of the Earth from the Sun) have been detected around Sun-like stars in the later phases of their gestation. Presumably they began forming in the earlier phases, while infalling material was still feeding the infant protostar, and so astronomers have been trying to probe younger protostars. Evidence for the existence of these early stage disks, however, has been slight: a few examples have been found with total radii less than about 150 AU. If there is smaller structure within them, it has not been measurable.

CfA astronomers Qizhou Zhang and Paul Ho and their four colleagues used the Atacama Large Millimeter/submillimeter Array (ALMA) to spatially resolve the dust disk around the young protostar known as HH-212. The disk is seen nearly edge-on with a radius of only sixty AU, and has a prominent equatorial dark lane sandwiched between two brighter layers. The astronomers tentatively estimate the mass of the disk to be about fourteen-thousandths of a solar-mass, and combining the results with earlier observations at other wavelengths they model the dust grains as being as large as a millimeter in size. The dark lane along the midplane of the disk has colder, denser dust than in the outer layers. The large grains signal that the coalescence of small grains into larger ones begins earlier in the lifetime of a star than many had previously thought. The new observations also show that direct detection and characterization of small disks around the youngest protostars is possible. The results provide constraints on theories of disk formation. If small disks turn out to be commonplace, then theoretical models in which magnetic effects inhibit disk formation would need significant revision.

First Detection of Equatorial Dark Dust Lane in a Protostellar Disk at Submillimeter Wavelength - Chin-Fei Lee et al

• Science Advances 3(4):e1602935 (19 Apr 2017) DOI: 10.1126/sciadv.1602935
  arXiv.org > astro-ph > arXiv:1704.08962 > 28 Apr 2017

http://asterisk.apod.com/viewtopic.php?p=270803#p270803
http://asterisk.apod.com/viewtopic.php?t=37277