SAO: Weekly Science Updates 2017

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The Shapes of Galaxies

Postby bystander » Sat Jun 24, 2017 1:58 pm

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

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
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The Puzzling Detection of X-Rays from Pluto

Postby bystander » Fri Jun 30, 2017 4:43 pm

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

Fig.1-EMLisse-etal-Icarus-May2017..png

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

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Y-Type Stars

Postby bystander » Mon Jul 10, 2017 6:12 pm

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

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
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Re: Y-Type Stars

Postby BDanielMayfield » Mon Jul 10, 2017 6:30 pm

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.

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Re: Y-Type Stars

Postby Ann » Mon Jul 10, 2017 7:44 pm

BDanielMayfield wrote: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: :arrow:

They don't form like this!

Or so I think anyway!

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Re: SAO: Weekly Science Updates 2017

Postby BDanielMayfield » Mon Jul 10, 2017 8:15 pm

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
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Re: Y-Type Stars

Postby neufer » Mon Jul 10, 2017 10:26 pm

BDanielMayfield wrote:
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 :?:
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Re: Y-Type Stars

Postby BDanielMayfield » Tue Jul 11, 2017 2:57 am

neufer wrote:
BDanielMayfield wrote:
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
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