Author Topic: Astronomy Thread  (Read 83144 times)

Online ExoExplorer

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Re: Astronomy Thread
« Reply #500 on: 07/14/2018 08:15 AM »
An asteroseismic view of the radius valley: stripped cores, not born rocky (arXiv accepted paper)

[Snipped abstract]

It seems that things have rapidly moved on from the question as to whether there is a 'Fulton gap' to questions about the characterisation of that gap! I also note that the gap itself is being used to clearly delineate super-Earths from sub-Neptunes. Previously it seemed that people used these as synonyms for exoplanets with sizes between Earth and Neptune.

Of course the use of Earth and Neptune in a classification scheme is, if not entirely arbitrary, somewhat anthropocentric. After all, there's no clear distinction between a super-Earth, Earth and, say, Venus, or between a sub-Neptune, Neptune and, say, Uranus. I suspect that sooner or later these terms will give way to classifications that span these objects and have the gap as their upper and lower bounds.

There is something different between Earth and super-Earths, and Neptune and sub-Neptunes. The ambiguity of these dictions lead people to think super-Earths are just the scaled up Earth and sub-Neptunes are just simply the scaled down Neptune with no clear compositional or formational distinctions, defining the boundary is just the matter of time.

In fact, they actually represent totally distinct planet populations which do not present in our solar system. Solar system planets can be compositionally categorized into three groups: rocky planets with little ice (<1%) and no gases (Mercury, Venus, Earth and Mars), gaseous (>80%) planets with little rocks and ice (Jupiter, Saturn), and icy (>50%) planets with little gases (<20%) and rocks (Neptune, Uranus).

Based on the location of the gap and atmospheric escape models, now we have a good reason to suspect that the majority of planets with radius under 1.6 (the so-called super-Earths) is actually rocky planets with >1% gases and no ice, but the atmospheres were blown away during later evolution. This population implies a formation process of rocky planets that is complete different from that of solar system rocky planets. Most planets with radius between 1.6 and 3 might share similar formation process (mainly rock with higher mass fraction of gases) with super-Earths but are enveloped by higher mass fraction of gases which could not be blown away. That is exactly why we need ARIEL to show us how alien these planets are.

Ah, the vagaries of astronomical terminology! :) It seemed to me (I could, of course, be under a misapprehension; but, that's how it seemed to me) that initially the term 'super-Earth' simply meant any planet with a mass bigger than Earth but less than Neptune and the term 'sub-Neptune' meant any planet with a mass less than Neptune but bigger than Earth. The overlap was obvious, but different astronomers preferred one or the other and as there was no scientific justification at the time to prefer one or the other both terms persisted. There was a tendency over time for some people to prefer 'super-Earth' for objects at the lower end of that mass range and 'sub-Neptune' for those at the upper, but nothing definitive until the demonstration of the Fulton or radius gap gave a clear(ish) boundary to hang such a distinction on. And that's where it seems to me the position is at present!

(One wrinkle is that people initially defined these terms with reference to the planet's mass (as given by radial velocity detections) but this is giving way to defining them with reference to the radius (as given by transit detections). There is some correlation of course, but both definitions seem to be persisting and should ideally be sorted out! Perhaps the use of r and m subscripts? :) )

I take your point about the different formation histories of the rocky planets of the solar system and those in the gap itself. Perhaps these should indeed be put into separate classes with (presumably!) the term super-Earth being reserved for the class which has the Earth as a member. And something similar for the term sub-Neptune? If so, this makes the mass or radius of the Earth and Neptune seem even more arbitrary as boundaries for these classes! What would be the real distinction between Venus, Earth and a super-Earth, etc?

My one caveat in having different classification classes for such objects based on their formation history is that although the distinction is clear conceptually, in practice how easy is it to determine the formation history of a particular object?
Complete understandable, some scientists have already begin to change the astronomical language and terms, like this one https://doi.org/10.1038/s41550-017-0042

Offline eeergo

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Re: Astronomy Thread
« Reply #501 on: 07/14/2018 10:21 AM »
MeerKAT radio telescope inaugurated in South Africa – reveals clearest view yet of centre of the Milky Way


http://www.ska.ac.za/media-releases/meerkat-radio-telescope-inaugurated-in-south-africa-reveals-clearest-view-yet-of-center-of-the-milky-way/
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Offline Star One

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Astronomy Thread
« Reply #502 on: 07/14/2018 10:38 AM »
Could those filaments be “cosmic strings” caught in the gravity well of the SMB?
« Last Edit: 07/14/2018 11:01 AM by Star One »

Offline eeergo

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Re: Astronomy Thread
« Reply #503 on: 07/14/2018 01:26 PM »
These filaments have been known since the 90s, and there's still debate about what causes them, but the current consensus seems to point toward magnetically-stabilized (rather than gravitationally) dust:

http://www.astronomy.com/news/2018/02/milky-way-core-revealed-in-clearest-infrared-image-yet (February)
https://gizmodo.com/new-south-african-telescope-releases-epic-image-of-the-1827572028 (July)
https://www.nrao.edu/pr/2004/filaments/ (2004)
https://link.springer.com/chapter/10.1007/978-94-009-1687-6_31 (1996)
« Last Edit: 07/14/2018 01:28 PM by eeergo »
-DaviD-

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Re: Astronomy Thread
« Reply #504 on: 07/16/2018 06:56 AM »
The Hunt for Earth’s Deep Hidden Oceans

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According to the standard tale, Earth’s water was imported. The region around the sun where the planet formed was too hot for volatile compounds like water to condense. So the nascent Earth started out dry, getting wet only after water-rich bodies from the distant solar system crashed into the planet, delivering water to the surface. Most of these were likely not comets but rather asteroids called carbonaceous chondrites, which can be up to 20 percent water by weight, storing it in a form of hydrogen like ringwoodite.

But if there’s a huge stockpile of water in the transition zone, this story of water’s origin would have to change. If the transition zone could store 1 percent of its weight in water — a moderate estimate, Jacobsen said — it would contain twice the world’s oceans. The lower mantle is much drier but also voluminous. It could amount to all the world’s oceans (again). There’s water in the crust, too. For subduction to incorporate that much water from the surface at the current rate, it would take much longer than the age of the planet, Jacobsen said.

https://www.quantamagazine.org/the-hunt-for-earths-deep-hidden-oceans-20180711/

Offline Star One

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Re: Astronomy Thread
« Reply #505 on: 07/17/2018 03:46 PM »
12 New Moons of Jupiter Discovered

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It may come as a surprise, then, that astronomers still find new moons orbiting the largest planet in the solar system. But 12 more moons were just discovered, bringing the total to 79, the most of any planet that orbits the sun. These small moons were not discovered by a spacecraft, but rather by powerful telescopes on Earth—and by a team, led by Carnegie Institution for Science astronomer Scott Sheppard, that didn't even set out to look for them.

"These moons are the last remnants of the objects that the planets were built from," Sheppard tells Popular Mechanics via email. "Most of the small objects that helped build the planets we see today were incorporated into the planets themselves, and these moons are all that remains."

https://www.popularmechanics.com/space/a22185640/dozen-new-moons-discovered-jupiter/


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Re: Astronomy Thread
« Reply #506 on: 07/17/2018 08:26 PM »
The Hawaii Infrared Parallax Program. III. 2MASS J0249-0557 c: A Wide Planetary-mass Companion to a Low-mass Binary in the beta Pic Moving Group

We have discovered a wide planetary-mass companion to the β Pic moving group member 2MASSJ02495639-0557352 (M6 VL-G) using CFHT/WIRCam astrometry from the Hawaii Infrared Parallax Program. In addition, Keck laser guide star adaptive optics aperture-masking interferometry shows that the host is itself a tight binary. Altogether, 2MASSJ0249-0557ABc is a bound triple system with an 11.6+1.0−1.3 MJup object separated by 1950±200 AU (40") from a relatively close (2.17±0.22 AU, 0.04") pair of 48+12−13 MJup and 44+11−14 MJup objects. 2MASSJ0249-0557AB is one of the few ultracool binaries to be discovered in a young moving group and the first confirmed in the β Pic moving group (22±6 Myr). The mass, absolute magnitudes, and spectral type of 2MASSJ0249-0557 c (L2 VL-G) are remarkably similar to those of the planet β Pic b (L2, 13.0+0.4−0.3 MJup). We also find that the free-floating object 2MASSJ2208+2921 (L3 VL-G) is another possible β Pic moving group member with colors and absolute magnitudes similar to β Pic b and 2MASSJ0249-0557 c. β Pic b is the first directly imaged planet to have a "twin," namely an object of comparable properties in the same stellar association. Such directly imaged objects provide a unique opportunity to measure atmospheric composition, variability, and rotation across different pathways of assembling planetary-mass objects from the same natal material.

https://arxiv.org/abs/1807.05235

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Re: Astronomy Thread
« Reply #507 on: 07/19/2018 04:41 AM »
A new planet transiting LHS 1140 is discovered with an orbital period 3.8 day. The real properties like radius are yet to be published. I will do some calculation to show a possible radius and compositional range of this planet.
A 3.8 day planet signal with less than 3 Earth-mass is also discovered in radial velocity date after reanalyzing the Dittmann et al's work.
Transmission spectroscopy study revises the LHS 1140 b radius to 1.63 Earth-radius.
« Last Edit: 07/19/2018 06:50 AM by ExoExplorer »

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Re: Astronomy Thread
« Reply #508 on: 07/19/2018 06:47 AM »
Just did a little calculation with the transit depth of LHS-1140c in the graph.

There are three sources of LHS-1140 radius available: 0.186±0.013 Rs (original study), ~0.2 Rs (new transmission spectroscopy), and 0.223±0.013 Rs (Gaia DR2).

planet radius (in Earth unit) = Transit depth^0.5 * stellar radius / Earth radius

According to the graph, I am pretty confident that the relative flux of LHS-1140 drops to ~0.997 when planet c transits, so the transit depth is 0.003.

Take 0.19 Rs (new transmission spectroscopy) and 0.23 Rs (Gaia DR2) as stellar radius lower limit and upper limit, I get 1.25±0.12 Rp for the radius of planet c.

The additional radial velocity of c is presented in arxiv.org/abs/1807.02483, showing that the mass likely falls somewhere in between 1.5 and 2 Earth-mass.
Therefore, the bulk density of c would be 5.4±2.2g/cm3.

Its composition lies between oceanworld and rocky planet (including Earth-like composition). Unless we can further constrain the planetary mass and radius, there is no a sure conclusion to its volatile inventory.

The short orbital distance makes this planet extremely hot and uninhabitable, contrasting to its neighbor b.
« Last Edit: 07/19/2018 06:53 AM by ExoExplorer »

Offline Star One

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Re: Astronomy Thread
« Reply #509 on: 07/19/2018 01:32 PM »
Wandering Star May Have Disrupted Outer Solar System's Order

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There's something strange about the outer solar system — and that could be the signature of a long-ago visit to our neighborhood, according to a new study that looked to simulate how the outer solar system might have ended up so oddly arranged.

https://www.space.com/41212-wandering-star-disturbed-outer-solar-system.html

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Re: Astronomy Thread
« Reply #510 on: 07/19/2018 02:04 PM »
Supersharp Images from New VLT Adaptive Optics

ESO’s Very Large Telescope (VLT) has achieved first light with a new adaptive optics mode called laser tomography — and has captured remarkably sharp test images of the planet Neptune, star clusters and other objects. The pioneering MUSE instrument in Narrow-Field Mode, working with the GALACSI adaptive optics module, can now use this new technique to correct for turbulence at different altitudes in the atmosphere. It is now possible to capture images from the ground at visible wavelengths that are sharper than those from the NASA/ESA Hubble Space Telescope. The combination of exquisite image sharpness and the spectroscopic capabilities of MUSE will enable astronomers to study the properties of astronomical objects in much greater detail than was possible before.

The MUSE (Multi Unit Spectroscopic Explorer) instrument on ESO’s Very Large Telescope (VLT) works with an adaptive optics unit called GALACSI. This makes use of the Laser Guide Star Facility, 4LGSF, a subsystem of the Adaptive Optics Facility (AOF). The AOF provides adaptive optics for instruments on the VLTs Unit Telescope 4 (UT4). MUSE was the first instrument to benefit from this new facility and it now has two adaptive optics modes — the Wide Field Mode and the Narrow Field Mode [1].

The MUSE Wide Field Mode coupled to GALACSI in ground-layer mode corrects for the effects of atmospheric turbulence up to one kilometre above the telescope over a comparatively wide field of view. But the new Narrow Field Mode using laser tomography corrects for almost all of the atmospheric turbulence above the telescope to create much sharper images, but over a smaller region of the sky [2].

With this new capability, the 8-metre UT4 reaches the theoretical limit of image sharpness and is no longer limited by atmospheric blur. This is extremely difficult to attain in the visible and gives images comparable in sharpness to those from the NASA/ESA Hubble Space Telescope. It will enable astronomers to study in unprecedented detail fascinating objects such as supermassive black holes at the centres of distant galaxies, jets from young stars, globular clusters, supernovae, planets and their satellites in the Solar System and much more.

Adaptive optics is a technique to compensate for the blurring effect of the Earth’s atmosphere, also known as astronomical seeing, which is a big problem faced by all ground-based telescopes. The same turbulence in the atmosphere that causes stars to twinkle to the naked eye results in blurred images of the Universe for large telescopes. Light from stars and galaxies becomes distorted as it passes through our atmosphere, and astronomers must use clever technology to improve image quality artificially.

To achieve this four brilliant lasers are fixed to UT4 that project columns of intense orange light 30 centimetres in diameter into the sky, stimulating sodium atoms high in the atmosphere and creating artificial Laser Guide Stars. Adaptive optics systems use the light from these “stars” to determine the turbulence in the atmosphere and calculate corrections one thousand times per second, commanding the thin, deformable secondary mirror of UT4 to constantly alter its shape, correcting for the distorted light.

MUSE is not the only instrument to benefit from the Adaptive Optics Facility. Another adaptive optics system, GRAAL, is already in use with the infrared camera HAWK-I. This will be followed in a few years by the powerful new instrument ERIS. Together these major developments in adaptive optics are enhancing the already powerful fleet of ESO telescopes, bringing the Universe into focus.

This new mode also constitutes a major step forward for the ESO’s Extremely Large Telescope, which will need Laser Tomography to reach its science goals. These results on UT4 with the AOF will help to bring ELT’s engineers and scientists closer to implementing similar adaptive optics technology on the 39-metre giant.

https://www.eso.org/public/news/eso1824/

Here’s a video as well.




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Re: Astronomy Thread
« Reply #511 on: 07/20/2018 07:55 PM »
How Disc Galaxies Work

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Disc galaxies like our own Milky Way, characterized by a flattened disc of stars and gas (often with a central bulge of material as well) have a wide range of masses, spatial extents, and stellar content. Nonetheless all disc galaxies, both locally and in the distant Universe, share some strikingly similar properties. Most notable is that the star formation rate correlates tightly with the galaxy’s gas content, the gas motions (the "velocity dispersion"), and the dynamical lifetime (roughly, the time it takes for the galaxy to rotate once). Moreover, this curiously universal rate is remarkably small: only about one per cent of the gas in disc galaxies turns into stars over that timescale, with much of the activity concentrated in the galaxies’ central regions. Most simple models of star formation predict that gravity should be much more effective in forming stars as it compresses the gas in molecular clouds. Observations indicate that both the correlations and the inefficiency extend down to the scale of individual molecular clouds.

https://www.cfa.harvard.edu/news/su201829