Recent

[Fred56]

So who's following any recent stuff about condensed matter and radio astronomy these days? All the white dwarf and AGN surveys?

[Martin]

So who's following any recent stuff about condensed matter and radio astronomy these days? All the white dwarf and AGN surveys?

 

How about giving some arxiv links? I'm not sure what you are talking about.

 

AGN are active galactic nuclei. A white dwarf is something very different, as I'm sure you know. It is a dead star that is still white-hot. I think it is the normal end state for an average mass star (like the sun for example) after it has done all the red giant stuff. A typcial white dwarf might be made mostly of carbon and oxygen which (because it is not massive enough to force them to fuse) it is unable to use as thermonuclear fuel.

 

I did see some articles about white dwarf end-state recently but nothing special comes to mind.

 

AGN are in distant galaxies, with supermassive BH millions of times more massive than a star.

 

White dwarf remnants are just star-size mass and would normally be seen only in our own galaxy because they would be too faint for us to see them in other galaxies.

 

AGN involve black holes with masses like million of solar. They cause ionized relativistic jets, cosmic rays, gammaray flares.

 

OH I BET I KNOW WHAT YOU ARE TALKING ABOUT! The AGN results from the Argentina Auger observatory which proved that AGN are the origin of ultrahigh energy COSMIC RAYS! UHECR

 

Yes, that was interesting. I would not think of it actually as a SURVEY of AGN because it involved only a dozen or so comparatively nearby galaxies.

 

What they did was make a map of where the UHECR were coming from and they found that within error of about 3 degrees they were all coming from a dozen or so nearby AGN.

 

In some sense this was not surprising. People had suspected for a long time that UHECR come from supermassive BH at center of active galaxies. they just did not have PROOF of that. But it made sense. Now, with the Auger Observatory results, it is pretty much proven, a closed book I'd say. Smoking gun evidence.

 

Was that part of what you were talking about?

 

If so, we have a thread about it already:

http://www.scienceforums.net/forum/showthread.php?t=29504

[Fred56]

I used that particular word (survey) instead of using "astronomy" again tbh.

 

The Sloan survey results have some good stuff about dwarfs, and there are truckloads of papers about unusual ones. Nothing more interesting than this occurred to me, but: there doesn't seem to be many threads about observational astronomy lately in this section (what's it called again?)...

 

Dwarfs of the White kind?

http://sciencenow.sciencemag.org/cgi/content/full/2007/1121/1

 

ScienceDaily (Nov. 22, 2007) — Astronomers have discovered white dwarf stars with pure carbon atmospheres. The discovery could offer a unique view into the hearts of dying stars.

 

These stars possibly evolved in a sequence astronomers didn't know before. They may have evolved from stars that are not quite massive enough to explode as supernovae but are just on the borderline. All but the most massive two or three percent of stars eventually die as white dwarfs rather than explode as supernovae.

 

When a star burns helium, it leaves "ashes" of carbon and oxygen. When its nuclear fuel is exhausted, the star then dies as a white dwarf, which is an extremely dense object that packs the mass of our sun into an object about the size of Earth. Astronomers believe that most white dwarf stars have a core made of carbon and oxygen which is hidden from view by a surrounding atmosphere of hydrogen or helium.

 

They didn't expect stars with carbon atmospheres.

 

"We've found stars with no detectable traces of helium and hydrogen in their atmospheres," said University of Arizona Steward Observatory astronomer Patrick Dufour. "We might actually be observing directly a bare stellar core. We possibly have a window on what used to be the star's nuclear furnace and are seeing the ashes of the nuclear reaction that once took place."

 

Dufour, UA astronomy Professor James Liebert and their colleagues at the Université de Montréal and Paris Observatory published the results in the Nov. 22 issue of Nature.

 

The stars were discovered among 10,000 new white dwarf stars found in the Sloan Digital Sky Survey. The survey, known as the SDSS, found about four times as many white dwarf stars previously known.

 

Liebert identified a few dozens of the newfound white dwarfs as "DQ" white dwarfs in 2003. When observed in optical light, DQ stars appear to be mostly helium and carbon. Astronomers believe that convection in the helium zone dredges up carbon from the star's carbon-oxygen core.

 

Dufour developed a model to analyze the atmospheres of DQ stars as part of his doctoral research at the Université de Montréal. His model simulated cool DQ stars, stars at temperatures between 5,000 degrees and 12,000 degrees Kelvin. For reference, our sun's surface temperature is around 5,780 degrees Kelvin.

 

When Dufour joined Steward Observatory in January, he updated his code to analyze hotter stars, stars as hot as 24,000 degrees Kelvin.

 

"When I first started modeling the atmospheres of these hotter DQ stars, my first thought was that these are helium-rich stars with traces of carbon, just like the cooler ones," Dufour said. "But as I started analyzing the stars with the higher temperature model, I realized that even if I increased the carbon abundance, the model still didn't agree with the SDSS data," Dufour said.

 

In May 2007, "out of pure desperation, I decided to try modeling a pure-carbon atmosphere. It worked," Dufour said. "I found that if I calculated a pure carbon atmosphere model, it reproduces the spectra exactly as observed. No one had calculated a pure carbon atmosphere model before. No one believed that it existed. We were surprised and excited."

 

Dufour and his colleagues have identified eight carbon-dominated atmosphere white dwarf stars among about 200 DQ stars they've checked in the Sloan data so far.

 

The great mystery is why these carbon-atmosphere stars are found only between about 18,000 degrees and 23,000 degrees Kelvin. "These stars are too hot to be explained by the standard convective dredge-up scenario, so there must be another explanation," Dufour said.

 

Dufour and Liebert say they these stars might have evolved from a star like the unique, much hotter star called H1504+65 that Pennsylvania State University astronomer John A. Nousek, Liebert and others reported in 1986. If so, carbon-atmosphere stars represent a previously unknown sequence of stellar evolution.

 

H1504+65 is a very massive star at 200,000 degrees Kelvin.

 

Astronomers currently believe this star somehow violently expelled all its hydrogen and all but a very small trace of its helium, leaving an essentially bare stellar nucleus with a surface of 50 percent carbon and 50 percent oxygen.

 

"We think that when a star like H1504+65 cools, it eventually becomes like the pure-carbon stars," Dufour said. As the massive star cools, gravity separates carbon, oxygen and trace helium. Above 25,000 degrees Kelvin, the trace helium rises to the top, forming a thin layer above the much more massive carbon envelope, effectively disguising the star as a helium-atmosphere white dwarf, Dufour and Liebert said.

 

But between 18,000 and 23,000 degrees Kelvin, convection in the carbon zone probably dilutes the thin helium layer. At these temperatures, oxygen, which is heavier than carbon, has probably sunk too deep to be dredged to the surface.

 

Dufour and his colleagues say that models of stars nine to 11 solar masses might explain their peculiar carbon stars.

 

Astronomers predicted in 1999 that stars nine or 10 times as massive as our sun would become white dwarfs with oxygen-magnesium-neon cores and mostly carbon-oxygen atmospheres. More massive stars explode as supernovae.

 

But scientists aren't sure where the dividing line is, whether stars eight, nine, 10 or 11 times as massive as our sun are required to create supernovae.

 

"We don't know if these carbon atmosphere stars are the result of nine-or-10 solar mass star evolution, which is a key question," Liebert said.

 

The UA astronomers plan making new observations of the carbon atmosphere stars at the 6.5-meter MMT Observatory on Mount Hopkins, Ariz., in December to better pinpoint their masses. The observations could help define the mass limit for stars dying as white dwarfs or dying as supernovae, Dufour said.

 

Adapted from materials provided by University of Arizona.

 

If anyone's interested, this is called AGB astronomy (Asymptotic Giant Branch). Stars that become white dwarfs (after a red giant stage, where they inflate and become somewhat rarified), are believed to start out as asymptotic stars, where the matter is spread over a larger area that contracts, eventually forming a compact object, about rocky planet size.

 

The first white dwarf was spotted (unknowingly) back in the 19th century, but nowadays there are thousands being studied, with some quite unusual ones (with odd-looking spectra, like the carbon-atmosphere ones above).

Stellar evolution and the models running on computers (and the supernova models) are big deals (if you're into cosmology).

 

So I s'pose I should include a link to some online doco (a google book)

http://books.google.com/books?hl=en&lr=&id=KpRH1mAl7HIC&oi=fnd&pg=PA325&dq=astronomy+white+dwarfs+unusual+arizona+%27p+dufour%27&ots=oDjm2qbtLT&sig=hhpTxd0g_uhlDn3FdPhhkzeWtCU

 

If you get into google books and do a search on 'AGB stars', there are quite a lot of entries.