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Fraser
2005-Nov-16, 05:58 AM
SUMMARY: This unique photograph, taken by NASA's Chandra X-Ray Observatory, contains not one, but two supernova remnants. The pair are known as DEM L316, and they're located in the Large Magellanic Cloud galaxy. The gas shell at the upper-left contains considerably much more iron, so it's probably the product of a Type 1a, triggered by the infall of matter from a companion star onto a white dwarf. The lower-right shell is a Type II supernova, the remains of a massive star that exploded a few million years into its short life.

View full article (http://www.universetoday.com/am/publish/two_supernova_remnants_chandra.html)
What do you think about this story? post your comments below.

RussT
2005-Nov-16, 11:15 AM
I don't understand how a white dwarf can steal matter from it's companion star!!!
On The Cosmos tonight with Carl Sagan, it said Red Giants that turn into white dwarfs, lose about half their mass when the outer shell is ejected, so how can the white dwarf accrete the other more massive stars matter?

Even if it didn't lose mass, neither star was able to accrete the other matter before.

Shouldn't the accretion disc be a black hole?

RussT

BillG
2005-Nov-16, 07:59 PM
Russ,

You wrote:

> I don't understand how a white dwarf can steal matter from it's companion star!!!

Any two stars that are close enough to share a Roche lobe across their inner Lagrange point can exchange mass. In the case of a white dwarf that is a close companion of a giant, the giant star swells up to the point that its outer atmosphere fills the Roche lobe and then starts to stream across into an accretion disk around the white dwarf. Matter in the accretion disk eventually either comes to rest on the white dwarf of gets flung away from it in a polar jet caused by the magnetic field of the white dwarf.

Mass transfer is also the reason behind the classical novae, and is present in contact binaries and semi-contact binaries.

> On The Cosmos tonight with Carl Sagan, it said Red Giants that turn into white dwarfs, lose about half their mass when the outer shell is ejected, so how can the white dwarf accrete the other more massive stars matter?

The white dwarf star still has significant mass. In the Kenichi-Nomoto model for SNe Ia it's about 1.2 solar masses. This is plenty enough mass to attract the outer atmosphere of a nearby red giant star if that red giant is a binary companion of the white dwarf.

> Even if it didn't lose mass, neither star was able to accrete the other matter before.

Why do you assume this? The two stars may have been exchanging mass for billions of years.

> Shouldn't the accretion disc be a black hole?

No, you can get accretion disks around any condensed star. White dwarves have them, as do neutron stars. For that matter the protoplanetary disks of young stars are a form of accretion disk.

BillG
2005-Nov-16, 08:51 PM
Fraser,

I know this will identify me as an old pedant, but supernova types are spectral types. Are there still hydrogen lines present in the spectrum of the SNR you identify as the remnant of a SN II? If not, what evidence to you have to show that it's not the remnant of an SN Ib or Ic?

Also, as much as I like Ken Nomoto and Stan Woosley's model for SNe Ia, I don't think it's considered to be proven yet. So saying that SNe Ia are the result of deflagration of white dwarf stars is a bit premature. I know that Astronomy magazine does it, because they're going after the GeeWhiz!!! market. I'm not yet sure what level of presentation Universe Today is shooting for.

Duane
2005-Nov-16, 09:57 PM
BillG, first Welcome to the forum! I hope you find it a wealth of information and a place to come to speak your mind with others who share your interest!

Second, understand that Fraser is not the person making these claims, rather he is posting news tidbits that he finds through lots of different sources. Further, while all of the news items carry his name, there is actually a team of writers. So it could have been written by any of them.

While questions and comments are certainly welcome in this area, if you have a specific question you may wish to go to the questions and answers area. If you would like to promote an idea that is off the "mainstream", then we have an area for that called (imagine!) Against the Mainstream.

A last point, if you have a question regarding a specific article, then present the question to the author. We have members who will attempt to appraoch the authjor on your behalf to ask them if they would answer the question directly, or at the least try to get a responce from someone in the know.

Again, welcome to the forum!

BillG
2005-Nov-17, 03:41 PM
Duane,

Thanks for the welcome. After posting my earlier questions, I chased down the source of the article and wrote a little note to the folks at CFA. They should know better.

This is a nice little service you have. I doubt I'll be posting anything against the mainstream, since I'm pretty firmly *in* the mainstream, but it's good to know about it.