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kordic
2007-Oct-16, 03:23 AM
Hey guys I have a question. I know how a white dwarf can accrete matter from a companion star, this phenomenon makes me think about the possibility of a white dwarf drawing just the right material to become a main sequence star again. Could this theoretically be possible? What if the orbit of the companion star was somehow shifted away from the white dwarf? Would this make the white dwarf stable with the accreting material? Thanks for any answers...

Thanatos
2007-Oct-16, 05:36 AM
Not possible. When a white dwarf accretes enough matter to reach the Chandreskhar limit [about 1.4 solar masses], the accreated material detonates creating a massive explosion. This is physics as we know it, not speculation. The rate of accreation is irrelevant.

Tim Thompson
2007-Oct-16, 04:03 PM
... this phenomenon makes me think about the possibility of a white dwarf drawing just the right material to become a main sequence star again.
It is not possible. Main sequence stars are stable because the core of the star supports proton-proton fusion in approximate thermal equilibrium. Once the star passes that phase, it leaves the main sequence. Red giant & AGB stars have dense cores that may not be actively fusing anything, but they are definitely not main sequence stars, and they are stable only for time periods no more than about 10% of their main sequence lifetimes.

in the case of an accreting white dwarf, the surface of the dwarf is much too intense. Any accreted material will immediately become part of the dense, degenerate dwarf, or explode in a thermonuclear nove. Once the entire becomes massive enough, it will obliterate iteself as a supernova (as Thanatos already points out). Cataclysmic variable stars (http://en.wikipedia.org/wiki/Cataclysmic_variable_star) are good examples of what happens as a white dwarf steadily accretes material from a companion star.

Jerry
2007-Oct-16, 04:28 PM
Not possible. When a white dwarf accretes enough matter to reach the Chandreskhar limit [about 1.4 solar masses], the accreated material detonates creating a massive explosion. This is physics as we know it, not speculation. The rate of accreation is irrelevant.
Almost. There have been a number of supernova events that don't fit the "accreting white dwarf" theory; they have the spectral signature normally associated with a white dwarf event, but explode to too great of magnitudes. This could be because they are rotating so fast the critical mass limit is surpassed. The high magnitude events may involve binary white dwarfs or supercritical dwarfs, (in which case theory may not be correct).

What will be helpful, is when we observe a supernova event, and we are able to pull up pre-event images that allow us to study what the star system was like just before the explosion. What would be ideal, is if one of the white dwarfs we have studied (or a binary pair) would go supernova.

Ken G
2007-Oct-16, 04:30 PM
Another thing to add to Tim Thompson's answer to the OP (that you can't get a main sequence star because you are not creating a core that is fusing), which might relate to what motivated the question, is that the big difference between a red giant (which essentially has a white dwarf for a core) and a white dwarf that is accreting matter is that the fusion occuring in shells in the red giant is stabilized by the large weight of the star around it. Fusion right at the surface of a white dwarf is not stable-- it erupts in explosive events (that's what Tim Thompson is talking about with cataclysmic variables). But in a red giant, the fusion is steady, until there are "thermal pulses" that play a role in converting the red giant to a planetary nebula later on. I forget why the red giant can have stable shell burning and the white dwarf cannot, but the bottom line is that the pre-existing structure of the object matters a lot in the difference between stable and unstable fusion. That, in a nutshell, is why the Sun doesn't explode like a hydrogen bomb, whereas we can make hydrogen bombs but not hydrogen fusion reactors.

kordic
2007-Oct-16, 05:36 PM
That makes a lot more sense, thanks for those answers!

Romanus
2007-Oct-16, 08:50 PM
You can get a fusing star out of a white dwarf--two white dwarfs, anyway. This is the currently-favored explanation of the origin of supergiant R Coronae Borealis stars: as the result of the merger of two white dwarfs with specific compositions. However, these bizarre, short-lived stars aren't main sequence by any stretch of the term.

jlhredshift
2007-Oct-16, 11:25 PM
Almost. There have been a number of supernova events that don't fit the "accreting white dwarf" theory; they have the spectral signature normally associated with a white dwarf event, but explode to too great of magnitudes. This could be because they are rotating so fast the critical mass limit is surpassed. The high magnitude events may involve binary white dwarfs or supercritical dwarfs, (in which case theory may not be correct).

What will be helpful, is when we observe a supernova event, and we are able to pull up pre-event images that allow us to study what the star system was like just before the explosion. What would be ideal, is if one of the white dwarfs we have studied (or a binary pair) would go supernova.

My bold

What happens to the other white dwarf when its companion goes SN?

neilzero
2007-Oct-17, 03:22 AM
I think the original post is almost correct. If a white dwarf accretes a main sequence class m star, in minutes, the class m may look like a class g star for a few million years. The pressure at the surface of the white dwarf supports rapid fusion of hydrogen to helium, even if 90% of the mass and a millionth % of the volume is not doing fusion.
I'm assuming the white dwarf is at the mass center of the class m star, and the fusion is preventing the white dwarf from converting the class m star to white star stuff. Neil

neilzero
2007-Oct-17, 03:36 AM
I'm unsure what a stable or unstable white dwarf is like. It may be improbable that a white dwarf will ditch the companion star that feeds it mass, but the white dwarf should be completely stable a few weeks after it stops significantly accreating matter, if it finds a way to depart the star that was feeding it. I suppose a very thin accreation disc might last millions of years, before it was better described as a planetary system. Neil