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White Dwarf Merger

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#1 earlyriser

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Posted 30 May 2019 - 09:54 AM

The thing I don't understand is how these white dwarfs could merge without instantly going supernova due to exceeding the Chandrasekhar Limit. Somehow, the star resulting from the merger started fusing carbon/oxygen/neon in a controlled fashion.

 

https://www.space.co...Obc7BItJVpyXLQ8



#2 Luca Brasi

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Posted 30 May 2019 - 11:09 AM

So long as they are not fusing to make iron they should be relatively stable. Though once they start the supernova process begins almost instantly.

#3 Astrola72

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Posted 30 May 2019 - 11:28 AM

I'm no astrophysicist, but I think the Chandrasekhar Limit only applies when there is no fusion going on. But in the case of the white dwarf merger, at least in this case, fusion of heavier elements has been re-ignited. Therefore the star avoids collapse because of thermal pressure resulting from the fusion reactions.

 

 

Joe


Edited by Astrola72, 30 May 2019 - 11:32 AM.


#4 earlyriser

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Posted 30 May 2019 - 11:42 AM

Normally, when a white dwarf accretes enough mass to exceed the Chandrasekhar limit, the result is collapse followed immediately by uncontrolled fusion and a type Ia supernova.

 

My guess is that with a merger of white dwarfs, the rapid rotation of the stars about their shared center of gravity as they begin coalesce prevents a sudden collapse and allows fusion to begin in a controlled manner.

 

The difficulty is in transitioning from a stellar core remnant composed of electron-degenerate matter into one that is fusing carbon without the whole thing blowing up.


Edited by earlyriser, 30 May 2019 - 11:44 AM.


#5 cfosterstars

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Posted 30 May 2019 - 12:12 PM

Some of this is due to the nature of the merger. If the system merges with enough angular momentum then even super Chandrasekhar limit white dwarfs are stable and will not collapse. If the system reignites fusion, then the thermal energy can lead to hydrostatic equilibrium without degeneracy pressure as the stablizing force. The collapse is prevented by simply thermal gas pressure like an ordinary star. The runaway fusion of a Type 1a is due to the fact that a white dwarf is not held up by thermal gas pressure but by degeneracy pressure. For degenerate matter, there is no increase in volume with temperature so that the temperature can dramatically increase with no increase in volume. With enough angular momentum, the combined super Chandrasekhar limit white dwarf could heat enough without runaway to lift degeneracy and get back to "normal" star fusion. This would not be like a main sequence star since there would be no hydrogen and the fusion would have to be heavier elements such as helium or carbon/oxygen fusion. So the resulting star would be more like a Wolf–Rayet star but with a lower mass. WR stars have no hydrogen or even helium envelope due to the massive solar wind blow these layer of the star away, but still have masses of 10s of solar masses. This white dwarf merger star would be only 1-2 solar mass core. So I really dont see how it could burn much heavier elements at non-degeneracy pressures and not have run away fusion.  


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#6 BillP

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Posted 30 May 2019 - 12:47 PM

The thing I don't understand is how these white dwarfs could merge without instantly going supernova due to exceeding the Chandrasekhar Limit.

 

The authors of your link referred to this as the answer to your question...

https://arxiv.org/pdf/1606.02300.pdf


Edited by BillP, 30 May 2019 - 12:47 PM.

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#7 cfosterstars

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Posted 30 May 2019 - 01:11 PM

The authors of your link referred to this as the answer to your question...

https://arxiv.org/pdf/1606.02300.pdf

Very cool paper. It is amazing how 3D effects are now modeled and lead to very unique conclusions. The effects of off center fusion ignition and deflagration (sub-sonic) fusion burning is so cool. Imagine a "white dwarf" now composed entirely of silicon and sulfur! Or how these object detonated to form neutron stars or do they just sort of flip. Its not clear how this happens and the authors state that it ongoing research. This is also interesting since this type of process is very similar to the evolution of main sequence stars right on the 8-10 solar masses boarder between white dwarfs and supernova. 



#8 BillP

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Posted 30 May 2019 - 06:47 PM

Imagine a "white dwarf" now composed entirely of silicon and sulfur! Or how these object detonated to form neutron stars or do they just sort of flip. Its not clear how this happens and the authors state that it ongoing research. This is also interesting since this type of process is very similar to the evolution of main sequence stars right on the 8-10 solar masses boarder between white dwarfs and supernova. 

 

Stars and stellar evolution are IMO the most amazing and interesting things in all of cosmology.


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