PDA

View Full Version : Can a helium white dwarf go supernova?



triclon
2008-Apr-01, 04:24 PM
Me and a friend were debating if helium white dwarfs can undergo something similar to a type Ia supernova. Normal white dwarfs are mainly made of carbon, and undergo nuclear fusion exploding as a type Ia supernova when they reach the Chandrasekhar Limit. Now I know helium white dwarfs are much rarer (and I think smaller) then normal white dwarfs but can they also undergo a similar type of supernova? And wouldn't helium, when it fuses, give off more energy then carbon resulting in a more powerful explosion?

Ken G
2008-Apr-01, 04:41 PM
The problem would be getting it up to the needed mass of 1.4 solar masses, but if you could do that, it doesn't matter if your white dwarf is helium or plutonium. Also, the energy emitted is mostly from gravitational collapse, not fusion, so again it wouldn't matter much except in the details like what the supernova looks like and how long it lasts.

korjik
2008-Apr-01, 05:22 PM
The problem would be getting it up to the needed mass of 1.4 solar masses, but if you could do that, it doesn't matter if your white dwarf is helium or plutonium. Also, the energy emitted is mostly from gravitational collapse, not fusion, so again it wouldn't matter much except in the details like what the supernova looks like and how long it lasts.

Last I checked, a SN Ia was carbon fusion, not gravitational collapse. The dwarf hits the fusion point just before degeneracy fails.

Spaceman Spiff
2008-Apr-01, 05:57 PM
Yep. When the density in a C/O white dwarf exceeds several x 10^9 g/cm^3, pycnonuclear carbon fusion ignites under highly degenerate conditions. As KenG alludes to this happens just under 1.4 solar masses - but apparently before gravitational instability arises to cause catastrophic collapse. But we still haven't answered the poster's question...

Nick Theodorakis
2008-Apr-01, 06:16 PM
This UT article (http://www.universetoday.com/2006/03/10/merging-white-dwarfs-create-helium-stars/?932006) says that a helium white dwarf merging with a carbon-oxygen white dwarf may either become a helium star or a Type Ia SN, depending on the mass of the combined star.

Nick

trinitree88
2008-Apr-01, 06:19 PM
Yep. When the density in a C/O white dwarf exceeds several x 10^9 g/cm^3, pycnonuclear carbon fusion ignites under highly degenerate conditions. As KenG alludes to this happens just under 1.4 solar masses - but apparently before gravitational instability arises to cause catastrophic collapse. But we still haven't answered the poster's question...

Spaceman Spiff. Pretty spiffy. There's a new word I had to look up...pyncnonuclear. Density sensitive. You never see it all. Thanks.:dance: pete

see:http://www.iop.org/EJ/abstract/0370-1328/84/2/304

korjik
2008-Apr-01, 06:35 PM
I was going to wait and hope that someone who actually knows would answer but...

I think that an He dwarf will start burning the helium at a much lower temp, and at a much lower rate, as it accretes mass. Prolly would have a recurring nova until there was enough carbon for a proper SN Ia

Ken G
2008-Apr-01, 10:44 PM
Last I checked, a SN Ia was carbon fusion, not gravitational collapse. The dwarf hits the fusion point just before degeneracy fails.I stand corrected! Thank you for pointing out the oversight, it was correctly stated in the OP. And thanks to Nick Theodorakis for pointing out that article-- I was imagining that if the mass went above 1.4 solar masses, the star would collapse and generate runaway fusion, but of course that's not right. If we add helium to it, after enough is added (maybe 0.01 to 0.1 solar masses) it will just have "helium flashes" similar to what will happen to the helium core of the Sun in 5 billion years, and will probably end up a nice normal helium-burning star, and will eventually make a carbon/oxygen white dwarf. If we add hydrogen, then the hydrogen will burn at lower temperatures than the helium, and that's the "recurrent nova" scenario. Neither leads to a supernova, unless you add so much helium that the helium star eventually makes a core collapse supernova.

Eroica
2008-Apr-12, 03:49 PM
I think the key issue is degeneracy. Degenerate matter does not expand when heated the way non-degenerate matter does. This has important consequences.

When a non-degenerate core of mainly carbon contracts and heats up to a temperature of about 5 x 108 K carbon burning begins. This generates a lot of energy, which tends to raise the temperature of the core even higher, but as it heats up the core expands and cools off, reducing the fusion rate (which is very sensitive to temperature). Thus, non-denerate matter has a natural built-in thermostat that keeps things at about 5 x 108 K.

Non-degenerate matter does not expand when it is heated, so it cannot cool itself. So there is no thermostat in the degenerate carbon-rich white dwarf to prevent runaway fusion. The temperature quickly rises to 1010 K, allowing neon burning, oxygen burning and silicon burning to occur in a matter of seconds, converting the core into iron group elements and blowing the star apart before core collapse can take place.

In helium-rich degenerate cores, the initial rise in temperature due to runaway fusion (the helium flash) removes the degeneracy, converting the core into non-degenerate matter and so restoring the natural thermostat. So I don't think you could get a supernova explosion - just a rejuvenated helium-burner, as Ken says.

antoniseb
2008-Apr-12, 04:46 PM
That's a good analysis Eroica.

It occurred to me that it might be possible for a remnant helium core to (collide or spiral in) to a near critical size Carbon/Oxygen core what dwarf, and collectively become a supernova... This is almost certainly NOT what the OP was asking about, but is a situation that might fit the letter if not spirit of the question.