Expert: Courtney Seligman - 7/20/2010

Question
Hi, I have a couple of questions about the role of angular momentum in type 1a supernovae. First, does it have any role? Second, does the white dwarf that is accreting mass slow down before it explodes (as bigger objects rotate slower no?)?  Are any of the two stars rotating faster or slower leading to the explosion?

Thank you very much for your time.

Answer
Type 1a supernovae result from the ignition of nuclear reactions in a white dwarf companion to a dying star. In a normal star, the rate of nuclear reactions is controlled by the tendency of gases to expand or contract when heated or cooled, naturally opposing any change in conditions. In the electron-degenerate material making up a white dwarf, this is not a factor, and the onset of nuclear reactions can lead to exponential energy increases, a large multiple of those increases in the rate of nuclear burning, and a rapid and violent burning of all available nuclear fuel in practically no time at all.

If the amount of fuel available is small, the result is a nova, which can recur after a period of decades or centuries, as fresh fuel is dumped onto the white dwarf by its companion. If the amount of fuel available is close to the mass of the white dwarf, the result is a type 1a supernova, which blows the white dwarf to smithereens.

There would be a natural transfer of rotational energy (from the orbital motion of the stars relative to each other) to the white dwarf, during the accretion phase prior to the nova or supernova, which would cause the white dwarf to spin faster and faster (more massive white dwarfs are smaller and would spin faster, even if the accreting gases didn't transfer rotational energy to them). That could conceivably slightly alter the point at which the explosion starts, but given the relatively large gravitational force at the surface of the white dwarf, is unlikely to have any significant effect on the timing. In any event, once the explosion starts, it is controlled by the amount of fuel available, and any other factors -- gravity, rotation, etc -- are unlikely to be of any importance. The rotation of the companion should have no effect on the situation, and even the rotation of the white dwarf, though faster and faster as more and more material is transferred to it, is unlikely to have any effect, save for perhaps delaying the exact moment of the explosion by some (very) small time. But once the nuclear reactions begin, not much is of any importance save how much fuel is available.

You might find it useful to take a look at Mass Transfer in Binary Star Systems, at http://cseligman.com/text/stars/binary.htm for a discussion of mass-transfer effects in such systems. (I'm not quite sure what it is that you're looking for, other than a discussion of how rotation affects the situation; and perhaps reading that will clear things up enough for you to rephrase the question in a way that allows me to give you a better answer.)