'White Widows' May Spawn Supernovae

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A new thought experiment by J. Craig Wheeler — an astronomer at the University of Texas, Austin — offers a possible alternative scenario for the origins of Type Ia supernovae. The paper appeared last month in The Astrophysical Journal.

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Most supernovae occur when a single star dies, specifically those of sufficiently large mass, namely, 1.4 times the mass of the sun. This is known as the Chandrasekhar limit. Stars of lesser mass usually end their lives as white dwarf stars.

Other supernovae occur in binary star systems, usually with one white dwarf and one normal star (a “single-degenerate” model). The latter sloughs off matter onto its white dwarf companion, which explodes when its mass hits the Chandrasekhar limit, leaving behind the normal stellar companion, which must live out the rest of its life alone, pining for its lost partner.

Wheeler has studied these exploding stars for 40 years and pioneered this idea back in 1971. Ever since, astronomers have bandied about various theories of what kind of star the companion in such a system might be.

Last month, astronomers announced that one of the most famous supernovae, SN 1006, may have resulted from a collision or merger of two white dwarf stars (a “double-degenerate” model). Such an event would also produce a supernova explosion — only one that leaves no trace, other than the glowing remnant we see today.

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Wheeler has proposed a third option, asserting that observational data of actual Type 1a supernovae don’t support either of those two models satisfactorily. Specifically, current models of supernova spectra — the light signatures from these very bright stars — as they change over time don’t match the data from actual supernovae.

So Wheeler suggests a modification of the first model is needed, in which a white dwarf pairs with a so-called M dwarf star in a binary system.

M dwarfs are quite common, but they are also very dim, and might not be detected by the large telescopes astronomers use to observe the remnants that remain after a supernova explosion. “One thing blows up as a supernova, the other thing’s got to be left behind,” Wheeler said via press release. “Where is it? We don’t see it.”

In this case, seeing nothing might be something — small red M dwarf stars too faint to be detected, or perhaps even devoured entirely by its companion star before the latter went supernova. That’s why Wheeler has dubbed his model the “white widow system” — similar to black widow systems where a neutron star devours its companion star.

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The tie that binds a white dwarf and an M dwarf star in Wheeler’s thought experiment is magnetism. Two magnetic stars in a binary system would cause them to be locked into a set rotation because their opposite magnetic poles would attract. So as the white dwarf slowly siphoned matter off the M dwarf star, it would pile up on a single spot. Wheeler suggests that this would further hasten the consumption of the M dwarf companion.

“It’s just a completely different part of parameter space to bring in the role of magnetic fields in the supernova game,” said Wheeler. “But it is the way nature works. Things are magnetic. The sun is magnetic. The Earth is magnetic. The magnetic fields are there. Are they big enough to do something?”