Type Ia Supernovae: A New Revelation

It has been a matter of uncertainty for quite some time as to what exactly goes into the making of a Type Ia supernova. There have been two competing models over the years as to how a Type Ia supernova occurs. The first model entails a semi-detached binary system in which a white dwarf primary accretes material from a normal-star secondary onto it’s surface. The second model entails two white dwarfs in orbit around one another, and a supernova occurs after the two white dwarfs merge. What has been generally agreed with respect to both models is that a white dwarf must exceed a certain mass, known as the Chandrasekhar limit, which for a non-rotating helium white dwarf would be 1.44 solar masses, 1.40 solar masses for a carbon white dwarf and 1.11 solar masses for an iron white dwarf, in order for it to go supernova. If the white dwarf is rotating very rapidly, then the limit goes up substantially.

White dwarf and stellar companion accretion model (Courtesy of Harvard-Smithsonian Center for Astrophysics.)

White dwarf merger model (Credit: Creative Commons License/Drew Taylor).

It should be noted at this time that the difference between Type I and Type II supernovae is not the explosive mechanism behind each type but rather the absence of (in the case of Type I) or presence of (in the case of Type II) hydrogen in the spectrum of each type. The majority of Type I supernovae (i.e. Type Ib and Ic) are believed to be core-collapse supernovae.

So how do astrophysicists resolve this question? Well for starters, it is known that if the process that produces Type Ia supernovae were a white dwarf accreting material from a stellar companion, that in itself would produce a significantly greater emission of X-rays prior to the white dwarf going supernova than if the supernova were produced by two merging white dwarfs. Type Ia supernovae produced in this fashion would also be 40 times brighter than if they were produced by the merger of two white dwarfs.

This composite image of M31 (also known as the Andromeda galaxy)
shows X-ray data from NASA’s Chandra X-ray Observatory in gold,
optical data from the Digitized Sky Survey in light blue and infrared
data from the Spitzer Space Telescope in red.

Using data from the Chandra X-ray Observatory, the Spitzer Space Telescope and the Digitized Sky Survey of the central region of M31, the Andromeda galaxy, (pictured above) has shown that the expected brightness for a white-dwarf/stellar- companion scenario wasn’t present. Five elliptical galaxies were surveyed as well and showed pretty much the same thing. And in both the Andromeda galaxy survey as well as the one done on the five elliptical galaxies, the Chandra X-ray observatory showed X-ray emissions 30 to 50 times smaller than what would have been expected in the white-dwarf/stellar-companion scenario.

“To many astrophysicists, the merger scenario seemed to be less likely because too few double-white-dwarf systems appeared to exist,” said Marat Gilfanov of the Max Planck Institute for Astrophysics in Germany. “Now this path to supernovae will have to be investigated in more detail.”

This was a rather unexpected revelation for researchers, to say the least. They just thought that orbiting white dwarfs would be too rare a thing in our universe for this to be possible.

“Our results suggest the supernovae in the galaxies we studied almost all come from two white dwarfs merging,” said co-author Akos Bogdan, also of Max Planck. “This is probably not what many astronomers would expect.”

Of course, this study is far from over. It may turn out that there may be a comparable number of white-dwarf/stellar-companion scenarios out there. But this is a significant finding, because it has implications on cosmological studies that have used Type Ia supernovae as a kind of “standard candle”.

Copyright © 2010 Eric F. Diaz

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