Found: Theoretical Supernova Actually Exists
Written by Nancy Atkinson
A new kind of supernova. Credit: Tony Piro
Astronomers have identified a type of supernova that appears to be a type predicted in theory but never actually observed before. Two years ago Lars Bildsten from UC Santa Barbara and his colleagues predicted a new type of supernova in distant galaxies which they dubbed the “.Ia” (point one a) mechanism, involving a helium detonation on a white dwarf, ejecting a small envelope of material. This theoretical explosion would be fainter than most other supernovae and its brightness would rise and fall in only a few weeks. Dovi Poznanski from Berkeley went back and looked at seven-year-old observations and found this unusual kind of supernova. Poznanski and colleagues say supernova 2002bj belongs in its own category, as its spectra suggest that it evolved extremely fast and produced an unusual combination of elements.
Supernovae are usually classified based on tell-tale lines in the spectrum of radiation they emit. The two main types are thought to develop from exploding white dwarfs and collapsing massive stars.
However, Bildsten's theory said that in rare instances, there is a binary star system where helium flows from one white dwarf onto another and accumulates on the more massive white dwarf.
It is this rare occurrence that leads to unique conditions of the explosive thermonuclear ignition and complete ejection of the accumulated helium ocean. The plethora of unusual radioactive elements made in the rapid fusion leads to a bright light show from the freshly synthesized matter that lasts a few weeks.
The "usual" explosions of white dwarfs are referred to as "Type Ia supernova." They are brighter than a whole galaxy for more than a month and are quite useful in cosmological studies. The predicted ".Ia" supernovae are only one-tenth as bright for one-tenth the time.
Poznanski and his team say 2002bj fits the bill for this never-seen-before type of supernova.
"This is the fastest evolving supernova we have ever seen," said Poznanski. "It was three to four times faster than a standard supernova, basically disappearing within 20 days. Its brightness just dropped like a rock."
Poznanski told Universe Today that he was actually looking at Type II supernovae for another purpose when he hit the spectrum of 2002bj. "My first reaction was great confusion," he said. "My second reaction, after showing it to other experts was greater confusion. After matching it against every object we know of, and finding nothing the confusion was topped with a lot of excitement. This kept rising until the .Ia idea came up and matched pretty well."
Then Poznanski and his team re-analyzed their data to make sure, and the rest is history.
This explosion was nothing like a regular Type Ia explosion, said team member Alex Filippenko, because the white dwarf survives the detonation of the helium shell. In fact, it has similarities to both a nova and a supernova. Novas occur when matter – primarily hydrogen – falls onto a star and accumulates in a shell that can flare up as brief thermonuclear explosions. SN 2002bj is a "super" nova, generating about 1,000 times the energy of a standard nova, he said.
"As we have talked about our work over the last years, most astronomers in the audience reminded us that they had never seen such an event," said Bildsten. "We told them to keep looking! With the sky the limit, the observers are usually ahead of theory, so I am really happy that we were able to make a prediction that allowed for a rapid interpretation of a new phenomena. Even though the supernova was observed in 2002, it took the keen eye of Dovi Poznanski to appreciate its import and relevance."
Source: Science
Filed under: Astronomy
Tags: SupernovaeRelated stories on Universe Today
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November 5th, 2009 at 4:50 pm
I seem to remember reading about this many years ago. The white dward accumulates material on its surface which then reaches criticality.
LC
November 6th, 2009 at 7:49 am
Helium fusion Generates Carbon and Oxygen so now we have an observed mechanism other than than a type II Supernova for distributing these elements across space, when they contaminate other stars we get Nitrgen and the building blocks of life
Since black holes have been found with a suspected Carbon atmosphere I guess a Carbon fusion nova would also be possible.
November 6th, 2009 at 9:08 am
Strange timing between this story and a simulation producing an underluminous SN that appears on the arXiv site ("Sub-luminous type Ia supernovae from the mergers of equal-mass white dwarfs with M~0.9 M_sun" @ http://arxiv.org/ftp/arxiv/papers/0911/0911.0926.pdf ). Both articles involve white dwarf binaries and result in underluminous, short maxima supernovae. In the linked paper, the authors explore several mechanisms leading to detonation but begin the simulation only a couple of orbits before disruption-detonation. Of course, only one white dwarf remains in this scenario.