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Ultra-luminous supernova sparked by merger
Posted: 17 March 2010

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More evidence that a significant proportion of Type Ia supernovae are the result of two white dwarfs merging together and exploding has been found in a study of an ultra-luminous supernova.

Standard theories of Type Ia supernovae are that they are the result of a white dwarf – the leftover core of a Sun-like star after it has reached the end of its lifetime and expanded out into a planetary nebula – gathering gas from a close-by companion star. Once it accumulates so much gas onto its surface that its total mass exceeds the Chandrasekhar limit (which is 1.4 times the Sun’s mass), conditions on the white dwarf become so serious that runaway nuclear fusion engulfs it and the white dwarf explodes. Because they all should explode at 1.4 solar masses, they should all have the same luminosity, which makes them perfect as standard candles for measuring intergalactic distances.

An artist’s impression of a Type Ia supernova. Image: ESO.

In recent years four Type Ia supernovae – SN 2003fg, SN 2006gz, SN 2007if and SN 2009dc – have all been brighter than they should have been. In a paper appearing in an upcoming issue of The Astrophysical Journal, an American and French team of physicists called the Nearby Supernova Factory describe how studies of SN 2007if reveal that whatever exploded had a total mass of around 2.1 solar masses, far exceeding the Chandrasekhar limit. This is based on the amount of nickel-56 that they detected in the supernova debris (nickel-56 is the isotope that the force of the blast transforms the white dwarf’s carbon and oxygen composition into), and also an unusual envelope of unprocessed carbon and oxygen surrounding the blast zone, which alone contributes up to half a solar mass. The absolute magnitude of SN 2007if was –20.39, which is about three times as luminous as a typical Type Ia supernova (about 1.2 magnitudes brighter).

A very high rotation rate could theoretically maintain a white dwarf that exceeds the Chandrasekhar limit, perhaps up to 2.4 solar masses, but lead author Richard Scalzo of Yale University plumps for the theory that these ‘super-Chandrasekhar’ supernovae are the product of two white dwarfs merging. This bolsters a hypothesis laid down last month that, based on X-ray observations, Type Ia supernovae are predominantly formed by mergers rather than accreting white dwarfs (see our related news story here for more information).

“Our paper gives a specific example of a white dwarf merger with what we think is a convincing signature, but doesn’t really address how many similar events there are,” Scalzo tells Astronomy Now. “To really see what’s going on, I think astronomers need to be able to tell which specific events are mergers and which are single white dwarf explosions. This means we need to establish an unambiguous merger signature, and I think our work helps with that.”

More evidence that it was a merger-based explosion comes in the shape of the carbon/oxygen envelope. According to Scalzo, as the two white dwarfs came close to one another, the smaller white dwarf began to be torn to shreds by the gravity of the larger white dwarf, forming a disc or envelope around it. When the merger process went supernova, the blast wave rammed into this envelope, creating a ‘shell’ as material piled up. It is this shell that has been directly observed, and from which the presence of the envelope has been inferred. Because the envelope is fairly low density, pressures never build up high enough to process the carbon and oxygen into the likes of nickel-56.

SN 2007if and other supernovae of its class (and potential supernovae – see our related news story here) are case studies upon which astronomers can build models to determine whether mergers are indeed the primary means of reaching Type Ia supernovae. If that’s true, then with luck the work done on SN 2007if will go some way to ensuring that the status of ‘ordinary’ Type Ia supernovae as standard candles is not diminished in the confusion.