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Explaining the explosion: rate of white dwarf mergers matches frequency of type Ia supernova

A new statistical survey suggests that the white dwarf binaries merge with …

A sample drawn from over 4000 white dwarfs in the Sloan Digital Sky Survey (SDSS) data. The number of binary white dwarf systems and the merger rate were determined from this data.
A sample drawn from over 4000 white dwarfs in the Sloan Digital Sky Survey (SDSS) data. The number of binary white dwarf systems and the merger rate were determined from this data.

Measuring vast distances in the Universe requires standard candles, objects with a known intrinsic brightness. Type Ia supernovae (SN Ia) are the brightest standard candles we know of, so they are useful for measuring the accelerating expansion of the Universe, as honored by the 2011 Nobel Prize in physics.

Most astronomers agree that type Ia supernovae have something to do with white dwarfs—compact remnants of stars similar to our Sun. However, the exact mechanism for turning these stars into a supernova is not at all certain. The models we use to explain the explosions all involve binary systems: either a white dwarf paired with an ordinary star, or two white dwarfs in mutual orbit. But no precursor star system has ever been identified, so it has been hard to determine if one or more of these models is correct. At least one supernova remnant shows no sign of an ordinary companion star, but we can't really conclude much from a single example.

Now, a study of white dwarfs by Carles Badenes and Dan Maoz indicates that the frequency of white dwarf mergers is very close to the rate of SN Ia events in spiral galaxies. Based on the statistics, the researchers argue that a significant number of type Ia supernova events may be due to mergers between pairs of low-mass white dwarfs.

White dwarfs are the cores of stars that were once similar to our Sun, but have since used up their hydrogen and helium fuel that makes them shine. The cores are degenerate: the material is dense enough that the atoms are packed tightly together, but the repulsion due to the Pauli exclusion principle prevents them from collapsing under their own gravity. This means white dwarfs are very small in comparison to their mass: they are approximately the size of Earth, with masses comparable to the Sun.

However, if enough mass is added, the degeneracy pressure will not be able to overcome gravity. If a white dwarf exceeds about 1.4 times the Sun's mass—a maximum mass known as the Chandrasekhar limit—it collapses. Type Ia supernova explosions occur as a result of this collapse.

The two major models for SN Ia explosions explain how white dwarfs hit the Chandrasekhar limit. The first, known as single-degenerate (SD), posits one white dwarf (the degenerate object) in mutual orbit with a normal star. In this scheme, the normal star transfers matter to the white dwarf until it exceeds the Chandrasekhar limit and explodes. The second model is double-degenerate (DD), which involves two white dwarfs. DD systems fall into two subtypes: super-Chandrasekhar, where the total mass of the system is greater than the maximum stable mass for a single white dwarf, and sub-Chandrasekhar, in which the mass is lower.

In both DD classes, the white dwarfs spiral inward, shedding energy in the form of gravitational radiation, until they merge. The difference is in how they explode when the merger actually happens: in the super-Chandrasekhar case, exceeding the mass limit obviously plays a role, while the sub-Chandrasekhar situation must rely on different physical mechanisms.

As mentioned before, white dwarfs are small, so it is rare to image them directly, much less resolve both objects in a binary. Given that SN Ia events occur when the binary has evolved to the point of contact, that means close binaries are the most important. Astronomers must rely on measuring the Doppler shift in white dwarf spectra as they orbit around each other: the magnitude of the shift determines how fast they are orbiting, which provides data about their masses. 

Badenes and Dan Maoz analyzed the spectra from about 4000 white dwarfs from the Sloan Digital Sky Survey (SDSS) data. They found a way to characterize the number of lower-mass white dwarf binaries, something that has been difficult to do before. They found 15 definite DD binaries, which enabled them to determine the frequency of mergers.

Extrapolating these results beyond the Milky Way, the researchers estimate that, on average, 1.4 white dwarf binaries merge per century for every 100 billion solar masses. In other words, for a galaxy 100 billion times the mass of our Sun, we would expect between one and (more rarely) two events each century. This rate is nearly the same as the frequency of SN Ia events as determined from observational data. 

From their statistical sample, Badenes and Maoz also find that about 90 percent of DD mergers are sub-Chandrasekhar, which is consistent both with the lack of higher-mass white dwarf pairs in other studies, and with the failure to find signs of ordinary companion stars in SN Ia remnants.

With the current data, it is still possible that at least some SN Ia events are from SD systems with very low-mass companion stars. However, both the SD and super-Chandrasekhar models are difficult to fit to the existing type Ia supernova data, and the current statistical study offers a way out. Perhaps nearly every SN Ia event is due to the merger of a sub-Chandrasekhar white dwarf binary.

This paper has not yet been through peer review, but the analysis appears sound. It is available at the arXiv. ArXiV:1202.5472

Listing image by Photograph by SDSS

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