U. PITTSBURGH (US) — Once a century an explosion in the Milky Way is caused by the merger of double white dwarf supernovae, a finding that may help measure the distances of galaxies.
In a paper to be published in Astrophysical Journal Letters, principal investigator Carlos Badenes, assistant professor of physics and astronomy at the University of Pittsburgh, details the ways in which his team used the Sloan Digital Sky Survey—a collection of multicolor images and more than a million spectra covering more than a quarter of the sky—to determine what kinds of starts produce the exploding stars, called Type Ia supernovae.
“We knew that two stars had to be involved in such an explosion, and that one of them had to be a white dwarf,” says co-author Dan Maoz, professor of physics and astronomy at Tel-Aviv University in Israel. “But there were two possibilities for what the second star is, which is what we sought to discover.”
This series of artist’s concepts shows two white dwarfs orbiting one another. In the future, their orbits will get smaller and smaller, and faster and faster, until someday they merge and explode. (Credit: NASA/GSFC/D.Berry)
There were two potential outcomes for the star’s type, Badenes says. It could be a “normal star,” like the sun, or it could be another white dwarf, which is a smaller, more dense faint star composed of electron-degenerate matter.
The team suspected the latter, as two white dwarfs within the same star system would revolve around one another at half a million miles an hour, speeding up and getting closer and closer until one day they merge, most likely producing the fireworks of Type Ia supernovae.
“There were obvious reasons to suspect that Type Ia supernovae come from the merging of a double white dwarf,” says Maoz. “But our biggest question was whether there were enough double white dwarfs out there to produce the number of supernovae that we see.”
Because white dwarfs are extremely small and faint, there is no hope of seeing them in distant galaxies. Therefore, Badenes and Maoz turned to the only place where they could be seen: the part of the Milky Way Galaxy within about a thousand light years of the sun.
To find the star’s companion, the team needed two spectra to measure the velocity between the two. However, SDSS only took one spectrum of most objects, so they decided to make use of a little-known feature in the SDSS spectra to separate each one into three or more subspectra. Although the reprocessing of the data was challenging, the team was able to compile a list of more than 4,000 white dwarfs within a year, each of which had two or more high-quality subspectra.
“We found 15 double white dwarfs in the local neighborhood and then used computer simulations to calculate the rate at which double white dwarfs would merge,” says Badenes. “We then compared the number of merging white dwarfs here to the number of Type Ia supernovae seen in distant galaxies that resemble the Milky Way.”
The result was that, on average, one double white dwarf merger event occurs in the Milky Way about once a century. “That number is remarkably close to the rate of Type Ia supernovae we observe in galaxies like our own,” says Badenes. “This suggests that the merger of a double white dwarf system is a plausible explanation for Type Ia supernovae.”
In addition to providing a key clue about the nature of these important events, the team’s discovery shows the potential of giant astronomical surveys like the SDSS.
“Twenty years ago we decided to take three subspectra for each spectrum. We did that for entirely practical reasons,” says Robert Lupton, senior research astronomer at Princeton and a colleague of Badenes. “We had no idea that it would someday give us an important clue to the mystery of the Type Ia supernovae.”
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