Washington: Scientists have unearthed the origin of an important type of exploding stars—Type Ia supernovae.
Studying supernovae of this type helps researchers measure galaxy distances and can lead to important astronomical discoveries.
Principal investigator Carlos Badenes, assistant professor of physics and astronomy in Pitt’s Kenneth P. Dietrich School of Arts and Sciences, detailed 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 Type Ia supernovae explosions.
“We knew that two stars had to be involved in such an explosion, and that one of them had to be a white dwarf,” said Dan Maoz, professor of physics and astronomy at Tel-Aviv University in Israel and coauthor of this soon-to-be-published paper on the discovery.
“But there were two possibilities for what the second star is, which is what we sought to discover.”
According to Badenes, there were two potential outcomes for the star’s type. 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.
“But our biggest question was whether there were enough double white dwarfs out there to produce the number of supernovae that we see,” said Maoz said.
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.
The team 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, said Badenes, 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,” said 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,” said Badenes.
“This suggests that the merger of a double white dwarf system is a plausible explanation for Type Ia supernovae,” Badenes added.
The study has been published in Astrophysical Journal Letters.