Universe’s “Standard Candles” Are White Dwarf Mergers

Berkeley — A new survey of distant Type Ia supernovae suggests that many if not most of these supernovae – key to astronomers’ conclusion that dark energy is accelerating the expansion of the universe – result when two white dwarf stars merge and annihilate in a thermonuclear explosion.

Evidence that Type Ia supernovae are caused by the merger of two white dwarfs – the so called double-degenerate theory – has been accumulating over the past two years, based on surveys by the Hubble Space Telescope and others. Before, astronomers favored the single-degenerate model: the idea that Type Ia’s result from the explosion of a white dwarf grown too fat by feeding on its normal stellar companion.

White dwarfs are dense, compact stars formed from normal stars like the sun once they exhaust their nuclear fuel and compress under their own weight.

The new survey using the Subaru Telescope in Hawaii and backed up by Keck Observatory observations, was conducted by a team of American, Israeli and Japanese astronomers and is the largest to date, having accumulated a sample of 150 distant supernovas that exploded between 5 and 10 billion years ago.

“The main goal of this survey was to measure the statistics of a large population of supernovae at a very early time, to get a look at the possible star systems,” said Dovi Poznanski, one of the main authors of the paper and a post-doctoral fellow at the University of California, Berkeley, and Lawrence Berkeley National Laboratory. “We need two white dwarfs merging to explain what we are seeing.”

The finding, when combined with previous surveys of closer Type Ia supernovae, suggests that astronomers surveying Type Ia supernovae may be seeing a mixture of single and double-degenerates. This does not, however, place in jeopardy the conclusion that the expansion of the universe is accelerating, said coauthor Alex Filippenko, UC Berkeley professor of astronomy.

“The tide is definitely turning, and these are the best data yet to support the double-degenerate theory,” he said. “But as long as Type Ia’s explode in the same way no matter what their origin, their intrinsic brightnesses should be the same and the distance calibrations would remain unchanged.”

Poznanski, Tel-Aviv University graduate student Or Graur, Filippenko and their colleagues will report their findings in the October 2011 issue of Monthly Notices of the Royal Astronomical Society (MNRAS).

“Over the past 14 years we used Type Ia supernovae to determine that the universe is actually accelerating in its expansion, under the influence of mysterious dark energy, but the nature of these events themselves is poorly understood and there is a fierce debate about how these explosions ignite,” said Poznanski.

“There are no good answers yet, and it could be that we are seeing a mix of the two types of explosions,” he said.

Though the two-faced nature of Type Ia supernovae still allows them to be used as calibratable candles to measure cosmic distance, Filippenko said, it might affect attempts to “quantify in detail the history of the expansion rate of the universe. The subtle differences between single- and double-degenerate models could introduce a systematic error that we’ll need to account for.”

The team also found that Type Ia supernovae were five times more common 5-10 billion years ago than today, probably because there were more young stars back then rapidly evolving into white dwarfs. Moreover, this study allowed the team to more accurately determine the production of iron over cosmic time, as Type Ia supernovae create iron through nuclear reactions when they explode.

To find their distant sample, the international team of astronomers exploited the enormous light collecting power of the Subaru Telescope’s Suprime-Camera on four separate occasions. They pointed the ground-based telescope, located atop Hawaii’s Mauna Kea volcano, toward a single field in the sky that was approximately the size of the full moon. Each run yielded about 40 supernovae among 150,000 galaxies.

Then they used the Keck telescopes on Mauna Kea to observe the galaxies where these explosions occurred. These observations were crucial for pinpointing the distance of these

Future observations with the Hyper Suprime-Camera, which will be mounted on the Subaru Telescope, will be able to discover even larger and more distant supernova samples to test this conclusion.

Other authors on the paper include Dan Maoz, Naoki Yasuda, Tomonori Totani, Masataka Fukugita, Ryan J. Foley, Jeffrey M. Silverman, Avishay Gal-Yam, Assaf Horesh, and Buell
T. Jannuzi. The research was supported in part by the National Science Foundation.

Read the paper at: http://arxiv.org/abs/1102.0005

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The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Big Island of Hawaii. The twin telescopes feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system which cancels out much of the interference caused by Earth’s turbulent atmosphere. The Observatory is a private 501(c) 3 organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.

(Adapted from a press release issued by U.C. Berkeley)