University of California, Berkeley | Robert Sanders | 2011 Oct 04
The largest survey to date of distant exploding stars is giving astronomers new clues to what’s behind the Type Ia supernovae they use to measure distances across the cosmos.
These stellar explosions helped astronomers conclude more than a decade ago that dark energy is accelerating the expansion of the universe, and today (Tuesday, Oct. 4) earned the discoverers – including UC Berkeley physicist Saul Perlmutter – the 2011 Nobel Prize in Physics. But what caused them was a mystery. Many astronomers thought white dwarf stars were pulling matter from their normal stellar companions and growing so fat they exploded.
But the new study by American, Israeli and Japanese astronomers instead suggests that many, if not most, of the Type Ia supernovae result when two white dwarf stars merge and annihilate in a thermonuclear explosion.
“The nature of these events themselves is poorly understood, and there is a fierce debate about how these explosions ignite,” 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.
“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,” he said. “Two white dwarfs merging can explain well what we are seeing.”
Poznanski, Tel-Aviv University graduate student Or Graur and their colleagues will report their findings in the October 2011 issue of the journal Monthly Notices of the Royal Astronomical Society (MNRAS).
The results do not place in jeopardy the conclusion that the expansion of the universe is accelerating, said coauthor Alex Filippenko, UC Berkeley professor of astronomy.
“As long as Type Ias explode in the same way, no matter what their origin, their intrinsic brightnesses should be the same, and the distance calibrations would remain unchanged,” he said.
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.
“The tide is definitely turning, and these are the best data yet to support the double-degenerate theory,” Filippenko said.
One white dwarf or two?
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, largest-ever survey using the Subaru Telescope in Hawaii accumulated a sample of 150 distant supernovas that exploded between 5 and 10 billion years ago.
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.
“There are no good answers yet, and it could be that we are seeing a mix of the two types of explosions,” Poznanski 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 events.
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.
Most Ancient Supernovas Are Discovered
American Friends of Tel Aviv University | 2011 Oct 05
Ten-billion-year-old exploding stars were a source of Earth's iron, TAU researchers say
Supernovas — stars in the process of exploding — open a window onto the history of the elements of Earth's periodic table as well as the history of the universe. All of those heavier than oxygen were formed in nuclear reactions that occurred during these explosions.
The most ancient explosions, far enough away that their light is reaching us only now, can be difficult to spot. A project spearheaded by Tel Aviv University researchers has uncovered a record-breaking number of supernovas in the Subaru Deep Field, a patch of sky the size of a full moon. Out of the 150 supernovas observed, 12 were among the most distant and ancient ever seen.
The discovery sharpens our understanding of the nature of supernovas and their role in element formation, say study leaders Prof. Dan Maoz, Dr. Dovi Poznanski and Or Graur of TAU's Department of Astrophysics at the Raymond and Beverly Sackler School of Physics and Astronomy. These "thermonuclear" supernovas in particular are a major source of iron in the universe.
The research, which appears in the Monthly Notices of the Royal Astronomical Society this month, was done in collaboration with teams from a number of Japanese and American institutions, including the University of Tokyo, Kyoto University, the University of California Berkeley, and Lawrence Berkeley National Laboratory.
A key element of the universe
Supernovas are nature's "element factories." During these explosions, elements are both formed and flung into interstellar space, where they serve as raw materials for new generations of stars and planets. Closer to home, says Prof. Maoz, "these elements are the atoms that form the ground we stand on, our bodies, and the iron in the blood that flows through our veins." By tracking the frequency and types of supernova explosions back through cosmic time, astronomers can reconstruct the universe's history of element creation.
In order to observe the 150,000 galaxies of the Subaru Deep Field, the team used the Japanese Subaru Telescope in Hawaii, on the 14,000-foot summit of the extinct Mauna Kea volcano. The telescope's light-collecting power, sharp images, and wide field of view allowed the researchers to overcome the challenge of viewing such distant supernovas.
By "staring" with the telescope at the Subaru Deep Field, the faint light of the most distant galaxies and supernovas accumulated over several nights at a time, forming a long and deep exposure of the field. Over the course of observations, the team "caught" the supernovas in the act of exploding, identifying 150 supernovas in all.
Sourcing man's life-blood
According to the team's analysis, thermonuclear type supernovas, also called Type-la, were exploding about five times more frequently 10 billion years ago than they are today. These supernovas are a major source of iron in the universe, the main component of the Earth's core and an essential ingredient of the blood in our bodies.
Scientists have long been aware of the "universal expansion," the fact that galaxies are receding from one another. Observations using Type-Ia supernovas as beacons have shown that the expansion is accelerating, apparently under the influence of a mysterious "dark energy" — the 2011 Nobel Prize in Physics will be awarded to three astronomers for this work. However, the nature of the supernovas themselves is poorly understood. This study improves our understanding by revealing the range of the ages of the stars that explode as Type-Ia supernovas. Eventually, this will enhance their usefulness for studying dark energy and the universal expansion, the researchers explain.
Supernovae in the Subaru Deep Field: the rate and delay-time distribution of type Ia supernovae out to redshift 2 - Or Graur et al
- Monthly Notices of the Royal Astronomical Society (online 19 Sep 2011) DOI: 10.1111/j.1365-2966.2011.19287.x
arXiv.org > astro-ph > arXiv:1102.0005 > 31 Jan 2011 (v1), 20 Jun 2011 (v3)
