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Keck Observatory looks at possible new type of supernovae

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This image, taken from a simulation of a supernova, shows a star that has ignited in a thermonuclear explosion and blown apart. Astronomers studying a calcium-rich supernova with the Keck telescopes think that they may have identified a new type of these stellar death explosions. Image by MIT

New data from several telescopes, including the W. M. Keck Observatory, suggest astronomers may have identified a new type of supernovae. The stellar death is thought to have originated in a star that was a low-mass white dwarf accumulating helium from a companion star. When the white dwarf exploded, about half of the mass ejected from the supernova was in the form of calcium. The finding suggests that a couple of supernovae like this exploding every 100 years would produce the high abundance of calcium observed in galaxies like the Milky Way, and the calcium present in life on Earth.

The supernova, SN 2005E, was discovered five years ago by the University of California, Berkeley’s Katzman Automatic Imaging Telescope (KAIT), and is one of only eight known “calcium-rich supernovae” that appear to be distinct from the two main classes of supernovae: the Type Ia supernovae, thought to be old, white dwarf stars that accrete matter from a companion until they undergo a thermonuclear explosion that blows them apart entirely; and Type Ib/c or Type II supernovae, thought to be hot, massive and short-lived stars that explode and leave behind black holes or neutron stars.

In the past decade, robotic telescopes have turned astronomers’ attention to scads of strange exploding stars, one-offs that may or may not point to new and unusual physics. “With the sheer numbers of supernovae we’re detecting, we’re discovering weird ones that may represent different physical mechanisms compared with the two well-known types, or may just be variations on the standard themes,” said Alex Filippenko, KAIT director and UC Berkeley professor of astronomy. “But SN 2005E was a different kind of ‘bang.’ It and the other calcium-rich supernovae may be a true suborder, not just one of a kind.”

One theory of this new exploding system, shown in the simulation images (above), is that a white dwarf steals helium from a companion until the mass thief becomes very hot and dense and a nuclear explosion occurs. The helium is transformed into elements such as calcium and titanium, eventually producing the building blocks of life for future generations of stars. Image by Avishay Gal-Yam, Weizmann Institute of Science

Filippenko is coauthor of a paper appearing in the May 20 issue of the journal Nature describing SN 2005E and presenting evidence that the original star was a low-mass white dwarf stealing helium from a binary companion until the temperature and pressure ignited a thermonuclear explosion – a massive fusion bomb – that blew off at least the outer layers of the star and perhaps blew the entire star to smithereens. The team of astronomers was led by Hagai Perets, now at the Harvard-Smithsonian Center for Astrophysics, and Avishay Gal-Yam of the Weizmann Institute of Science in Rehovot, Israel.

In November 2009, Filippenko and former UC Berkeley post-doctoral fellow Dovi Poznanski, currently at Lawrence Berkeley National Laboratory and also coauthor on the Nature paper, reported another supernova, SN 2002bj, that they believe explodes by a similar mechanism: ignition of a helium layer on a white dwarf.

“SN 2002bj is arguably similar to SN 2005E, but has some clear observational differences as well,” Filippenko said. “It was likely a white dwarf accreting helium from a companion star, though the details of the explosion seem to have been different because the spectra and light curves differ.” Astronomers have so far found only one example of this supernova.

Filippenko and UC Berkeley research astronomer Weidong Li first reported an unusual calcium-rich supernova in 2003, and since then, KAIT has discovered several more, including SN 2005E on Jan. 13, 2005. Because these supernovae, like Type Ib, show evidence for helium in their spectra shortly after they explode, and because in the later stages they show strong calcium emission lines, the UC Berkeley astronomers were the first to refer to them as “calcium-rich Type Ib supernovae.”

It was SN 2005E, which went off about 110 million years ago in the spiral galaxy NGC 1032 in the constellation Cetus, that initially drew the attention of Perets, Gal-Yam and their colleagues. Using data provided by Filippenko and Li, and taken by the W. M. Keck Observatory in Hawaii, the Palomar Observatory in California and the Liverpool Observatory in the United Kingdom, they created a detailed picture of the explosion. The small amount of mass ejected in the explosion, estimated at 30 percent the mass of the Sun, and the fact that the galaxy in which the explosion occurred was old with few hot, giant stars, led them to the conclusion that a low-mass white dwarf was involved.

The newly discovered supernova threw off unusually high levels of the elements calcium and radioactive titanium, which are the products of a nuclear reaction involving helium rather than carbon and oxygen that are involved in Type Ia supernovae.

“We know that SN 2005E came from the explosion of an old, low-mass star because of its specific location in the outskirts of a galaxy devoid of recent star formation,” Filippenko said. “And the presence of so much calcium in the ejected gases tells us that helium must have exploded in a nuclear runaway.”

The paper’s authors note that, if these eight calcium-rich supernovae are the first examples of a common, new type of supernovae, they could explain two puzzling observations: the abundance of calcium in galaxies and in life on Earth, and the concentration of positrons – the anti-matter counterpart of the electron – in the center of galaxies. The latter could be the result of the decay of radioactive titanium-44, produced abundantly in this type of supernova, to scandium-44 and a positron, prior to scandium’s decay to calcium-44. The most popular explanation for this positron presence is the decay of putative dark matter at the core of galaxies.

“Dark matter may or may not exist,” says Gal-Yam, “but these positrons are perhaps just as easily accounted for by the third type of supernova.”

Filippenko and Li hope that KAIT and other robotic telescopes scanning distant galaxies every night in search of new supernovae will turn up more examples of calcium-rich or even stranger supernovae, which can then be observed with larger telescopes such as Keck.

“The research field of supernovae is exploding right now, if you’ll pardon the pun,” Filippenko said. “Many supernovae with peculiar new properties have been found, pointing to a greater richness in the physical mechanisms by which nature chooses to explode stars.”

The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i and is a scientific partnership of the California Institute of Technology, the University of California, and NASA. For more information please call 808.881.3827 or visit www.keckobservatory.org

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