Rapid Supernova Could be New Class of Exploding Star

BERKELEY, CA—An unusual supernova rediscovered in seven-year-old data taken at the W. M. Keck Observatory and Lick Observatory may be the first example of a new type of exploding star, possibly in a binary star system where helium flows from one white dwarf onto another and detonates in a thermonuclear explosion.

In a paper first published online Nov. 5 in Science Express, astronomer Dovi Poznanski, of the University of California, Berkeley (UCB) and Lawrence Berkeley National Laboratory (LBNL), and his colleagues describe supernovae SN 2002bj and review the data that suggest it is a new type of stellar explosion.

The supernova was detected in 2002 in the galaxy NGC 1821, in the constellation Lepus, by UC Berkeley astronomer Alex Filippenko’s Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory near San Jose, as well as by amateur astronomers. The exploding star’s spectrum was obtained seven days after its discovery using the Keck I telescope with its Low Resolution Imaging Spectrograph.

The supernova was erroneously classified by the astronomical community as a common Type II supernova and filed away.

In June 2009, Poznanski reanalyzed the spectrum while reviewing data of Type II supernovae, which he wants to use as distance indicators to confirm the accelerating expansion of the Universe. When he carefully examined a high-quality spectrum of SN 2002bj, he realized that the supernova was not a Type II at all, but an unusual kind of supernova more akin to a Type Ia.

According to follow-up images made by KAIT, SN 2002bj disappeared 20 days after its discovery. An image of that area of the sky taken seven days prior to its discovery showed no supernova, so it had brightened and dimmed into obscurity in less than 27 days. Most supernovae brighten and dim over three to four months.

“This is the fastest evolving supernova we have ever seen,” said Poznanski, a UC Berkeley post-doctoral fellow who recently joined LBNL’s Computational Cosmology Center. “It was three to four times faster than a standard supernova.”

This rapid drop, coupled with the supernova’s faintness, the strong signature of helium in the spectrum of the explosion, the absence of hydrogen, and the possible presence of vanadium – an element never previously identified in supernova spectra – points toward helium detonation on a white dwarf.

“We think this may well be a new physical explosion mechanism, not just a minor variation of ones already known,” said Filippenko, a coauthor on the study. “This supernova is qualitatively different from the complete disruption of a white dwarf, known as a Type Ia supernova, or the collapse of an iron core and rebound of the surrounding material, so-called ‘core-collapse supernovae.’”

Co-author Joshua Bloom, of UCB, added that astronomers have seen great diversity in those two main supernova mechanisms, “but even within that diversity, observationally, there is a limited range of variation spectrally and in how events evolve in time,” he said. “This object (SN 2002bj) falls outside that range.”

Based on the available images and spectra SN 2002bj, Poznanski and graduate student Ryan Chornock – now a post-doctoral fellow at Harvard University – determined that the theory involving AM Canum Venaticorum (AM CVn) binary systems best matches the data.

Proposed by Lars Bildsten, a professor of physics at the Kavli Institute for Theoretical Physics at UC Santa Barbara, and colleagues, the theory of AM CVn binary systems describes them as composed of two white dwarfs, one of which is primarily made of helium that is being slowly pulled by gravity onto its companion. White dwarfs are the remnants of stars that burned their hydrogen down to carbon and oxygen or, in some particular cases, to helium.

In a 2007 Astrophysical Journal Letters paper, Bildsten and colleagues proposed that in AM CVn systems, when enough helium has been accumulated on the surface of the primary white dwarf, an explosion will occur that can “power a faint … and rapidly rising (few days) thermonuclear supernova.” The event is now called a .Ia (point one A) supernova, because it is one-tenth as bright for one-tenth the time as a Type Ia supernova.

Filippenko added, however, that this explosion is nothing like a regular Type Ia explosion because the white dwarf survives the detonation of the helium shell. In fact, it has similarities to both a nova and a supernova. Novas occur when matter – primarily hydrogen – falls onto a star and accumulates in a shell that can flare up as brief thermonuclear explosions. SN 2002bj is, however, a “super” nova because it generated about 1,000 times the energy of a standard nova, he said.

The past few years have yielded a bonanza of weird supernovae, Filippenko said. “A lot of us who have studied supernovae for several decades are amazed at the quality and quantity of data coming in recently, showing interesting new subclasses or even strange new physical classes of supernovae,” he said. “It whets my appetite for what else we might find out there.”

The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i. The twin telescopes feature a suite of advanced instrumentation including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system. The Observatory 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 http://www.keckobservatory.org.