Astronomers have finally identified the progenitor star system of a Type Ia supernova. The culprit that triggered the stellar explosion is a surprisingly normal star just a few times more massive than the Sun.

Type Ia supernovae are luminous objects that have been used as cosmological beacons to study the expansion and geometry of the Universe. Two separate teams have reported on-going observations of distant supernovae of this type that indicate the presence of a repulsive force, termed dark energy, causing the expansion of the Universe to accelerate.

This image of Supernova 2002ic was taken at the Swope 1-meter telescope at Las Campanas Observatory in Chile on January 7, 2003. The supernova is the bright point of light in the center, at the intersection of two background galaxies. The host galaxy of the supernova is extremely faint and is not visible. Credit: Carnegie Observatories and NOAO/AURA/NSF

"A great worry in this work has been that astronomers do not understand exactly how these supernovae are produced," says Mario Hamuy, a Hubble fellow at the Carnegie Observatories in Pasadena, CA, lead author of a paper in the August 7 issue of Nature magazine. "We have been searching for a progenitor system for two decades, and now we finally have a suspect clearly identified."

The generally accepted model for a Type Ia supernova consists of a binary system with a white dwarf star (a dense and dim stellar object near the end of its life) that undergoes a sudden thermonuclear explosion. But no one had observed anything about the companion star or the processes which ultimately lead to the explosion.

"In theory, some transfer of mass must occur between the companion star and the white dwarf before the explosion," Hamuy explains, "yet no evidence has ever been found for gas around a Type Ia supernova."

Based on a strong signal of hydrogen in ground-based spectroscopic observations of Supernova 2002ic from Las Campanas Observatory, scientists in Chile and the U.S. have concluded that the source of this Type Ia supernova consisted of a white dwarf that suddenly exploded after gaining hydrogen gas blown off by its gravitational partner.

"The amount of hydrogen-rich circumstellar gas detected is considerable," says Nicholas Suntzeff, an astronomer at the Cerro Tololo Inter-American Observatory in Chile and a co-author of the paper.

"The most reasonable conclusion is that the stellar system was made up of a white dwarf and a common asymptotic giant branch star about three to seven times the mass of the Sun, which proceeded to lose a large amount of its hydrogen in the form of a wind, due to instabilities in the late stages of its evolution," Suntzeff adds. "Some of this hydrogen settled on the companion white dwarf, driving its mass up to the point where the white dwarf incinerated within just a few seconds in a potent thermonuclear event."

Type Ia supernovae are important explosions in the buildup of heavy elements in the Universe. More than half of the iron peak elements in the Solar System, such as iron and nickel, come from the ashes of previous Type Ia explosions.

"We can't understand the buildup of the elements in the stars in galaxies unless we know precisely what is blowing up - what it is made of, and how long it takes to get to the point of the explosion," explains astronomer Mark Phillips of Carnegie Observatories, a co-author of the paper.

So far, the extremely faint galaxy that spawned the system which led to Supernova 2002ic has not been identified.

The redshift of the light from the supernova, however, allowed the astronomers to measure the distance to the explosion at 945 million light-years (290 megaparsecs) from Earth.

"The search for a progenitor for Type Ia supernovae has gone on for so long that it almost became a point of embarrassment for scientists in the field," Suntzeff notes. "Supernova 2002ic may not be the prototype for all Type Ia's, but it is certainly the first crack in the puzzle."

The first identified progenitor of any supernova was massive star Sn-69 202, which was the source of the famous Supernova 1987A, a nearby example of the "core collapse" class of supernovae known as Type II, which begin with stars at least 10 times as massive as the Sun. Five more progenitors of these core collapse-type of supernovae have been found since then.

Las Campanas Observatory is operated by the Carnegie Observatories, founded in 1904 by George Ellery Hale. It is one of six departments of the private, nonprofit Carnegie Institution of Washington, a pioneering force in basic scientific research since 1902.

Cerro Tololo Inter-American Observatory is part of the National Optical Astronomy Observatory (NOAO) in Tucson, which is operated by the Association of Universities for Research in Astronomy (AURA) Inc., under a cooperative agreement with the NSF.