News Release->

Hubble Finds Unexpected Source of Heavy Elements in Explosive Double Star Systems

Hubble Space Telescope observations of an extremely close double-star system, called a dwarf nova, has uncovered astonishing, totally unexpected evidence of an ancient, violent thermonuclear explosion which synthesized huge amounts of heavy elements vital to the formation of solar systems and provides a missing evolutionary link between two classes of exploding stars, dwarf novae, whose outbursts are powered by gravitational energy release and the classical novae whose much more energetic stellar explosions are thermonuclear. At the heart of the ancient fireworks was an extremely compact white dwarf star, a burned-out stellar corpse collapsed to the size of a planet no bigger than earth. "This is a major challenge to our understanding of proton capture nuclear reactions and suggests an important source for the production of nuclei vitally important to the formation of the solar system. Among these elements are short-lived radioactive nuclei like Al^26 whose decay products are seen in meteorites and whose heat release is required for the rapid heating of small bodies in solar systems, says Ed Sion of Villanova University, Villanova, Pennsylvania, leader of the research team which includes Fuhua Cheng, Villanova University, Paula Szkody, the University of Washington, Ivan Hubeny, the Goddard Space Flight Center, and Warren Sparks of the Los Alamos National Laboratory.

The presence of these heavy elements at the surface of the white dwarf star offers a veritable nuclear physics laboratory for the study of these reactions, called proton captures, which are poorly understood and difficult to study experimentally in nuclear physics laboratories on Earth. The chemical abundances are far in excess of what the theoretical predictions say should be produced in a classical nova. They are the hallmark products of nuclear reactions accompanying extremely hot, explosive hydrogen fusion into helium, the process that powers the classical nova explosion. This is the first time direct chemical evidence of a classical nova explosion (the second most violent stellar explosion, exceeded only by a supernova) has been seen in a dwarf nova and thus provides the first evolutionary link between the two types of explosive objects.

It is also intriguing that such overabundances of proton capture elements like Phosphorus and Aluminum are seen in gas associated with quasars and AGN's which appear to have massive accretion disks surrounding supermassive black holes. in this scenario, proposed by Greg Shields of the University of Texas, single white dwarfs can accrete dense gas in the disk as they orbit the hole, undergoing nova explosions which produce the huge quantities of phosphorus and aluminum.

Each dwarf nova is in a compact binary system, where its companion is a normal star. The stars whirl around each other in less than three hours and are so close together the entir binary system would fit inside our Sun. This allows gas to flow from the normal star onto the dwarf where it swirls into a pancake-shaped disk. When the disk of gas periodically collapses onto the white dwarf, it unleashes kinetic energy. This is called a dwarf nova outburst. Once dumped onto the surface, hydrogen accumulates until it undergoes thermonuclear fusion reactions that trigger the classical nova explosion. After the explosion the "fueling" of the white dwarf starts again. However, until their Hubble detection of a past thermonuclear runaway, no dwarf nova had ever been chemically linked to a classical nova.

"Though these extremely faint white dwarfs have been known for about 30 years, Hubble allows astronomers to observe them directly for the first time and provide observation evidence to test nuclear reaction theory", said Sion. Sion and co-investigators studied one of the best known cataclysmic variable dwarf novae, VW Hydri. Hubble was used to make spectroscopic observations of the dwarf novae just days after its eruption, before another gas disk formed and block directed observation of the white dwarf.

Future Hubble observations by the team during will attempt to resolve these mysteries. Their work appears in the May 1 issue of the Astrophysical Journal Letters.

For More Information Contact Dr. Edward M. Sion (610) 519-6132, emsion@ucis.vill.edu.