Peering into the heart of two recently exploded double-star systems, called cataclysmic variables, NASA's Hubble Space Telescope has surprised researchers by finding that the white dwarf stars at the heart of the fireworks are cooler than expected and spin more slowly than thought.
"This calls for revision of theory," says Prof. Edward Sion of Villanova University, Villanova, PA. "Though these extremely faint explosive white dwarfs have been known about for 30 years, Hubble allows astronomers to observe them directly for the first time and provide observation evidence to test theories."
Each dwarf incredibly dense, burned-out stars that have collapsed to the size of Earth is in a compact binary system, called a cataclysmic variable, where its companion is a normal star similar to, but smaller than the Sun. The stars orbit each other in less than three hours and are so close together the entire 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 a burst of kinetic energy, called a dwarf nova outburst, equivalent to 100 million times the energy of all the warheads in the U.S. and Soviet nuclear arsenal, at the peak of the Cold War. Once dumped onto the dwarf s surface, hydrogen accumulates until it undergoes thermonuclear fusion reactions that eventually trigger the classical nova explosion, which is 10,000 times even more energetic than the dwarf nova outburst. After the detonation, the "fueling" of the white dwarf starts again.
Sion and co-investigators studied the two best known cataclysmic variables, VW Hydri and U Geminorum. Hubble was used to make spectroscopic observations of the dwarf novae just days after their eruption, before another gas disk formed and obscured direct observation of the white dwarf.
The biggest surprise is that the spin rates of the white dwarf stars, as measured by Hubble (slightly less than four minutes for U Geminorum and approximately once a minute for VW Hydri) are so slow there should be violent collisions where the gas disk crashes onto the slower moving white dwarf surface. Since the predicted x-rays from the hot (several hundred thousand to a million degrees centigrade, or greater) colliding gas has never been observed, astronomers thought that the white dwarf was spinning as fast as the disk, so that contact between the disk and surface was less violent. However, the Hubble results contradict this conclusion.
"Despite the fact that several million years of accumulating the swirling gas disks should spin-up the white dwarfs, we just don't see it," says Sion. "Perhaps other mechanisms might be at work to carry away momentum, removing the spin.
Their Hubble observations have also provided the first direct measurements of the cooling of the white dwarfs in response to the heating by the dwarf nova explosion. The researchers found that, even though the gaseous disk heats the white dwarf star surfaces by thousands of degrees Kelvin, this is still well below the predicted heating, according to standard theory. "Somehow this energy is dissipated across the dwarf's surface, rather than being concentrated at the zone where the disk crashes," says Sion.
The Hubble results also show that the proportion of chemical elements in the dwarfs' atmospheres are significantly different from the observed proportions in the Sun's atmosphere. This is probably due to the fact that heavier elements falling onto the dwarf are pulled quickly below the surface layers by the dwarf's enormous gravitational field and turbulence associated with the accumulation of the gas disk.
Further Hubble observations by the team during 1995-96 will attempt to resolve these mysteries. Their work appears in the May 10 and May 20 issues of the Astrophysical Journal Letters.
The research team includes: E.M. Sion and Min Huang (Villanova University); Paula Szkody (University of Washington); Ivan Hubeny (NASA Goddard Space Flight Center); and Fuhua Cheng (University of Maryland).
Space Telescope Science Institute