White dwarf in pas de deux: Cooler, slower.

Sometimes a youthful companion makes all the difference. Consider the elderly, shrunken star known as a white dwarf. Left alone, it slowly radiates heat away and eventually drops out of sight. But if it attracts a less mature partner, it won't fade away. Drawing fresh material from its companion, the dense star will refuel, generating recurrent fireworks bright enough to be visible to the naked eye from a distance of several million light-years.

Astronomers have known for decades about such dynamic duos, known as cataclysmic variables. But new findings suggest that a white dwarf locked in a gravitational embrace with a younger partner spins more slowly and has a much lower temperature than predicted.

Edward M. Sion of Villanova (Pa.) University and his colleagues base their conclusions on Hubble Space Telescope spectra of two Milky Way white dwarfs at the heart of the cataclysmic variables VW Hydri and U Geminorum.

If observations of other cataclysmic variables confirm these results, the work "calls for revision of theory," says Sion. He adds that the findings, reported in the May 10 and May 20 Astrophysical Journal Letters, may shed light on the dynamics of other stellar twosomes, such as a normal star delivering gas to an object much denser than a white dwarf--a neutron star or black hole.

The material that a white dwarf draws from a younger, less dense companion forms a swirling disk around the dwarf's equator. Gravity pulls material from the disk to the white dwarf's surface. When enough has accumulated, it erupts in a thermonuclear explosion.

Researchers had predicted that the swirling motion of the gas would rev up the white dwarf's spin to several thousand kilometers per second--about half the rotation speed that would break it apart. Astronomers had reasoned that if these stars spun any more slowly, the rapidly rotating disks would crash violently into them, generating intense X-ray emissions. The absence of these predicted X rays suggested that white dwarfs indeed rotate rapidly.

Nonetheless, the Hubble measurements reveal that the white dwarfs in VW Hydri and U Geminorum are downright sluggish--spinning at one-tenth the expected rate. "It's not often that you get a result as decisive as this," says Joseph Patterson of Columbia University. "This means that the problem for the past 15 years in understanding where the energy goes [when matter is dumped onto a white dwarf] . . . can't be blamed on a rapidly rotating white dwarf."

Astronomers had also thought that the periodic explosions would raise a dwarf's temperature to several hundred thousand or perhaps a million kelvins. But the Hubble observations show that the surface temperature of the white dwarf in VW Hydri is 20,000 kelvins, while that of the white dwarf in U Geminorum reaches about 40,000 kelvins.

To account for the slower spin and lower temperatures, Sion speculates that instead of all the violence happening at the star's equator, where the gaseous disk crashes into the dwarf, some of the energy burrows in deeper or spreads across the star's surface.