The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/hst/95_23.txt

Text File | 1996-01-12 | 5KB | 96 lines

RELEASE DATE:  May 22, 1995

CONTACT:  Ray Villard
          Space Telescope Science Institute
          (Phone:  [phone])

PRESS RELEASE NO.:  STScI-PF95-23


HUBBLE PROBES THE WORKINGS OF A STELLAR HYDROGEN-BOMB

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).

                             * * * * * *

The Space Telescope Science Institute is operated by the Association
of Universities for Research in Astronomy, Inc. (AURA) for NASA,
under contract with the Goddard Space Flight Center, Greenbelt, MD.
The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA).