Date: May 22 21:08 UTC
Subject: HST Probes Stellar H-Bomb
From: NASA Space News
HUBBLE PROBES THE WORKINGS OF A STELLAR HYDROGEN-BOMB
Peering into the heart of two recently exploded double-star systems,
NASA's Hubble Space Telescope has surprised researchers by finding that
the white dwarf stars at the heart of the fireworks are cooler and spin
more slowly than expected.
"This calls for revision of theory," said 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 observational 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 Earth's 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 explosive energy of all the
warheads which were in U.S. and Soviet arsenals 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 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 Celsius, 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," said Sion. "Perhaps other mechanisms might be at work to carry
away rotational 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," said Sion.
The Hubble results also show that the proportion of chemical elements
in the dwarfs' atmospheres is 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, Greenbelt, MD.; and Fuhua Cheng, University of Maryland.
The Space Telescope Science Institute is operated by AURA (the
Association of Universities for Research in Astronomy, Inc.) 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.
NOTE TO EDITORS: Images and a video deepicting this event are available
to media representatives by calling NASA's Broadcast & Imaging Branch
at 202/358-1900. NASA photo numbers are: Color - 93-HC-375; B&W -
93-H-401.
Artist conception can be accessed on the Internet:
Image files in GIF and JPEG format may be accessed on Internet via
anonymous ftp:
GIF: "ftp://ftp.stsci.edu/pubinfo/gif/DiskIllus.gif"
JPEG: "ftp://ftp.stsci.edu/pubinfo/jpeg/DiskIllus.jpg"
The same images are available via World Wide Web from
"http://www.stsci.edu/EPA/Latest.html", or via links in
"http://www.stsci.edu/public.html".
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- T o d d E. V a n H o o s e a r -
``'''vanhoose@lalaland.cl.msu.edu - vanhoose@msu.edu - vanhoose@lalaland.cl.msu.edu
(._.) Michigan State University - East Lansing, MI USA
(_) Computer Laboratory - Department of Communication
`---' <A HREF="http://lalaland.cl.msu.edu/~vanhoose/">My Home Page</A>
"Error, no keyboard - press F1 to continue."
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
