Pre-Supernova White Dwarf Uncovered by Hubble Team (Forwarded)

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January 9, 2006

Pre-Supernova White Dwarf Uncovered by Hubble Team, Led by Villanova
University Astronomer Edward Sion

Astronomers today are reporting the first detection of direct radiation
from the surface of a white dwarf star in a pre-supernova binary star
system using the Hubble Space Telescope. This is a major step forward in
identifying the type of star that will become a Type Ia supernova, the
type of supernova that is being used to show that the expansion of the
universe is accelerating. These binary objects, called AM Cvn stars, have
virtually pure helium in their outer layers, and are considered among the
strongest progenitor candidates for the occurrence of Type Ia supernovae,
the kind of supernova which are used as "standard candles" to measure the
size of our Universe, and being used to show that the expansion of the
Universe is speeding up instead of slowing down. The Hubble team is led by
Dr. Edward Sion of Villanova University and includes Dr. Paula Szkody of
the University of Washington, Seattle, Dr.Jan-Erik Solheim of the
University of Oslo, Norway, Dr. Boris Gaensicke, the University of
Warwick, England and Dr. Steve Howell of the National Optical Astronomical
Observatories, Tucson.

The team is presenting the results of their computer modeling of the
Hubble data at the 207th meeting of the American Astronomical Society in
Washington, DC. Their work has been accepted for publication in an
upcoming issue of the Astrophysical Journal Letters.

The extremely dense, planet-sized white dwarf star is normally surrounded
by a swirling disk of helium gas from a very close helium-rich lighter
weight donor star. The Hubble Team observed the object during a brief time
when the helium disk hiding the white dwarf, temporarily goes away.

A mere teaspoonful of the white dwarf's matter weighs over 100 tons. This
heavy weight white dwarf and the lighter weight helium-rich donor star
whirl around each other in a stellar dance every 28 minutes. In order for
Type Ia supernovae to be used as proper reliable standard candles, we must
understand what kind of star exploded. This white dwarf detection helps
provide this information.

The Team demonstrates that the pre-supernova object is much cooler and
more slowly spinning than predicted by theory. The white dwarf's surface
chemistry is laden with heavy metals. In the pre-supernova binary, helium
is being transferred from the lightest, largest star to the heaviest,
smallest star as they orbit each other every 28 minutes. In most cases the
accretion of mass by the heaviest star proceeds via a nearly pure helium
accretion disk, At present, only about 10 such double nearly pure helium
white dwarf systems are known. But recent estimates predict enough to
account for the observed rate of Type Ia supernovae. This first
spectroscopic detection of the white dwarf in a AM CVn system allows us to
directly find for the first time the chemical makeup, spin rate, and mass
of the white dwarf as well as estimating how fast helium is accumulating
onto the primary white dwarf.

These objects can undergo a Type Ia supernova explosions without requiring
that the white dwarf star first reach its maximum possible mass, the
so-called Chandrasekhar limit. If the incoming helium accumulates slowly
enough onto the heavier white dwarf, it will gradually compress or crush
the matter below, triggering a helium thermonuclear explosion which will
cause the carbon in the core of the white dwarf to detonate 10,000,000
times more violently as a Type Ia supernova. This process called edge-lit
detonation (ELD) will occur in an AM CVn system at low enough accretion
rates and slow enough rotational velocities.

The AM CVn binary systems are also the only known source of low frequency
gravitational waves predicted by Einstein's theory of general relativity.

When the two compact stars in an AM CV binary system revolve around each
other, they lose energy and angular momentum through the emission of
gravitational waves, at the expense of their own orbital energy. This
causes their orbits to shrink ever further. The orbit shrinkage has been
observed in binary radiopulsars, e.g. in the famous Hulse-Taylor pulsar
PSR B1913+16 (Nobel Prize in Physics 1993) but there has never yet been a
direct detection of gravitational waves. AM CVn systems binary white
dwarfs are expected to be the dominant sources of gravitational waves to
be detected by LISA, the laser interferometer in space which is due for
launch be launch early in the next decade.