Final Death Throes of Nearby Star Witnessed First-Hand

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Whitney Clavin (626) 395-1877
Jet Propulsion Laboratory, Pasadena, Calif.

NEWS RELEASE: 2003-154 Nov. 21, 2003

Final Death Throes of Nearby Star Witnessed First-Hand

It takes only a few hundred to a thousand years for a dying Sun-like
star, many billions of years old, to transform into a dazzling,
glowing cloud called a planetary nebula. This relative blink in a long
lifetime means that a Sun-like star's final moments - the crucial
phase when its planetary nebula takes shape - have, until now, gone
undetected.

In research reported in the Nov. 20 issue of Nature, astronomers led
by Dr. Raghvendra Sahai of NASA's Jet Propulsion Laboratory, Pasadena,
Calif., have caught one such dying star in the act. This nearby star,
called V Hydrae, has been captured by the Space Telescope Imaging
Spectrograph onboard NASA's Hubble Space Telescope in the last stages
of its demise, just as material has begun to shoot away from it in a
high-speed jet outflow.

While previous studies have indicated the role of jet outflows in
shaping planetary nebulae, the new findings represent the first time
these jets have been directly detected.

"The discovery of a newly launched jet outflow is likely to have a
significant impact on our understanding of this short-lived stage of
stellar evolution and will open a window onto the ultimate fate of our
Sun," said Sahai.

Other institutions contributing to this paper include: University of
California, Los Angeles; Princeton University, Princeton, New Jersey;
Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts;
and Valdosta State University, Valdosta, Georgia.

Low-mass stars like the Sun typically survive around ten billion years
before their hydrogen fuel begins to run out and they start to die.
Over the next ten to hundred thousand years, the stars slowly eject
nearly half of their mass in expanding, spherical winds. Then - in a
poorly understood phase lasting just 100 to 1,000 years - the stars
evolve into a stunning array of geometrically shaped glowing clouds
called planetary nebulae.

Just how these extraordinary "star-clouds" are shaped has remained
unclear, though Sahai, in several previous papers, put forth a new
hypothesis. Based on results from a recent Hubble Space Telescope
imaging survey of young planetary nebulae, he proposed that two-sided,
or bipolar, high-speed jet-like outflows are the primary means of
shaping these objects. The latest study will allow Sahai and his
colleagues to test this hypothesis with direct data for the first
time.

"Now, in the case of V Hydrae, we can observe the evolution of the jet
outflow in real-time," said Sahai, who together with his colleagues
will study the star with the Hubble Space Telescope for three more
years.

The new findings also suggest what may be driving the jet outflows.
Past models of dying stars predict that accretion discs - swirling
rings of matter encircling stars - may trigger jet outflows. The V
Hydrae data support the presence of an accretion disc surrounding, not
V Hydrae itself, but a companion object around the star. This
companion is likely to be another star or even a giant planet too dim
to be detected. The authors have also found evidence for an outlying
large dense disc in V Hydrae, which could enable the formation of the
accretion disc around the companion.

Further support in favor of a companion-driven jet outflow comes from
the scientists' observation that the jet fires in bursts: because the
companion orbits the star in a periodic fashion, the accretion disc
around it is expected to produce regular spurts of material rather
than a steady stream.
The Space Telescope Imaging Spectrograph is managed by NASA's Goddard
Space Flight Center in Greenbelt, Maryland. The Hubble Space Telescope
is a project of international cooperation between NASA and the
European Space Agency. The California Institute of Technology,
Pasadena manages JPL for NASA.

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