A "vampire" star that underwent an outburst in 2000 after
gulping down part of a companion star looks to be a ticking time bomb: It is
poised to become the long-sought fuse to a certain type of supernova.
The gas-sucking star is part of a double star system known
as V445 in the constellation of Puppis ("the Stern").
In November 2000, this system underwent a nova
outburst, becoming 250 times brighter than before and ejecting a large
quantity of matter into space.
To study the system further, a team of astronomers used the
NACO adaptive optics instrument on the European Southern Observatory's Very
Large Telescope (VLT) in Chile to obtain very sharp images of V445 Puppis over
a time span of two years.
Helium, not hydrogen
The images show a bipolar shell, initially with a very
narrow waist, with lobes on each side. Two knots are also seen at both the
extreme ends of the shell, which appear to move at about 19 million mph (30
million kph).
The shell — unlike any previously observed for a nova — is itself
moving at about 15 million mph (24 million kph). A thick disk of dust, which
must have been produced during the last outburst, obscures the two central
stars.
The VLT data also suggest that V445 Puppis could one day
explode as a Type 1a
supernova.
Type 1a supernovas are a particular family of these
life-ending stellar
explosions, one prominent feature of which is a lack of a hydrogen
signature in their spectrum, despite the fact that hydrogen is the most common
element in the universe.
V445 Puppis is the first, and so far only, nova showing no
evidence at all for hydrogen. It provides the first evidence for an outburst on
the surface of a white dwarf dominated by helium.
"This is critical, as we know that Type Ia supernovae
lack hydrogen," said team member Danny Steeghs of the University of Warwick in
the United Kingdom, "and the companion star in V445 Pup fits this nicely by
also lacking hydrogen, instead dumping mainly helium gas onto the white dwarf."
A crucial question
Type 1a supernovas are thought to arise in stellar systems composed
of two stars, one of them being the end product of the life of sun-like stars —
white dwarfs. When such white dwarfs, acting as stellar vampires that suck
matter from their companion, become heavier than a given limit, they become
unstable and explode.
But the process isn't quite as simple as that: As the white
dwarf cannibalizes its prey, matter accumulates on its surface. If this layer
becomes too dense, it becomes unstable and erupts as a nova. These controlled,
mini-explosions eject part of the accumulated matter back into space.
The crucial question is thus to know whether the white dwarf
can manage to gain weight despite the outburst — that is, if some of the matter
taken from the companion stays on the white dwarf, so that it will eventually
become heavy enough to explode as a supernova.
Combining the NACO images with data obtained with several
other telescopes allowed the astronomers to determine the distance to the
system — about 25,000 light-years from the Sun — and its intrinsic brightness —
over 10,000 times brighter than the Sun.
This implies that the vampire white dwarf in this system has
a high mass that is near its fatal limit and is still simultaneously being fed
by its companion at a high rate.
"Whether V445 Puppis will eventually explode as a supernova,
or if the current nova outburst has pre-empted that pathway by ejecting too
much matter back into space is still unclear," said team member Patrick Woudt
of the University of Cape Town in South Africa. "But we have here a pretty good
suspect for a future Type Ia supernova!"
The findings for V445 Puppis are detailed in the Nov. 20
issue of the Astrophysical Journal.
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