Vampire Star Is a Ticking Time Bomb
These time lapse images of V445 Puppis (taken with the NACO adaptive optics instrument on the ESO's Very Large Telescope in Chile) span two years. They show the expansion of the bipolar shell ejected by a "vampire star," which underwent an outburst after gulping down part of its companion's matter.
Credit: ESO/P.A. Woudt

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.