Using ESO's Very Large Telescope, astronomers have made the first time-lapse movie of an unusual shell of material ejected by a 'vampire star' that is a prime candidate progenitor star of a Type Ia supernova.

In November 2000 V445 Puppis underwent an outburst after consuming some of its companion's matter and over a period of two years, ESO's VLT monitored this cannibalistic act, revealing some dramatic changes. Initially a bipolar shell with a narrow waist and lobes on each side is present. Two knots are seen at the extreme ends of the shell moving at some 30 million kilometres per hour, while the shell itself races through space at about 24 million miles per hour. Thick dust obscures the two central stars.

The double star is a prime contender to be a much sought-after progenitor of the breed of exploding star known as a Type Ia supernovae. “One of the major problems in modern astrophysics is the fact that we still do not know exactly what kinds of stellar system explode as a Type Ia supernova,” says Patrick Woudt, of the University of Cape Town and lead author of the paper reporting the results. “As these supernovae play a crucial role in showing that the Universe’s expansion is currently accelerating, pushed by a mysterious dark energy, it is rather embarrassing.”

V445 Puppis is also the first, and so far only nova showing no evidence for hydrogen, providing the first evidence for an outburst on the surface of a white dwarf dominated by helium. White dwarfs are the evolutionary end product of stars up to a few solar masses that have exhausted their fuel supply. “This is critical, as we know that Type Ia supernovae lack hydrogen,” says co-author Danny Steeghs of the University of Warwick. “And the companion star in V445 Pup fits this nicely by also lacking hydrogen, instead dumping mainly helium gas onto the white dwarf.”

A lack of hydrogen is a defining characteristic of Type Ia supernovae. It is long suspected that supernovae like these arise in systems composed of two stars, one of which is a white dwarf. These white dwarfs act as stellar vampires, sucking material from their companion until they have gorged on so much matter they become unstable and explode. Sometimes, controlled mini-explosions eject part of the accumulated matter back into space to regulate the star's mass, but astronomers strive to understand if some of the material taken from the companion stays on the white dwarf, so that it will eventually become heavy enough to explode as a supernova.

“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,” says Woudt. “But we have here a pretty good suspect for a future Type Ia supernova!”

Type Ia supernovae are of particular interest to cosmologist because they can be used as 'standard candles' to measure distances in the Universe, and thus can be used to calibrate the accelerating expansion of the Universe driven by dark energy. Using the VLT adaptive optics images and combining them with data from ESO's New Technology Telescope, the IMACS spectrograph on the 6.5-metre Magellan Baade telescope, and the Infrared Survey Facility and the SIRIUS camera at the South African Astronomical Observatory, astronomers determined the system as 25,000 light years from the Sun. The system is also 10,000 times brighter than the Sun, suggesting that the white dwarf has a high mass that is near its fatal limit, but is still feeding on its companion at a runaway rate.

The research is presented in the 20 November issue of the Astrophysical Journal.

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