“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, from 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.”
The astronomers studied the object known as V445 in the constellation of Puppis (“the Stern”) in great detail. 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,” says co-author Danny Steeghs, from the University of Warwick, UK, “and the companion star in V445 Pup fits this nicely by also lacking hydrogen, instead dumping mainly helium gas onto the white dwarf.”
In November 2000, this system underwent a nova outburst, becoming 250 times brighter than before and ejecting a large quantity of matter into space.
The team of astronomers used the NACO adaptive optics instrument  on ESO’s Very Large Telescope (VLT) to obtain very sharp images of V445 Puppis over a time span of two years. 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 30 million kilometres per hour. The shell — unlike any previously observed for a nova — is itself moving at about 24 million kilometres per hour. A thick disc of dust, which must have been produced during the last outburst, obscures the two central stars.
“The incredible detail that we can see on such small scales — about hundred milliarcseconds, which is the apparent size of a one euro coin seen from about forty kilometres ? is only possible thanks to the adaptive optics technology available on large ground-based telescopes such as ESO’s VLT,” says Steeghs.
A supernova is one way that a star can end its life, exploding in a display of grandiose fireworks. One family of supernovae, called Type Ia supernovae, are of particular interest in cosmology as they can be used as “standard candles” to measure distances in the Universe  and so can be used to calibrate the accelerating expansion that is driven by dark energy.
One defining characteristic of Type Ia supernovae is the lack of hydrogen in their spectrum. Yet hydrogen is the most common chemical element in the Universe. Such supernovae most likely arise in systems composed of two stars, one of them being the end product of the life of sun-like stars, or 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 .
The build-up is not a simple process. As the white dwarf cannibalises 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  the astronomers could determine the distance of 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,” says Woudt. “But we have here a pretty good suspect for a future Type Ia supernova!”
 White dwarfs represent the evolutionary end product of stars with initial masses up to a few solar masses. A white dwarf is the burnt-out stellar core that is left behind when a star like the Sun sheds its outer layers towards the end of its active life. It is composed essentially of carbon and oxygen. This process normally also leads to the formation of a surrounding planetary nebula.
 Adaptive optics is a technique that allows astronomers to obtain an image of an object free from the blurring effect of the atmosphere. See the adaptive optics page at ESO: http://www.eso.org/public/astronomy/technology/adaptive_optics.html
 See for example http://www.eso.org/~bleibund/papers/EPN/epn.html
 This Chandrasekhar limit, named after the Indian physicist Subrahmanyan Chandrasekhar, is nearly 1.4 times the mass of the Sun. When a white dwarf reaches a mass above this limit, either by sucking matter from a companion or merging with another white dwarf, it will turn itself into a thermonuclear bomb that will burn carbon and oxygen explosively.
 The team also used the SOFI instrument on 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 Sutherland station of the South African Astronomical Observatory.
This research was presented in a paper to appear in the 20 November 2009 issue of the Astrophysical Journal, vol. 706, p. 738 (“The expanding bipolar shell of the helium nova V445 Puppis”, by P. A. Woudt et al.).
The team is composed of P. A. Woudt and B. Warner (University of Cape Town, South Africa), D. Steeghs and T. R. Marsh (University of Warwick, UK), M. Karovska and G. H. A. Roelofs (Harvard-Smithsonian Center for Astrophysics, Cambridge MA, USA), P. J. Groot and G. Nelemans (Radboud University Nijmegen, the Netherlands), T. Nagayama (Kyoto University, Japan), D. P. Smits (University of South Africa, South Africa), and T. O’Brien (University of Manchester, UK).
ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 14 countries: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 42-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.
– Research paper: http://arxiv.org/abs/0910.1069
University of Cape Town,
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University of Warwick, UK
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