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DISCOVERY OF A TYPE IA SUPERNOVA PROGENITOR INT+IDS Type Ia supernovae (SNe Ia) are one of the most important tools for observational cosmology because there appears to be a relatively small spread in their peak optical brightness and they can be seen out to cosmological distances so they can be used to measure cosmological parameters. However, the peak optical brightnesses of SNe Ia are not uniform; they are correlated with the shape of the light curve. Meaningful measurements of cosmological parameters require this variation to be calibrated. The corrections to peak brightnesses have to be empirical because it is still not yet clear what causes SNe Ia. All the most likely models for progenitors of SNe Ia feature an accreting white dwarf which ignites carbon in its core either because it has reached the Chandrasekhar mass or because ignition of accumulated helium causes compression of the core and a so-called 'edge-lit detonation'. This explains the fast rise times for SNeIa, the lack of hydrogen and helium and the fairly uniform peak brightness. To initiate the explosion, the white dwarf must accrete material from a companion star. Two models for the companion star which have gained popularity in recent times are supersoft sources and double degenerates. However, the possibility of a helium star companion to a white dwarf has not been widely considered as a source of SNe Ia. In particular, sub-dwarf B (sdB) star binaries might be good candidates. There are many white dwarfs which are known to begaining mass from a normal star, but these are made of hydrogen which causes a series of small explosions before the Chandresekhar limit is reached. This is what causes a nova explosion. To make a supernova, the white dwarf has to be supplied with helium, which explodes less easily but releases much more energy. KPD1930+2752 is a sdB star. It is about one fifth the size of the Sun and is about half as massive. Unlike normal stars, which are composed almost entirely of hydrogen, KPD1930+2752 is made of helium. It is not entirely clear how sdB stars are made, but recent work suggests they are the remains of stars like the Sun which lose half their mass just before they complete the end of the red giant phase of their evolution. Only some small fraction of stars evolve this way and this is thought to be related to the fact that most sdB stars are binary stars. KPD1930+2752 was observed with the INT as part of a programme to study sdB stars to understand how they are formed. The Doppler shift shows that the star is orbiting an unseen companion every 137 minutes at a speed of 350 km/s. The unseen companion has almost the same mass as the Sun, but it is much smaller and fainter. The unseen companion star could be a neutron star or blackhole, but it is much more likely to be a white dwarf star. When binary stars have orbital periods as short as two hours, they produce "gravitational waves" which drain energy from the orbit, so the stars gradually spiral in towards each other. KPD1930+2752 will merge within 200 million years. The white dwarf will then gain extra mass from the sdB star and will exceed the Chandresekhar critical mass. This is thought to lead to a Type Ia supernova explosion. KPD1930+2752 is the first star to be discovered that is a good candidate for the progenitor of a Type Ia supernova of this type, which may explode on an astrophysically interesting time-scale. Some references:
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