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The fastest nova in town

posted Oct 27, 2015, 9:05 AM by Jeremy Drake   [ updated Oct 27, 2015, 9:15 AM ]
A nova outbursts results from a thermonuclear explosion on the surface of a white dwarf star in a close binary with a non-degenerate (more "normal") star. The gravitational pull of each star in such systems is finely balanced and about equal at a point very near the surface of the "normal" companion such that material in its outer atmosphere is lost to the gravity of the white dwarf.  The white dwarf accretes this material until the surface density and temperature of gas are are high enough to initiate a thermonuclear runaway.

The sizes of white dwarf stars are related to their mass in a way opposite to that of "normal" stars - more massive white dwarfs have smaller radii than less massive ones, and consequently, stronger surface gravities. It takes less accreted material to reach thermonuclear trigger densities on a more massive white dwarf.  Consequently, their nova explosions generally involve less debris that can be flung off at higher velocities.  There is also less material left behind on the white dwarf surface that continues to undergo steady nuclear burning, sustaining a "supersoft X-ray" phase and driving a massive outflow through radiation pressure - see an earlier post on the supersoft phase of the eclipsing nova U Scorpii. Explosions and the supersoft phase on more massive white dwarf nova progenitors then evolve more rapidly and are over more quickly than for lower mass white dwarfs. 

V745 Scorpii is a "recurrent" nova - a nova observed to have more than a single outburst. Its latest explosion was discovered by amateur variable star observer Rod Stubbings on 2014 February 6. It is an especially interesting nova because the white dwarf companion is a red giant star and the white dwarf accretes material from its dense wind. An explosion within such a medium initiates a blast wave that propagates outward  - like a Type II supernova explosion of a massive star at the end of its life, only evolving much more quickly. The blast was detected in gamma rays by the Fermi gamma ray satellite, and in visual light V745 Sco faded by 3 magnitudes in only 4 days. A team lead by Kim Page from Leicester University, UK, followed the X-ray signature of the nova using NASA's Swift satellite. The supersoft source phase emerged on day 3 and was over by the end of day 10. To put this in context, some novae do not enter the supersoft phase for several months to a year, and this steady burning phase can last more than a year.  V745 Sco is, in fact, the fastest known Galactic nova. The speed with which the nova evolved points to the white dwarf being particularly massive, and likely very close to the Chandrasekhar limiting mass of about 1.4 times the mass of the Sun.  This suggests V745 Sco could be a supernova type 1a progenitor destined to end its life as a binary in the conflagration of the white dwarf when it finally accretes enough material to exceed the  Chandrasekhar limitThis study was published in volume 454 of the Monthly Notices of the Royal Astronomical Society.
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