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Blast in Scorpius: an overture to a supernova?

posted Jul 27, 2016, 3:03 AM by Jeremy Drake
The fastest nova in town featured in an earlier posting detailed the fascinatingly rapid evolution of the V745 Scorpii event followed in the UV and X-rays by NASA's Swift satellite.  Nova explosions are mini versions of supernova explosions that occur on the surface of a white dwarf.  The fuel for the explosion is hydrogen-rich gas accreted from a close companion star.  V745 Sco is special because it is a "recurrent" nova - a nova observed to have had more than a single outburst. All novae likely go through many explosion cycles, but for the great majority the time between events is many thousands of years.  The explosion cycle time is controlled by the speed with which new fuel is accreted, and by the mass of the white dwarf star.  

More massive white dwarfs are more compact and have stronger surface gravitational fields. It takes less fuel to set off the thermonuclear runaway that causes the explosion - and therefore less time to gather the fuel and less time between nova events. V745 Sco likely hosts a white dwarf close to the maximum mass possible - a star with a mass close to the "Chandrasekhar Limit" of about 1.4 times the mass of the Sun.  Add too much more mass and the star will no longer be able to support its own weight and will collapse and explode in the conflagration of a Type 1a supernova. 

V745 Sco is also special because it is a symbiotic nova - a white dwarf accreting from the extended atmosphere and wind of an evolved giant star rather than from a K or M dwarf star like most novae. The explosion sets off a blast wave through the gas.  The shock wave heats the gas to millions of degrees and we can observe it in the X-rays with sophisticated instruments such as the Chandra X-ray Observatory High Energy Transmission Grating Spectrometer. Careful analysis of data obtained by Chandra two weeks after the nova discovery enabled us to study the blast wave in detail.  We found that the blast was quite aspherical, and could deduce the energy of the explosion and verify estimates of the density of the red giant wind in the immediate vicinity.  We could also estimate the mass thrown off in the explosion and compare it with the mass needed to trigger the blast for various white dwarf masses. The mass ejected  - a few millionths of a solar mass - appears to be less than that required to trigger an explosion even for the most massive white dwarf.  The white dwarf must be close to the Chandrasekhar Limit to have undergone a nova at all.  But also some mass must have been retained on the white dwarf.  This means the white dwarf is growing in mass, and creeping inexorably toward a supernova demise.  This work was published in the 2016 July 10 edition of the Astrophysical Journal.