Several postings within these pages have discussed new observations and insights into nova explosions. The term "nova" derives from 16th century astronomer Tycho Brahe's nova stella - "new star" - description of supernova SN 1572 in Cassiopeia. Unable to see the progenitor of the explosion, from his perspective it appeared like a bright new star where before there was none. Novae seemed similar - new stars appearing where nothing was visible to the naked eye before. Classical novae are actually like mini supernova
explosions, but rather than the detonation of an entire star the explosion originates on the surface of a white dwarf that has accreted material from a close companion star - see the postings on U Scorpii and V407 Cygni for further details.
The "Fastest Nova in Town", V745 Sco, is a special member of this class. The white dwarf orbits within the dense wind of its red giant companion from which its strong gravity scavenges the hydrogen fuel that powers thermonuclear outbursts at intervals of about 25 years. One persistent problem in understanding observations of nova explosions has been evidence for distinctly aspherical blasts. Why would an explosion over the surface of a very spherical white dwarf be aspherical? We have pursued this problem for several recent nova events using supercomputer hydrodynamical simulations.
The key to the form and evolution of a nova explosion turns out to lie in its immediate environment. The expanding blast wave propagates at different speeds through gas of different density. The denser the gas the slower it moves, but more dense gas also looks brighter when it has been heated by the blast. Salvo Orlando, of the Palermo Astronomical Observatory in Sicily, has lead a study based on simulations of the V745 Sco 2014 event to try to understand Doppler shift observations that revealed a very aspherical explosion. Earlier studies have indicated that a white dwarf orbiting in a dense wind tends to attract higher density gas in the plane of its orbit. The supercomputer simulations confirmed that we cannot reproduce a sufficiently aspherical explosion without this. The result is a blast and ejecta that shoot out poleward, expanding rapidly northward and southward of the white dwarf. The dense equatorial gas instead lights up more brightly in X-rays and UV light, producing a ring-like emission structure. Careful comparison of different simulations with X-ray observations indicate the explosion threw off about 1/10 of an Earth mass, with an energy equivalent to about 1,000,000,000,000,000,000,000,000,000 tonnes of TNT (4x1043 erg). This work was published in the 2017 February 1 edition of Monthly Notices of the Royal Astronomical Society.
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