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The blast wave of a nova explosion inside the wind of a red giant

posted May 24, 2013, 9:17 AM by Jeremy Drake   [ updated Jun 3, 2013, 11:57 AM ]
V407 Cygni is a symbiotic star - a binary system in which a main-sequence or degenerate star (a white dwarf or neutron star) is orbiting within the wind of an evolved red giant.  In the case of V407 Cyg, the degenerate star is a white dwarf and it is accreting gas from the wind of the red giant, which is a Mira variable - a class of pulsating "asymptotic giant branch" stars. 

Like in the nova U Scorpiiover time the accretion of material builds up a layer on the white dwarf sufficiently hot and dense that nuclear fusion is ignited, resulting in thermonuclear runaway and an explosion - essentially a giant hydrogen bomb going off over the stellar surface.  The explosion generates a blast wave that propagates outward, but not in a uniform, spherically-symmetric way.  The shock wave speed depends on the density of the gas into which it is expanding.  Since the white dwarf is situation in the red giant wind, the gas density increases toward the red giant, and decreases going away from it.  

The image shows a 3D rendering of the particle number density from one of our models at day 60 after the explosion. The ejecta from the blast - the material thrown off from the white dwarf surface by the explosion - are highlighted in red. The blue indicates the extent of the forward-propagating shock. The orbit of the central binary system lies in the (x, y) plane, and the red giant is situated close to the left edge of the red ejecta on this plane. The blast wave is refracted around the red giant, resulting in the density enhancement, indicated by the lighter blue, to the left. The ejecta are strongly collimated, like polar jets, in the z direction because of the presence of a higher gas density in the orbital plane in this simulation. The simulations enabled us to understand the blast morphology and place limits on the explosion energy and mass of ejected material. This work was lead by Salvo Orlando of the Osservatorio Astronomical di Palermo, and was published in Volume 419, 2012 of the Monthly Notices of the Royal Astronomical Society.