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Nova V2672 Ophiuchi: an exploding swiss cheese

posted Jul 15, 2014, 9:04 AM by Jeremy Drake
A bright optical nova, or "new star", was discovered on 2009 August in the constellation Ophiuchus. Later named V2672 Oph, this nova evolved extremely rapidly compared with common nova types. Novae are tremendous thermonuclear explosions on the surface of a white dwarf, fueled by the accretion of hydrogen from a very close orbital companion - see earlier posts on nova explosions for details on other aspects of this interesting type of astrophysical phenomena.  The speed of evolution of a nova, as judged largely by the rapidity with which the optical light fades, is thought to be related to the white dwarf mass.  The more massive the white dwarf, the less hydrogen is needed to reach the pressure and temperature required to initiate thermonuclear runaway.  The small amount of accreted mass can be flung off at greater speed - 12,000 km/s in the case of V2672 Oph - leaving little material behind to fuel residual, steady nuclear burning. This steady nuclear burning is often revealed in soft X-rays once the material from the initial blast and subsequent burning has dissipated. 

V2672 Oph entered this "super-soft source" X-ray phase only eleven days after outburst and maintained burning for only ten days or so - both periods to be compared with a few to many months for typical novae.  We obtained target of opportunity X-ray observations using the Japanese Suzaku satellite on days 12 and 22 of the outburst. The data on day 12 showed a rapid change in the amount of absorbing material in the line-of-sight to the central white dwarf. The absorption at that time would be largely due to material driven off from the white dwarf by the radiation pressure of the extremely bright nuclear burning source. The variation in absorption betrayed a highly inhomogeneous and porous "wind", likely resulting from the formation of photon-blown bubbles and the accumulation of denser blobs within outward flowing gas - perhaps a bit like a swiss cheese, albeit one expanding at several thousand km/s.  This work was lead by Japanese scientist and ex-SAO postdoc Dai Takei, and was published in the 2014 April 2 issue of the Publications of the Astronomical Society of Japan.