paper

A limit cycle model for long-term optical variations of V Sagittae: The second example of accretion wind evolution

arXiv:astro-ph/0308065 · doi:10.1086/378848

Abstract

V Sagittae shows quasi-periodic optical high (soft X-ray off) and low (soft X-ray on) states with the total period of ~300 days. A binary model is presented to explain orbital light curves both for the high and low states as well as the transition mechanism between them. The binary model consists of a white dwarf (WD), a disk around the WD, and a lobe-filling main-sequence (MS) companion. In the optical high state, the mass transfer rate to the WD exceeds the critical rate of ~1 x 10^{-6} Msun/yr, and the WD blows an optically thick, massive wind. Surface layers of the disk are blown in the wind and the disk surface extends to the companion or over. As a result, optical luminosity of the disk increases by a magnitude because of its large irradiation effect. The massive wind completely obscures soft X-rays. This corresponds to the optical high/soft X-ray off state. The transition between optical high and low states is driven by an attenuation of the mass transfer from the secondary. As the mass supply stops, the WD wind weakens and eventually stops. The disk shrinks to a Roche lobe size and the optical magnitude drops. This phase corresponds to the optical low/soft X-ray on state. This cycle is repeated like a limit cycle. The WD can grow in mass at the critical rate and eventually reach the Chandrasekhar mass limit. This process is called ``accretion wind evolution,'' which is a key evolutionary process in a recently developed evolutionary scenario of Type Ia supernovae. This evolutionary process was first confirmed in the LMC supersoft X-ray source RX J0513.96951. Thus, V Sge is the second example of accretion wind evolution.

to appear in ApJ, 33 pages including figures

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