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V Sge new interpretation (Hachisu and Kato)
This paper will be interesting to all readers of this field:
astro-ph/0308065
"A limit cycle model for long-term optical variations of V Sagittae:
The second example of accretion wind evolution"
by Izumi Hachisu (Univ. of Tokyo) and Mariko Kato (Keio Univ.)
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~J0513.9$-$6951. Thus, V~Sge is
the second example of accretion wind evolution.
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