A Limit Cycle Model For Long‐Term Optical Variations of V Sagittae: The Second Example of Accretion Wind Evolution

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 for both 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 companion. In the optical low state, the mass-transfer rate to the WD is small and the size of the disk is as small as its Roche lobe size. In the optical high state, the mass-transfer rate to the WD exceeds the critical rate of ~1 × 10-6 M☉ yr-1 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. During the optical high state, the wind from the WD hits the companion and strips off its surface layer. The mass transfer from the companion gradually reduces and stops. 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. Then a rapid mass transfer resumes. The mass of the WD envelope increases and blows a wind again. This cycle is repeated like a limit cycle. During this intermittent wind phase, the WD can grow in mass at the critical rate and eventually reach the Chandrasekhar mass limit. This process is called 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, although it commonly occurs in the supersoft X-ray sources when the mass-transfer rate exceeds the critical rate. Thus, V Sge is the second example of accretion wind evolution.