A Model for the Quiescent Phase of the Recurrent Nova U Scorpii
Abstract
A theoretical light curve is constructed for the quiescent phase of the recurrent nova U Scorpii in order to resolve the existing distance discrepancy between the outbursts (6 kpc) and the quiescences (14 kpc). Our U Sco model consists of a very massive white dwarf (WD), an accretion disk (ACDK) with a flaring-up rim, and a lobe-filling, slightly evolved, main-sequence star (MS). The model properly includes an accretion luminosity of the WD, a viscous luminosity of the ACDK, a reflection effect of the MS and the ACDK irradiated by the WD photosphere. The B light curve is well reproduced by a model of 1.37 Msun WD + 1.5 Msun MS (0.8---2.0 Msun MS is acceptable) with an ACDK having a flaring-up rim, and the inclination angle i~80 degree. The calculated color is rather blue (B-V~0.0) for a suggested mass accretion rate of 2.5 x 10-7 Msun yr-1, thus indicating a large color excess of E(B-V)~0.56 with the observational color of B-V=0.56 in quiescence. Such a large color excess corresponds to an absorption of AV~1.8 and AB~2.3, which reduces the distance to 6-8 kpc. This is in good agreement with the distance estimation of 4-6 kpc for the latest outburst. Such a large intrinsic absorption is very consistent with the recently detected period change of U Sco, which is indicating a mass outflow of ~3 x 10-7 Msun yr-1 through the outer Lagrangian points in quiescence.
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