A model for the quiescent phase of the recurrent nova U Scorpii

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 (d ∼ 6 kpc) and the quiescences (d ∼ 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, and 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 M_⊙ WD + 1.5 M_⊙ MS (0.8-2.0 M_⊙ MS is acceptable) with an ACDK having a flaring-up rim and the inclination angle of the orbit i ∼ 80°. The calculated color is rather blue (B-V ∼ 0.0) for a suggested mass accretion rate of 2.5 × 10^-7 M_⊙ 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 A_v ∼ 1.8 and A_B11 ∼ 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 × 10^-7 M_⊙ yr_-1 through the outer Lagrangian points in quiescence.