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 (d approximately 6 kpc) and the quiescences (d approximately 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 middle dot in circle WD + 1.5 M middle dot in circle MS (0.8-2.0 M middle dot in circle MS is acceptable) with an ACDK having a flaring-up rim and the inclination angle of the orbit i approximately 80&j0;. The calculated color is rather blue (B-V approximately 0.0) for a suggested mass accretion rate of 2.5x10-7 M middle dot in circle yr-1, thus indicating a large color excess of E(B-V) approximately 0.56 with the observational color of B-V=0.56 in quiescence. Such a large color excess corresponds to an absorption of AV approximately 1.8 and AB approximately 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 approximately 3x10-7 M middle dot in circle yr-1 through the outer Lagrangian points in quiescence.

The Change of the Orbital Periods Across Eruptions and the Ejected Mass for Recurrent Novae Ci Aquilae and U Scorpii

“…For U Sco, the mass of the WD must be near the Chandrasekhar mass, with Hachisu et al (2000b) giving M WD = 1.37 ± 0.01 M . The companion star was measured by Thoroughgood et al (2001) to have M comp = 0.88 ± 0.17 M , and this is consistent with the requirement that q 5/6.…”

“…The companion star was measured by Thoroughgood et al (2001) to have M comp = 0.88 ± 0.17 M , and this is consistent with the requirement that q 5/6. Hachisu et al (2000b) give a = 6.87 R and R comp = 2.66 R , so that β = 0.04. These values give A = 1.11 ± 0.05 with the uncertainty dominated by M comp .…”

U Scorpii 2010 outburst: a new understanding of the binary accretion disk and the secondary star

“…This is even more likely if the U Sco accretion disk has raised edges downstream from the hot spot which remain uneclipsed at the inclination of ∼83 • of the binary (see Sect. 4, as well as Hachisu et al 2000).…”

High-Velocity Jets in Recurrent Nova Outbursts

“…U Sco is an eclipsing binary with a high inclination angle of ∼80Њ (Hachisu et al 2000b). If the Ha line observed originates mainly from two jets emanating at the north/south poles, the jet velocity required for a 5000 km s Ϫ1 component is estimated to be ∼10,000 km s Ϫ1 .…”

“…Therefore, a light curve fitting of U Sco (Kato 1999;Hachisu et al 2000aHachisu et al , 2000b gives us a WD mass of ∼1.37 M , . Even in such a massive WD, large expansion velocities observed cannot be reproduced by theories presented so far.…”