Recurrent Novae as a Progenitor System of Type Ia Supernovae. I. RS Ophiuchi Subclass: Systems with a Red Giant Companion

Theoretical light curves of four recurrent novae in outburst are modeled to obtain various physical parameters. The four objects studied here are those with a red giant companion, i.e., T Coronae Borealis, RS Ophiuchi, V745 Scorpii, and V3890 Sagittarii. Our model consists of a very massive white dwarf (WD) with an accretion disk and a red giant companion. Light-curve calculation includes reflection effects of the companion star and the accretion disk together with a shadowing effect on the companion by the accretion disk. We also include a radiation-induced warping instability of the accretion disk to reproduce the second peak of T CrB outbursts. The early visual light curves are well reproduced by applying a thermonuclear runaway model to a very massive white dwarf close to the Chandrasekhar mass limit, i.e., M_WD=1.37+/-0.01 M_solar for T CrB and 1.35+/-0.01 M_solar for RS Oph with solar metallicity (Z=0.02), but 1.377+/-0.01 M_solar for RS Oph with low metallicity (Z=0.004), 1.35+/-0.01 M_solar for V745 Sco, and 1.35+/-0.01 M_solar for V3890 Sgr. Optically thick winds, which blow from the WDs during the outbursts, play a key role in determining the nova duration and the speed of decline because the wind quickly reduces the envelope mass on the WD. The envelope mass at each optical maximum is also estimated to be ΔM~3×10^-6 M_solar (T CrB), 2×10^-6 M_solar (RS Oph), 5×10^-6 M_solar (V745 Sco), 3×10^-6 M_solar (V3890 Sgr), indicating average mass accretion rates of M_acc~0.4×10^-7 M_solar yr^-1 (80 yr; T CrB), 1.2×10^-7 M_solar yr^-1 (18 yr; RS Oph), 0.9×10^-7 M_solar yr^-1 (52 yr; V745 Sco), and 1.1×10^-7 M_solar yr^-1 (28 yr; V3890 Sgr) during the quiescent phase before the last outburst. Although a large part of the envelope mass is blown off in the wind, each WD retains a substantial part of the envelope mass after hydrogen burning ends. Thus, we have obtained net mass-increasing rates of the WDs as M_He~0.1×10^-7 M_solar yr^-1 (T CrB), 0.12×10^-7 M_solar yr^-1 (RS Oph), 0.05×10^-7 M_solar yr^-1 (V745 Sco), 0.11×10^-7 M_solar yr^-1 (V3890 Sgr). These results strongly indicate that the WDs in the recurrent novae have now grown to near the Chandrasekhar mass limit and will soon explode as a Type Ia supernova if the WDs consist of carbon and oxygen. We have also clarified the reason that T CrB shows a secondary maximum but the other three systems do not.