A theoretical light-curve model for the 1999 outburst of U Scorpii

A theoretical light curve for the 1999 outburst of U Scorpii Is presented in order to obtain various physical parameters of the recurrent nova. Our U Sco model consists of a very massive white dwarf (WD) with an accretion disk and a lobe-filling, slightly evolved, main-sequence star (MS). The model includes a reflection effect by the companion and the accretion disk together with a shadowing effect on the companion by the accretion disk. The early visual light curve (with a linear phase of t similar to 1-15 days after maximum) is well reproduced by a thermonuclear runaway model on a very massive WD close to the Chandrasekhar limit (M-WD = 1.37 +/- 0.01 M.), in which optically thick winds blowing from the ND play a key role in determining the nova duration. The ensuing plateau phase (t similar to 15-30 days) is also reproduced by the combination of a slightly irradiated MS and a fully irradiated flaring-up disk with a radius similar to 1.4 times the Roche lobe size. The cooling phase (t similar to 30-40 days) is consistent with a low-hydrogen content of X approximate to 0.05 of the envelope for the 1.37 M. WD. The best-fit parameters are the WD mass of M-WD similar to 1.37 M., the companion mass of M-MS similar to 1.5 M. (0.8-2.0 M. is acceptable), the inclination angle of the orbit (i similar to 80 degrees), and the flaring up edge, the vertical height of which is similar to 0.30 times the accretion disk radius. The duration of the strong wind phase (t similar to 0-17 days) is very consistent with the BeppoSAX supersoft X-ray detection at t similar to 19-20 days because supersoft X-rays are self-absorbed by the massive wind. The envelope mass at the peak is estimated to be similar to 3 x 10(-6) M., which is indicates an average mass accretion rate of similar to 2.5 x 10(-7) M. yr(-1) during the quiescent phase between 1987 and 1999. These quantities are exactly the same as those predicted in a new progenitor model of Type Ia supernovae.