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 i n 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 dis k and a lobe-filling, slightly evolved, main-sequence star (MS). The model includes a reflection effect by the companion and the accretion disk togeth er with a shadowing effect on the companion by the accretion disk. The earl y visual light curve (with a linear phase of t similar to 1-15 days after m aximum) is well reproduced by a thermonuclear runaway model on a very massi ve 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 th e nova duration. The ensuing plateau phase (t similar to 15-30 days) is als o reproduced by the combination of a slightly irradiated MS and a fully irr adiated flaring-up disk with a radius similar to 1.4 times the Roche lobe s ize. The cooling phase (t similar to 30-40 days) is consistent with a low-h ydrogen 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 c ompanion mass of M-MS similar to 1.5 M. (0.8-2.0 M. is acceptable), the inc lination angle of the orbit (i similar to 80 degrees), and the flaring up e dge, the vertical height of which is similar to 0.30 times the accretion di sk radius. The duration of the strong wind phase (t similar to 0-17 days) i s 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 win d. 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 quan tities are exactly the same as those predicted in a new progenitor model of Type Ia supernovae.