A theoretical light-curve model for the 1999 outburst of U Scorpii.
HACHISU I., KATO M., KATO T. and MATSUMOTO K.
Abstract (from CDS):
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∼1-15 days after maximum) is well reproduced by a thermonuclear runaway model on a very massive WD close to the Chandrasekhar limit (MWD=1.37±0.01 M☉), in which optically thick winds blowing from the WD play a key role in determining the nova duration. The ensuing plateau phase (t∼15-30 days) is also reproduced by the combination of a slightly irradiated MS and a fully irradiated flaring-up disk with a radius ∼1.4 times the Roche lobe size. The cooling phase (t∼30-40 days) is consistent with a low-hydrogen content of X~0.05 of the envelope for the 1.37 M☉ WD. The best-fit parameters are the WD mass of MWD∼1.37 M☉, the companion mass of MMS∼1.5 M☉ (0.8-2.0 M☉ is acceptable), the inclination angle of the orbit (i∼80°), and the flaring-up edge, the vertical height of which is ∼0.30 times the accretion disk radius. The duration of the strong wind phase (t∼0-17 days) is very consistent with the BeppoSAX supersoft X-ray detection at t∼19-20 days because supersoft X-rays are self-absorbed by the massive wind. The envelope mass at the peak is estimated to be ∼3x10–6M☉, which is indicates an average mass accretion rate of ∼2.5x10–7M☉.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.