THE SECONDARY OUTBURST MAXIMUM OF T-CORONAE BOREALIS - IMPLICATIONS FOR THE PHYSICS OF ACCRETION DISKS

We examine Webbink's hypothesis that the accretion of a torus of matter onto a main-sequence star caused secondary maxima during the 1866 and 1946 eruptions of the recurrent nova T CrB. Our simple one-dimensional hydrodynamical calculations show that the accretion disk viscosity must increase with time to produce light curves resembling observations. We adopt a model in which the viscosity parameter-alpha increases from a seed value-alpha(i) to a final value-alpha(f) via the relation alpha(t) = alpha(f)/[1 + (alpha(f)/alpha(i)) exp (-t/tau)]. The observed light curve requires alpha(i) = 10(-3) and alpha(f) = 3 if the initial torus mass is 10(-4) M. and the viscous growth time, tau, is the orbital time at the initial radius of the torus. Our model implies that the physical mechanism responsible for producing the viscous dissipation in the accretion disk has a fast growth rate and saturates to an alpha of order unity. Balbus & Hawley identify a MHD instability with the same growth rate and saturation viscosity required by our phenomenological model. The good agreement between parameters estimated from observations and those derived from a physical viscosity mechanism suggests that this instability is a promising source for the accretion disk viscosity.