The V-EUV delay for dwarf nova outbursts:: A case study for VW Hydri, U Geminorum, and SS Cygni

We present a parameter study using time-dependent calculations of the thermal limit cycle model for dwarf nova outbursts. Our goal is to delineate the dependence of the delay between the initial rapid rise of the visual and EUV fluxes during the start of an outburst on model parameters, concentrating on three bright nearby systems for which complete optical and EUV observations exist: VW Hydri, U Geminorum, and SS Cygni. For each system we compute 15 models spanning the early part of an outburst, taking the ratio of the instigation radius to the outer disk radius to be 0.5, 0.7, or 1.0 and adopting a value for the alpha viscosity parameter in the ionized disk of 0.1, 0.15, 0.2, 0.25, or 0.3. We confirm Smak's findings, which show a consistency of the standard model with observations. For these systems we infer that the outburst must be triggered at similar to0.7-0.8 of the outer disk radius to produce delays that are in accord with observations. We show that the level of EUV flux in outburst is dictated by the amount of material carried within the inward-moving heating front spike as it reaches the inner edge of the disk, and we reaffirm earlier work by Meyer et al. that found the heating front speed to be given by the alpha parameter times the sound speed within the heating spike. We also see a stagnation, or period of slow warming (first noted by Mineshige), during the early stages of thermal instability but find that it does not influence the V-EUV delay since it precedes the rapid rise in V at the start of the outburst. In studying the sensitivity of our results to the initial mass distribution, we find that if one decreases the surface density in the inner disk, interior to the instigation radius, the V-EUV delay can be lengthened by as much as a factor of 2. In addition, we find there to be a weak relation between the V-EUV delay and the value of the alpha viscosity parameter in quiescence.