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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 Hyi, U Gem, and SS Cyg. For these systems we infer that the outburst must be triggered at ~0.7-0.8 of the outer disk radius to produce delays which are in accord with observations. We also see a stagnation or period of slow warming (first noted by Mineshige) during the early stages of thermal instability, but find it does not influence the V-EUV delay since it precedes the rapid rise in V at the start of the outburst. 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 two. 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.