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J.K. Cannizzo (Emergent Information Technologies, Inc.)
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 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 either 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 ~0.7-0.8 of the outer disk radius to produce delays which are in accord with observations. We also show that the level of the 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 re-affirm earlier work by Meyer and Lin et al. which found that the heating front speed is given by the alpha parameter times the sound speed within the heating spike.
The author(s) of this abstract have provided an email address for comments about the abstract: cannizzo@stars.gsfc.nasa.gov