The nature of dwarf nova outbursts

We show that if the dwarf-nova disc instability model includes the effects of heating by stream impact and tidal torque dissipation in the outer disc, the calculated properties of dwarf-nova outbursts change considerably, and several notorious deficiencies of this model are repaired. In particular: (1) outside-in outbursts: occur for mass transfer rates lower than in the s tandard model as required by observations; (2) the presence of long (wide) and short (narrow) outbursts with similar peak luminosities is a natural pr operty of the model. Mass-transfer fluctuations by factors similar to2 can explain the occurrence of both long and short outbursts above the cataclysm ic variable period gap, whereas below 2 hr only short normal outbursts are expected tin addition tu superoutbursts which are not dealt with in this ar ticle). With additional heating by the stream and tidal torques, such fluct uations can also explain the occurrence of both outside-in and inside-out o utbursts in SS Cyg and similar systems. The occurrence of outside-in outbur sts in short orbital-period, low mass-transfer-rate systems requires the di sc to be much smaller than the tidal-truncation radius. In this case the re currence time of both inside-out and outside-in outbursts have a similar de pendence on the mass-transfer rate M-2.