SHORTEST RECURRENCE PERIODS of FORCED NOVAE

We revisit hydrogen shell burning on white dwarfs (WDs) with higher mass accretion rates than the stability limit, Ṁstable, above which hydrogen burning is stable. Novae occur with mass accretion rates below the limit. For an accretion rate >Ṁstable, a first hydrogen shell flash occurs followed by steady nuclear burning, so the shell burning will not be quenched as long as the WD continuously accretes matter. On the basis of this picture, some persistent supersoft X-ray sources can be explained by binary models with high accretion rates. In some recent studies, however, the claim has been made that no steady hydrogen shell burning exists even for accretion rates >Ṁstable. We demonstrate that, in such cases, repetitive flashes occurred because mass accretion was artificially controlled. If we stop mass accretion during the outburst, no new nuclear fuel is supplied, so the shell burning will eventually stop. If we resume mass accretion after some time, the next outburst eventually occurs. In this way, we can design the duration of outburst and interpulse time with manipulated mass accretion. We call such a controlled nova a forced nova. These forced novae, if they exist, could have much shorter recurrence periods than natural novae. We have obtained the shortest recurrence periods for forced novae for various WD masses. Based on the results, we revisit WD masses of some recurrent novae, including T Pyx.