Faint Thermonuclear Supernovae from AM Canum Venaticorum Binaries

Helium that accretes onto a Carbon/Oxygen white dwarf in the double white dwarf AM Canum Venaticorum (AM CVn) binaries undergoes unstable thermonuclear flashes when the orbital period is in the 3. 5-25 minute range. At the shortest orbital periods (and highest accretion rates, Mdot > 10^-7 Msol/yr), the flashes are weak and likely lead to classical novae outbursts. However, as the orbit widens and the accretion rate drops, the mass required for the unstable ignition increases, leading to progressively more violent flashes up to a final flash with Helium shell mass ~ 0. 02-0. 1 Msol. The high pressures of these last flashes allow the burning to produce the radioactive elements 48Cr, 52Fe, and 56Ni that power a faint (MV in the range of -15 to -18) and rapidly evolving (few days) thermonuclear supernova. We estimate one such explosion every 1000 years in a 10¹1 Msol elliptical galaxy (~ 25% of the Type Ia supernovae rate). These ``. Ia'' supernovae (one-tenth as bright for one-tenth the time as a Type Ia supernovae) are excellent targets for deep (e. g. V=24) searches with nightly cadences, potentially yielding an all-sky rate of 10, 000 per year.