Faint Thermonuclear Supernovae from AM Canum Venaticorum Binaries

Helium that accretes onto a Carbon/Oxygen white dwarf in the double whitedwarf AM Canum Venaticorum (AM CVn) binaries undergoes unstable thermonuclearflashes when the orbital period is in the 3.5-25 minute range. At the shortestorbital periods (and highest accretion rates, Mdot > 10^-7 Msol/yr), theflashes are weak and likely lead to the Helium equivalent of classical novaoutbursts. However, as the orbit widens and Mdot drops, the mass required forthe unstable ignition increases, leading to progressively more violent flashesup to a final flash with Helium shell mass ~ 0.02-0.1 Msol. The high pressuresof these last flashes allow the burning to produce the radioactive elements48Cr, 52Fe, and 56Ni that power a faint (M_V in the range of -15 to -18) andrapidly rising (few days) thermonuclear supernova. Current galactic AM CVnspace densities imply one such explosion every 5,000-15,000 years in 10^11 Msolof old stars (~ 2-6% of the Type Ia rate in E/SO galaxies). 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 1,000 per year.