Faint Thermonuclear Supernovae from AM Canum Venaticorum Binaries

Helium that accretes onto a carbon/oxygen white dwarf in 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, > 10^-7 M_☉ yr^-1), the flashes are weak and likely lead to the helium equivalent of classical nova outbursts. However, as the orbit widens and 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 M_☉. The high pressures of these last flashes allow the burning to produce the radioactive elements ⁴⁸Cr, ⁵²Fe, and ⁵⁶Ni that power a faint (M_V = -15 to -18) and rapidly rising (few days) thermonuclear supernova. Current galactic AM CVn space densities imply one such explosion every 5,000-15,000 years in 10¹¹ M_☉ of 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 1000 per year.