2011ApJ...734...38W

For carbon-oxygen white dwarfs accreting hydrogen or helium at rates in the range ~(1-10)x10^–8 M_☉/yr, a variety of explosive outcomes is possible well before the star reaches the Chandrasekhar mass. These outcomes are surveyed for a range of white dwarf masses (0.7-1.1 M_☉), accretion rates ((1-7)x10^–8 M_☉/yr), and initial white dwarf temperatures (0.01 and 1 L_☉). The results are particularly sensitive to the convection that goes on during the last few minutes before the explosion. Unless this convection maintains a shallow temperature gradient and unless the density is sufficiently high, the accreted helium does not detonate. Below a critical helium ignition density, which we estimate to be (5-10)x10⁵ g/cm3, either helium novae or helium deflagrations result. The hydrodynamics, nucleosynthesis, light curves, and spectra of a representative sample of detonating and deflagrating models are explored. Some can be quite faint indeed, powered at peak for a few days by the decay of ⁴⁸Cr and ⁴⁸V. Only the hottest, most massive white dwarfs considered with the smallest helium layers, show reasonable agreement with the light curves and spectra of common Type Ia supernovae (SNe Ia). For the other models, especially those involving lighter white dwarfs, the helium shell mass exceeds 0.05 M_☉ and the mass of the ⁵⁶Ni that is synthesized exceeds 0.01 M_☉. These explosions do not look like ordinary SNe Ia or any other frequently observed transient.