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Thermonuclear .Ia Supernovae from Helium Shell Detonations: Explosion Models and Observables

During the early evolution of an AM CVn system, helium is accreted onto the surface of a white dwarf under conditions suitable for unstable thermonuclear ignition. The turbulent motions induced by the convective burning phase in the He envelope become strong enough to influence the propagation of burning fronts and may result in the onset of a detonation. Such an outcome would yield radioactive isotopes and a faint rapidly rising thermonuclear ".Ia" supernova. In this paper, we present hydrodynamic explosion models and observable outcomes of these He shell detonations for a range of initial core and envelope masses. The peak UVOIR bolometric luminosities range by a factor of 10 (from 5e41 - 5e42 erg/s), and the R-band peak varies from M_R,peak = -15 to -18. The rise times in all bands are very rapid (<10 d), but the decline rate is slower in the red than the blue due to a secondary near-IR brightening. The nucleosynthesis primarily yields heavy alpha-chain elements (40Ca through 56Ni) and unburnt He. Thus, the spectra around peak light lack signs of intermediate mass elements and are dominated by CaII and TiII features, with the caveat that our radiative transfer code does not include the non-thermal effects necessary to produce He features.