  Detonations in helium-rich envelopes surrounding white dwarfs have garnered
attention as triggers of faint thermonuclear ".Ia" supernovae and double
detonation Type Ia supernovae. However, recent studies have found that the
minimum size of a hotspot that can lead to a helium detonation is comparable
to, or even larger than, the white dwarf's pressure scale height, casting doubt
on the successful ignition of helium detonations in these systems. In this
paper, we examine the previously neglected effects of C/O pollution and a full
nuclear reaction network, and we consider hotspots with spatially constant
pressure in addition to constant density hotspots. We find that the inclusion
of these effects significantly decreases the minimum hotspot size for
helium-rich detonation ignition, making detonations far more plausible during
turbulent shell convection or during double white dwarf mergers. The increase
in burning rate also decreases the minimum shell mass in which a helium
detonation can successfully propagate and alters the composition of the shell's
burning products. The ashes of these low mass shells consist primarily of
silicon, calcium, and unburned helium and metals and may explain the
high-velocity spectral features observed in most Type Ia supernovae.
