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We present non-LTE time-dependent radiative transfer simulations for ejectaproduced by the detonation of an helium shell at the surface of a low-masscarbon/oxygen white dwarf (WD). This mechanism is one possible origin forsupernovae (SNe) with faint and fast-decaying light curves, such as .Ia SNe andCa-rich transients. Our initial ejecta conditions at 1d are given by the 0.18Bexplosion model COp45HEp2 of Waldman et al.. The 0.2Msun ejecta initiallycontains 0.11Msun of He, 0.03Msun of Ca, and 0.03Msun of Ti. We obtain a ~5drise to a bolometric maximum of 3.59x10^41 erg/s, primarily powered by 48Vdecay. Multi-band light curves show distinct morphologies, with a rise tomaximum magnitude (-14.3 to -16.7mag) that varies between 3 to 9d from the U tothe K bands. Near-IR light curves show no secondary maximum. Because of thepresence of both HeI and SiII lines at early times we obtain a hybrid TypeIa/Ib classification. During the photospheric phase line blanketing is causedprimarily by TiII. At nebular times, the spectra show strong CaII lines in theoptical (but no [OI] 6300--6364A emission), and TiII in the near-IR. Overall,these results match qualitatively the very disparate properties of .Ia SNe andCa-rich transients. Although the strong TiII blanketing and red colors that wepredict are rarely observed, they are seen, for example, in OGLE-2013- SN-079.Furthermore, we obtain a faster light-curve evolution than, for example,PTF10iuv, indicating an ejecta mass >0.2Msun. An alternate scenario may be themerger of two WDs, one or both composed of He.

One-dimensional non-LTE time-dependent radiative transfer of an He-detonation model and the connection to faint and fast-decaying supernovae