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

We present non-LTE time-dependent radiative transfer simulations for ejecta produced by the detonation of an helium shell at the surface of a low-mass carbon/oxygen white dwarf (WD). This mechanism is one possible origin for supernovae (SNe) with faint and fast-decaying light curves, such as .Ia SNe and Ca-rich transients. Our initial ejecta conditions at 1d are given by the 0.18B explosion model COp45HEp2 of Waldman et al.. The 0.2Msun ejecta initially contains 0.11Msun of He, 0.03Msun of Ca, and 0.03Msun of Ti. We obtain a ~5d rise to a bolometric maximum of 3.59x10^41 erg/s, primarily powered by 48V decay. Multi-band light curves show distinct morphologies, with a rise to maximum magnitude (-14.3 to -16.7mag) that varies between 3 to 9d from the U to the K bands. Near-IR light curves show no secondary maximum. Because of the presence of both HeI and SiII lines at early times we obtain a hybrid Type Ia/Ib classification. During the photospheric phase line blanketing is caused primarily by TiII. At nebular times, the spectra show strong CaII lines in the optical (but no [OI] 6300--6364A emission), and TiII in the near-IR. Overall, these results match qualitatively the very disparate properties of .Ia SNe and Ca-rich transients. Although the strong TiII blanketing and red colors that we predict 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 the merger of two WDs, one or both composed of He.