Type Ia Supernovae in the First Few Days: Signatures of Helium Detonation versus Interaction

Abstract

Abstract The mechanism for the blue and excessive early-phase emission reported within the first few days of several Type Ia supernovae (SNe Ia) has been suggested to be the interaction of the SN ejecta with either a non-degenerate companion star or circumstellar media (CSM). Recently, another mechanism has been suggested within the context of the He-detonation-triggered SN scenario (i.e., the double-detonation scenario or He-ignited violent merger), in which the radioactive isotopes in the outermost layer of the SN ejecta produce the early emission. In this paper, we investigate the properties of the excessive early-phase emission predicted by these different scenarios. The early flash in the He-detonation scenario shows behaviors different from those in the companion/CSM interaction scenarios. Especially clear diagnostics are provided once the behaviors in the UV and in the optical are combined. The spectra synthesized for the He-detonation scenario are characterized by absorptions due to He-detonation products, which develop in the decay phase. We further expect a relation between the properties of the early-phase flash and those of the maximum SN emission, in such a way that the brighter and slower initial flash is accompanied by a more substantial effect from the additional absorptions (and reddening). This relation, however, should be considered together with the maximum luminosity of the SN, since the larger luminosity suppresses the effect of the additional absorption. With these expected features, we address the possible origins of the excessive early-phase emission observed in several SNe.