The diversity of late-time interaction in Type II supernovae

Abstract

Studying core-collapse supernovae (SNe) beyond one year after explosion, when ejecta interaction with circumstellar material (CSM) progressively becomes the dominant power source, offers critical insights into the progenitor mass loss history. Using binary progenitors that underwent different levels of H-rich envelope stripping, we present radiative transfer calculations of the ejecta with CSM interaction in Type IIb, II-L and II-P SN scenarios covering epochs of 1.5 to 3-6yr, with a focus on the UV and optical properties. The impact of CSM interaction is evaluated for a cold-dense shell (CDS) of 0.1-2.0Msun (CDS velocity of ~5000-10000km/s) and shock powers of 1e39-1e41erg/s, producing a grid of about 200 models. We find that Lyalpha, MgII2800, and Halpha are typically the strongest individual lines, MgII2800 being the most constant relative to changes in time and in CDS mass. Lyalpha thrives at low CDS mass and is strongly attenuated otherwise. A large fraction of the emerging UV and optical flux is, however, influenced by a forest of Fe lines, which introduces strong optical depth effects. A myriad of other lines, generally forbidden, are predicted that each contain less than a percent of the total flux. N enrichment yield stronger N lines in the UV and optical. A subset of these models roughly match UV and optical observations of SNe 1993J, 1998S, 2013ej, and 2023ixf at 1.5-3yr, although matching the full range consistently is a challenge, in particular when dust must be invoked. Blue-red asymmetry of line profiles is typical of all lines in the UV, the more so at earlier times or higher CDS mass, even in the absence of dust. In our simulations, optical depth effects, associated with the gas or dust or both, quenches the emission of Lyalpha or [OIII]4958-5007A from regions within or below the CDS.