Final moments. II. Observational properties and physical modeling of circumstellar-material-interacting Type II supernovae

Abstract

We present ultraviolet/optical/near-infrared observations and modeling of Type II supernovae (SNe II) whose early time (δt < 2 days) spectra show transient, narrow emission lines from shock ionization of confined(r < 10¹⁵ cm) circumstellar material (CSM). The observed electron-scattering broadened line profiles (i.e., IIn-like)of H I, He I/II, C IV, and N III/IV/V from the CSM persist on a characteristic timescale (tIIn) that marks a transition to a lower-density CSM and the emergence of Doppler-broadened features from the fast-moving SN ejecta. Our sample, the largest to date, consists of 39 SNe with early time IIn-like features in addition to 35 “comparison” SNe with no evidence of early time IIn-like features, all with ultraviolet observations. The total sample includes 50unpublished objects with a total of 474 previously unpublished spectra and 50 multiband light curves, collected primarily through the Young Supernova Experiment and Global Supernova Project collaborations. For all sample objects, we find a significant correlation between peak ultraviolet brightness and both tIIn and the rise time, as well as evidence for enhanced peak luminosities in SNe II with IIn-like features. We quantify mass-loss rates and CSM density for the sample through the matching of peak multiband absolute magnitudes, rise times, t_IIn, and optical SN spectra with a grid of radiation hydrodynamics and non-local thermodynamic equilibrium radiative-transfer simulations. For our grid of models, all with the same underlying explosion, there is a trend between the duration of the electron-scattering broadened line profiles and inferred mass-loss rate: t_IIn ≈3.8 [M/ (0.01 M_⊙ yr⁻¹)] days.

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