Diversity of supernovae and impostors shortly after explosion

Abstract

Observational surveys are now able to detect an increasing number of transients, such as core-collapse supernovae (SN) and powerful non-terminal outbursts (SN impostors). Dedicated spectroscopic facilities can follow up these events shortly after detection. Here we investigate the properties of these explosions at early times. We use the radiative transfer code CMFGEN to build an extensive library of spectra simulating the interaction of supernovae and their progenitor’s wind or circumstellar medium (CSM). We have considered a range of progenitor mass-loss rates (Ṁ = 5 × 10−4−10−2 M⊙ yr−1), abundances (solar, CNO-processed, and He-rich), and SN luminosities (L = 1.9 × 108 − 2.5 × 1010 L⊙). The models simulate events approximately one day after explosion, and we assume a fixed location of the shock front as Rin = 8.6 × 1013 cm. We show that the large range of massive star properties at the pre-SN stage causes a diversity of early-time interacting SN and impostors. We identify three main classes of early-time spectra consisting of relatively high-ionisation (e.g. He II and O VI), medium-ionisation (e.g. C III and N III), and low-ionisation lines (e.g. He I and Fe II/III). They are regulated by L and the CSM density. Given a progenitor wind velocity υ∞, our models also place a lower limit of Ṁ ≳ 5 × 10−4 (υ∞/150 km s−1) M⊙ yr−1 for detection of CSM interaction signatures in observed spectra. Early-time SN spectra should provide clear constraints on progenitors by measuring H, He, and CNO abundances if the progenitors come from single stars. The connections are less clear considering the effects of binary evolution. Nevertheless, our models provide a clear path for linking the final stages of massive stars to their post-explosion spectra at early times, and guiding future observational follow-up of transients with facilities such as the Zwicky Transient Facility.