SN 2015as: a low-luminosity Type IIb supernova without an early light-curve peak

Abstract

We present results of the photometric (from 3 to 509 days past explosion) and spectroscopic (up to 230 days past explosion) monitoring campaign of the He-rich Type IIb supernova (SN) 2015as. The (B-V) colour evolution of SN 2015as closely resemble those of SN 2008ax, suggesting that SN 2015as belongs to the SN IIb subgroup that does not show the early, short-duration photometric peak. The light curve of SN 2015as reaches the B-band maximum about 22 days after the explosion, at an absolute magnitude of -16.82 ± 0.18 mag. At ∼ 75 days after the explosion, its spectrum transitions from that of a SN II to a SN Ib. P Cygni features due to He I lines appear at around 30 days after explosion, indicating that the progenitor of SN 2015as was partially stripped. For SN 2015as, we estimate a 56Ni mass of ∼ 0.08 M⊙ and ejecta mass of 1.1–2.2 M⊙, which are similar to the values inferred for SN 2008ax. The quasi bolometric analytical light curve modelling suggests that the progenitor of SN 2015as has a modest mass (∼ 0.1 M⊙), a nearly-compact (∼ 0.05× 1013 cm) H envelope on top of a dense, compact (∼ 2× 1011 cm) and a more massive (∼ 1.2 M⊙) He core. The analysis of the nebular phase spectra indicates that ∼ 0.44 M⊙ of O is ejected in the explosion. The intensity ratio of the [Ca II]/[O I] nebular lines favours either a main sequence progenitor mass of ∼ 15 M⊙ or a Wolf Rayet star of 20 M⊙.