2018MNRAS.476.3611G

We present results of the photometric (from 3 to 509 d post-explosion) and spectroscopic (up to 230 d post-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 d after the explosion, at an absolute magnitude of -16.82 ± 0.18 mag. At ∼75 d 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 d 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 x 10¹³ cm) H envelope on top of a dense, compact (∼2 x 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_☉.