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 \pm 0.18 mag .
At \sim 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 ^ { 56 } Ni mass of \sim 0.08 M _ { \odot } and ejecta mass of 1.1–2.2 M _ { \odot } , 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 ( \sim 0.1 M _ { \odot } ) , a nearly-compact ( \sim 0.05 \times 10 ^ { 13 } cm ) H envelope on top of a dense , compact ( \sim 2 \times 10 ^ { 11 } cm ) and a more massive ( \sim 1.2 M _ { \odot } ) He core .
The analysis of the nebular phase spectra indicates that \sim 0.44 M _ { \odot } 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 \sim 15 M _ { \odot } or a Wolf Rayet star of 20 M _ { \odot } .