A Study Of The Observational Properties Of Stripped-envelope Supernovae And Their Progenitors

Stripped-envelope supernovae(SESNe)are a group of supernovae(SNe)whose pro-genitors are envelope stripped prior to core-collapse.Their spectra are lack of H or have less H than those H-rich SNe.The classical theories of stellar evolution predict that stars with larger initial masses can be more stripped,so they end up with different types of SNe.However,many recent studies has thown doubt on these traditional theories.SNe with different observational properties might arise from stars with similar initial masses.Their diversity can provide valuble information on evolution of massive stars.The most effective approach to the study of the SNe progenitors is to obeserve them before they die.SN 2017ein filled the gap in observations of the progenitors of SNe Ic.We did optical observations on SN 2017ein.Our observations of SN 2017ein show that it is a quite normal SN Ic in spetral evolution,and also in light curves.Very early photom-etry(within 1 day after explosion)might have catched its shock breakout cooling signal.With pure radiation decay(RD)powering,the early light curve data can not be well fit-ted.While the early energy excess can be modeled by a shock cooling model in which the breakout happens at the surface of the progenitor star.Using this we can constrain the radius of the progenitor as?8M_?,which is consistent with Wolf-Rayet stars(WR).Meanwhile,HST had taken a shot at the SN location before the explosion.We find the progenitor candidate of SN 2017ein in these images,and then do photometry on it.Based on the phototmetry results,the properties of the candidate can be obtained using stellar spectral models and stellar evolution models.We find that the candidate has a very high luminosity as well as high temperature,consistent with the properties of WR stars.Asum-ming that the candidate is a single star,this candidate has a stellar radius in agree with that derived from the shock cooling model.The progenitor candidate is most likely to be a massive star with initial mass>40M_?,although as predicted as the progenitors of SNe Ic,the low ejecta mass of SN 2017ein is much less than expected from such stars.The cause of such low mass is not clear yet.Systematic study of SE-SNe shows that they are a group of optical transient with large diversity in their light curves.Some of them can be connected to the recently discovered fast blue optical transients(FBOTs),showing similar light curve properties.AT2018cow is a member of FBOTs.We did spectroscopic and photometric observations on AT2018cow.AT2018cow has distinct observational features from other SNe:fast evolving light curves,blue colors,high expansion velocities,weak spectral lines,etc.Af-ter 10 days since discovery,the spectra of AT2018cow show emission lines which should be signitures of circumstellar interaction(CSI).We apply a model including contribution of CSI and RD to the bolometric light curves,and find plausible fitting results that can match the observations very well.The best-fit model requires a CSM mass of?0.04M_?.The CSM can not be formed by the steady stellar wind of the progenitor star,but probably by the episodic mass ejection from the stellar surface shortly prior to explosion.Many evidences suggest that AT2018cow is originated from the core-collapse explosion of a massive star in dense CSM.