2018A&A...618A..37F

We present an analysis of 507 spectra of 173 stripped-envelope (SE) supernovae (SNe) discovered by the untargeted Palomar Transient Factory (PTF) and intermediate PTF (iPTF) surveys. Our sample contains 55 TypeIIb SNe (SNeIIb), 45 TypeIb SNe (SNeIb), 56 TypeIc SNe (SNeIc), and 17 TypeIb/c SNe (SNeIb/c). We have compared the SE SN subtypes via measurements of the pseudo-equivalent widths (pEWs) and velocities of the HeIλλ5876, 7065 and OIλ7774 absorption lines. Consistent with previous work, we find that SNeIc show higher pEWs and velocities in OIλ7774 compared to SNeIIb andIb. The pEWs of the HeIλλ5876, 7065 lines are similar in SNeIb andIIb after maximum light. The HeIλλ5876, 7065 velocities at maximum light are higher in SNeIb compared to SNeIIb. We identify an anticorrelation between the HeIλ7065 pEW and OIλ7774 velocity among SNeIIb andIb. This can be interpreted as a continuum in the amount of He present at the time of explosion.It has been suggested that SNeIb andIc have similar amounts of He, and that lower mixing could be responsible for hiding He in SNeIc. However, our data contradict this mixing hypothesis. The observed difference in the expansion rate of the ejecta around maximum light of SNeIc (V_m=sqrt(2)E_k/M_ej~=15000km/s) and SNeIb (V_m~=9000km/s) would imply an average He mass difference of ∼1.4M_☉, if the other explosion parameters are assumed to be unchanged between the SE SN subtypes. We conclude that SNeIc do not hide He but lose He due to envelope stripping.