A family of type II supernovae with signatures of a dense circumstellar wind with an outflow rate M > 10^-4u_10 (where u_10 is the wind velocity in units of 10 km/s) is identified. The relative occurrence rate of these supernovae (designated type IIdw) is estimated to be 0.05 of the rate of all type II supernovae. The anomalous IIdw spectra are described using qualitative models involving a collision with either a smooth or a clumpy wind. The main properties of these supernovae are summarized. An analysis of the deceleration of the envelope of SN 1979C indicates the ejected mass to be ~ 2 M_solar, which is consistent with IIdw supernovae having relatively low masses. Two possible origins for IIdw supernovae are considered: (1) intense mass loss from a binary system that is embedded in a common envelope; (2) a superwind from a single star. The IIdw/II ratio predicted by the first model is too low (~10^-3), and this model must therefore be rejected. The second mechanism can work if IIdw supernovae are born of stars with some interval of initial masses delta M_ms ~ 1 M_solar at the boundary for intermediate mass stars: i.e., stars with masses M_ms ~ 8_10 M_solar. The superwind model predicts a low mass for IIdw envelopes and a low abundance of oxygen and ^56Ni. Some IIdw supernovae may have a narrow optical maximum and low luminosity at the radioactive stage. The case of a moderate-density wind may correspond to SN1987B-type supernovae, which show signatures of interaction with a wind and have relatively low luminosity.
- Pub Date:
- September 1997