The peculiar Type I supernova in NGC 991

A spectrum of the recent supernova SN 1984l in NGC 991 taken near maximum light is presented. The spectrum is very similar to that of SN 1983n in M83, thus establishing a class of precisely defined peculiar Type I supernova. These supernovae are less luminous than classical Type I events, by perhaps a factor of 4, but seem to have similar photospheric velocities and light curves with peaks of similar width, and they may all be associated with Population I regions. These features lead to the conclusion that peculiar Type I supernovae eject less radioactive Ni-56, but a similar total mass to classical Type I supernovae, and hence that they derive most of their kinetic energy from core collapse rather than thermonuclear burning. If this is the case, their progenitors are probably moderately massive stars, 10-20 solar masses, which explode by the same mechanism as Type II supernovae but which have lost their hydrogen envelopes by winds or mass exchange.