THE PECULIAR TYPE-II SUPERNOVA 1997D - A CASE FOR A VERY-LOW NI-56 MASS

SN 1997D in NGC 1536 is possibly the least luminous and energetic Type II supernova discovered to date. The entire light curve is subluminou s, never reaching M-v = -14.65. The radioactive tail follows the Co-56 decay slope. In the case of a nearly complete trapping of the gamma-r ays, the Ni-56 mass derived from the tail brightness is extremely smal l, similar to 0.002 M.. At discovery, the spectra showed a red continu um and line velocities on the order of 1000 km s(-1). The luminosity a nd the photospheric expansion velocity suggest that the explosion occu rred about 50 days before discovery and that a plateau probably follow ed. Model light curves and spectra of the explosion of a 26 M. star su ccessfully fitted the observations. Low-mass models are inconsistent w ith the observations. The radius of the progenitor, constrained by the prediscovery upper limits, is R-0 less than or similar to 300 R.. A l ow explosion energy of similar to 4 x 10(50) ergs is then required in the modeling. The strong Pa II lines in the photospheric spectra are r eproduced with a solar abundance and low T-eff. A scenario in which th e low Ni-56 mass observed in SN 1997D is due to fallback of material o nto the collapsed remnant of the explosion of a 25-40 M. star appears to be favored over the case of the explosion of an 8-10 M. star with l ow Ni-56 production.