The Peculiar Type II Supernova 1997D: A Case for a Very Low [TSUP]56[/TSUP]N[CLC]i[/CLC] Mass

Abstract

SN 1997D in NGC 1536 is possibly the least luminous and energetic Type II supernova discovered to date. The entire light curve is subluminous, never reaching MV=-14.65. The radioactive tail follows the 56Co decay slope. In the case of a nearly complete trapping of the γ-rays, the 56Ni mass derived from the tail brightness is extremely small, ~0.002 M☉. At discovery, the spectra showed a red continuum and line velocities on the order of 1000 km s−1. The luminosity and the photospheric expansion velocity suggest that the explosion occurred about 50 days before discovery and that a plateau probably followed. Model light curves and spectra of the explosion of a 26 M☉ star successfully fitted the observations. Low-mass models are inconsistent with the observations. The radius of the progenitor, constrained by the prediscovery upper limits, is R0 300 R☉. A low explosion energy of ~4 × 1050 ergs is then required in the modeling. The strong Ba II lines in the photospheric spectra are reproduced with a solar abundance and low Teff. A scenario in which the low 56Ni mass observed in SN 1997D is due to fallback of material onto 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 low 56Ni production.