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 M V =-14.65. The radioactive tail follows the 56 Co decay slope. In the case of a nearly complete trapping of the γ-rays, the 56 Ni 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 R 0 ≲ 300 R ☉ . A low explosion energy of ~4 × 10 50 ergs is then required in the modeling. The strong Ba II lines in the photospheric spectra are reproduced with a solar abundance and low T eff . A scenario in which the low 56 Ni 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 56 Ni production.