The peculiar type II supernova 1997D: A case for a very low⁵⁶Ni mass

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 ⁵⁶Co decay slope. In the case of a nearly complete trapping of the γ-rays, the ³⁶Ni 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 M̈ 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 n lines in the photospheric spectra are reproduced with a solar abundance and low T_eff. 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.