The nature of SN 1997D: low-mass progenitor and weak explosion

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We analyzed the spectra and light curve of the peculiar type II-P supernova 1997D to recover ejecta parameters. The optimal hydrodynamical model of SN 1997D, which meets observational constraints at the photospheric epoch, suggests a low explosion energy of about 10⁵⁰ erg, ejecta mass around 6 M, and presupernova radius near 85 R. We confirm the previous result by Turatto et al. (tmy98) that the ejecta contain a very low amount of radioactive ⁵⁶Ni (≈ 0.002 M). Modelling the nebular spectrum supports the hydrodynamical model and permits us to estimate the mass of freshly synthesized oxygen (0.02--0.07 M). Combined with the basic results of stellar evolution theory the obtained parameters of SN 1997D imply that the progenitor was a star from the 8--12 M mass range at the main sequence. The fact that at least some progenitors from this mass range give rise to core-collapse supernovae with a low kinetic energy (≈ 10⁵⁰ erg) and low amount of radioactive ⁵⁶Ni (≈ 0.002 M) has no precedent and imposes important constraints on the explosion mechanism. We speculate that the galactic supernovae 1054 and 1181 could be attributed to SN 1997D-like events.