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

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 $10^{50}$ erg, ejecta mass around $6 M_{\odot}$, and presupernova radius near $85 R_{\odot}$. We confirm the previous result by Turatto et al. (\cite{tmy98}) that the ejecta contain a very low amount of radioactive $^{56}$Ni ($\approx 0.002 M_{\odot}$). Modelling the nebular spectrum supports the hydrodynamical model and permits us to estimate the mass of freshly synthesized oxygen (0.02--0.07 $M_{\odot}$). 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 $M_{\odot}$ 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 ($\approx 10^{50}$ erg) and low amount of radioactive $^{56}$Ni ($\approx 0.002 M_{\odot}$) 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.