A very low mass of 56 Ni in the ejecta of SN 1994

We present spectroscopic and photometric observations of the luminous narrow-line Type IIP (plateau) supernova 1994W. After the plateau phase (t ∼> 120 days), the light curve dropped by ∼ 3.5 mag in V in only 12 days. Between 125 and 197 days after explosion the supernova faded substantially faster than the decay rate of Co, and by day 197 it was 3.6 magnitudes less luminous in R compared to SN 1987A. The low R-luminosity could indicate ∼< 0.0026 +0.0017 −0.0011 M⊙ of Ni ejected at the explosion, but the emission between 125 and 197 days must then have been dominated by an additional power source, presumably circumstellar interaction. Alternatively, the late light curve was dominated by Co decay. In this case, the mass of the ejected Ni was 0.015 −0.008 M⊙, and the rapid fading between 125 and 197 days was most likely due to dust formation. Though this value of the mass is higher than in the case with the additional power source, it is still lower than estimated for any previous Type II supernova. Only progenitors with MZAMS ∼ 8− 10 M⊙ and MZAMS ∼> 25 M⊙ are expected to eject such low masses of Ni. If MZAMS ∼ 8 − 10 M⊙, the plateau phase indicates a low explosion energy, while for a progenitor with MZAMS ∼> 25 M⊙ the energy can be the canonical ∼ 10 ergs. As SN 1994W was unusually luminous, the low-mass explosion may require an uncomfortably high efficiency in converting explosion energy into radiation. This favors a MZAMS ∼> 25 M⊙ progenitor. The supernova’s narrow (∼ 1000 km s) emission lines were excited by the hot supernova spectrum, rather than a circumstellar shock. The thin shell from which the lines originated was most likely accelerated by the radiation from the supernova. Subject headings: supernovae: individual (SN 1994W) – abundances – gamma rays: theory – circumstellar matter – stars: neutron – black hole physics Stockholm Observatory, SE-133 36 Saltsjöbaden, Sweden. Send offprint requests to Peter Lundqvist; E-mail: [email protected]