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

We present spectroscopic and photometric observations of the luminous narrow-line Type IIP (plateau) supernova 1994W. After the plateau phase (t greater than or similar to 120 days), the light curve dropped by similar to 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-56, and by day 197 it was 3.6 mag less luminous in R than SN 1987A. The low R luminosity could indicate less than or similar to 0.0026(-0.0011)(+0.0017) M-. of Ni-56 ejected at the explosion. The emission between 125 and 197 days would in this case be dominated by diffusion of emission from the mantle region, or by an additional power source, presumably circumstellar interaction. Alternatively, the late light curve could have been dominated by Co-56 decay. In this case, the mass of the ejected Ni-56 would have been 0.015(-0.008)(+0.012) M-., and the rapid fading between 125 and 197 days most likely due to dust formation. Although this value of the mass is higher than in the first case, it is still lower than estimated for any previous Type II supernova. Only progenitors with M(ZAMS)similar to 8-10 M-. and M(ZAMS)greater than or similar to 25 M-. are expected to eject such low masses of Ni-56. If M(ZAMS)similar to 8-10 M-., the plateau phase indicates a low explosion energy, while for a progenitor with M(ZAMS)greater than or similar to 25 M-., the energy can be the canonical similar to 10(51) 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 M(ZAM)greater than or similar to 25 M-. progenitor. The supernova's narrow (similar to 1000 km s(-1)) emission lines were excited by the hot supernova spectrum, rather than by a circumstellar shock. The thin shell from which the lines originated was most likely accelerated by radiation from the supernova.