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


Authors:
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Jesper Sollerman, Robert J. Cumming, & Peter Lundqvist


Abstract:
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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 roughly 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
magnitudes less luminous in R compared to SN 1987A. The low R-luminosity could
indicate less than 0.0026{+0.0017}{-0.0011} solar masses 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 was dominated by Co-56 decay. In this case, the mass of the ejected 
Ni-56 was 0.015{+0.012}{-0.008} solar masses, 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 first case, it is still lower than estimated for any 
previous Type II supernova.

Only progenitors with M(ZAMS) = 8-10 solar masses and M(ZAMS) > 25 solar
masses are expected to eject such low masses of Ni-56. If M(ZAMS) = 8-10 solar
masses, the plateau phase indicates a low explosion energy, while for a
progenitor with M(ZAMS) > 25 solar masses the energy can be the canonical
1.0E{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(ZAMS) > 25 solar mass progenitor.

The supernova's narrow (roughly 1000 km s^{-1}) 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.


To appear in:
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ApJ (Main Journal); accepted September 10, 1997; scheduled for Feb. 1, 1998,
						 Vol. 493.