A spectroscopically normal type Ic supernova from a very massive
  progenitor
  We present observations of the Type Ic supernova (SN Ic) 2011bm spanning a
period of about one year. The data establish that SN 2011bm is a
spectroscopically normal SN Ic with moderately low ejecta velocities and with a
very slow spectroscopic and photometric evolution (more than twice as slow as
SN 1998bw). The Pan-STARRS1 retrospective detection shows that the rise time
from explosion to peak was 40 days in the R band. Through an analysis of the
light curve and the spectral sequence, we estimate a kinetic energy of 7-17 foe
and a total ejected mass of 7-17 Mo, 5-10 Mo of which is oxygen and 0.6-0.7 Mo
is 56Ni. The physical parameters obtained for SN 2011bm suggest that its
progenitor was a massive star of initial mass 30-50 Mo. The profile of the
forbidden oxygen lines in the nebular spectra show no evidence of a bi-polar
geometry in the ejected material.
