Signs of a Massive Progenitor
Figure 4.
Day 807 spectrum
(purple) of SN 2011ja
showing enhanced
blue emission.
Comparison with the
light echo spectrum
created from an
integrated fluency
of the first 84 days
(red) indicates that
a light echo cannot
be responsible for
the flux bluewards of
6,000 Å. The orange
and yellow spectra
are synthesized stellar
populations created
with Starburst99 for 3
and 6 Myr. It is possible
that the late-time
luminosity has a large
component of the
parental stellar cluster.
While we had found evidence for early dust
formation within the first 100 days, we needed to continue observing the object as long
as possible to search for additional grain formation in the ejecta. After 400 days or so, we
noticed little change in its optical luminosity
(Figure 3). Normally we would expect to see a
fading of about 1 magnitude every 100 days
due to the radioactive decay of 56Co which
powers the late-time lightcurves of SNe.
Light echoes scattering off dust clouds between us and the SN, or radiative shocks
plowing into nearby CSM, may have caused
this late-time brightness. But most likely in
this case the SN continuum had faded below the brightness of the parent star cluster
from which the massive progenitor star was
born. This scenario would allow both the
strong broad Hα emission line, and a bright
blue continuum.
Comparing the day 807 spectra with Starburst99 stellar synthesis models of young
massive star clusters (Figure 4) indicates that
the late-time luminosity most likely has a
large component of the parental stellar cluster, which is between 3-6 million years (Myr)
young, corresponding to a SN progenitor
mass of 20-30 Suns.
The absolute magnitude at maximum
(I = ~ -18.3) in tandem with the short plateau duration and the steep drop into the
radioactive decay phase of the optical light
curve, all point to a CCSN with a small hydrogen envelope.
Combined with the estimated age of the
parent cluster, this would suggest S