GeminiFocus April 2016 | Page 8

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