Signs of a Massive Progenitor
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 56 Co 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 SN 2011ja likely went through a strong mass-loss phase not long before eruption .
The Future
Our group is continuing to study CCSNe at late times to look for increased dust formation . Specifically we are monitoring the dust as the ejecta cools and expands , to determine how the progenitor mass of the SN correlates with dust mass . We currently have a project underway using GMOS to look at SNe with ages between 4-60 years in order to model their Hα emission .
Tests done on SN 1987A have shown that the shape and strength of the broad Hα line can be correlated to dust mass . This will be amazingly useful , especially in the era before James Webb Space Telescope , since data suggest the peak dust production may occur in the cooler dust regimes only accessible by long-wavelength instruments .
Jennifer Andrews is a postdoctoral researcher at the University of Arizona . She can be reached at : jandrews @ as . arizona . edu
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 .
January 2017 | 2016 Year in Review GeminiFocus
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