Figure 3 . Optical and NIR light curves of SN 2011ja . The NIR curves have been shifted down a magnitude for clarity , and the dashed line indicates 56 Co decay .
ing out of the plateau phase . Most interestingly , the strength of the blue peak of Hα increases relative to the red , and the line peaks themselves begin to flatten over time .
Multipeaked emission lines are mostly attributed to a toroidal or disk geometry of surrounding CSM material , while the flattening is caused by the ejecta interacting with the CSM . From the optical spectra alone , we can therefore infer that the SN is running into asymmetric mass-loss from the SN progenitor . Chandra X-ray observations provide further support as the SN ’ s early X-ray emission only increased over the first 100 days .
The degrading of the red peak with time was our first clue that dust was forming early on , as the grains were obscuring the receding side of the ejecta more than the approaching side . While this in itself may have been enough to determine dust was forming within a few months of explosion in SN 2011ja , the IR and optical light curves also added credence : there is a 0.4 magnitude brightening in the K-band between day 121 and 243 , and a simultaneous drop of ~ 0.5 magnitude in the optical brightness as can be seen in Figure 3 .
All observational signs pointed to dust formation occurring sometime around day 100 . Together with the spectral signatures of CSM interaction , this would seem to indicate that the dust is in the CDS , formed between the forward and reverse shocks created as the ejecta plows into the pre-exisitng gas and dust lost by the progenitor before the end of its life .
Modeling the Dust
Now that we had observational evidence of dust grains forming in SN 2011ja , we could turn our attention to modeling the dust : how much , what kind , and where was it located .
Using our 3D Monte Carlo radiative transfer code MOCASSIN and our optical and IR observations , we modeled various geometries and compositions of dust , including a spherical shell of smooth and clumped dust , as well as a smooth distribution of dust in a torus of increasing inclinations around the system . We limited our dust composition to carbon grains only , since 10.8 micron Very Large Telescope observations did not detect strong silicon emission .
The modeling of four GMOS and Spitzer Infrared Array Camera epochs revealed about 1 x 10 -5 solar masses of pre-existing dust located about 3,500 AU away from the center of the SN , and up to 6 x 10 -4 solar masses of newly formed dust in a torus inclined roughly 45 º from edge on and closely surrounding the SN .
This dust mass is still much less than that observed from SN 1987A and other SN remnants . By continuing to follow SN 2011ja as it expands and cools , we will likely see more and more dust being formed , albeit at a much lower temperature .
18 GeminiFocus January 2017 | 2016 Year in Review