top) that appeared as the SN was transitioning 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.
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 45º 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.
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 56Co decay.
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.
April 2016
GeminiFocus
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