Extending the Search with GMOS
Over the past decade , our team has been using ground- and space-based optical and IR imaging and spectroscopy to look for signatures of dust formation in young CCSNe . In particular the size and sensitivity of GMOS has allowed us to follow a collection of objects for years after explosion in a search for the three telltale signs of grain condensation .
First , as dust forms , the optical luminosity will decrease while almost simultaneously the near-infrared ( NIR ) will increase , as the dust grains absorb the shorter wavelength light and re-emit it in the IR . Grain formation will also alter the optical spectrum , creating asymmetric and blue-shifted lines as the dust grains attenuate the red ( receding ) side of the ejecta preferentially .
And while we initially believed that the dust grains could only condense 300-600 days after explosion ( when the ejecta had expanded and cooled ) there have been more and more confirmed cases of dust forming much earlier , within 100 days of explosion .
An early onset of dust formation can occur when shocks interact with nearby circumstellar material ( CSM ), creating an area known as the cool dense shell ( CDS ) with temperatures and densities appropriate for grain growth . This not only allows a separate channel for dust formation in CCSNe , but can also reveal important properties of SN evolution and progenitor mass loss .
In February 2012 , we also began using GMOS in an extensive observing campaign on SN 2011ja in NGC 4945 ( Figure 1 ). This “ normal ” Type II-P SN , located ~ 11 million light years away , has an absolute I-band magnitude of ~ -18.3 . Our goal was to follow SN 2011ja from near peak to well past 600 days . This allowed us to look for both channels of dust formation , and to quantify the mass and composition of any forming dust .
January 2017 | 2016 Year in Review
The Evolution of SN 2011ja
Using the new B600 grating and 0.75 arcsecond slit , we obtained GMOS-South mediumresolution spectra of SN 2011ja — 84 , 112 , 159 , 450 , and 807 days after explosion ( Figure 2 ). We also obtained g ’, r ’, and i ’ imaging at the same time ( Figure 1 ). Both datasets are supplemented by European Southern Observatory optical and NIR photometry ( Figure 3 ), as well as optical spectra .
We immediately noticed the strange multipeaked shape of the hydrogen lines ( Figure 2 , top ) that appeared as the SN was transition-
GeminiFocus
Figure 2 . ( Top ): Hα evolution of SN 2011ja during the first 8 months . The degredation of the red peak at ~ 2,500 km / s is a sign of dust formation . ( Bottom ): Full spectroscopic evolution of SN 2011ja over the first two years .
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