Outgassing Behavior of C/2012 S1 (ISON) From September 2011 to June 2013

TL;DR

This study tracks the outgassing behavior of comet C/2012 S1 (ISON) from 2011 to 2013, revealing a CO-driven activity with a long outburst likely caused by CO-induced ejection of water ice grains.

Contribution

It provides the first comprehensive multi-wavelength observational analysis of ISON's outgassing, including photometry and sub-mm data, and models the sublimation processes involved.

Findings

Outgassing was dominated by CO sublimation at large distances.

A long, slow outburst began in late 2011 and peaked in early 2013.

No gas was detected at the observed wavelengths, setting upper limits.

Abstract

We report photometric observations for comet C/2012 S1 (ISON) obtained during the time period immediately after discovery (r=6.28 AU) until it moved into solar conjunction in mid-2013 June using the UH2.2m, and Gemini North 8-m telescopes on Mauna Kea, the Lowell 1.8m in Flagstaff, the Calar Alto 1.2m telescope in Spain, the VYSOS-5 telescopes on Mauna Loa Hawaii and data from the CARA network. Additional pre-discovery data from the Pan STARRS1 survey extends the light curve back to 2011 September 30 (r=9.4 AU). The images showed a similar tail morphology due to small micron sized particles throughout 2013. Observations at sub-mm wavelengths using the JCMT on 15 nights between 2013 March 9 (r=4.52 AU) and June 16 (r=3.35 AU) were used to search for CO and HCN rotation lines. No gas was detected, with upper limits for CO ranging between (3.5-4.5)E27 molec/s. Combined with published water production rate estimates we have generated ice sublimation models consistent with the photometric light curve. The inbound light curve is likely controlled by sublimation of CO2. At these distances water is not a strong contributor to the outgassing. We also infer that there was a long slow outburst of activity beginning in late 2011 peaking in mid-2013 January (r~5 AU) at which point the activity decreased again through 2013 June. We suggest that this outburst was driven by CO injecting large water ice grains into the coma. Observations as the comet came out of solar conjunction seem to confirm our models.