Chemical Composition of Outbursting Comet C/2015 ER61 (PanSTARRS)

Comet C/2015 ER61 (PanSTARRS) is a long-period Oort cloud comet whose favorable approach to the inner parts of the solar system in 2017 April-May enabled us to characterize its primary volatile composition using the iSHELL spectrograph mounted on the 3 m NASA Infrared Telescope Facility (IRTF) on Maunakea, Hawaii. We used three iSHELL settings (covering similar to 2.8-5.2 mu m) to sample fluorescent emissions from H2O, OH, CH3OH, HCN, C-2 H-2, NH3, CO, CH4, C-2 H-6, H2CO, and OCS on multiple dates ranging from UT 2017 April 15 (shortly after its April 4 outburst) to May 13, nearly 30 days before the detection of its double nucleus. Our observations also offered the opportunity to obtain sensitive 3 sigma upper limits for cyanoacetylene (HC3N). We report rotational temperatures (T-rot), production rates (Qs), abundance (mixing) ratios (relative to H2O and C-2 H-6), and spatial distributions in the coma. ER61 exhibits variability in production rates of many species on short (day-to-day) and long (pre- versus post-perihelion) timescales. The relative abundances of these volatile species remained consistent within uncertainties during our pre-perihelion observations but tended to decrease during our post-perihelion observations (with the exception of CH3OH and HCN). The short-timescale variability in the production rates of these volatiles could be due to diurnal effects (over the course of the rotation of the nucleus) and/or the effect of its outburst. The decrease in the production rates and hence the mixing ratios in some volatiles in post-perihelion dates could be due to the presence of seasonal effects in ER61.

Automated summary

Using the iSHELL spectrograph, researchers characterized the volatile composition and spatial distribution of Comet C/2015 ER61, finding variability in production rates and mixing ratios during pre- and post-perihelion observations. The relative abundances remained consistent before perihelion but tended to decrease afterwards, possibly due to seasonal effects in the comet.