The Compositional Evolution of C/2012 S1 (ISON) from Ground-Based High-Resolution Infrared Spectroscopy as Part of a Worldwide Observing Campaign

Abstract Volatile production rates, relative abundances, rotational temperatures, and spatial distributionsin the coma were measured in C/2012 S1 (ISON) using long-slit high-dispersion ( λ /Δ λ ∼ 2.5 × 10 4 ) infrared spectroscopy as part of a worldwide observing campaign. Spectra were obtained on UT 2013 October 26 and 28 with NIRSPEC at the W.M. Keck Observatory, and UT 2013 November 19 and 20 with CSHELL at the NASA IRTF. H 2 O was detected on all dates, with production rates increasing markedly from (8.7 ± 1.5) × 10 27 molecules s −1 on October 26 ( R h = 1.12 AU) to (3.7 ± 0.4) × 10 29 molecules s −1 on November 20 ( R h = 0.43 AU). Short-term variability of H 2 O production is also seen as observations on November 19 show an increase in H 2 O production rate of nearly a factor of two over a period of about 6 h. C 2 H 6 , CH 3 OH and CH 4 abundances in ISON are slightly depleted relative to H 2 O when compared to mean values for comets measured at infrared wavelengths. On the November dates, C 2 H 2 , HCN and OCS abundances relative to H 2 O appear to be within the range of mean values, whereas H 2 CO and NH 3 were significantly enhanced. There is evidence that the abundances with respect to H 2 O increased for some species but not others between October 28 ( R h = 1.07 AU) and November 19 ( R h = 0.46 AU). The high mixing ratios of H 2 CO/CH 3 OH and C 2 H 2 /C 2 H 6 on November 19, and changes in the mixing ratios of some species with respect to H 2 O between October 28 to November 19, indicates compositional changes that may be the result of a transition from sampling radiation-processed outer layers in this dynamically new comet to sampling more pristine natal material as the outer processed layer was increasingly eroded and the thermal wave propagated into the nucleus as the comet approached perihelion for the first time. On November 19 and 20, the spatial distributionfor dust appears asymmetric and enhanced in the antisolar direction, whereas spatial distributionsfor volatiles (excepting CN) appear symmetric with their peaks slightly offset in the sunward direction compared to the dust. Spatial distributionsfor H 2 O, HCN, C 2 H 6 , C 2 H 2 , and H 2 CO on November 19 show no definitive evidence for significant contributions from extended sources; however, broader spatial distributionsfor NH 3 and OCS may be consistent with extended sources for these species. Abundances of HCN and C 2 H 2 on November 19 and 20 are insufficient to account for reported abundances of CN and C 2 in ISON near this time. Differences in HCN and CN spatial distributionsare also consistent with HCN as only a minor source of CN in ISON on November 19 as the spatial distributionof CN in the coma suggests a dominant distributed source that is correlated with dust and not volatile release. The spatial distributionsfor NH 3 and NH 2 are similar, suggesting that NH 3 is the primary source of NH 2 with no evidence of a significant dust source of NH 2 ; however, the higher production rates derived for NH 3 compared to NH 2 on November 19 and 20 remain unexplained. This suggests a more complete analysis that treats NH 2 as a distributed source and accounts for its emission mechanism is needed for future work.