TY - JOUR
T1 - The compositional evolution of C/2012 S1 (ISON) from ground-based high-resolution infrared spectroscopy as part of a worldwide observing campaign
AU - Dello Russo, N.
AU - Vervack, R. J.
AU - Kawakita, H.
AU - Cochran, A.
AU - McKay, A. J.
AU - Harris, W. M.
AU - Weaver, H. A.
AU - Lisse, C. M.
AU - DiSanti, M. A.
AU - Kobayashi, H.
AU - Biver, N.
AU - Bockelée-Morvan, D.
AU - Crovisier, J.
AU - Opitom, C.
AU - Jehin, E.
N1 - Funding Information: Data were obtained at the NASA Infrared Telescope Facility operated by the University of Hawaii under cooperative agreement number NNX-08AE38A with the NASA Science Mission Directorate, Planetary Astronomy Program. Data presented herein were also obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation . We thank NASA, the Comet ISON Observing Campaign (CIOC), and the IRTF and Keck Observatories for setting aside campaign time for these observations, and the former NASA PAST and PATM Programs for their financial support of this work. MAD thanks the NASA SSW Program, grant NNX15AH29G for support. We note that all raw data from observations presented here are publicly available through IRTF and Keck archives. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Publisher Copyright: © 2015 Elsevier Inc.
PY - 2016/3/1
Y1 - 2016/3/1
N2 - Volatile production rates, relative abundances, rotational temperatures, and spatial distributions in the coma were measured in C/2012 S1 (ISON) using long-slit high-dispersion (λ/δλ~2.5×104) 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. H2O was detected on all dates, with production rates increasing markedly from (8.7±1.5)×1027moleculess-1 on October 26 (Rh=1.12AU) to (3.7±0.4)×1029moleculess-1 on November 20 (Rh=0.43AU). Short-term variability of H2O production is also seen as observations on November 19 show an increase in H2O production rate of nearly a factor of two over a period of about 6h. C2H6, CH3OH and CH4 abundances in ISON are slightly depleted relative to H2O when compared to mean values for comets measured at infrared wavelengths. On the November dates, C2H2, HCN and OCS abundances relative to H2O appear to be within the range of mean values, whereas H2CO and NH3 were significantly enhanced. There is evidence that the abundances with respect to H2O increased for some species but not others between October 28 (Rh=1.07AU) and November 19 (Rh=0.46AU). The high mixing ratios of H2CO/CH3OH and C2H2/C2H6 on November 19, and changes in the mixing ratios of some species with respect to H2O 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 distribution for dust appears asymmetric and enhanced in the antisolar direction, whereas spatial distributions for volatiles (excepting CN) appear symmetric with their peaks slightly offset in the sunward direction compared to the dust. Spatial distributions for H2O, HCN, C2H6, C2H2, and H2CO on November 19 show no definitive evidence for significant contributions from extended sources; however, broader spatial distributions for NH3 and OCS may be consistent with extended sources for these species. Abundances of HCN and C2H2 on November 19 and 20 are insufficient to account for reported abundances of CN and C2 in ISON near this time. Differences in HCN and CN spatial distributions are also consistent with HCN as only a minor source of CN in ISON on November 19 as the spatial distribution of CN in the coma suggests a dominant distributed source that is correlated with dust and not volatile release. The spatial distributions for NH3 and NH2 are similar, suggesting that NH3 is the primary source of NH2 with no evidence of a significant dust source of NH2; however, the higher production rates derived for NH3 compared to NH2 on November 19 and 20 remain unexplained. This suggests a more complete analysis that treats NH2 as a distributed source and accounts for its emission mechanism is needed for future work.
AB - Volatile production rates, relative abundances, rotational temperatures, and spatial distributions in the coma were measured in C/2012 S1 (ISON) using long-slit high-dispersion (λ/δλ~2.5×104) 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. H2O was detected on all dates, with production rates increasing markedly from (8.7±1.5)×1027moleculess-1 on October 26 (Rh=1.12AU) to (3.7±0.4)×1029moleculess-1 on November 20 (Rh=0.43AU). Short-term variability of H2O production is also seen as observations on November 19 show an increase in H2O production rate of nearly a factor of two over a period of about 6h. C2H6, CH3OH and CH4 abundances in ISON are slightly depleted relative to H2O when compared to mean values for comets measured at infrared wavelengths. On the November dates, C2H2, HCN and OCS abundances relative to H2O appear to be within the range of mean values, whereas H2CO and NH3 were significantly enhanced. There is evidence that the abundances with respect to H2O increased for some species but not others between October 28 (Rh=1.07AU) and November 19 (Rh=0.46AU). The high mixing ratios of H2CO/CH3OH and C2H2/C2H6 on November 19, and changes in the mixing ratios of some species with respect to H2O 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 distribution for dust appears asymmetric and enhanced in the antisolar direction, whereas spatial distributions for volatiles (excepting CN) appear symmetric with their peaks slightly offset in the sunward direction compared to the dust. Spatial distributions for H2O, HCN, C2H6, C2H2, and H2CO on November 19 show no definitive evidence for significant contributions from extended sources; however, broader spatial distributions for NH3 and OCS may be consistent with extended sources for these species. Abundances of HCN and C2H2 on November 19 and 20 are insufficient to account for reported abundances of CN and C2 in ISON near this time. Differences in HCN and CN spatial distributions are also consistent with HCN as only a minor source of CN in ISON on November 19 as the spatial distribution of CN in the coma suggests a dominant distributed source that is correlated with dust and not volatile release. The spatial distributions for NH3 and NH2 are similar, suggesting that NH3 is the primary source of NH2 with no evidence of a significant dust source of NH2; however, the higher production rates derived for NH3 compared to NH2 on November 19 and 20 remain unexplained. This suggests a more complete analysis that treats NH2 as a distributed source and accounts for its emission mechanism is needed for future work.
KW - Comets
KW - Infrared observations
KW - Spectroscopy
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U2 - 10.1016/j.icarus.2015.11.030
DO - 10.1016/j.icarus.2015.11.030
M3 - Article
SN - 0019-1035
VL - 266
SP - 152
EP - 172
JO - Icarus
JF - Icarus
ER -