THE PARENT VOLATILE COMPOSITION OF 6P/d’ARREST AND A CHEMICAL COMPARISON OF JUPITER-FAMILY COMETS MEASURED AT INFRARED WAVELENGTHS

The volatile abundances in Jupiter-family comet (JFC) 6P/d'Arrest were measured on two dates (UT 2008 July 28 and August 11) using high-dispersion (λ/Δλ ∼ 2.8 × 10 4 ) infrared spectroscopy with NIRSPEC at the W. M. Keck Observatory. H 2 O, CH 3 OH, and C 2 H 6 were detected on both dates and H 2 CO and NH 3 were detected on UT August 11. A tentative detection of HCN and an upper limit for C 2 H 2 are also reported on UT August 11. CH 3 OH, H 2 CO, and NH 3 abundances relative to H 2 O are in the typical range, while C 2 H 6 , C 2 H 2 , and HCN are all moderately to severely depleted compared to other comets. Gas production increased by about a factor of 2 from July 28 to August 11. Relative abundances of C 2 H 6 and CH 3 OH with respect to H 2 O are consistent on these dates within the limitations of the measurements. Rotational temperatures were determined for H 2 O on UT August 11 and found to be about 40 K. The spatial distributions of H 2 O and C 2 H 6 are both asymmetric with peak flux about 100 km in the anti-sunward direction compared to the more symmetric dust distribution. A comparison between the abundances of daughter fragments (e.g., C 2 , CN, NH, and NH 2 ) and their suspected parents (e.g., C 2 H 2 , C 2 H 6 , HCN, and NH 3 ) shows that while the abundance of NH 3 in 6P is consistent with it being the primary source of NH and NH 2 , there is insufficient C 2 H 2 , C 2 H 6 , and HCN to account for C 2 and CN in 6P. This is in contrast to other measured JFCs where abundances of C 2 and CN are consistent with C 2 H 2 , C 2 H 6 , and HCN as their primary parents to within the accuracy of the measurements. Thus, optical taxonomic classifications of comets may not correlate well to taxonomic classifications based on parent volatile chemistry in some cases.