Volatile Composition and Outgassing in C/2018 Y1 (Iwamoto): Extending Limits for High-resolution Infrared Cometary Spectroscopy between 2.8 and 5.0 mu m

We used the powerful high spectral resolution cross-dispersed facility spectrograph, iSHELL, at the NASA Infrared Telescope Facility (IRTF) to observe C/2018 Y1 (Iwamoto), a long-period comet from the Oort cloud. We report production rates for water and eight other parent molecules (native ices), C2H6, CO, CH4, H2CO, CH3OH, HCN, NH3, and C2H2, on three preperihelion UT dates, 2019 January 13 and (near perihelion) February 4-5. We present abundance ratios relative to both C2H6 (a nonpolar molecule) and H2O (a polar molecule), thereby providing a more complete picture of the parent volatile composition of C/2018 Y1 and potential associations of ices in its nucleus. Overall, the measured spatial distributions for polar molecules (in particular, H2O and CH3OH) were broader, exhibiting more complex structure compared with nonpolar or weakly polar species (CH4, C2H6, and CO). Our January 13 results permitted quantitatively assessing the significant improvement in sensitivity delivered by iSHELL compared with previous capabilities at the IRTF. The efficient spectral coverage of iSHELL plus the favorable geocentric Doppler shift of C/2018 Y1 allowed for measuring >50% of CH4 nu (3)-band emission intensity on both January 13 and February 5. Compositionally, compared to their respective mean abundances among comets from the Oort cloud, C2H6 and CH3OH were enriched, CH4 and HCN were near normal, and all other species were depleted. The abundance ratio CH3OH/C2H6 was higher by 45% +/- 8% on January 13 versus February 5, whereas CH4/C2H6 was unchanged within the uncertainty, suggesting nonhomogeneous composition among regions of the nucleus dominating activity on these dates.