Quantifying the Evolution of Molecular Production Rates of Comet 21P/Giacobini-Zinner with iSHELL/NASA-IRTF

This paper presents results from our molecular characterization of comet 21P/Giacobini-Zinner during its 2018 apparition. We followed the comet during four observing runs, for a total of eight nights. The high-resolution spectra of 21P were acquired using iSHELL-the near-infrared high-resolution immersion echelle spectrograph on NASA/IRTF (Maunakea, Hawaii). We detected many cometary emission lines across four customized iSHELL settings in the (2.9-5.2) mu m range. CO abundances relative to water displayed a relatively constant behavior during the observing campaign, suggesting a possible correlation between the outgassing of water and CO. While CO mixing ratios in 21P are consistent with the reference median value for Jupiter family comets (JFCs), ethane was depleted relative to the reference median value for JFCs, except for one observing run (August 8 and 9) in which ethane appeared overabundant. We consider several possible causes: an ethane outburst, decreasing outgassing of water, or a seasonal effect owing to nucleus rotation. In the pre-perihelion runs, methanol was overabundant compared with the reference median value for JFCs, however it decreased steadily to a depleted value during 21P's post-perihelion phase, suggesting a possible seasonal effect. We report 3 sigma upper limits for acetylene, formaldehyde, ammonia, and methane. The ratios of our measured 3 sigma upper limits for acetylene and detections of hydrogen cyanide are consistent with the depletion of C-2 relative to CN in 21P observed at optical wavelengths. This result confirms that 21P is depleted in the carbon-chain primary volatile responsible for C-2 production.