Spectroscopy of Comet Hale–Bopp in the infrared

High resolution infrared spectra of Comet C/1995 O1 (Hale–Bopp) were obtained during 2–5 March 1997 UT from the NASA Infrared Telescope Facility on Mauna Kea, Hawaii, when the comet was at r≈1.0 AU from the Sun pre-perihelion. Emission lines of CH4, C2H6, HCN, C2H2, CH3OH, H2O, CO, and OH were detected. The rotational temperature of CH4 in the inner coma was Trot=110±20 K. Spatial profiles of CH4, C2H6, and H2O were consistent with release solely from the nucleus. The centroid of the CO emission was offset from that of the dust continuum and H2O. Spatial profiles of the CO lines were much broader than those of the other molecules and asymmetric. We estimate the CO production rate using a simplified outflow model: constant, symmetric outflow from the peak position. A model of the excitation of CO that includes optical depth effects using an escape probability method is presented. Optical depth effects are not sufficient to explain the broad spatial extent. Using a parent+extended-source model, the broad extent of the CO lines can be explained by CO being produced mostly (∼90% on 5 March) from an extended source in the coma. The CO rotational temperature was near 100 K. Abundances relative to H2O (in percent) were 1.1±0.3 (CH4), 0.39±0.10 (C2H6), 0.18±0.04 (HCN), 0.17±0.04 (C2H2), 1.7±0.5 (CH3OH), and 37–41 (CO, parent+extended source). These are roughly comparable to those obtained for other long-period comets also observed in the infrared, though CO appears to vary.