Carbon Monoxide Production and Excitation in Comet C/1995 O1 (Hale-Bopp): Isolation of Native and Distributed CO Sources

Abstract

The release of carbon monoxide from Comet C/1995 O1 Hale-Bopp was studied between June 1996 and September 1997 using high resolution infrared spectroscopy near 4.7 μm. The excitation of CO molecules in the coma was assessed through measurement of the rotational temperature on several dates at an angular resolution of ∼1 arcsecond. An increase in T rot with distance from the nucleus was revealed, most likely because of photolytic heating by fast H-atoms. Observed temperature profiles varied from date to date, but overall the degree of heating was most pronounced near perihelion. The similar rotational temperatures observed for CO and HCN may indicate control of rotational populations by collisions with electrons. The spatial distribution of CO molecules in the coma revealed two distinct sources for CO, one being CO ice native to the nucleus, and another being CO released from a progenitor distributed in the coma. Only the native source was seen when the comet was beyond 2 AU from the Sun. Based on pre- and post-perihelion observations on five dates with heliocentric distance R h between 4.10 and 2.02 AU, a heliocentric dependence Q CO,native=(1.06±0.44)×10 30 R h −1.76±0.26 molecules s −1 was obtained. Within R h∼1.5 AU, however, both native and distributed sources were consistently present on all dates of observation. The total CO produced was the sum of the two sources and, based on seven dates, obeyed Q CO,total=(2.07±0.20)×10 30 R h −1.66±0.22 molecules s −1. This heliocentric dependence was consistent with that found for water (Q H 2O αR h −1.88±0.18 between 0.93 and 1.49 AU) and for mm-sized dust (R h −1.7±0.2 between 0.9 and 2.5 AU). Our derived total mixing ratio for CO was Q CO,total/Q H 2O =0.241±0.009, with native and distributed sources each contributing an abundance of approximately 12 percent that of water. This was the case even after correcting measured CO and H 2O column densities, and hence production rates, for opacity in the solar pump. The distributed source exhibited behavior consistent with thermal destruction of a precursor material. The observed variations in its production rate and spatial distribution along the slit suggested contributions from both a diffuse source in the coma and possibly from one or more jets enriched in CO or CO-containing material, such as CHON grains.