Carbon monoxide production and excitation in Comet C/1995 O1 (Hale-Bopp): Isolation of native and distributed CO sources

The release of carbon monoxide from Comet C/1995 O1 Hale-Bopp was studied b etween June 1996 and September 1997 using high resolution infrared spectros copy near 4.7 mum. The excitation of CO molecules in the coma was assessed through measurement of the rotational temperature on several dates at an an gular resolution of similar to 1 arcsecond. An increase in Trot with distan ce from the nucleus was revealed, most likely because of photolytic heating by fast H-atoms. Observed temperature profiles varied from date to date, b ut overall the degree of heating was most pronounced near perihelion. The s imilar 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 C O released from a progenitor distributed in the coma. Only the native sourc e was seen when the comet was beyond 2 AU from the Sun. Based on pre- and p ost-perihelion observations on five dates with heliocentric distance Rh bet ween 4.10 and 2.02 AU, a heliocentric dependence Q(CO.native) = (1.06 +/- 0 .44) x 10(30) R-h (-1.76 +/-0.26) molecules s(-1) was obtained. Within R-h similar to 1.5 AU, however, both native and distributed sources were consis tently present on all dates of observation. The total CO produced was the s um of the two sources and, based on seven dates, obeyed Q(CO.total) = (2.07 +/- 0.20) x 10(30) R-h (-1.66 +/-0.22) molecules s(-1). This he. liocentri c dependence was consistent with that found for water (QH(2)O alpha R-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)/QH(2)O = 0.241 +/- 0.009, with native and distributed sources each contributing an abundance of approximately 12 percent that of water. This w as the case even after correcting measured CO and H2O column densities, and hence production rates, for opacity in the solar pump. The distributed sou rce exhibited behavior consistent with thermal destruction of a precursor m aterial. The observed variations in its production rate and spatial distrib ution 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. (C) 2001 Academic Press.