Molecular investigations of comets C/2002?X5 (Kudo-Fujikawa), C/2002?V1 (NEAT), and C/2006 P1 (McNaught) at small heliocentric distances

摘要

We present unique spectroscopic radio observations of comets C/2002?X5 (Kudo-Fujikawa), C/2002?V1 (NEAT), and C/2006?P1 (McNaught), which came within rh?≈? 0.2?AU of the Sun in 2003 and 2007. The molecules OH, HCN, HNC, CS, and CH3OH were detected in each of these comets when they were exposed to strong heating from the Sun. HC3N and HCO+ were detected in comets C/2002?X5 and C/2006 P1, respectively. We show that in these very productive comets close to the Sun screening of the photodissociation by the Sun UV radiation plays a non-negligible role. Acceleration of the gas expansion velocity and day-night asymmetry is also measured and modeled. The CS photodissociation lifetime was constrained to be about 2.5?×?10-5?s-1 at rh?=?1 AU. The relative abundances are compared to values determined from more distant observations of C/2002?X5 or other comets. A high HNC/HCN production-rate ratio, in the range 10–30% between 0.5 and 0.1 AU from the Sun, is measured. The trend for a significant enrichment in CS in cometary comae (CS/HCN ) is confirmed in all three comets. The CH3OH/HCN production rate ratio decreases at low rh. The HC3N/HCN production rate ratio in comet C/2002?X5 is four times higher than measured in any other comet.