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

¹ LESIA, CNRS UMR 8109, UPMC, Université Paris-Diderot, Observatoire de Paris, 5 pl. Jules Janssen, 92195 Meudon, France
e-mail: Nicolas.Biver@obspm.fr
² IRAM, Avd. Divina Pastora, 7, 18012 Granada, Spain
³ Max Planck Institute for Radio Astronomy, PO Box 20 24, 53010 Bonn, Germany

Received: 2 December 2010
Accepted: 14 January 2011

Abstract

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 r_h ≈  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 r_h = 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 $\mathrm{\propto }{\mathit{r}}_{\mathrm{h}}^{-0.8}$) is confirmed in all three comets. The CH₃OH/HCN production rate ratio decreases at low r_h. The HC₃N/HCN production rate ratio in comet C/2002 X5 is four times higher than measured in any other comet.