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

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. Both 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 x 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 to 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 proportional to r_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.