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

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 CH 3 OH were detected in each of these comets when they were exposed to strong heating from the Sun. HC 3 N 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 ∝ r −0.8 h ) is confirmed in all three comets. The CH 3 OH/HCN production rate ratio decreases at low r h . The HC 3 N/HCN production rate ratio in comet C/2002 X5 is four times higher than measured in any other comet.

En Route to Destruction: The Evolution in Composition of Ices in Comet D/2012 S1 (Ison) Between 1.2 and 0.34 Au From the Sun as Revealed at Infrared Wavelengths*

“…A notable study extending to small R h is represented by the observations of C/2002 X5 Kudo-Fujikama at R h =1.2, 0.5, and 0.21 AU, using the IRAM 30m and Nançay radio telescopes (Biver et al 2011). Abundance ratios for both HCN and HNC relative to H 2 O increased by approximately twofold at 0.21 AU compared to the mean of their respective values at R h =0.5 and 1.2 AU, while that of CH 3 OH remained constant.…”

“…In our calculations we adopt a gas outflow speed v gas =800 R h −0.5 m s −1 , based on velocity-resolved observations of several moderately bright comets at radio wavelengths (Biver et al 2006(Biver et al , 2011Agúndez et al 2014;Cordiner et al 2014). The parameter f(x) represents the fraction of all molecules in the coma contained in the beam, assuming release solely from the nucleus and uniform gas outflow in the comaf(x) was originally formulated for a circular aperture (Yamamoto 1982) and later revised for square pixels (see Appendix in Hoban et al 1991).…”

Submillimetric spectroscopic observations of volatiles in comet C/2004 Q2 (Machholz)

“…Neutrals in cometary atmospheres are excited by collisions with other molecules and electrons, which are the dominant effects in the inner coma, and radiative pumping of the fundamental vibrational levels by the solar infrared flux. Infrared pumping of vibrational bands by solar radiation contributes to the excitation in the outer coma where the gas and electron densities are low (Bockelée-Morvan 1987). Most of the detected emission in comet C/2004 Q2 (Machholz) originates from a region in which molecules are in an excitation state intermediate between fluorescence and collision-dominated equilibrium.…”

“…We obtained the photodissociative lifetimes for CH 3 OH, HCN, HNC, H 2 CO and CO from Crovisier (1994) assuming the quiet-Sun reference spectrum, and the CS radical lifetime was taken from Biver et al (2011). The photodissociation rates were scaled by the heliocentric distance of C/2004 Q2 (Machholz) at the time of the observations.…”

“…We adopted a molecular excitation and radiative transfer model based on the Sobolev escape probability method, which includes collisional effects and infrared fluorescence by solar radiation to derive the production rates (Bockelée-Morvan 1987;Biver 1997;Biver et al 1999). In the outer coma, solar infrared pumping of vibrational bands followed by instantaneous spontaneous decay establishes a fluorescence equilibrium.…”