2013A&A...559A..48D

The chemical composition of comets can be inferred using spectroscopic observations in submillimeter and radio wavelengths. We aim to compare the production rates ratio of several volatiles in two comets, C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR), which are generally regarded as dynamically new and likely to originate in the Oort cloud. This type of comets is considered to be composed of primitive material that has not undergone considerable thermal processing. The line emission in the coma was measured in the comets, C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR), that were observed on five consecutive nights, 7-11 May 2004, at heliocentric distances of 1.0 and 0.7AU, respectively, by means of high-resolution spectroscopy using the 10-m Submillimeter Telescope at the Arizona Radio Observatory. Both objects became very bright and reached naked-eye visibility during their perihelion passage in the spring of 2004. We present a search for six parent- and product-volatile species (HCN, H2CO, CO, CS, CH3OH, and HNC) in both comets. Multiline observations of the CH3OH J=5-4 series allow us to estimate the rotational temperature using the rotation diagram technique. We derive rotational temperatures of 54(9)K for C/2001 Q4 (NEAT) and 119(34)K for C/2002 T7 (LINEAR). The gas production rates are computed using the level distribution obtained with a spherically symmetric molecular excitation code that includes collisions between neutrals and electrons. The effects of radiative pumping of the fundamental vibrational levels by infrared photons from the Sun are considered for the case of HCN. We find an HCN production rate of 2.96(5)x10²⁶molec/s for comet C/2001 Q4 (NEAT), corresponding to a mixing ratio with respect to H₂O of 1.12(2)x10^–3^. The mean HCN production rate during the observing period is 4.54(10)x10²⁶molec/s for comet C/2002 T7 (LINEAR), which gives a mixing ratio of 1.51(3)x10^–3. Relative abundances of CO, CH₃OH, H₂CO, CS, and HNC with respect to HNC are 3.05(83)x10¹, 1.50(25)x10¹, 1.16(27), 7.02(3)x10^–1, and 5.75(73)x10^–2 in comet C/2001 Q4 (NEAT) and <4.12x10¹, 4.07(44)x10¹, 4.72(73), 1.32(6), and 1.09(8)x10^–1 in comet C/2002 T7 (LINEAR). With systematically lower mixing ratios in comet C/2001 Q4 (NEAT), production rate ratios of the observed species with respect to H₂O lie within the typical ranges of dynamically new comets in both objects. We find a relatively low abundance of CO in C/2001 Q4 (NEAT) compared to the observed range in other comets based on millimeter/submillimeter observations, and a significant upper limit on the CO production in C/2002 T7 (LINEAR) is derived. Depletion of CO suggests partial evaporation from the surface layers during previous visits to the outer solar system and agrees with previous measurements of dynamically new comets. Rotational temperatures derived from CH₃OH rotational diagrams in both C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR) are roughly consistent with observations of other comets at similar distances from the Sun.