Submillimetric spectroscopic observations of volatiles in comet C/2004 Q2 (Machholz)
Context. Submillimeter spectroscopic observations of comets provide an important tool for understanding their chemical composition and enable a taxonomic classification. Aims. We aim to determine the production rates of several parent- and product volatiles and the 12 C/ 13 C isotopic carbon ratio in the long-period comet C/2004 Q2 (Machholz), which is likely to originate from the Oort Cloud. Methods. The line emission from several molecules in the coma was measured with high signal-to-noise ratio in January 2005 at heliocentric distance of 1.2 AU by means of high-resolution spectroscopic observations using the Submillimeter Telescope (SMT) at the Arizona Radio Observatory (ARO). Results. We have obtained production rates of several volatiles (CH 3 OH, HCN, H 13 CN, HNC, H 2 CO, CO, and CS) by comparing the observed and simulated line-integrated intensities. We calculated the synthetic profiles using a radiative transfer code that includes collisions between neutrals and electrons, and the e ects of radiative pumping of the fundamental vibrational levels by solar infrared radiation. Furthermore, multiline observations of the CH 3 OH J = 7‐6 series allow us to estimate the rotational temperature using the rotation diagram technique. We find that the CH 3 OH population distribution of the levels sampled by these lines can be described by a rotational temperature of 40 3 K. Derived mixing ratios relative to hydrogen cyanide are CO/CH 3 OH/H 2 CO/CS/HNC/H 13 CN/HCN = 30.9/24.6/4.8/0.57/0.031/0.013/1 assuming a pointing o set of 8 00 due to the uncertain ephemeris at the time of the observations and the telescope pointing error. Conclusions. The measured relative molecular abundances in C/2004 Q2 (Machholz) are between low- to typical values of those obtained in Oort Cloud comets, suggesting that it has visited the inner solar system previously and undergone thermal processing. The HNC/HCN abundance ratio of 3:1% is comparable to that found in other comets, accounting for the dependence on the heliocentric distance, and could possibly be explained by ion-molecule chemical processes in the low-temperature atmosphere. From a tentative H 13 CN detection, the measured value of 97 30 for the H 12 CN/H 13 CN isotopologue pair is consistent with a telluric value. The outgassing variability observed in the HCN production rates over a period of two hours is consistent with the rotation of the nucleus derived using di erent observational techniques.