A Search for the CO–H2Dimer in the Galaxy

We have used the 30 m IRAM millimeter radio telescope to search for two low-lying rotational transitions of the CO-H 2 dimer in the emission spectra of three Galactic sources and in the absorption spectrum of a low-latitude extragalactic continuum source. We first summarize the structure of this dimer as it can be inferred currently from laboratory infrared spectroscopy. The transitions we have observed and their initial estimated frequencies are the 111-↔000+ line at ~109 GHz and the 110+↔101- line at ~91 GHz. The search targets were the TMC-1 and L134 dark clouds, the L1157 bipolar outflow, and the compact radio continuum source 2013+370. The spectral search covered the frequency ranges from 90.58 to 91.10 GHz and from 108.95 to 109.46 GHz. Several known emission lines were found, including HC 3 N (12-11), HC 3 N (10-9), OCS, SO, and HNC, but no other features were seen in the spectra. The rms noise was typically 5 mK per 1 MHz channel (≈ 3 km s -1 ). Improved laboratory values for the frequencies were obtained after the 30 m observations were made and are also given here for future reference. The frequency of the 111-↔000+ line may now fall outside of our present search range, but the 110+↔101- frequency is likely to be within it. We conclude that the fractional abundance of the CO-H 2 dimer with respect to the CO monomer is typically ≤1 × 10 -4 , if the excitation temperature is at least a few degrees above the cosmic background. This is the most stringent upper limit yet placed on the presence of a dimer in the interstellar medium.