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Description
Almost 200 different species have been detected in the interstellar medium (ISM) during the last decades, revealing not only simple species but complex molecules with more than 6 atoms. Other exotic compounds, like the weakly-bound dimer (H_2_)_2_, have also been detected in astronomical sources like Jupiter. We aim at detecting for the first time the CO-H_2_ van der Waals complex in the ISM, which if detected can be a sensitive indicator for low temperatures. We use the IRAM30m telescope, located in Pico Veleta (Spain), to search for the CO-H_2_ complex in a cold, dense core in TMC-1C (with a temperature of 10K). All the brightest CO-H_2_ transitions in the 3mm (80-110GHz) band have been observed with a spectral resolution of 0.5-0.7km/s, reaching a rms noise level of 2mK. The simultaneous observation of a broad frequency band, 16GHz, has allowed us to conduct a serendipitous spectral line survey. No lines belonging to the CO-H_2_ complex have been detected. We have set up a new, more stringent upper limit for its abundance to be [CO-H_2_]/[CO]=5x10^-6^, while we expect the abundance of the complex to be in the range 10^-8^-10^-3^. The spectral line survey has allowed us to detect 75 lines associated with 41 different species (including isotopologues). We detect a number of complex organic species, e.g. methyl cyanide (CH_3_CN), methanol (CH_3_OH), propyne (CH_3_CCH) and ketene (CH_2_CO), associated with cold gas (excitation temperatures about 7K), confirming the presence of these complex species not only in warm objects but also in cold regimes.
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