| Abstract
| - Context. 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 six atoms. Other exotic compounds, like the weakly-bound dimer (H 2) 2, have also been detected in astronomical sources like Jupiter. Aims. We aim to detect, for the first time, the CO-H 2 van der Waals complex in the ISM, which could be a sensitive indicator for low temperatures if detected. Methods. We used the IRAM 30 m 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 ~10 K). All the brightest CO-H 2 transitions in the 3 mm (80-110 GHz) band were observed with a spectral resolution of 0.5-0.7 km s -1, reaching a rms noise level of ~2 mK. The simultaneous observation of a broad frequency band, 16 GHz, allowed us to conduct a serendipitous spectral line survey. Results. We did not detected any lines belonging to the CO-H 2 complex. We set up a new, more stringent upper limit for its abundance to be [CO-H 2]/[CO] ~5 × 10 -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, for example methyl cyanide (CH 3CN), methanol (CH 3OH), propyne (CH 3CCH), and ketene (CH 2CO), associated with cold gas (excitation temperatures ~7 K), confirming the presence of these complex species not only in warm objects but also in cold regimes.
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