The CO-H2 van der Waals complex and complex organic molecules in cold molecular clouds: a TMC-1C survey

TL;DR

This study conducted a sensitive search for the CO-H2 van der Waals complex in a cold molecular cloud using the IRAM30m telescope, setting new upper limits on its abundance and detecting various complex organic molecules in cold gas.

Contribution

First observational attempt to detect CO-H2 complex in the ISM, establishing more stringent upper limits and confirming presence of complex molecules in cold environments.

Findings

No detection of CO-H2 complex, setting an upper limit of 5x10^{-6} for its abundance.

Detected 75 lines from 41 species, including complex organic molecules like CH3CN, CH3OH, CH3CCH, and CH2CO.

Confirmed complex molecules exist in cold, dense interstellar regions at about 7 K.

Abstract

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 (H2)2, have also been detected in astronomical sources like Jupiter. We aim at detecting for the first time the CO-H2 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-H2 complex in a cold, dense core in TMC-1C (with a temperature of 10 K). All the brightest CO-H2 transitions in the 3 mm (80-110 GHz) band have been observed with a spectral resolution of 0.5-0.7 km/s, reaching a rms noise level of 2 mK. The simultaneous observation of a broad frequency band, 16 GHz, has allowed us to conduct a serendipitous spectral line survey. No lines belonging to the CO-H2 complex have been detected. We have set up a new, more stringent upper limit for its abundance to be [CO-H2]/[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 (CH3CN), methanol (CH3OH), propyne (CH3CCH) and ketene (CH2CO), associated with cold gas (excitation temperatures about 7 K), confirming the presence of these complex species not only in warm objects but also in cold regimes.