Protonated HCN in molecular clouds.

Observations of protonated HCN, HCNH(+), in a selection of galactic molecular clouds are reported. This species plays a key role in understanding the chemistry of the important high density tracer HCN. HCNH(+) has been detected in the nearby cold dust cloud TMC-1 with a ratio relative to HCN of HCNH(+)/HCN between 0.015 and 0.26 (preferred value 0.03) and tentatively in DR21 (OH) with a ratio of approximately 0.01. This is about 100 times higher than the ratio of protonated carbon monoxide to CO, but comparable to the HCS(+)/CS ratio. Possible explanations of these high abundance ratios are discussed in the light of model calculations.