2016A&A...588L...8B

Complex organic molecules (COMs) are detected in many regions of the interstellar medium, including prestellar cores. However, their formation mechanisms in cold (∼10K) cores remain to this date poorly understood. The formyl radical HCO is an important candidate precursor for several O-bearing terrestrial COMs in cores, as an abundant building block of many of these molecules. Several chemical routes have been proposed to account for its formation: on grain surfaces, as an incompletely hydrogenated product of H addition to frozen-out CO molecules; and in the gas phase, either as the product of the reaction between H₂CO and a radical or as a product of dissociative recombination of protonated formaldehyde H₂COH⁺. The detection and abundance determination of H₂COH⁺, if present, could provide clues as to whether this latter scenario might apply. We searched for protonated formaldehyde H₂COH⁺ in the prestellar core L1689B using the IRAM 30m telescope. The H_2COH⁺ ion is unambiguously detected, for the first time, in a cold (∼10K) source. The derived abundance agrees with a scenario in which the formation of H₂COH⁺ results from the protonation of formaldehyde. We use this abundance value to constrain the branching ratio of the dissociative recombination of H₂COH⁺ towards the HCO channel to ∼10-30%. This value could however be lower if HCO were efficiently formed from neutral-neutral reactions in the gas phase, and we stress the need for laboratory measurements of the rate constants of these reactions at 10K. Given the experimental difficulties in measuring branching ratios experimentally, observations can place valuable constraints on these values and provide useful input for chemical networks.