  Complex organic molecules (COMs) are detected in many regions of the
interstellar medium, including prestellar cores. However, their formation
mechanisms in cold (~10 K) 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, both on grain surfaces, as an incompletely hydrogenated product of H
addition to frozen-out CO molecules, or in the gas phase, either 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 H2COH+ ion is
unambiguously detected, for the first time in a cold (~10 K) 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 smaller if HCO
can be efficiently formed from gas-phase neutral-neutral reactions, and we
stress the need for laboratory measurements of the rate constants of these
reactions at 10 K. Given the experimental difficulties in measuring branching
ratios experimentally, observations can bring valuable constraints on these
values, and provide a useful input for chemical networks.
