Detection of protonated formaldehyde in the prestellar core L1689B

TL;DR

This study reports the first detection of protonated formaldehyde H2COH+ in a cold prestellar core, providing insights into its formation mechanisms and constraining the chemical pathways leading to complex organic molecules in space.

Contribution

The paper presents the first unambiguous detection of H2COH+ in a cold core and uses this to constrain the dissociative recombination branching ratio, advancing understanding of interstellar chemistry.

Findings

H2COH+ detected in L1689B at ~10 K

Abundance supports formation via protonation of formaldehyde

Branching ratio of H2COH+ recombination estimated at 10-30%

Abstract

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.