Protonated CO2 in massive star-forming clumps

TL;DR

This study reports new detections of HOCO+ in 11 high-mass star-forming regions, providing insights into its formation pathways and increasing the known interstellar occurrences of this molecule.

Contribution

It presents the first extensive detection of HOCO+ in multiple high-mass star-forming clumps, expanding understanding of its chemistry and formation mechanisms.

Findings

Detected HOCO+ in 11 star-forming regions, tripling previous detections.

Found positive correlation between HOCO+ and H13CO+ abundances.

Suggested gas-phase formation route from HCO+ is significant for HOCO+ formation.

Abstract

Interstellar CO2 is an important reservoir of carbon and oxygen, and one of the major constituents of the icy mantles of dust grains, but it is not observable directly in the cold gas because has no permanent dipole moment. Its protonated form, HOCO+, is believed to be a good proxy for gaseous CO2. However, it has been detected in only a few star-forming regions so far, so that its interstellar chemistry is not well understood. We present new detections of HOCO+ lines in 11 high-mass star-forming clumps. Our observations increase by more than three times the number of detections in star-forming regions so far. We have derived beam-averaged abundances relative to H2 in between 0.3 and 3.8 x 10^{-11}. We have compared these values with the abundances of H13CO+, a possible gas-phase precursor of HOCO+, and CH3OH, a product of surface chemistry. We have found a positive correlation with H13CO+, while with CH3OH there is no correlation. We suggest that the gas-phase formation route starting from HCO+ plays an important role in the formation of HOCO+, perhaps more relevant than protonation of CO2 (upon evaporation of this latter from icy dust mantles).