2018MNRAS.481L..79F

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 cold gas because it 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 to date, so its interstellar chemistry is not well understood. We present new detections of HOCO⁺ lines in 11 high-mass star-forming clumps. Our observations more than treble the number of detections in star-forming regions to date. We derived beam-averaged abundances relative to H₂ of between 0.3 and 3.8 x 10^–11. We compared these values with the abundances of H¹³CO⁺, a possible gas-phase precursor of HOCO⁺, and of CH₃OH, a product of surface chemistry. We found a positive correlation with H¹³CO⁺, but no correlation with CH₃OH. We suggest that the gas-phase formation route starting from HCO⁺ plays an important role in the formation of HOCO⁺, perhaps more important than protonation of CO₂ (upon evaporation of CO₂ from icy dust mantles).