Protonated CO2 in massive star-forming clumps

Abstract

Interstellar CO<SUB>2</SUB> 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<SUP>+</SUP>, is believed to be a good proxy for gaseous CO<SUB>2</SUB>. 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<SUP>+</SUP> 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 H<SUB>2</SUB> of between 0.3 and 3.8 × 10<SUP>-11</SUP>. We compared these values with the abundances of H<SUP>13</SUP>CO<SUP>+</SUP>, a possible gas-phase precursor of HOCO<SUP>+</SUP>, and of CH<SUB>3</SUB>OH, a product of surface chemistry. We found a positive correlation with H<SUP>13</SUP>CO<SUP>+</SUP>, but no correlation with CH<SUB>3</SUB>OH. We suggest that the gas-phase formation route starting from HCO<SUP>+</SUP> plays an important role in the formation of HOCO<SUP>+</SUP>, perhaps more important than protonation of CO<SUB>2</SUB> (upon evaporation of CO<SUB>2</SUB> from icy dust mantles).