2008ApJ...675L..89S

The millimeter-wave rotational emission lines (4₀₄-3₀₃ and 5₀₅-4₀₄) of protonated carbon dioxide, HCO⁺₂(HOCO⁺), have been detected toward the low-mass Class 0 protostar IRAS 04368+2557 in L1527 with the IRAM 30 m telescope. This is the first detection of HCO⁺₂ except for the Galactic center clouds. The column density of HCO⁺₂ averaged over the beam size (29") is determined to be 7.6x10¹⁰/cm2, assuming a rotational temperature of 12.3 K. The fractional abundance of gaseous CO₂ relative to H₂is estimated from the column density of HCO⁺₂ with the aid of a simplified chemical model. If the HCO⁺₂ emission only comes from the evaporation region of CO₂ near the protostar (T≳50 K), the fractional abundance of CO₂ is estimated to be higher than 6.6x10^–4. This is comparable to the elemental abundance of carbon in interstellar clouds, and hence, the direct evaporation of CO₂ from dust grain is unrealistic as a source of gaseous CO₂ in L1527. A narrow line width of HCO⁺₂also supports this. On the other hand, the fractional abundance of CO₂ is estimated to be 2.9x10^–7, if the source size is comparable to the beam size. These results indicate that gaseous CO₂ is abundant even in the low-mass star-forming region. Possible production mechanisms of gaseous CO₂ are discussed.