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Detection of HCO2+ toward the Low-Mass Protostar IRAS 04368+2557 in L1527

The millimeter-wave rotational emission lines (4(04)-3(03) and 5(05)-4(04)) of protonated carbon dioxide, HCO2+(HOCO+), has 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 HCO2+ except for the Galactic Center clouds. The column density of HCO2+ averaged over the beam size (29") is determined to be 7.6x10^10 cm^-2, assuming the rotational temperature of 12.3 K. The fractional abundance of gaseous CO2 relative to H2 is estimated from the column density of HCO2+ with an aid of a simplified chemical model. If the HCO2+ 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 HCO2+ also 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.