Abundance of HOCO+ and CO2 in the outer layers of the L1544 prestellar core

The L1544 prestellar core has been observed as part of the ASAI IRAM Large Program at 3 mm. These observations led to the detection of many complex molecules. In this Letter, we report the detection of two lines, at 85.5 GHz (4(0,4)-3(0,3)) and 106.9 GHz (5(0,5)-4(0,4)), respectively, of the protonated carbon dioxide ion, HOCO+. We also report the tentative detection of the line at 100.4 GHz (5(0,5)-4(0,4)) of DOCO+. The non-LTE analysis of the detected lines shows that the HOCO+ emission originates in the external layer where non-thermal desorption of other species has previously been observed. Its abundance is (5 +/- 2) x 10(-11). Modelling of the chemistry involved in the formation and destruction of HOCO+ provides a gaseous CO2 abundance of 2 x 10(7) (with respect to H-2) with an upper limit of 2 x 10(-6).