J1717-3342 and J1744-3116 CO spectra

Previously detected at the outer edges of photodissociation regions (PDRs) and formed from the reaction of C^+^ and OH, CO^+^ is the main chemical precursor of HCO^+^ and CO in a thermal, cosmic-ray, and UV-driven chemistry. Our aim was to test whether the thermal cosmic-ray and UV-driven chemistry is producing CO in diffuse interstellar molecular gas through the intermediate formation of CO^+^ We searched for CO^+^ absorption with the Atacama Large Millimeter Array (ALMA) toward two quasars with known Galactic foreground absorption from diffuse interstellar gas, J1717-3342 and J1744-3116, targeting the two strongest hyperfine components of the J=2-1 transition near 236GHz.

We could not detect CO^+^ but obtained sensitive upper limits toward both targets. The derived upper limits on the CO^+^ column densities represent about 4% of the HCO^+^ column densities. The corresponding upper limit on the CO^+^ abundance relative to H_2_ is <1.2x10^-10^.