Search for C2− in Diffuse Clouds

A search has been carried out for the C2 ion in diffuse clouds toward HD 23180, HD 24912, HD 24398, HD 46711, and HD 50064 using the HIDES spectrometer on the Okayama 188-cm telescope. An upper limit of 8.3× 1010–1.8× 1012 cm−2 was obtained for the C2 column density. The upper limit value (3.8× 1011 cm−2) toward HD 23180 is two orders of magnitude smaller than that of the C2 radical. Possible production mechanisms for C2 are discussed.