Search for CCH^-, NCO^-, and NCS^- negative ions in molecular clouds

Despite several suggestions concerning the existence of negatively charged molecules and negative atomic ions in molecular clouds, few attempts have been made to search for negative ions in molecular clouds. In the present study, we extensively searched for three negative molecular ions (CCH^-, NCO^-, and NCS^-) in a dark cloud, L134N, using the Nobeyama 45-m radio telescope. The three negative ions are molecules whose rotational constants have been reported based on laboratory experiments. After a long accumulation, a trace amount of an unidentified emission line was detected at a frequency close to the theoretical prediction of the J = 1-0 transition of CCH^-. If this unidentified line is attributable to CCH^-, the observed emission intensity would provide an estimation of the column density of CCH^- in L134N to be 1.0 × 10¹¹ cm ^-2, which corresponds to a fractional abundance of 5 × 10 ^-12 relative to hydrogen. However, our recent observation using the IRAM 30-m telescope did not reproduce the J = 1-0 signal, nor detect any trace of the J = 2-1 transition. Thus, the identification of CCH^- in L134N is not yet confirmed. As for other ions, signals of neither NCO^- nor NCS^- were detected in L134N. Although we also searched for the three negative ions in a translucent cloud, CB228, and a star-forming region, SgrB2, no signals were detected. The upper limit abundances of the ions in these clouds are discussed.