Search for CCH—, NCO—, and NCS— Negative Ions in Molecular Clouds

Abstract

Abstract 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 ($\mathrm{CCH}^{-}, \mathrm{NCO}^{-}$, and $\mathrm{NCS}^{-}$) 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 $J = 1 \hbox{--} 0$ transition of $\mathrm{CCH}^{-}$. If this unidentified line is attributable to $\mathrm{CCH}^{-}$, the observed emission intensity would provide an estimation of the column density of $\mathrm{CCH}^{-}$ in L134N to be $1.0 \times 10^{11} \,\mathrm{cm}^{-2}$, which corresponds to a fractional abundance of $5 \times 10^{-12}$ relative to hydrogen. However, our recent observation using the IRAM 30-m telescope did not reproduce the $J = 1 \hbox{--} 0$ signal, nor detect any trace of the $J = 2 \hbox{--} 1$ transition. Thus, the identification of $\mathrm{CCH}^{-}$ in L134N is not yet confirmed. As for other ions, signals of neither $\mathrm{NCO}^{-}$ nor $\mathrm{NCS}^{-}$ 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.