First detection of radio recombination lines of ions heavier than helium

<jats:p>We report the first detection of radio recombination lines (RRLs) of ions heavier than helium. In a highly sensitive multi-band (12–50 GHz) line survey toward Orion KL with the TianMa 65-m Radio Telescope (TMRT), we successfully detected more than fifteen unblended<jats:italic>α</jats:italic>lines of RRLs of singly ionized species (X<jats:sc>II</jats:sc>) recombined from X<jats:sc>III</jats:sc>. The<jats:italic>Ka</jats:italic>-band (26–35 GHz) spectrum also shows tentative signals of<jats:italic>β</jats:italic>lines of ions. The detected lines can be successfully crossmatched with the rest frequencies of RRLs of C <jats:sc>II</jats:sc>and/or O <jats:sc>II</jats:sc>. This finding greatly expands the scope of our understanding of ion RRLs, since prior to this work, only two blended lines (105<jats:italic>α</jats:italic>and 121<jats:italic>α</jats:italic>) of He <jats:sc>II</jats:sc>had been reported. Our detected lines can be fitted simultaneously under assumption of local thermodynamic equilibrium (LTE). An abundance of C <jats:sc>III</jats:sc>and O <jats:sc>III</jats:sc>of 8.8×10<jats:sup>−4</jats:sup>was obtained, avoiding the complexities of optical and infrared observations and the blending of RRLs of atoms. It is consistent with but approaches the upper bound of the value (10<jats:sup>−4</jats:sup>–10<jats:sup>−3</jats:sup>) estimated from optical and infrared observations. The effects of dielectronic recombination may contribute to enhancing the level populations even at large<jats:italic>n</jats:italic>. We expect future observations using radio interferometers could break the degeneracy between C and O, while also helping to reveal the ionization structure and dynamical evolution of various ionized regions.</jats:p>