First detection of radio recombination lines of ions heavier than helium

Abstract

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 unblendedαlines of RRLs of singly ionized species (XII) recombined from XIII. TheKa-band (26–35 GHz) spectrum also shows tentative signals ofβlines of ions. The detected lines can be successfully crossmatched with the rest frequencies of RRLs of C IIand/or O II. This finding greatly expands the scope of our understanding of ion RRLs, since prior to this work, only two blended lines (105αand 121α) of He IIhad been reported. Our detected lines can be fitted simultaneously under assumption of local thermodynamic equilibrium (LTE). An abundance of C IIIand O IIIof 8.8×10−4was 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 (10−4–10−3) estimated from optical and infrared observations. The effects of dielectronic recombination may contribute to enhancing the level populations even at largen. 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.