First Detection of Radio Recombination Lines of Ions Heavier than Helium

TL;DR

This study reports the first detection of radio recombination lines from ions heavier than helium, specifically CII and OII, in Orion KL, expanding the understanding of ion RRLs and their astrophysical implications.

Contribution

It is the first to detect RRLs of ions heavier than helium, specifically CII and OII, using a multi-band survey with the TianMa 65-m Radio Telescope.

Findings

Detected over fifteen unblended RRLs of singly ionized species (XII).

Fitted lines under LTE assumptions, estimating CIII and OIII abundance.

Results align with optical/infrared estimates, suggesting ionization structures.

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 unblended $\alpha$ lines of RRLs of singly ionized species (XII) recombined from XIII. The Ka-band (35--50 GHz) spectrum also shows tentative signals of $\beta$ lines of ions. The detected lines can be successfully crossmatched with the the rest frequencies of RRLs of CII and/or OII. This finding greatly expands the connotation of ion RRLs, since before this work only two blended lines (105$\alpha$ and 121$\alpha$) of HeII had been reported. Our detected lines can be fitted simultaneously under assumption of local thermodynamic equilibrium (LTE). An abundance of CIII and OIII of 8.8$\times$10$^{-4}$ is obtained, avoiding the complexities of optical/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/infrared observations. The effects of dielectronic recombination may contribute to enhancing the level populations even at large $n$. We expect future observations using radio interferometers could break the degeneracy between C and O, and help to reveal the ionization structure and dynamical evolution of various ionized regions.