Astronomers detected radio recombination lines of Carbon/Oxygen Ion for the first time

Recently, a research team successfully detected radio recombination lines (RRLs) of ions heavier than helium for the first time using the TianMa 65-m Radio Telescope (TMRT), operated by the Shanghai Astronomical Observatory (SHAO). Those lines were assigned to carbon and/or oxygen ions allowing measurements of elemental abundances with unprecedented accuracy, indicating that ion RRLs could be used to determine elemental abundances in places like the inner parts of the Milky Way, where high extinction makes it hard or impossible to do so with visible emission lines. This discovery has been published on 28th February 2023 in Astronomy & Astrophysics as a Letter to the Editor.
Figure 1: The white line shows the RRLs of ions of C and/or O detected by TMRT [1]. The green areas are the modeled emission of ion RRLs. The green dotted lines are the modeled emission, taking into account all RLLs and molecular lines. The background is the image of the heart-shaped Orion nebula (M42), with Orion KL located within the lower-left bright region of M42 (photo by Shawn Nielsen).
Ionized gas is the most widely distributed interstellar gas component, and an important laboratory of measuring elemental abundances. These abundances have generally been measured using optical/infrared lines, with uncertainties being sometimes as high as one order of magnitude. Radio recombination lines (RRLs) can avoid the difficulties of optical line observations (e.g., extinction, limited spectral resolution, an emission model highly dependent on densities and temperatures), since RRLs are usually optically thin and with well understood emission mechanisms. However, so far, the types of detected RRL emitters were nearly all neutral atoms [2]. Only two RRL transitions (121α and 115α) of helium ions in planetary nebulae had been previously reported [3,4]. Line blending makes the RRLs of atoms heavier than helium difficult to spectrally resolve. In contrast, the RRLs of ions are not generally blended with RRLs of neutral atoms, making them much more powerful ways to measure abundances. However, previous observations did not have the combination of wide spectral coverage and high sensitivity needed to detect them. 
Recently, astronomers of SHAO successfully detected RRLs of ions heavier than helium for the first time with the TMRT. This new discovery happened when they searched for interstellar emission lines through an on-going TMRT spectral line survey toward Orion KL. That line survey aims to provide an unprecedently sensitive (mK) spectral template covering 1-50 GHz, in preparation for the forthcoming era of low-frequency radio interferometers (e.g., ALMA band 1, SKA). More than one thousand line features have been extracted from the already completed Ka/Q-band (26-50 GHz) pioneer surveys, including many previously undetected transitions of complex organic molecules (COMs) and RRLs [5].
While identifying the Ka-band (26-35 GHz) spectral lines of Orion KL, astronomers found that there were several broad line features that could not be assigned to any molecular species nor RRLs of atoms. Those line features have weak intensities (~10 mK), but are already significant enough to be distinguished because of the high sensitivity of the spectrum. Because their line widths (~15 km/s) are similar to those of H/He RRLs, astronomers realized that those line features could be RRLs of ions. The principal quantum number is large for RRLs and it is thus valid to treat atoms and ions as hydrogenic emitters with rest frequencies described by the Rydberg formula. To confirm this, astronomers further conducted follow-up Ku-band (12-18 GHz) observations using TMRT to search for signals of ion RRLs at the expected frequencies, and eight more alpha lines (RRLs with Δn=1) of ions were detected. Further, marginal signals of alpha lines in Q band and beta lines (Δn=2) in Ka band were found. Astronomers compared the spectra obtained in different days, and found the frequencies of the line features remain unchanged when corrected for the motion of the Earth, confirming that those ion RRLs origin from the space.
In total, tens of RRLs of interstellar ions were detected by TMRT, and many of them are not blended with any transitions of molecules nor with RRLs of atoms. Those lines detected by TMRT are more than 20 kilometers per second bluer than the expected frequencies of helium ion RRLs, and were thus assigned to ions heavier than helium. The abundance of the doubly ionized elements associated with those ion RRLs were accurately fitted to be 8.8 parts in 10000, and it is consistent with the value of carbon/oxygen estimated from optical/infrared observations.
Xunchuan Liu, a postdoc of SHAO and the corresponding author and first author of the study, led the TMRT spectral line survey and the discovery. “Previously, RRLs were commonly defined as radio spectral lines resulting from transitions of high-n levels of atoms, appearing after the recombination of singly ionized ions and electrons. We detected tens of unblended ion RRLs simultaneously, implying that RRLs of ions deserve more attention.” said Dr. Liu. Since the frequencies of the same RRL transitions of oxygen and carbon are pretty close, it is difficult to distinguish them at this stage through single-dish observations. “Future observations using the state-of-the-art radio interferometers such as ALMA, SKA and even some day ngVLA could break the degeneracy between oxygen and carbon, and detect RRLs of different kinds of ions in nebulas or even high redshift galaxies .” added by Prof. Paul F. Goldsmith, a senior researcher at NASA/JPL. “Such a technique would be very valuable to study the abundances of carbon and oxygen, the most important constituents of carbon monoxide and interstellar complex organic molecules, in the inner Galaxy, where optical observations are very difficult.” Said Prof. Neal J. Evans II, a professor in the University of Texas at Austin.
This work presents clear evidence of an important discovery, as commented by the referee. “The 65-m large radio telescope TMRT, with its high sensitivity and a receiving system covering a full frequency range of 1-50 GHz, will play a more important role in searching for weak lines such as ion RRLs.” said Zhiqiang Shen, the director of SHAO and a co-corresponding author of the study. “This new discovery demonstrates the huge potential of TMRT in astrochemistry studies. The on-going TMRT spectral line survey towards Orion KL and other Galactic objects will reaches an unprecedented line sensitivity (~mK), which will lead to more new discoveries such as RRLs of heavy ions, new transitions of molecular lines and even new molecule species.” concluded by Tie Liu, a researcher in SHAO and co-corresponding author of the study. 
[1] Liu, X., Liu, T., Shen, Z., et al. 2023, A&A, 671, L1
[2] Gordon, M. A. & Sorochenko, R. L. 2002, Radio Recombination Lines
[3] Chaisson, E. J. & Malkan, M. A. 1976, ApJ, 210, 108
[4] Terzian, Y. 1980, Radio Recombination Lines
[5] Liu, X., Liu, T., Shen, Z., et al. 2022, ApJS, 263, 13

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