Iron is one of the most abundant heavy elements in the universe. Its spectral features stand out in many astronomical spectra, especially in those of stars and galaxies. As a dominant emitter in many X-ray sources, iron ions have been the focus of decades of spectral studies, offering valuable insights into cosmic plasmas.

The researchers from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) and their collaborators have measured absolute rate coefficients for dielectronic recombination (DR) of sodium-like iron Ions,  providing a benchmark data set for astrophysical modeling. Their findings were published in The Astrophysical Journal Supplement Series.

The emission and absorption lines of iron ions can reveal key information about celestial objects, including their temperature, density, chemical composition, and dynamics. Interpreting these spectra requires a good understanding of plasma charge-state distributions, which are determined by DR, a predominant electron–ion recombination process in plasmas. The DR rate coefficients for multi-electron ions are among the most urgently needed data for astrophysical applications.

In this study, researchers carried out a precision DR experiment at the cooling storage ring of the Heavy Ion Research Facility in Lanzhou (HIRFL). Using the electron–ion merged-beams technique, they measured DR rate coefficients for sodium-like Fe¹⁵⁺ forming magnesium-like Fe¹⁴⁺.

The DR spectroscopy experiments conducted at heavy-ion storage rings offer high energy resolution with precise, wide-range modulation capabilities for electron-ion relative energies. This enables accurate measurement of critical low-energy collision recombination cross-section data.

Researchers compared the experimental results with theoretical calculations by using three independent state-of-the-art perturbative techniques. The results are in good agreement with the experimental data in the energy range of 0–40 eV.

Furthermore, researchers derived temperature-dependent plasma recombination rate coefficients from the measured DR rate coefficients over the temperature range of 10³-10⁸ K.

The study was in collaboration with the University of Science and Technology of China, Fudan University, the University of Strathclyde, GSI Helmholtzzentrum für Schwerionenforschung, and the University of Giessen. The first author of the paper is HUANG Houke, a PhD student at IMP, and Prof. WEN Weiqiang and Prof. MA Xinwen are the co-corresponding authors.

The research was supported by the National Key Research and Development Project of China, the Strategic Priority Research Program of CAS, the National Natural Science Foundation of China.

Article link: https://iopscience.iop.org/article/10.3847/1538-4365/adcc25

Figure 1. Experimental DR rate coefficients (black filled symbols) for sodium-like Fe¹⁵⁺. (Image from IMP)

Figure 2. The plasma rate coefficients for sodium-like Fe¹⁵⁺. (Image from IMP)