First evidence of enhanced recombination in astrophysical environments and the implications for plasma diagnostics

TL;DR

This paper presents the first observational evidence of Rydberg Enhanced Recombination (RER) in astrophysical plasmas, demonstrating its significant impact on ionization balance and elemental abundance calculations in low-temperature photoionized environments.

Contribution

It introduces RER as a new recombination mechanism, provides observational evidence, and discusses its implications for plasma diagnostics and astrophysical modeling.

Findings

Detection of RER features in planetary nebulae spectra

Identification of RER-related ultraviolet transition in a symbiotic star

Recombination rate enhancement up to a factor of 7.5 at 3500 K

Abstract

We report the first unambiguous observational evidence of Rydberg Enhanced Recombination (RER), a potentially important recombination mechanism that has hitherto been unexplored in low-temperature photoionized plasmas. RER shares similarities with dielectronic recombination, with the difference that the electron is captured into a highly excited state below the ionization threshold -- rather than above the threshold -- of the recombining ion. We predict transitions of carbon and oxygen ions that are formed via the RER process, and their relative strengths with collisional-radiative spectral models. Optical C II RER features are detected in published high-resolution spectra of eight planetary nebulae, and a C III transition has been found in the ultraviolet spectrum in a symbiotic star system. The relative intensities of these lines are consistent with their production by this recombination mechanism. Because RER has not previously been accounted for in photoionized plasmas, its inclusion in models can significantly impact the predicted ionization balance and hence abundance calculations of important astrophysical species. Calculations for C+ suggest that the enhancement in the total recombination rate can amount to a factor of 2.2 at 8100 K, increasing to 7.5 at T = 3500 K. These results demonstrate the importance of including RER in models of photoionized astrophysical plasmas and in elemental abundance determinations.