Charge transfer in C + O⁺ collisions and its impact on supernova spectra

¹ Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden
² The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden

^★ Corresponding author: deboki.reja@physics.uu.se

Received: 17 September 2025
Accepted: 17 October 2025

Abstract

The charge-transfer process C + O⁺ → C⁺ + O (¹D) has been found in previous modelling to be important in neutralising oxygen ions in supernova (SN) ejecta, as well as in determining the strength of the [O I] doublet at 6300 and 6364 Å. This conclusion is, however, based on a highly uncertain simple estimate of the rate coefficient. In this work, calculations of the cross-sections at low energy (0–10 eV) are performed using quantum mechanical methods. The rate coefficients at temperatures up to 10 000 K are determined and found to be significantly lower than the simple estimate. Using spectral modelling of SN ejecta, we show that the new rates change model predictions for [O I] λλ6300, 6364 and [C I] λλ9824, 9850 by ~10% in the early, warm nebular phases, and by yet larger factors in the late, cold phases, with direct impact on inferences of carbon and oxygen masses.