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

The charge-transfer process C + O + → C + + O ( 1 D) 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 ] λλ 6300, 6364 and [C I ] λλ 9824, 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.

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