2020PASP..132g4201R

We have detected CH⁺ and CH molecular absorption lines from the youngcompact planetary nebula IC 4997 from high resolution optical spectra. A high-resolution infra-red (H and K bands) spectrum provides detection of H₂ emission lines among many other lines. The H₂ lines provide an excitation temperature of 2100 K which may result from UV fluorescence in the envelope or from shocks formed at the interface between an expanding outflow of ionized gas and the neutral envelope ejected when the star was on the AGB. It is suggested that the CH⁺ may result from the endothermic reaction C + H₂ - CH⁺ + H. Intriguingly, CH⁺ and also CH show a higher expansion velocity than H₂ emission suggesting they may be part of the post-shocked gas.