The first interstellar molecular cation, and for a long time the only one known, was CH~+. The molecule proved to be interesting since the observed large abundances of the methylidyne cation are still unexplained, many decades after its first detection of the 4232.54? line in the visible spectra of nearby stars by Dunham [11], identified then by Douglas & Herzberg [1] in 1941. The radial velocities of CH~+ lines differ in certain cases from those, measured in other interstellar features. It is thus likely that the methylidyne cations occupy different places than, say, neutral methylidyne or neutral atoms, like KI (see the plot below – data from the UVES spectrograph of the Paranal Observatory).
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