April 2007

Methylidyne ion (CH+)

The earliest detections of interstellar and cometary species were made via electronic spectroscopy in the UV-visible region rather than rotational spectroscopy in the microwave and sub-mm regions. The brief report in 1941 of the identification of CH+ by Alex Douglas and Gerhard Herzberg alludes to an informal gathering at Yerkes Observatory at which Pol Swings reported that several lines near 4000 Å could not be matched with known spectra. The first two lines, at 4233 and 3958Å, had been observed by Dunham toward three stars: ξ Persei, χ2 Orionsis, and 55 Cygni. Subsequently, Adams observed both of these lines and a third, fainter one at 3579 Å in ζ Ophiuchi (see p. 23). All of these bands lie in the violet region of visible light. At the Yerkes meeting, Herzberg and Edward Teller noted the similarity of these lines to the isoelectronic species BH and suggested that CH+ could be the source of the lines. New spectra taken by Douglas and Herzberg at the University of Saskatchewan confirmed the identification. The lines are associated with excitations between the ground vibration level of the 1Σ+ ground state of CH+ and the first three vibrational levels the 1Π excited state, the (0,0), (1,0), and (2,0) transitions. Ledoux described parts of this story in an article in Popular Astronomy.

Later, Swings used the same spectra to determine that CH+ was also present in comets. In 1987, Magain & Gillet reported the detection of CH+ in the Large Magellanic Cloud. CH+ has also been detected with rotational spectroscopy by Cernicharo et al.

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