An optical measurement of the population inversion of the ground state Lambda doublet of interstellar CH

Abstract

We have performed very high signal-to-noise ratio optical observations of interstellar CH toward three stars and have measured an inversion temperature of -1.30 ± 0.4 K for the ground-state Λ doublet levels leading to the 9 cm radio emission of CH toward ζ Per. This result is consistent with the inversion indicated by the radio measurements in this direction.