Discovery of far-infrared pure rotational transitions of CH+ in NGC 7027

We report the discovery of the rotational spectrum of CH+ in the Infrared Space Observatory Long Wavelength Spectrometer (LWS) spectrum of the planetary nebula NGC 7027. The identification relies on a 1996 reanalysis of the LWS spectrum by Liu et al. and on neu LWS data. The strong line at 179.62 mu m (coinciding with the 2(12)-1(01) transition of water vapor) and the lines at 119.90 and 90.03 mu m (reported as unidentified by Liu et al.), whose frequencies are in the harmonic relation 2:3:4, are shown to arise from the J = 2-1, 3-2, and 4-3 rotational transitions of CH+. This identification is strengthened by the new LWS spectra of NGC 7027, which clearly show the next two rotational lines of CH+ at 72.140 and 60.247 mu m. This is the first time that the pure rotational spectrum of CH+ has been observed. This discovery opens the possibility of probing the densest and warmest zones of photodissociation regions. We derive a rotational temperature for the CH+ lines of 150 +/- 20 K and a CH+/CO abundance ratio of 2-6 x 10(-4).