An ISO Long Wavelength Spectrometer detection of CH in NGC 7027 and an HeH^+ upper limit

We have detected an emission line at 149.18+/-0.06 μm in the ISO Long Wavelength Spectrometer (LWS) grating spectrum of the planetary nebula NGC 7027. This line coincides in wavelength with both the HeH^+ J=1-0 fundamental pure rotational line at 149.14 μm and the CH ^2Pi_3/2 (F_2) J=3/2-^2Pi_1/2 (F_2) J=1/2 fundamental pure rotational lines at 149.09 and 149.39 mum. Another feature of similar strength at 180.7 μm is well-fitted by the CH doublet ^2Pi_1/2 (F_1) J=5/2-3/2 at 180.48 and 180.93 mum. We therefore attribute both the 149.18- and 180.7-μm lines to CH, the first evidence of this molecule in NGC 7027 and the first detection anywhere of the far-infrared lines of CH in emission. We estimate a CH/CO abundance ratio of ~0.06 and a CH/CH^+ ratio of ~0.2, the latter being more than an order of magnitude lower than predicted for photodissociation regions. The contribution from the HeH^+ J=1-0 line to the 149.18-μm feature is at least a factor of three lower than the predictions of Cecchi-Pestellini & Dalgarno (1993).