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 mu m in the ISO L ong Wavelength Spectrometer (LWS) grating spectrum of the planetary ne bula NGC 7027. This line coincides in wavelength with both the HeH+ J= 1-0 fundamental pure rotational line at 149.14 mu m and the CH (2) Pi( 3/2) (F-2) J=3/2-(2) Pi(1/2) (F-2) J=1/2 fundamental pure rotational l ines at 149.09 and 149.39 mu m. Another feature of similar strength at 180.7 mu m is well-fitted by the CH doublet (2) Pi(1/2) (F-1) J=5/2-3 /2 at 180.48 and 180.93 mu m. We therefore attribute both the 149.18- and 180.7-mu m 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 similar to 0.06 a nd a CH/CH+ ratio of similar to 0.2, the latter being more than an ord er of magnitude lower than predicted for photodissociation regions. Th e contribution from the HeH+ J=1-0 line to the 149.18-mu m feature is at least a factor of three lower than the predictions of Cecchi-Pestel lini & Dalgarno (1993).