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

Abstract

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 2Π3/2 (F2) J = 3/2-2Π1/2 (F2) J = 1/2 fundamental pure rotational lines at 149.09 and 149.39 μm. Another feature of similar strength at 180.7 Mm is well-fitted by the CH doublet 2Π1/2 (F1) J = 5/2-3/2 at 180.48 and 180.93 μm. 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). © 1997 RAS.