2002A&A...391..693L

We observed the 3335MHz (λ 9cm) F=1-1 line of CH toward a sample of diffuse clouds occulting compact extragalactic mm-wave continuum sources, using the old NRAO 43m telescope. Because radiofrequency observations of CH really must be calibrated with reference to a known CH abundance, we begin by deriving the relationships between CH, E_B–V, H₂ and other hydrides found by optical spectroscopy. No simple relationship exists between N(CH) and E_B–V, since N(CH) is strongly bimodal with respect to reddening for E_B–V<0.3mag and the typical range in the N(CH)/E_B–V ratio is an order of magnitude or more at any given E_B–V>0.3mag. However, N(CH)/N(H₂)=4.3±1.9x10^–8 in the mean and N(CH) ∝N(H₂)^1.00±0.06 for 10¹⁹<N(H₂)<10²¹cm^–2. If CH is a good predictor of H₂, 40%-45% of the hydrogen in the local diffuse/translucent ISM is in the molecular form at the accepted mean density, higher than previous estimates found in samples of lower-than-average mean density. Optical observations of the population ratios in the upper and lower halves of the CH lambda-doublet suggest that the brightness of the 3335 MHz CH line should be double-valued at a given CH column density in diffuse gas: double-valuedness is noticeable in our data when comparing CH with CO or HCO⁺. The CH brightness at 3335 MHz is mildly bimodal with respect to CO emission in our diffuse cloud data but much more strongly bimodal when comparing diffuse or translucent gas and dark gas. The CH Λ-doublet is generally inverted in diffuse gas but we did not succeed in measuring the excitation temperature except toward 3C 123 where we confirm one older value T_exc≃-10K.