EDP Sciences
Free Access
Volume 391, Number 2, August IV 2002
Page(s) 693 - 704
Section Diffuse matter in space
DOI https://doi.org/10.1051/0004-6361:20020849
Published online 02 August 2002

A&A 391, 693-704 (2002)
DOI: 10.1051/0004-6361:20020849

Comparative chemistry of diffuse clouds. IV: CH

H. Liszt1 and R. Lucas2

1  National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
2  Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, 38406 Saint Martin d'Hères, France

(Received 21 March 2002 / Accepted 6 June 2002 )

We observed the 3335 MHz ( $\lambda$ 9 cm) 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, EB-V, H 2 and other hydrides found by optical spectroscopy. No simple relationship exists between N(CH) and EB-V, since N(CH) is strongly bimodal with respect to reddening for EB-V  < 0.3 mag and the typical range in the N(CH)/ EB-V ratio is an order of magnitude or more at any given EB-V  > 0.3 mag. However, N(CH)/ N(H 2 $) = 4.3 \pm 1.9 \times 10^{-8}$ in the mean and N(CH $) \propto N$(H 2) $^{1.00 \pm 0.06}$ for 1019 < N(H 2 $) < 10^{21}~{\rm cm}^{-2}$. If CH is a good predictor of H 2, 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 $\Lambda$-doublet is generally inverted in diffuse gas but we did not succeed in measuring the excitation temperature except toward 3C123 where we confirm one older value $T_{\rm exc} \approx$ -10 K.

Key words: ISM: molecules

Offprint request: H. S. Liszt, hliszt@nrao.edu

© ESO 2002

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