Issue |
A&A
Volume 391, Number 2, August IV 2002
|
|
---|---|---|
Page(s) | 693 - 704 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20020849 | |
Published online | 02 August 2002 |
Comparative chemistry of diffuse clouds. IV: CH
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
Corresponding author: H. S. Liszt, hliszt@nrao.edu
Received:
21
March
2002
Accepted:
6
June
2002
We observed the 3335 MHz (λ 9 cm) –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, , and other hydrides found by
optical spectroscopy. No simple relationship exists between
N(CH) and , since N(CH) is strongly bimodal with respect to
reddening for < 0.3 mag and the typical range in the N(CH)/ ratio
is an order of magnitude or more at any given > 0.3 mag. However,
N(CH)/N(
in the mean and
N(CH
()
for
(
. If CH is a good predictor of ,
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 3C123 where we confirm one older
value
-10 K.
Key words: ISM: molecules
© ESO, 2002
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