A&A Supplement Series, Vol. 125, October II 1997, 303-312
Received March 15, 1996; accepted January 10, 1997
G.P. Remy
, I.A. Grenier
,
,
G. Duvert
,
- P. Thaddeus
Send offprint request: I.A. Grenier
EUROPA/Université Paris 7,
Observatoire de Paris, Meudon, France
Université de Cergy-Pontoise,
Cergy-Pontoise, France - CEA/DAPNIA/Service d'Astrophysique, Centre
d'Études de Saclay, France
Observatoire de Grenoble, St. Martin
d'Hères, France
Harvard-Smithsonian Center for Astrophysics,
Cambridge, MA, U.S.A.
The eastern cloud of the nearby Cepheus complex, located at
and 
, has been mapped
in the CO(
) transition at a resolution of 0.8 pc with the
CfA 1.2 m telescope. This massive, but fairly diffuse cloud, with visual
extinction 
, has also been sampled in the
and 2
1 rotational transitions of CO and 
at the
same resolution, using the CfA and POM-2 millimeter telescopes.
The radiative transfer of the lines has been treated in the LTE and LVG
approximations, the two yielding comparable results. In CO, low excitation
temperatures have been found in the range of 
, with moderate
optical depths (
) and CO column-densities up to 
. In , the gas is optically thin with slightly
lower excitation temperatures of 
, for 
column-densities up to 
. Under these
conditions, the observed ratios of 
over 
velocity-integrated intensities, 
, have been found to be consistent with a uniform value over
the cloud of 
in CO and 
in ,
as typical of many clouds.
The ratios of CO over velocity-integrated intensities,
), have been found to decrease with
intensity as expected from the progressive saturation of
the CO lines. The large scatter about this relation, observed at scales of
0.8 and 0.2 pc, cannot be accounted for by instrumental error, beam
dilution, or the dispersion of excitation temperatures and line widths
measured in the cloud. The 
fluctuations are therefore indicative of intrinsic variations in the
molecular abundances. In particular, very low ratios may result from efficient isotopic
fractionation in this cold environment. Similar variations in intensity have been reported at a scale of 0.2 pc in two other dark
clouds, HCL 2 (Cernicharo & Guélin 1987) and IC 5146 (Lada
et al. 1994), from the behaviour of their or 
ratios with visual
extinction. Together with the present results, they suggest that the abundance integrated along the line of sight largely varies
inside the diffuse envelopes of molecular clouds.
keywords: ISM: Cepheus cloud -- ISM: abundances -- ISM: clouds -- ISM: molecules -- radio lines: ISM