Issue |
Astron. Astrophys. Suppl. Ser.
Volume 125, Number 2, October II 1997
|
|
---|---|---|
Page(s) | 303 - 312 | |
DOI | https://doi.org/10.1051/aas:1997372 | |
Published online | 15 October 1997 |
The Cepheus molecular cloud
I. Multi-transition observations in CO and
1
EUROPA/Université Paris 7, Observatoire de Paris, Meudon, France
2
Université de Cergy-Pontoise, Cergy-Pontoise, France
3
CEA/DAPNIA/Service d'Astrophysique, Centre d'Études de Saclay, France
4
Observatoire de Grenoble, St. Martin d'Hères, France
5
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, U.S.A.
Send offprint request to: I.A. Grenier
Received:
15
March
1996
Accepted:
10
January
1997
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. 1994s), 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.
Key words: ISM: Cepheus cloud / ISM: abundances / ISM: clouds / ISM: molecules / radio lines: ISM
© European Southern Observatory (ESO), 1997