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A&A Supplement Series, Vol. 125, October II 1997, 303-312

Received March 15, 1996; accepted January 10, 1997

The Cepheus molecular cloud

I. Multi-transition observations in CO and tex2html_wrap_inline1715

G.P. Remytex2html_wrap1787, I.A. Greniertex2html_wrap1789,gif, G. Duverttex2html_wrap1791, - P. Thaddeustex2html_wrap1793

Send offprint request: I.A. Grenier
tex2html_wrap1795 EUROPA/Université Paris 7, Observatoire de Paris, Meudon, France
tex2html_wrap1795  Université de Cergy-Pontoise, Cergy-Pontoise, France - CEA/DAPNIA/Service d'Astrophysique, Centre d'Études de Saclay, France
tex2html_wrap1797  Observatoire de Grenoble, St. Martin d'Hères, France
tex2html_wrap1799  Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, U.S.A.


The eastern cloud of the nearby Cepheus complex, located at tex2html_wrap_inline1719 and tex2html_wrap_inline1721, has been mapped in the CO(tex2html_wrap_inline1723) transition at a resolution of 0.8 pc with the CfA 1.2 m telescope. This massive, but fairly diffuse cloud, with visual extinction tex2html_wrap_inline1725, has also been sampled in the tex2html_wrap_inline1723 and 2tex2html_wrap_inline17291 rotational transitions of CO and tex2html_wrap_inline1731 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 tex2html_wrap_inline1733, with moderate optical depths (tex2html_wrap_inline1735) and CO column-densities up to tex2html_wrap_inline1737. In tex2html_wrap_inline1731, the gas is optically thin with slightly lower excitation temperatures of tex2html_wrap_inline1741, for tex2html_wrap_inline1743 column-densities up to tex2html_wrap_inline1745. Under these conditions, the observed ratios of tex2html_wrap_inline1747 over tex2html_wrap_inline1749 velocity-integrated intensities, tex2html_wrap_inline1751, have been found to be consistent with a uniform value over the cloud of tex2html_wrap_inline1753 in CO and tex2html_wrap_inline1755 in tex2html_wrap_inline1743, as typical of many clouds.

The ratios of CO over tex2html_wrap_inline1743 velocity-integrated intensities, tex2html_wrap_inline1761), have been found to decrease with tex2html_wrap_inline1743 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 tex2html_wrap_inline1765 fluctuations are therefore indicative of intrinsic variations in the molecular abundances. In particular, very low tex2html_wrap_inline1765 ratios may result from efficient isotopic fractionation in this cold environment. Similar variations in tex2html_wrap_inline1743 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 tex2html_wrap_inline1765 or tex2html_wrap_inline1773 ratios with visual extinction. Together with the present results, they suggest that the tex2html_wrap_inline1743 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

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