The Cepheus molecular cloud. I. Multi-transition observations in CO and 13CO

Details The Cepheus molecular cloud. I. Multi-transition observations in CO and 13CO The Cepheus molecular cloud. I. Multi-transition observations in CO and 13CO

Oct 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997a%26as..125..303r&link_type=abstract

A & A Supplement series, Vol. 125, October II 1997, 302-312.

Astronomy and Astrophysics

Astronomy

Ism: Cepheus Cloud, Ism: Abundances, Ism: Clouds, Ism: Molecules, Radio Lines: Ism}

Scientific paper

The eastern cloud of the nearby Cepheus complex, located at 111d0) transition at a resolution of 0.8 pc with the CfA 1.2 m telescope. This massive, but fairly diffuse cloud, with visual extinction < 2 mag, has also been sampled in the J=1-->0 and 2-->1 rotational transitions of CO and 13CO 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 5-11 K, with moderate optical depths (tau<3) and CO column-densities up to 4 x 1016 cm^{-2}. In 13CO, the gas is optically thin with slightly lower excitation temperatures of 4-8 K, for 13CO column-densities up to 6 x 1015 cm^{-2}. Under these conditions, the observed ratios of 2-->1 over 1-->0 velocity-integrated intensities, W_{2 --> 1}/W_{1 --> 0}, have been found to be consistent with a uniform value over the cloud of 0.88+/-0.09 in CO and 0.60+/-0.07 in 13CO, as typical of many clouds. The ratios of CO over 13CO velocity-integrated intensities, W(12{CO})/W(13CO), have been found to decrease with 13CO 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 W(12{CO})/W(13CO) fluctuations are therefore indicative of intrinsic variations in the molecular abundances. In particular, very low W(12{CO})/W(13CO) ratios may result from efficient isotopic fractionation in this cold environment. Similar variations in 13CO intensity have been reported at a scale of 0.2 pc in two other dark clouds, HCL 2 (\cite[Cernicharo \& Guelin 1987]{ref5}) and IC 5146 (\cite[Lada et al. 1994]{ref13}), from the behaviour of their W(12{CO})/W(13CO) or W(13{CO})/AV ratios with visual extinction. Together with the present results, they suggest that the 13CO abundance integrated along the line of sight largely varies inside the diffuse envelopes of molecular clouds.

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