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Astron. Astrophys. 362, 1109-1121 (2000)

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6. Summary

Investigating the global properties of the sequential star forming region Cepheus B, we mapped the molecular cloud in 6 lower-J transitions of CO and its isotopomers. Multi line analysis with two radiative transfer codes were conducted and revealed the following results:

  • We find a temperature gradient perpendicular to the line of sight from around 70 K near the HII region down to 20 K at the south-eastern end of the mapped region.

  • At the column density peak position - at 1.7 pc distance to the hot core - the data are consistent with a decreasing temperature gradient along the line of sight.

  • These temperature gradients imply that the whole cloud is still in a relatively calm state. Star formation takes place just around the hot core, maybe triggering further star formation deeper inside Cepheus B in the future.

  • The exciting FUV flux is supplied mostly by an O7 and a B1 star in the west with little contributions from other members of the OB association in the north-east. The flux varies between [FORMULA] near the HII region and around a few at the south-eastern edge of the mapped region (in units of the average interstellar radiation field).

  • Local H2 densities are about [FORMULA] cm-3 while the average H2 density is less than [FORMULA] cm-3, thus, volume filling factors range between 2% and 4%, and the average clump diameter is estimated to be 0.02 pc.

  • CI observations indicate lower limits for the abundance ratio of atomic to molecular carbon between 0.15 and 0.3.

  • 12CO/13CO ratios are well correlated with the corresponding integrated 13CO intensities, most likely this is due to optical depths effects.

  • 13CO/C18O ratios rise at the cloud edges. Recent investigtions by Zielinsky et al. (2000) reveal that a clumpy cloud PDR model is capable to naturally explain the observed line ratios.

  • The homogenous Escape Probability model is only partly able to reproduce the observed data set, while the more sophisticated PDR model, with only the average density and the clump radius as free parameters, of Störzer et al. (2000) reproduces most observed line ratios. An exception is the high 12CO ratio at the hot core position. Probably, models which include inner heating sources are capable to explain such ratios in the future.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000