2002A&A...392.1081R

We study the formation of CO molecules at the edge of dense molecular clouds. As shown by van Dishoeck & Black (1988ApJ...334..771V) the CO photodissociation process is dominated by line rather than continuous absorption. Hence, a turbulent velocity field, modifying the line shape, strongly affects the CO density distribution. We investigate these effects in detail. To describe the turbulent velocity field we use the statistical approach by G. Traving and collaborators (cf. Gail et al., 1974A&A....32...65G) which accounts for a finite correlation length for the velocity field. We solve the radiative transfer equation selfconsistently with the rate equations describing the chemical reactions. One main goal of the investigation is an improvement of molecular cloud models used to analyze observational data. To bring the observational data into agreement with the model of an isothermal spherical cloud being stabilized by turbulent and thermal pressure it turned out to be neccessary to implement a cut off radius for the CO density distribution in order to define a cloud edge (Piehler & Kegel, 1995A&A...297..841P). This radius depends heavily on the intensity and density distribution in the outer parts of the cloud. Our calculations show that turbulence has substantial influence on the penetration of UV radiation into a molecular cloud. Even turbulent velocities in the order of a few thermal velocities are sufficient to allow the radiation to penetrate significantly deeper into the cloud than in a nonturbulent medium. On the other hand correlation length effects may lead to a decrease in photodissociation efficiency. By accounting for a finite correlation length of the stochastic velocity field the self-shielding of CO absorption bands is considerably enhanced and CO molecules can effectively form in depths that have a much stronger UV intensity in standard radiative transfer models.