The regulatory role of Rv in the photochemistry of dark clouds

The penetration of UV radiation inside dark clouds is discussed; and detailed calculations of Rv-dependent photorates are described. Using an accurate method for solving radiative transfer in the presence of coherent anisotropic. non-conservative scattering by dust grains, the role of the parameter Rv as the main regulatory agent of the penetration of radiation inside clouds is evidenced. Local variations in optical properties of dust and their possible link with the evolution of the cloud are also discussed.