Penetration of FUV radiation into clouds and disks: the spherical harmonics method revisited

Details Penetration of FUV radiation into clouds and disks: the spherical harmonics method revisited Penetration of FUV radiation into clouds and disks: the spherical harmonics method revisited

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007msl..confe.105g&link_type=abstract

"Molecules in Space and Laboratory, meeting held in Paris, France, May 14-18, 2007. Editors: J.L. Lemaire, F. Combes. ISBN: 9782

Astronomy and Astrophysics

Astrophysics

Modelling_Environments

Scientific paper

Far-UV photons (FUV; h.nu < 13.6 eV) strongly affect the physical and chemical state of molecular clouds, protoplanetary disks and entire galaxies (e.g., Goicoechea et al. 2005; 2006). The solution of the FUV radiative transfer equation can be complicated if dust scattering and gas line absorption are included, and if optical properties are depth dependent. We have extended the spherical harmonics method (Flannery et al. 1980; Roberge 1983) to solve for the FUV radiation field in externally illuminated clouds taking into account gas absorption and coherent, nonconservative and anisotropic scattering by dust grains. The new formulation has been introduced in the Meudon PDR code (Le Petit et al. 2006; Goicoechea & Le Bourlot 2007). Our formalism allows us to consistently include: (i) varying dust populations and (ii) gas lines in the FUV radiative transfer. As a result, the FUV penetration depth rises for increasing dust albedo and anisotropy of the scattered radiation (e.g. when grains grow towards cloud interiors). For dense and embedded sources (Av >1) we show that the FUV radiation field inside molecular clouds can differ by orders of magnitude depending on the assumed grain properties and growth. Our models reveal significant different physical and chemical structures for steep vs. flat extinction curves towards molecular clouds. In particular, we show that the photochemical and thermal gradients can be very different depending on grain growth. Therefore, the assumption of uniform dust properties and averaged extinction curves can be a crude approximation to determine the resulting scattering properties, prevailing chemistry and atomic/molecular abundances in ISM clouds and protoplanetary disks.

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