Full SED fitting with the KOSMA-\tau\ PDR code - I. Dust modelling
  We revised the treatment of interstellar dust in the KOSMA-\tau\ PDR model
code to achieve a consistent description of the dust-related physics in the
code. The detailed knowledge of the dust properties is then used to compute the
dust continuum emission together with the line emission of chemical species. We
coupled the KOSMA-\tau\ PDR code with the MCDRT (multi component dust radiative
transfer) code to solve the frequency-dependent radiative transfer equations
and the thermal balance equation in a dusty clump under the assumption of
spherical symmetry, assuming thermal equilibrium in calculating the dust
temperatures, neglecting non-equilibrium effects. We updated the calculation of
the photoelectric heating and extended the parametrization range for the
photoelectric heating toward high densities and UV fields. We revised the
computation of the H2 formation on grain surfaces to include the Eley-Rideal
effect, thus allowing for high-temperature H2 formation. We demonstrate how the
different optical properties, temperatures, and heating and cooling
capabilities of the grains influence the physical and chemical structure of a
model cloud. The most influential modification is the treatment of H2 formation
on grain surfaces that allows for chemisorption. This increases the total H2
formation significantly and the connected H2 formation heating provides a
profound heating contribution in the outer layers of the model clumps. The
contribution of PAH surfaces to the photoelectric heating and H2 formation
provides a boost to the temperature of outer cloud layers, which is clearly
traced by high-J CO lines. Increasing the fraction of small grains in the dust
size distribution results in hotter gas in the outer cloud layers caused by
more efficient heating and cooler cloud centers, which is in turn caused by the
more efficient FUV extinction.
