A photon dominated region code comparison study
Röllig M., Abel N.P., Bell T., Bensch F., Black J., Ferland G.J., Jonkheid B., Kamp I., Kaufman M.J., Le Bourlot J., Le Petit F., Meijerink R., Morata O., Ossenkopf V., Roueff E., Shaw G., Spaans M., Sternberg A., Stutzki J., Thi W.-F., van Dishoeck E.F., van Hoof P.A.M., Viti S., Wolfire M.G.
published in: A&A, 467, 187 (2007)
Aims. We present a comparison between independent computer codes,
modeling the physics and chemistry of interstellar photon dominated regions
(PDRs). Our goal was to understand the mutual differences in the PDR codes and
their effects on the physical and chemical structure of the model clouds, and
to converge the output of different codes to a common solution.
Methods. A number of benchmark models have been created, covering low
and high gas densities n = 103 , 105.5 cm-3
and far ultraviolet intensities χ = 10, 105 in units of the
Draine field (FUV: 6 < hν < 13.6 eV). The benchmark models were computed in
two ways: one set assuming constant temperatures, thus testing the consistency
of the chemical network and photo-processes, and a second set determining the
temperature self consistently by solving the thermal balance, thus testing the
modeling of the heating and cooling mechanisms accounting for the detailed
energy balance throughout the clouds.
Results. We investigated the impact of PDR geometry and agreed on the
comparison of results from spherical and plane-parallel PDR models. We
identified a number of key processes governing the chemical network which have
been treated differently in the various codes such as the effect of PAHs on
the electron density or the temperature dependence of the dissociation of CO
by cosmic ray induced secondary photons, and defined a proper common
treatment. We established a comprehensive set of reference models for ongoing
and future PDR model bench-marking and were able to increase the agreement in
model predictions for all benchmark models significantly. Nevertheless, the
remaining spread in the computed observables such as the atomic fine-structure
line intensities serves as a warning that there is still a considerable
uncertainty when interpreting astronomical data with our models.
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Peter van Hoof
Royal Observatory of Belgium
Ringlaan 3
1180 Brussel
Belgium
email: p DOT vanhoof AT oma DOT be
