Observations of the high-mass star forming region AFGL 2591 reveal a large
abundance of CO+, a molecule known to be enhanced by far UV (FUV) and X-ray
irradiation. In chemical models assuming a spherically symmetric envelope, the
volume of gas irradiated by protostellar FUV radiation is very small due to the
high extinction by dust. The abundance of CO+ is thus underpredicted by orders
of magnitude. In a more realistic model, FUV photons can escape through an
outflow region and irradiate gas at the border to the envelope. Thus, we
introduce the first 2D axi-symmetric chemical model of the envelope of a
high-mass star forming region to explain the CO+ observations as a prototypical
FUV tracer. The model assumes an axi-symmetric power-law density structure with
a cavity due to the outflow. The local FUV flux is calculated by a Monte Carlo
radiative transfer code taking scattering on dust into account. A grid of
precalculated chemical abundances, introduced in the first part of this series
of papers, is used to quickly interpolate chemical abundances. This approach
allows to calculate the temperature structure of the FUV heated outflow walls
self-consistently with the chemistry.
Synthetic maps of the line flux are calculated using a raytracer code.
Single-dish and interferometric observations are simulated and the model
results are compared to published and new JCMT and SMA observations. The
two-dimensional model of AFGL 2591 is able to reproduce the JCMT single-dish
observations and also explains the non-detection by the SMA. We conclude that
the observed CO+ line flux and its narrow width can be interpreted by emission
from the warm and dense outflow walls irradiated by protostellar FUV radiation.Comment: Accepted by ApJ. 17 pages, 11 figures. A version with higher
resolution images is available from
http://www.astro.phys.ethz.ch/staff/simonbr/papgridII.pdf . First paper of
this series of papers available at arXiv:0906.058