000002169 001__ 2169
000002169 005__ 20160701171700.0
000002169 0247_ $$2DOI$$a10.1088/0004-637X/779/2/122
000002169 037__ $$aASTROimport-326
000002169 100__ $$aChatzikos, M.
000002169 245__ $$aEffects of External Radiation Fields on Line Emission—Application to Star-forming Regions
000002169 260__ $$c2013
000002169 520__ $$aA variety of astronomical environments contain clouds irradiated by a combination of isotropic and beamed radiation fields. For example, molecular clouds may be irradiated by the isotropic cosmic microwave background, as well as by a nearby active galactic nucleus. These radiation fields excite atoms and molecules and produce emission in different ways. We revisit the escape probability theorem and derive a novel expression that accounts for the presence of external radiation fields. We show that when the field is isotropic the escape probability is reduced relative to that in the absence of external radiation. This is in agreement with previous results obtained under ad hoc assumptions or with the two-level system, but can be applied to complex many-level models of atoms or molecules. This treatment is in the development version of the spectral synthesis code CLOUDY. We examine the spectrum of a Spitzer cloud embedded in the local interstellar radiation field and show that about 60% of its emission lines are sensitive to background subtraction. We argue that this geometric approach could provide an additional tool toward understanding the complex radiation fields of starburst galaxies. 
000002169 700__ $$a Ferland, G. J.
000002169 700__ $$a Williams, R. J. R.
000002169 700__ $$a Porter, R.
000002169 700__ $$a van Hoof, P. A. M.
000002169 773__ $$c122$$i2$$pThe Astrophysical Journal$$v779$$y2013
000002169 85642 $$ahttp://esoads.eso.org/abs/2013ApJ...779..122C
000002169 905__ $$apublished in
000002169 980__ $$aREFERD