Carbon monoxide line emission from photon dominated regions.
Abstract
We present a theoretical study of ^12^CO and ^13^CO rotational line emission from photon dominated regions (PDRs). We incorporate the effects of clumpy cloud structure by computing the physical structures of plane-parallel PDRs with finite thickness which are illuminated by UV-radiation fields from either one or both sides. We examine the influence of the gas density (n_0_(H) = 10^4^cm^-3^ to 10^7^cm^-3^), the UV intensity (χ = 10^3^ to 10^6^ times the intensity of the average interstellar UV field), the cloud thickness (measured in units of the visual extinction A_V_, 2 <= A_V_ <= 10) and the doppler width (1 km/s and 3 km/s) on the emergent CO line center brightness temperatures. We explicitly include the effects of the ^13^C chemistry on the line intensities. The high brightness temperatures of the ^13^CO J=6->5 line observed in several sources can be explained as originating in high density PDRs (n(H) >= 10^6^cm^-3^) which are illuminated from two sides and under the assumption that several PDR clumps lie along the line of sight. To model the observed low-J ^12^CO and ^13^CO line ratios the models require densities of close to 10^5^cm^-3^ or higher. Due to chemical fractionation the isotopic line intensity ratios for ^12^CI/^13^CI can be a factor 2 to 3 lower than the intrinsic isotopic ^12^C/^13^C ratio. The high-J ^12^CO brightness temperatures that we find are in general agreement with earlier PDR models.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- April 1994
- Bibcode:
- 1994A&A...284..545K
- Keywords:
-
- Abundance;
- Carbon Isotopes;
- Carbon Monoxide;
- Emission Spectra;
- Interstellar Matter;
- Line Spectra;
- Photons;
- Rotational Spectra;
- Brightness Distribution;
- Molecular Clouds;
- Radiative Transfer;
- Ultraviolet Radiation;
- Astrophysics;
- LINE: FORMATION;
- RADIATIVE TRANSFER;
- ISM: CLOUDS;
- ISM: MOLECULES;
- RADIO LINES: ISM