May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280e..67m&link_type=abstract
The Molecular Universe, Proceedings of the 280th Symposium of the International Astronomical Union held in Toledo, Spain, May 30
Physics
Scientific paper
I will present an overview of the theory and observations of Photon Dominated Regions (far-ultraviolet; 6 < E < 13.6 eV) and X-ray dominated regions (E > 1 keV). In PDRs and XDRs, the chemical structure and thermal balance are completely determined by the radiation field. Therefore, PDRs and XDRs are direct manifestations of the energy balance of interstellar gas, and their study allows one to determine how the ISM survives the presence of stars and AGN. I will discuss the important processes in XDR and PDRs. UV and X-ray irradiation give rise to different gas cloud thermal/chemical structures, and therefore also show different line emission spectra. X-rays penetrate through a larger column of gas (up to NH ˜ 1024 cm-2) than UV photons (NH ˜ 1021- 1022 cm-2), and the heating efficiency of X-rays (10-50 percent) is much larger than that of UV irradiation (less than 1 percent). X-rays also give rise to significant ionization fractions (up to xe ˜ 10-22 - 10-1), driving an efficient ion-molecule chemistry, while UV photons are more efficient in dissociating molecules. Therefore, for comparable energy inputs, XDRs have much larger column densities of warm molecular gas than PDRs. This is illustrated in Fig. 1, where the CO spectral line energy distribution of a PDR and an XDR with density n = 105 cm-3 and the same impinging flux F = 160 erg s-1 cm-2 (equivalent to G0 = 105, where G0 = 1 is the observed radiation field in the Solar Neighborhood) are compared. I will conclude my talk by showing the application of PDR and XDR physics to observations of the ISM in nearby (U)LIRGs as observed in the Herschel Open Time Key Program HerCULES.
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