期刊
PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC
卷 127, 期 949, 页码 299-310出版社
UNIV CHICAGO PRESS
DOI: 10.1086/680342
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资金
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [1140030, 1410190] Funding Source: National Science Foundation
The optically thin critical densities and the effective excitation densities to produce a 1Kkm/s (or 0.818Jykm/s >) spectral line are tabulated for 12 commonly observed dense gas molecular tracers. The dependence of the critical density and effective excitation density on physical assumptions (i.e., gas kinetic temperature and molecular column density) is analyzed. Critical densities for commonly observed dense gas transitions in molecular clouds (i.e., HCN 1-0, HCO+ 1-0, N2H+ 1-0) are typically 1-2 orders of magnitude larger than effective excitation densities because the standard definitions of critical density do not account for radiative trapping and 1Kkm/s lines are typically produced when radiative rates out of the upper energy level of the transition are faster than collisional depopulation. The use of effective excitation density has a distinct advantage over the use of critical density in characterizing the differences in density traced by species such as NH3, HCO+, N2H+, and HCN, as well as their isotopologues; but, the effective excitation density has the disadvantage that it is undefined for transitions when E-u/k >> T-k, for low molecular column densities, and for heavy molecules with complex spectra (i.e., CH3CHO).
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