Commonality of the CO- conversion factors in diffuse and dark clouds can be understood from considerations of radiative transfer and CO chemistry. There is unavoidable confusion between CO emission from diffuse and dark gas and misattribution of CO emission from diffuse to dark or giant molecular clouds. The character of the ISM is different from what has been believed if CO and that have been attributed to molecular clouds on the verge of star formation are actually in more tenuous, gravitationally-unbound diffuse gas.
Conversely, it is also the case that molecular gas is detected in the local ISM even when CO emission is not. Lines of sight with , have long been detectable in surveys of uv absorption (Sheffer et al. 2008; Burgh et al. 2007; Sonnentrucker et al. 2007; Sheffer et al. 2007), with expected integrated CO brightnesses as low as (Liszt 2007b). And, as discussed here, mm-wave and CO absorption from clouds with are also more common than CO emission along the same lines of sight (see Liszt & Lucas 2000; Lucas & Liszt 1996, and Appendix A).
Find a constancy of the CO-H2 conversion factor due to radiative transfer and chemistry.
Note that many lines of sight lacking molecular absorption data show CO emission well beyond the galactic extent of the dense gas layer.
The similarity of the CO-H2 conversion factors in diffuse and fully molecular gas must have lead to confusion whereby CO emission arising in diffuse gas has been attributed to "molecular clouds".
The CO sky is mostly an image of the CO chemistry.
Kutner, M. L. 1984, Fundamentals of Cosmic Physics, 9, pp. 233-316 "Probing Molecular Clouds"
Goldsmith, P. F. 1996, in "Millimeter-Wave Astronomy: Molecular Chemistry & Physics in Space", Proceedings of the 1996 INAOE Summer School of Millimeter-Wave Astronomy held at INAOE, Tonantzintla, Puebla, Mexico, 15-31 July 1996. Edited by W. F. Wall, A. Carramiñana, and L. Carrasco. Kluwer Academic Publishers, 1999., p.57 "Probing Molecular Clouds - Their Density and Structure"
NOTE: Cannot explicitly model masing lines, but can handle maser emission (pretty much the same limitation as with LVG). Problem is when tau becomes very negative and the intensity runs-off to infinity.
Excellent comparison of two viable HF modelling procedures
Contains appendix of N2H+ statistical weights, A-values, frequencies, and collisional rates.
Compare simulations of N2H+ hyperfine lines made with approximate and more exact rates and find that satisfactory results are obtained.
Collisional rates between the individual hyperfine levels themselves have been calculated for only three molecules: HCN (Monteiro & Stutzki 1986), NH3 (Chen et al. 1998), and N2H+ (Daniel et al. 2005) and even then for only a limited number of hyperfine levels.
The hyperfine levels of molecules that emit in the millimeter radio spectrum are typically separated by energies in the milli-Kelvin range whereas the separations between rotational levels are several tens to hundreds of Kelvin. Therefore, the hyperfine levels may sometimes be populated approximately in statistical equilibrium even if the rotational levels are not.
We do not need to actually compute or store the populations of the hyperfine levels. The assumption of HSE is equivalent to the assumption that the spectral line profile function of the rotational transition including the hyperfine structure is the sum of the spectra of the individual hyperfine lines with the same relative intensities as in optically thin emission. Because these relative intensities depend only on the dipole matrix elements of the hyperfine radiative transitions, we compute the composite profile function once and then replace the simple line profile function of the rotational transition with the composite profile function everywhere in the calculation.
The proportional approximation represents a better match to the data, yet for some purposes the HSE approximation may be good enough.
Based on this example, the proportional approximation is adequate for N2H+ and could be useful for other molecules with unknown hyperfine collision rates.
Models H2CO emission with abundance jumps of approximately a factor of 100 at point where Tk = 100 K (ice evaporation point).
Studies various ways to reproduce the abundance jump with other physical effects (velocity profile, density profile, ortho-para ratio, evaporation temperature, etc.) and finds no variations in these parameters that can better fit models with abundance jumps.
Rotational (de)-excitation state-to-state and effective rate coefficients for temperatures up to 1500 K.
The 45 lowest energy levels of o-H2O with:
H2 (j2 = 0) and delta(j2) = 0, +2
H2 (j2 = 2) and delta(j2) = 0, -2
The 10 lowest energy levels of o-H2O with:
H2 (j2 = 4) and delta(j2) = 0, -2
H2 (j2 = 2) and delta(j2) = +2
Estimates for effective rate coefficients for j2 = 6 and 8
For the given model the accuracy of the quantum rate coefficients, explicitly given for different temperatures and transitions, is rather homogeneous and lies between 5% and 40% for the first 40 levels of o-H2O.
Strongly recommend using these new rates over previous (Green 1993 or Faure 2007) calculated rates.
Note that the coupled state (CS) approximation does extremely badly even at high energy for j2 not equal to 0. This is an issue for some previously calculated rates.