The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers

By Y. Shirley. 2015

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The optically thin critical densities and the effective excitation densities to produce a 1 K km/s (or 0.818 Jy km/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 1 K km/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 Eu/k≫Tk, for low molecular column densities, and for heavy molecules with complex spectra (i.e., CH3CHO).

Subfields: Astronomy and Astrophysics, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/4df86c2b925f2a34746307c378582db4d390c3ef

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In Astronomy and Astrophysics

1. T. Pillai
2. E. Bergin
3. Jens Kauffmann
4. J. Kruijssen
5. S. Longmore
6. K. Oberg
7. L. Testi
8. J. Bally

In Spectroscopy

1. E. Bergin
2. T. Pillai
3. K. Oberg
4. M. Requena-Torres
5. J. Kruijssen
6. Jens Kauffmann
7. S. Longmore
8. J. Darling

Citations in the corpus

• Astrochemistry and compositions of planetary systems (2020, methodology): “The reader is referred to the following references: Hogerheijde and van der Tak (2000), Brinch and Hogerheijde (2010), van der Tak (2011),Dullemond et al. (2012), and Shirley (2015) 1.3.2.”
• Dense gas in the Galactic central molecular zone is warm and heated by turbulence (2015, methodology): “…measurements in the CMZ primarily used the popular ammonia metastable inversion transition thermometer (NH3 (1,1) - (7, 7); Güsten et al. 1981; Mauersberger et al. 1986; Hüttemeister et al. 1993; Ott et al. 2014), which is sensitive to moderate density gas (n(H2) ∼ 103 − 104 cm−3; Shirley 2015).”