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Diffuse Molecular Cloud Densities from UV Measurements of CO Absorption

Goldsmith, Paul F. (2013) Diffuse Molecular Cloud Densities from UV Measurements of CO Absorption. Astrophysical Journal, 774 (2). Art. No. 134. ISSN 0004-637X. doi:10.1088/0004-637X/774/2/134.

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We use UV measurements of interstellar CO toward nearby stars to calculate the density in the diffuse molecular clouds containing the molecules responsible for the observed absorption. Chemical models and recent calculations of the excitation rate coefficients indicate that the regions in which CO is found have hydrogen predominantly in molecular form and that collisional excitation is by collisions with H_2 molecules. We carry out statistical equilibrium calculations using CO-H_2 collision rates to solve for the H_2 density in the observed sources without including effects of radiative trapping. We have assumed kinetic temperatures of 50 K and 100 K, finding this choice to make relatively little difference to the lowest transition. For the sources having T^(ex)_(10) only for which we could determine upper and lower density limits, we find 〈n(H2) 〉 = 49 cm^(–3). While we can find a consistent density range for a good fraction of the sources having either two or three values of the excitation temperature, there is a suggestion that the higher-J transitions are sampling clouds or regions within diffuse molecular cloud material that have higher densities than the material sampled by the J = 1-0 transition. The assumed kinetic temperature and derived H_2 density are anticorrelated when the J = 2-1 transition data, the J = 3-2 transition data, or both are included. For sources with either two or three values of the excitation temperature, we find average values of the midpoint of the density range that is consistent with all of the observations equal to 68 cm^(–3) for T^k = 100 K and 92 cm^(–3) for T^k = 50 K. The data for this set of sources imply that diffuse molecular clouds are characterized by an average thermal pressure between 4600 and 6800 K cm^(–3).

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Goldsmith, Paul F.0000-0002-6622-8396
Additional Information:© 2013 The American Astronomical Society. Received 2013 January 30; accepted 2013 July 23; published 2013 August 26. We thank Drs. N. Balakrishnan and L. Wiesenfeld for very helpful information about collision rate coefficients and potential energy surfaces. We thank Nicolas Flagey and Jorge Pineda for useful discussions about dealing with the uncertainties in the column densities of CO and the rotational excitation temperatures, and Bill Langer for clarifying a number of points and a careful reading of the manuscript. The anonymous referee also made significant contributions by pointing out particular aspects of UV studies of diffuse clouds that would otherwise have been missed, and by carefully checking of the data presented here. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
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Subject Keywords:ISM: molecules - radio lines: ISM
Issue or Number:2
Record Number:CaltechAUTHORS:20130812-105453627
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Official Citation:Paul F. Goldsmith 2013 ApJ 774 134
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:39874
Deposited By: Ruth Sustaita
Deposited On:12 Aug 2013 18:24
Last Modified:09 Nov 2021 23:47

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