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Title: C{sup +} in the interstellar medium: Collisional excitation by H{sub 2} revisited

C{sup +} is a critical constituent of many regions of the interstellar medium, as it can be a major reservoir of carbon and, under a wide range of conditions, the dominant gas coolant. Emission from its 158 μm fine structure line is used to trace the structure of photon-dominated regions in the Milky Way and is often employed as a measure of the star formation rate in external galaxies. Under most conditions, the emission from the single [C II] line is proportional to the collisional excitation rate coefficient. We here used improved calculations of the deexcitation rate of [C II] by collisions with H{sub 2} to calculate more accurate expressions for interstellar C{sup +} fine structure emission, its critical density, and its cooling rate. The collision rates in the new quantum calculation are ∼25% larger than those previously available, and narrow the difference between rates for excitation by atomic and molecular hydrogen. This results in [C II] excitation being quasi-independent of the molecular fraction and thus dependent only on the total hydrogen particle density. A convenient expression for the cooling rate at temperatures between 20 K and 400 K, assuming an LTE H{sub 2} ortho to para ratio is Λ=(11.5+4.0more » e{sup −100} {sup K/T{sup k{sup i{sup n}}}}) e{sup −91.25} {sup K/T{sup k{sup i{sup n}}}} n(C{sup +}) n(H{sub 2})×10{sup −24} erg cm{sup −3} s{sup −1}. The present work should allow more accurate and convenient analysis of the [C II] line emission and its cooling.« less
Authors:
 [1] ;  [2]
  1. UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), UMR 5274, Grenoble (France)
  2. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91125 (United States)
Publication Date:
OSTI Identifier:
22348213
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 780; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CARBON; CARBON IONS; COLLISIONS; DE-EXCITATION; DENSITY; EMISSION; EXCITATION; FINE STRUCTURE; HYDROGEN; LTE; MILKY WAY; PHOTONS; STARS