EDP Sciences
Free Access
Issue
A&A
Volume 448, Number 2, March III 2006
Page(s) 425 - 432
Section Astrophysical processes
DOI https://doi.org/10.1051/0004-6361:20053837
Published online 24 February 2006
A&A 448, 425-432 (2006)
DOI: 10.1051/0004-6361:20053837

Determining the cosmic ray ionization rate in dynamically evolving clouds

C. J. Lintott and J. M. C. Rawlings

Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
    e-mail: cjl@star.ucl.ac.uk

(Received 15 July 2005 / Accepted 1 November 2005)

Abstract
The ionization fraction is an important factor in determining the chemical and physical evolution of star forming regions. In the dense, dark starless cores of such objects, the ionization rate is dominated by cosmic rays; it is therefore possible to use simple analytic estimators, based on the relative abundances of different molecular tracers, to determine the cosmic ray ionization rate. This paper uses a simple model to investigate the accuracy of two well-known estimators in dynamically evolving molecular clouds. It is found that, although the analytical formulae based on the abundances of ${\rm H_3^+}$, ${\rm H_2}$, CO, O, ${\rm H_2O}$ and  ${\rm HCO^+}$ give a reasonably accurate measure of the cosmic ray ionization rate in static, quiescent clouds, significant discrepancies occur in rapidly evolving (collapsing) clouds. As recent evidence suggests that molecular clouds may consist of complex, dynamically evolving sub-structure, we conclude that simple abundance ratios do not provide reliable estimates of the cosmic ray ionization rate in dynamically active regions.


Key words: astrochemistry -- stars: formation

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© ESO 2006

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