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The Mass Spectrum of Interstellar Clouds

Published online by Cambridge University Press:  12 April 2016

John M. Dickey  and
R. W. Garwood
John M. Dickey
Affiliation:
Sterrewacht Leiden and University of Minnesota
R. W. Garwood
Affiliation:
University of Pittsburgh

Abstract

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The abundance of 21-cm absorption lines seen in surveys at high latitudes can be translated into a line of sight abundance of clouds vs. column density using an empirical relationship between temperature and optical depth. As VLA surveys of 21-cm absorption at low latitudes are now becoming available, it is possible to study the variation of this function with galactic radius. It is interesting to compare the abundance of these diffuse atomic clouds (with temperatures of 50 to 100 K and masses of 1 to 10 M⊙) to the abundance of molecular clouds. To do the latter we must make assumptions about cloud cross-sections in order to convert the line of sight abundance of diffuse clouds into a number per unit volume, and to convert from cloud column density to mass. The spectrum of diffuse clouds matches fairly well the spectrum of molecular clouds, although observationally there is a gap of several orders of magnitude in cloud mass. Optical absorption studies also agree well with the 21-cm results for clouds of column density a few times 1020 M⊙.

Type
VII. The Structure of the Interstellar Medium
Information
International Astronomical Union Colloquium , Volume 120: Structure and Dynamics of the Interstellar Medium , 1989 , pp. 511 - 517
Copyright
Copyright © Springer-Verlag 1989

References

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