Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26a....94..162c&link_type=abstract
Astronomy and Astrophysics, Vol. 94, P. 162, 1981
Astronomy and Astrophysics
Astrophysics
57
Scientific paper
The samples of 21-cm absorption spectra from (Crovisier et al., 1978) and from Arecibo (Dickey et al., 1978; Crovisier et al., 1980) are analysed in order to derive the spatial distribution and statistical properties of HI interstellar clouds. Biases caused by velocity blending and spurious features in the Nançay sample are estimated.
The distribution of the velocity dispersion inside the clouds peaks around 1 km s-1, but its first moment is ˜1.7 km s-1. The "apparent spin temperature" of the clouds, as deduced from the comparison of 21-cm emission and absorption, is negatively correlated with optical depth, as already known from former studies; several proposed explanations include observational biases and cloud physics. The comparison of the spin temperatures and the velocity dispersions inside the clouds suggests that turbulent and bulk motions inside the clouds are mildly super- sonic, and may contribute to the heating.
The mean number of clouds on a line of sight, with optical depth greater than a given limit, is derived: there is for instance an average of 0.25 cloud with τ > 0.12 on a line of sight reduced to |b| = 90°, which corresponds to one such cloud every 700 pc. Spectra at different latitudes were compared assuming a cosecant |b| law, but large-scale local inhomogeneities (especially a deficiency of cold H I at high latitudes) limit the validity of this plane-parallel model. The Arecibo sample contains a relative excess of weak clouds with respect to the sample, which could be due to a more complete Gaussian decomposition of the Arecibo profiles. The distribution of the cloud column densities is found to be proportional to N-1.3HI, and consistent with a mass spectrum proportional to M-1.8. An analysis of the integrated optical depth of the spectra is made, and compared with the analyses of the stellar color excesses. The parameters of the HI interstellar clouds have wide-spread distributions which prevent any description of these objects in terms of a "standard cloud" endowed with average properties.
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