Figures 1, 2 and Table 1 are only available in electronic form at http://www.aanda.org

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IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.

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... $\beta _{\rm spec}=1$

$\beta_{\rm spec}=\sigma_{v}^2/(kT/\mu m_{\rm p})$ with $\mu=0.58$ the mean molecular weight and $m_{\rm p}$ the proton mass.

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