This assumption of similar dust and gas temperatures is only valid if the densities are higher than $\sim$ 10⁵ ${{\rm cm}}^{-3}$ (e.g. Goldsmith 2001). However, in the range of temperatures typical of low-mass dense cores, this approximation does not significantly change the results of our model.

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At a density of 2 $\times$ 10⁶ cm^-3, temperature $T_{\rm gas}$ = 8 K, grain size 0.1 $\mu$ m, the o/p- ${\rm H}_{2}{\rm D}^{+}$ is about 1.2 (Pineau des Forêts, priv. comm.)

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... RADEX

The RADEX code is available on-line at the URL http://www.sron.rug.nl/vdtak/radex/radex.php.

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