Exclusion plot for cloud of radius ﹩R_{\mathrm{cl}\,}﹩ and mass ﹩M_{\mathrm{cl}\,}﹩, showing region of allowed (unshaded regions) and prohibited (shaded regions) parameters for clouds with polytropic index ﹩n=1.5﹩ (left) and ﹩n=4.5﹩ (right). Dotted lines represent the limitation set by the requirement that the atmosphere be gravitationally bound. Dashed line shows mass restriction by Wardle & Walker (1999). The region above the dot‐dashed lines contains models with central temperatures larger than 50 and 100 K, respectively, which might exclude them (hatching). Note that another allowed region appears for small cloud radii in the ﹩n=4.5﹩ case, bounded by the dashed line roughly corresponding to the event rate ﹩3\times 10^{-8}\,\mathrm{events}\,\,\mathrm{yr}\,^{-1}﹩ and line ﹩M=10^{-1.7}\,M_{\odot }﹩. As discussed in the text, the ﹩n=1.5﹩ polytropic model is the most conservative assumption in the absence of detailed thermal models for these clouds. The star on the ﹩n=1.5﹩ plot shows the position of the cloud model with ﹩R_{\mathrm{cl}\,}=10﹩ AU and ﹩M_{\mathrm{cl}\,}=10^{-3}\,M_{\odot }﹩.