Results of model calculations. Panel (a) shows the most plausible profiles in accordance with observations, where ﹩\alpha =1.5﹩, ﹩n_{0}=7\times 10^{3}\ \mathrm{cm}\,^{-3}﹩, ﹩r_{0}=0.4R﹩, and ﹩\beta =1﹩. Profiles in the right panels illustrate how they respond to input parameters, where the parameters are kept the same as in (a), except where otherwise specified. Only half of the profiles are presented to save space. (b) ﹩\alpha =2﹩ (solid lines) and ﹩\alpha =1﹩ (dotted lines) (c) ﹩n_{0}=1\times 10^{4}\ \mathrm{cm}\,^{-3}﹩ (solid lines) and ﹩n_{0}=5\times 10^{3}\ \mathrm{cm}\,^{-3}﹩ (dotted lines). (d) ﹩r_{0}=0.6R﹩ (solid lines) and ﹩r_{0}=0.2R﹩ (dotted lines). (e) ﹩\beta =0.5﹩ (solid lines) and ﹩\beta =1.5﹩ (dotted lines). ¹²CO and ¹³CO lines in (e) are indistinguishable from those in (a). In (e), ¹²CO lines from a conventional model with a smooth density distribution of ﹩n_{\mathrm{H}\,_{2}}( r) =n_{0}( r/ 0.4R) ^{-\alpha }﹩ are also presented for ﹩\alpha =1﹩ (upper one) and 1.5 (lower one) as dashed lines. See the text for details.