Example of the variation of |$\vert \tau _{4.8}\vert$| (upper panel) and |$\vert \tau _{14.5}\vert$| (bottom panel) on the |$n_{\mathrm{H_2}}$|–|$T_k$| plane for EM  = |$2.46 \times 10^9\, \mathrm{pc\, cm^{-6}}$| and |$\alpha = 20$|⁠. The case for |$\alpha = 10$| does not differ substantially from what is shown.