Weighted fluxes for HCN(1−0) for different density PDFs. In the first three rows the median density of the PDFs corresponds to that of the SPH simulation, whereas the dispersion is increased from 2.1 to 2.7 to 3.5. In the fourth row, the median density is increased by a factor of 100 compared to that of the SPH simulation, but the dispersion is kept fixed at 2.1. In the last row we show the weighted fluxes for the PDF obtained by the Wada Wada (2001) simulation. In the first column the FUV flux is varied, from 100 times the flux at the solar neighborhood to 10⁶G₀  corresponding to the FUV flux in extreme starbursts (A_V = 10 mag, log ₁₀Γ_mech = −22). In the middle column we vary Γ_mech  from the heating rate corresponding to quiescent disks to rates typical to violent starbursts with a SFR of 1000 M_⊙ per year (A_V = 10 mag, log ₁₀G = 2). In the last column the A_V is varied from 0.1 to 10 mag, corresponding respectively to the typical values for a transition zone from H⁺ to H and for dark molecular clouds (log ₁₀G = 2, log ₁₀Γ_mech = −22). The onset of emission is determined mainly by Γ_mech; for instance in looking at the middle column, we see that this onset of emission corresponds to n ~ 10² cm^-3, where for lower densities H₂ does not form, which is essential for other molecules such as HCN to form. For each curve in every panel we also plot (with filled circles) the density, n₉₀, where 90% of the emission emanates from n <n₉₀. For example, in the top row, even for the most intense FUV flux n₉₀ ~ 10^3.5 cm^-3. On the other hand, in bottom row, n₉₀ > 10⁵ cm^-3. We note that the curves corresponding to Γ_mech  10^-50 (a pure PDR) and 10^-30 erg cm^-3 s^-1  in the second column overlap.