 Grey-atmosphere convection onset with seventeenth-power opacity

For a grey atmosphere with $T^4=T_e^4(2+3\tau )/4$, an ideal gas and $\kappa ={\kappa }_0\rho T^{17}$, hydrostatic equilibrium in optical depth integrates to $P^2=512\mathcal{R}g[1/8-(2+3\tau {)}^{-3}]/(9\mu {\kappa }_0{T}_e^{16})$, taking $P(0)=0$. The logarithmic radiative gradient is $[(2+3\tau {)}^3-8]/48$. Equating it to the monatomic adiabatic temperature gradient $2/5$ gives $(2+3{\tau }_b{)}^3=136/5$ and the displayed onset temperature. Beyond this point the radiative profile must be replaced by convective transport.