Pressure-gradient parameter of a protoplanetary disk
= Pressure-gradient parameter of a protoplanetary disk
{title2=$\eta=-\frac12(c_s/v_K)^2\,d\ln P_g/d\ln r$}
A convention for the small fractional reduction of azimuthal gas speed due to radial <pressure> support. With $c_s^2=P_g/\rho_g$, radial balance gives $v_{g\phi}^2=v_K^2(1-2\eta)$ and hence $v_{g\phi}\simeq v_K(1-\eta)$. Other authors absorb the factor of two into the definition; the convention must accompany a drift formula.