 Oblate-rotator flattening proxy

The simplified moment prescription $q={\sigma }_z^2/({\sigma }_x^2+v_p^2)$, with $e=1-q$ and $\beta =1-{\sigma }_z^2/{\sigma }_x^2$, gives the displayed relation by solving for $v_p^2$. The prescription is an approximation, not a consequence of the tensor virial theorem for arbitrary oblate density profiles. For constant azimuthal streaming $v_{\mathrm{az}}$, the one-component streaming second moment is $v_p^2=v_{\mathrm{az}}^2/2$.