somehow dropped "frac"

-  Project :math:`U_g^{\ast}` by solving

.. math:: \nabla \cdot \frac{\varepsilon_g}{\rho_g} \nabla \phi = \nabla \cdot \left( \frac{1}{\Delta t} (\varepsilon_g  U_g)^{\ast}+ {\varepsilon_g}{\rho_g} \nabla {p_g}^{n-1/2} \right)

.. math:: \nabla \cdot \frac{\varepsilon_g}{\rho_g} \nabla \phi = \nabla \cdot \left( \frac{1}{\Delta t} 

          (\varepsilon_g  U_g)^{\ast}+ \frac{\varepsilon_g}{\rho_g} \nabla {p_g}^{n-1/2} \right)

then defining