Hydrostatic Balance

Brant Robertson bio photo By Brant Robertson

Hydrostatic Balance of an Adiabatic Fluid in a Background Potential

Consider an adiabatic fluid with an adiabatic index . We want to compute the density structure in a potential . The gas pressure is .

First, note that or

The equation of vertical hydrostatic equilibrium is

If we define the central midplane gas temperature at to be such that the sound speed is , then . Given the above, we then require

We can arrange this if we define and write

and our density profile becomes

A numerical integral sets by demanding

Hydrostatic disks with varying surface densities

So we can use the above to set the central density if the surface density is not a function of radius. But the surface density is declining with radius and the potential is varying as the radius increases.

Once we set at , , we are stuck with that equation of state if the disk gas is all on the same adiabat.

Now, reconsider a different place in the disk. The central density in the midplane will be lower because of the surface density, but also lower if the potential has changed radially.

Clearly , so the constant of integration in determining the density by integrating the vertical force is not the same as at .

Instead we can write


such that . The density where the sound speed is is a constant, so the surface density constraint must be used to set . This has to be done iteratively, but a first reasonable guess is

Note .