On the horseshoe drag of a low-mass planet. I - Migration in isothermal disks

Understand

We investigate the unsaturated horseshoe drag exerted on a low-mass planet by an isothermal gaseous disk.

• In the globally isothermal case, we use a formal- ism, based on the use of a Bernoulli invariant, that takes into account pressure effects, and that extends the torque estimate to a region wider than the horse- shoe region.
• We find a result that is strictly identical to the standard horseshoe drag.
• This shows that the horseshoe drag accounts for the torque of the whole corotation region, and not only of the horseshoe region, thereby deserving to be called corotation torque.