Dynamical friction in an isentropic gas
  When a gravitating object moves across a given mass distribution, it creates
an overdense wake behind it. Here, we performed an analytical study of the
structure of the flow far from object when the flow is isentropic and the
object moves subsonically within it. We show that the dynamical friction force
is the main drag force on the object and by using a perturbation theory, we
obtain the density, velocity and pressure of the perturbed flow far from the
mass. We derive the expression of the dynamical friction force in an isentropic
flow and show its dependence on the Mach number of the flow and on the
adiabatic index. We find that the dynamical friction force becomes lower as the
adiabatic index increases. We show analytically that the wakes are less dense
in our isentropic case in comparison to the isothermal ones.
