On the Minimum Energy Configuration of a Rotating Barotropic Fluid

In a recent paper (astro-ph/0207561, posted on July 25, 2002), Fromang & Balbus state that ``a uniformly rotating barotropic fluid in an external potential attains a true energy minimum if and only if the rotation profile is everywhere subsonic. If regions of supersonic rotation are present, fluid variations exist that could take the system to states of lower energy.'' We present a simple counter-example which demonstrates that the stablity criterion presented by Fromang & Balbus is incorrect.