CONSEQUENTIAL ANGULAR-MOMENTUM LOSS AND THE PERIOD GAP OF CATACLYSMIC VARIABLES

We consider cataclysmic variable evolution driven partially by angular momentum losses which are themselves the result of mass transfer (e.g., via mass loss from an accretion disk). We show that such consequential angular momentum loss (CAML) mechanisms have great difficulty in reproducing the main features of the CV period gap. To interrupt mass transfer at all, CAML-driven systems must be unstable to dynamical-timescale mass transfer above the gap. Further, extremely tight constraints on the system parameters (e.g., the masses) are required to account for the observed width of the CV period gap: the CAML mechanism must essentially bring the system to the brink of thermal-timescale mass transfer instability. It is not obvious that these requirements can be satisfied in any given case, and they make the system very vulnerable to unstable mass transfer through small perturbations of the CAML process. A magnetic disk wind, as recently suggested by Livio and Pringle, cannot provide a large enough orbital torque to make a gap of the observed width except for unacceptably small white dwarf masses.