The Distribution of Secondary Masses in Post-Common Envelope Binaries: A Potential Test of Disrupted Magnetic Braking

Abstract

The distribution of secondary star masses in present-day post-common envelope binaries (PCEBs) is calculated using four different models for angular momentum loss (AML) during the post-CE phase: only gravitational radiation (GR), GR + disrupted magnetic braking (DMB), GR + reduced MB, and GR + intermediate MB. For the DMB model, we find that the number of PCEBs decreases abruptly by 38% once MB begins to operate for non-fully convective secondaries. We do not find a similar feature in the distributions calculated using any of the other three AML models in which MB is not disrupted. This percentage decrease in the number of present-day PCEBs predicted using the DMB model is easily large enough so that an observed distribution of secondary masses or even spectral types in PCEBs can provide an important test of whether magnetic braking is indeed substantially reduced in secondary stars that are fully convective. We discuss briefly the feasibility of such observations.