An Halpha-selected sample of cataclysmic variables -- II. Implications for CV evolution

TL;DR

This study uses a new Halpha-selected sample of cataclysmic variables to test and challenge existing models of CV evolution, suggesting that additional angular momentum loss mechanisms may be needed below the period gap.

Contribution

It provides new observational constraints on CV populations and highlights discrepancies with standard evolution models, proposing the need for revised angular momentum loss rates.

Findings

Observed CV period distribution differs from predictions

Standard models underestimate short-period CVs

Additional angular momentum loss mechanisms are implied

Abstract

We use an independent new sample of cataclysmic variables (CVs), constructed by selecting objects for Halpha emission, to constrain the properties of the intrinsic CV population. This sample is restricted to systems that are likely to be non-magnetic and unevolved; it consists of 17 CVs, of which at least 10 have orbital periods above 3 h. We find that even very generous allowance for selection effects is not sufficient to reconcile the large ratio of short- to long-period CVs predicted by standard CV evolution theory with the observed sample, possibly implying that short-period systems evolve faster than predicted by the disrupted magnetic braking model. This would require that an angular momentum loss mechanism, besides gravitational radiation, acts on CVs with orbital periods below the period gap. To bring the model into agreement with observations, the rate of angular momentum loss below the period gap must be increased by a factor of at least 3, unless the model also over estimates the angular momentum loss rate of long-period CVs.