The Secondary Stars of Cataclysmic Variables

TL;DR

This review summarizes current knowledge about donor stars in cataclysmic variables, highlighting observational confirmations of theoretical models, the empirical mass-radius relationship, and implications for CV evolution and angular momentum loss.

Contribution

It provides a comprehensive semi-empirical donor sequence for CVs and links observed donor properties to the evolution of these systems.

Findings

Donor stars are slightly inflated compared to isolated main-sequence stars.

Empirical mass-radius relation shows a discontinuity at M_2 = 0.2 M_sun.

Angular momentum loss rates below the period gap may be higher than gravitational radiation alone suggests.

Abstract

I review what we know about the donor stars in cataclysmic variables (CVs), focusing particularly on the close link between these binary components and the overall secular evolution of CVs. I begin with a brief overview of the "standard model" of CV evolution and explain why the key observables this model is designed to explain - the period gap and the period minimum -- are intimately connected to the properties of the secondary stars in these systems. CV donors are expected to be slightly inflated relative to isolated, equal-mass main-sequence (MS) stars, and this "donor bloating" has now been confirmed observationally. The empirical donor mass-radius relationship also shows a discontinuity at M_2 = 0.2 M_sun which neatly separates long- and short-period CVs. This is strong confirmation of the basic disrupted magnetic braking scenario for CV evolution. The empirical M_2-R_2 relation can be combined with stellar models to construct a complete, semi-empirical donor sequence for CVs. This sequence provides all physical and photometric properties of "normal" CV secondaries along the standard CV evolution track. The observed donor properties can also be used to reconstruct the complete evolution track followed by CVs, i.e. the mass-transfer rate and angular-momentum-loss rate as a function of orbital period. Such a reconstruction suggests that angular momentum loss rates below the period gap are too high to be driven solely by gravitational radiation.