[Paper Review] The Evolution of Cataclysmic Variables

C. Knigge, Baraffe, I.+1 more|arXiv (Cornell University)|Jan 1, 2011

Pulsars and Gravitational Waves Research3 citations

TL;DR

This paper reviews the evolution of cataclysmic variables (CVs), testing the standard 'disrupted magnetic braking' model using observational data on white dwarf temperatures and donor star radii. It finds that gravitational radiation alone cannot explain mass transfer rates below the period gap, suggesting enhanced angular momentum loss, and presents an empirically calibrated alternative model to the standard theory.

ABSTRACT

I review our current understanding of the evolution of cataclysmic variables (CVs). I first provide a brief introductory "CV primer", in which I describe the physical structure of CVs, as well as their astrophysical significance. The main part of the review is divided into three parts. The first part outlines the theoretical principles of CV evolution, focusing specifically on the standard "disrupted magnetic braking" model. The second part describes how some of the most fundamental predictions this model are at last being test observationally. Finally, the third part describes recent efforts to actually reconstruct the evolution path of CVs empirically. Some of these efforts suggest that angular momentum loss below the period gap must be enhanced relative to the purely gravitational-radiation-driven losses assumed in the standard model.

Motivation & Objective

• . The paper aims to test long-standing theoretical predictions of CV evolution using new observational data.
• It investigates whether the standard model of angular momentum loss via magnetic braking and gravitational radiation accurately describes CV evolution.
• The objective includes reconstructing the evolutionary path of CVs empirically using primary and secondary star properties.
• It seeks to resolve discrepancies between theory and observations, particularly regarding mass transfer rates below the period gap.
• The study aims to provide a benchmark for population synthesis by calibrating a revised model of angular momentum loss.

Proposed method

• . The authors use white dwarf effective temperatures as tracers of mass accretion rates, assuming known white dwarf masses.
• They apply theoretical mass-radius relations for low-mass stars to infer donor mass and radius from observed properties.
• The study compares observed donor star radii and white dwarf temperatures to predictions from standard and modified evolutionary models.
• It employs a non-standard model with adjustable angular momentum loss (AML) strengths above and below the period gap to fit observational data.
• The analysis uses a sample of CVs with reliable temperature and radius measurements, including quiescent WD temperatures from Townsley & G"ansicke (2009).
• A joint fit to donor mass-radius and WD temperature data is performed to derive the best-fitting AML model, deviating from the standard assumption of pure gravitational radiation below the gap.

Experimental results

Research questions

• RQ1 . Does the standard model of CV evolution, based on magnetic braking above and gravitational radiation below the period gap, accurately reproduce observed properties of CVs?
• RQ2 . Are the observed mass transfer rates below the period gap consistent with gravitational radiation alone, or is additional angular momentum loss required?
• RQ3 . Can the evolutionary path of CVs be reconstructed empirically using white dwarf temperatures and donor star radii?
• RQ4 . How do discrepancies between theoretical predictions and observations constrain the strength of angular momentum loss in CVs?
• RQ5 . What is the empirical calibration of angular momentum loss below the period gap, and how does it differ from the standard model?

Key findings

• . Observational data on white dwarf temperatures suggest that gravitational radiation alone is insufficient to drive the observed mass transfer rates below the period gap.
• . The donor star radius data show significant inflation compared to standard models, indicating stronger mass loss than predicted by the standard model.
• . A non-standard model with enhanced angular momentum loss below the period gap provides a better fit to the observed donor mass-radius relation than the standard model.
• . The best-fitting empirical model implies that angular momentum loss below the period gap must be significantly stronger than predicted by gravitational radiation alone.
• . The study demonstrates that both white dwarf temperature and donor radius measurements can be used to empirically reconstruct CV evolution, offering a new benchmark for population synthesis.
• . The results support the idea that CVs are universal laboratories for accretion physics, with phenomena like variability and disk winds mirroring those in neutron star and black hole systems.