Photometric masses for long period CVs: the case study of CSS131106

TL;DR

This study uses high-speed photometry to determine the system parameters of the long-period CV CSS131106, revealing unusually small and cool donor stars, and demonstrates the reliability of eclipse modeling despite blended ingress features.

Contribution

It introduces a method to reliably constrain component masses in long-period CVs using eclipse lightcurve modeling, even with blended ingress features.

Findings

Mass ratio q = 0.81 ± 0.06 and inclination i = 78.5 ± 0.7°

White dwarf mass M_w = 0.72 ± 0.04 M_sun, donor mass M_d = 0.58 ± 0.06 M_sun

Donors are unusually small and cool for their mass, compared to typical M-dwarfs.

Abstract

We present high-speed photometry of the eclipsing cataclysmic variable CSS131106 J052412+004148. We determine the system parameters by modelling the eclipse lightcurve using the photometric eclipse method, in which the mass ratio is determined from the relative timings of the white dwarf and bright spot eclipses. Despite the blended white dwarf and bright spot ingress, typical of longer period cataclysmic variables, we perform simulations that show we are able to reliably constrain the component masses. We find a mass ratio of $q = 0.81 \pm 0.06$ and inclination $i = 78.5 \pm 0.7$ degrees. The white dwarf and donor masses were found to be $M_{w} = 0.72 \pm 0.04 \, M_{\odot}$ and $M_{d} = 0.58 \pm 0.06 \, M_{\odot}$ respectively. The white dwarf temperature was estimated to be $T_{\rm eff} = 18~500 \pm 2~000$ K, implying a moderate accretion rate of $\dot{M} = 3 \pm 1 \times 10^{-10} M_{\odot}$ yr$^{-1}$. The donor in CSS131106 J052412+004148 joins two other long-period cataclysmic variables (IP Peg and HS 0220+0031) in being unusually small for its mass, even when compared to detached M-dwarfs. The donors in all three systems are also unusually cool for their mass. We discuss possible explanations for the small radii and cool temperatures of the donors in these systems, but find no viable explanation for their properties.