The Identification of Hydrogen-Deficient Cataclysmic Variable Donor Stars

TL;DR

This study uses synthetic spectra to identify hydrogen deficiency and other abundance anomalies in donor stars of certain cataclysmic variables, revealing new insights into their compositions and properties.

Contribution

The paper introduces a method combining synthetic spectra and light curve modeling to detect and analyze chemical abundance anomalies in CV donor stars, including hydrogen deficiency.

Findings

Four CV systems show significant hydrogen deficits.

Common abundance anomalies include carbon and magnesium deficits, sodium enhancements.

Some donor stars are cooler than expected for their orbital periods.

Abstract

We have used ATLAS12 to generate hydrogen-deficient stellar atmospheres to allow us to construct synthetic spectra to explore the possibility that the donor stars in some cataclysmic variables (CVs) are hydrogen deficient. We find that four systems, AE Aqr, DX And, EY Cyg, and QZ Ser have significant hydrogen deficits. We confirm that carbon and magnesium deficits, and sodium enhancements, are common among CV donor stars. The three Z Cam systems we observed are found to have solar metallicities and no abundance anomalies. Two of these objects, Z Cam and AH Her, have M-type donor stars; much cooler than expected given their long orbital periods. By using the combination of equivalent width measurements and light curve modeling, we have developed the ability to account for contamination of the donor star spectra by other luminosity sources in the binary. This enables more realistic assessments of secondary star metallicities. We find that the use of equivalent width measurements should allow for robust metallicities and abundance anomalies to be determined for CVs with M-type donor stars.