Abundance Derivations for the Secondary Stars in Cataclysmic Variables from Near-Infrared Spectroscopy

TL;DR

This study derives metallicities and elemental abundances for 41 cataclysmic variables using near-infrared spectroscopy, revealing generally sub-solar metallicities and carbon deficits, with implications for understanding donor star properties and system evolution.

Contribution

It provides the first comprehensive metallicity and abundance analysis of CV donors using synthetic spectra and phase-resolved data, highlighting the prevalence of sub-solar metallicities and unusual compositions.

Findings

Most CV donors have sub-solar metallicities.

Many systems exhibit carbon deficits, especially those with orbital periods > 5 hr.

Two systems show sodium excesses and extreme carbon deficits.

Abstract

We derive metallicities for 41 cataclysmic variables (CVs) from near-infrared spectroscopy. We use synthetic spectra that cover the 0.8 $\mu$m $\leq \lambda \leq$ 2.5 $\mu$m bandpass to ascertain the value of [Fe/H] for CVs with K-type donors, while also deriving abundances for other elements. Using calibrations for determining [Fe/H] from the $K$-band spectra of M-dwarfs, we derive more precise values for T$_{\rm eff}$ for the secondaries in the shortest period CVs, and examine whether they have carbon deficits. In general, the donor stars in CVs have sub-solar metallicities. We confirm carbon deficits for a large number of systems. CVs with orbital periods $>$ 5 hr are most likely to have unusual abundances. We identify four CVs with CO emission. We use phase-resolved spectra to ascertain the mass and radius of the donor in U Gem. The secondary star in U Gem appears to have a lower {\it apparent} gravity than a main sequence star of its spectral type. Applying this result to other CVs, we find that the later-than-expected spectral types observed for many CV donors is mostly an affect of inclination. All of the magnetic CVs, except the low accretion rate polar MQ Dra, have donors with subsolar metallicities. We find that two systems with unusual spectra, EI Psc and QZ Ser, that have large excesses of sodium, and extreme deficits of carbon. Synthetic spectra that have a reduced abundance of hydrogen are best able to explain the spectra of these two objects.