K-Band Spectroscopy of (Pre-)Cataclysmic Variables: Are Some Donor Stars Really Carbon Poor?

TL;DR

This study investigates the carbon abundance in secondary stars of cataclysmic variables using K-band spectroscopy, revealing diverse chemical signatures and challenging previous claims of universal carbon deficiency.

Contribution

It provides new K-band spectral data for a diverse sample of CVs, showing that carbon deficiency is not universal among secondary stars and highlighting the complexity of chemical compositions.

Findings

Some CVs show normal CO absorption, contradicting the carbon deficiency claim.

Presence of non-solar abundances and enhanced NV/CIV ratios in certain systems.

CO absorption anomalies are not solely due to secondary star evolution.

Abstract

We present a new sample of $K$-band spectral observations for CVs: non-magnetic and magnetic as well as present day and pre CVs. The purpose of this diverse sample is to address the recent claim that the secondary stars in dwarf novae are carbon deficient, having become so through a far more evolved evolution than the current paradigm predicts. Our new observations, along with previous literature results, span a wide range of orbital period and CV type. In general, dwarf novae in which the secondary star is seen show weak to no CO absorption while polar and pre-CV donor stars appear to have normal CO absorption for their spectral type. However, this is not universal. The presence of normal looking CO absorption in the dwarf nova SS Aur and the hibernating CV QS Vir and a complete lack of CO absorption in the long period polar V1309 Ori cloud the issue. A summary of the literature pointing to non-solar abundances including enhanced NV/CIV ratios is presented. It appears that some CVs have non-solar abundance material accreting onto the white dwarf suggesting an evolved secondary star while for others CO emission in the accretion disk may play a role. However, the exact mechanism or combination of factors causing the CO absorption anomaly in CVs is not yet clear.