Infrared Spectroscopic Observations of the Secondary Stars of Short Period Sub-Gap Cataclysmic Variables

TL;DR

This study presents K-band infrared spectroscopy of nine short-period cataclysmic variable systems, detecting secondary stars in most cases, and compares their spectral features to understand chemical abundances and their relation to ultraviolet anomalies.

Contribution

First direct detection of secondary stars in several short-period CVs and a comprehensive IR spectral analysis of 61 systems, revealing spectral and abundance patterns.

Findings

Most secondary stars show normal CO band strengths for their spectral types.

Detected secondary stars in V2051 Oph, V436 Cen, and provided new classifications for others.

Potential link between IR abundance anomalies and ultraviolet C IV/N V ratio.

Abstract

We present K-band spectroscopy of short period, "sub-gap" cataclysmic variable (CV) systems obtained using ISAAC on the VLT. We show the infrared spectra (IR) for nine systems below the 2-3 hour period gap: V2051 Oph, V436 Cen, EX Hya, VW Hyi, Z Cha, WX Hyi, V893 Sco, RZ Leo, and TY PsA. We are able to clearly detect the secondary star in all but WX Hyi, V893 Sco, and TY PsA. We present the first direct detection of the secondary stars of V2051 Oph, V436 Cen, and determine new spectral classifications for EX Hya, VW Hyi, Z Cha, and RZ Leo. We find that the CO band strengths of all but Z Cha appear normal for their spectral types, in contrast to their longer period cousins above the period gap. This brings the total number of CVs and pre-CVs with moderate resolution (R >~ 1500) IR spectroscopy to sixty-one systems: nineteen pre-CVs, thirty-one non-magnetic systems, and eleven magnetic or partially magnetic systems. We discuss the trends seen in the IR abundance patterns thus far, and highlight a potential link between anomalous abundances seen in the IR with the C IV/N V anomaly seen in the ultraviolet. We present a compilation of all systems with sufficient resolution IR observations to assess the CO band strengths, and, by proxy, obtain an estimate on the C abundance on the secondary star.