Quantifying the Carbon Abundances in the Secondary Stars of SS Cygni, RU Pegasi, and GK Persei

TL;DR

This study uses synthetic spectral modeling to measure carbon and iron abundances in secondary stars of three cataclysmic variables, revealing significant carbon deficits and demonstrating the effectiveness of lower resolution spectra for such analyses.

Contribution

It introduces a modified MOOG approach to derive elemental abundances from moderate and low-resolution spectra of CV secondary stars, highlighting the detection of carbon deficits.

Findings

Secondary stars have solar or sub-solar [Fe/H]

Significant carbon deficits are present in all three systems

Lower resolution spectra can reliably quantify carbon deficits

Abstract

We use a modified version of MOOG to generate large grids of synthetic spectra in an attempt to derive quantitative abundances for three CVs (GK Per, RU Peg, and SS Cyg) by comparing the models to moderate resolution (R $\sim$ 25,000) $K$-band spectra obtained with NIRSPEC on Keck. For each of the three systems we find solar, or slightly sub-solar values for [Fe/H], but significant deficits of carbon: for SS Cyg we find [C/Fe] = $-$0.50, for RU Peg [C/Fe] = $-$0.75, and for GK Per [C/Fe] = $-$1.00. We show that it is possible to use lower resolution (R $\sim$ 2,000) spectra to quantify carbon deficits. We examine realistic veiling scenarios and find that emission from H I or CO cannot reproduce the observations.