Direct Detection of the L-Dwarf Donor in WZ Sagittae

TL;DR

This study provides direct evidence of an L-dwarf donor in WZ Sagittae by analyzing phase-resolved K-band spectra, revealing absorption features that confirm the donor's spectral type and measure its orbital motion.

Contribution

The paper presents the first direct detection of an L-dwarf donor in a cataclysmic variable through spectral analysis, demonstrating a novel method to identify donor types.

Findings

Detection of absorption from the L-dwarf donor's CO bandhead.

Measured radial velocity amplitude of the donor consistent with previous estimates.

Confirmed the spectral type of the donor as an L-dwarf.

Abstract

Analysis of a large set of phase-resolved $K$-band spectra of the cataclysmic variable WZ Sge shows that the secondary star of this system appears to be an L-dwarf. Previous $K$-band spectra of WZ Sge found that the CO overtone bandheads were in emission. We show that absorption from the $^{\rm 12}$CO$_{\rm (2,0)}$ bandhead of the donor star creates a dip in the $^{\rm 12}$CO$_{\rm (2,0)}$ emission feature. Measuring the motion of this feature over the orbital period, we construct a radial velocity curve that gives a velocity amplitude of K$_{\rm abs}$ = 520 $\pm$ 35 km s$^{\rm -1}$, consistent with the previously published values for this parameter.