Far-Ultraviolet Spectroscopy of the Nova-Like Variable KQ Monocerotis: A New SW Sextantis Star?

TL;DR

This study analyzes optical and far-ultraviolet spectra of KQ Monocerotis, revealing characteristics consistent with SW Sextantis stars and suggesting the white dwarf may be magnetic, based on synthetic spectral modeling and spectral features.

Contribution

First synthetic spectral analysis of KQ Mon's archival IUE spectra using advanced accretion disk and photosphere models, indicating its likely classification as an SW Sex star.

Findings

Spectra show strong Balmer and He I absorption lines.

FUV spectra exhibit deep absorption lines without wind outflow signatures.

Disk models suggest a white dwarf mass of ~0.6 Msun and a distance of 144-165 pc.

Abstract

New optical spectra obtained with the SMARTS 1.5m telescope and archival IUE far-ultraviolet spectra of the nova-like variable KQ Mon are discussed. The optical spectra reveal Balmer lines in absorption as well as He I absorption superposed on a blue continuum. The 2011 optical spectrum is similar to the KPNO 2.1m IIDS spectrum we obtained 33 years earlier, except that the Balmer and He I absorption is stronger in 2011. Far-ultraviolet IUE spectra reveal deep absorption lines due to C II, Si III, Si IV, C IV, and He II, but no P Cygni profiles indicative of wind outflow. We present the results of the first synthetic spectral analysis of the IUE archival spectra of KQ Mon with realistic optically thick, steady-state, viscous accretion-disk models with vertical structure and high-gravity photosphere models. We find that the photosphere of the white dwarf (WD) contributes very little FUV flux to the spectrum and is overwhelmed by the accretion light of a steady disk. Disk models corresponding to a WD mass of ~0.6 Msun, with an accretion rate of order 10^{-9} Msun/yr and disk inclinations between 60 and 75 deg, yield distances from the normalization in the range of 144 to 165 pc. KQ Mon is discussed with respect to other nova-like variables. Its spectroscopic similarity to the FUV spectra of three definite SW Sex stars suggests that it is likely a member of the SW Sex class and lends support to the possibility that the WD is magnetic.