Hubble Space Telescope STIS Spectroscopy of Three Peculiar Nova-Like Variables: BK Lyn, V751 Cygni and V380 Oph

TL;DR

This study uses Hubble STIS spectra to analyze three nova-like variables, revealing that accretion disk models fit some systems well but not others, highlighting limitations in current spectral modeling and data coverage.

Contribution

First spectral analysis of three nova-like variables during high states, demonstrating the applicability and limitations of steady state accretion disk models.

Findings

BK Lyn fits accretion disk models with specific accretion rates

V751 Cygni and V380 Oph spectra do not fit steady state models

Lack of FUV data below Lyman limit limits white dwarf characterization

Abstract

We obtained Hubble STIS spectra of three nova-like variables: V751 Cygni, V380 Oph, and - the only confirmed nova-like variable known to be below the period gap - BK Lyn. In all three systems, the spectra were taken during high optical brightness state, and a luminous accretion disk dominates their far ultraviolet (FUV) light. We assessed a lower limit of the distances by applying the infrared photometric method of \citet{Knigge2006}. Within the limitations imposed by the poorly known system parameters (such as the inclination, white dwarf mass, and the applicability of steady state accretion disks) we obtained satisfactory fits to BK Lyn using optically thick accretion disk models with an accretion rate of $\dot{M} = 1\times10^{-9} M_{\odot}$ yr$^{-1}$ for a white dwarf mass of $M_{wd} = 1.2 M_{\odot}$ and $\dot{M} = 1 \times 10^{-8} M_{\odot}$ yr$^{-1}$ for $M_{wd} = 0.4 M_{\odot}$. However, for the VY Scl-type nova-like variable V751 Cygni and for the SW Sex star V380 Oph, we are unable to obtain satisfactory synthetic spectral fits to the high state FUV spectra using optically thick steady state accretion disk models. The lack of FUV spectra information down to the Lyman limit hinders the extraction of information about the accreting white dwarf during the high states of these nova-like systems.