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

📝 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.

💡 Analysis

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.

📄 Content

arXiv:0908.2094v1 [astro-ph.SR] 14 Aug 2009 Hubble Space Telescope STIS Spectroscopy of Three Peculiar Nova-Like Variables: BK Lyn, V751 Cygni and V380 Oph Robert Zellem, Nicholas Hollon, Ronald-Louis Ballouz, Edward M. Sion, Patrick Godon1 Dept of Astronomy & Astrophysics, Villanova University, Villanova, PA 19085: robert.zellem@villanova.edu, nicholas.hollon@villanova.edu, ronald-louis.ballouz@villanova.edu, edward.sion@villanova.edu, patrick.godon@villanova.edu Boris T. G¨ansicke Department of Physics, University of Warwick, Coventry, CV4 9BU, UK: Boris.Gaensicke@warwick.ac.uk Knox Long Space Telescope Science Institute, Baltimore, MD 21218: Long@stsci.edu 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 ul- traviolet (FUV) light. We assessed a lower limit of the distances by applying the infrared photometric method of Knigge (2006). Within the limitations im- posed 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 ˙M = 1 × 10−9M⊙yr−1 for a white dwarf mass of Mwd = 1.2M⊙ and ˙M = 1 × 10−8M⊙yr−1 for Mwd = 0.4M⊙. 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 in- formation down to the Lyman limit hinders the extraction of information about the accreting white dwarf during the high states of these nova-like systems. Subject headings: Stars: cataclysmic variables, stars: white dwarfs, stars: nova- like variables, accretion disks 1Visiting at the Space Telescope Science Institute, Baltimore, MD 21208; godon@stsci.edu – 2 – 1. Introduction Cataclysmic variables (CVs) are short-period binaries in which a late-type, Roche-lobe- filling main-sequence dwarf transfers gas through an accretion disk onto a rotating, accretion- heated white dwarf (WD). The nova-like variables are a subclass of CVs in which the mass- transfer rate is high and the light of the system is dominated by a very bright accretion disk. The spectra of nova-like variables resemble those of classical novae (CN) that have settled back to quiescence. However, nova-like variables have never had a recorded CN outburst. Hence their evolutionary status is unknown. They could be close to having their next CN explosion, or they may have had an unrecorded explosion, possibly hundreds or thousands of years ago. Adding to the mystery of nova-like variables is that some of them (known as the VY Sculptoris stars after their prototype) show the curious behavior of being in a high optical brightness state most of the time, but then, plummeting into a deep low-brightness state with little or no ongoing accretion. Then, just as unpredictably, they return to the high-brightness state. These drops are possibly related to cessation of mass transfer from the K-M dwarf secondary star either by starspots positioned under L1 (Livio & Pringle 1994) or irradiation feedback in which an inflated outer disk can modulate the mass transfer from the secondary by blocking irradiation of the hot inner accretion disk region (Wu et al. 1995). As part of a Hubble Space Telescope Snapshot project extending over three Hubble observing cycles (B. G¨ansicke, PI), several nova-like variables were observed among dozens of non-magnetic and magnetic cataclysmic variables. In this paper we report on an analysis of the HST STIS spectra of three of these objects, BK Lyn, V751 Cygni and V380 Oph. 1.1. BK Lyn BK Lyn (PG0917+342) is an extraordinary nova-like variable with an orbital period placing it below the CV period gap. As such, it is the only bona fide nova-like variable below the CV period gap (Dobrzycka & Howell 1992). It was classified a short period dwarf nova by Dhillon et al. (2000) but long term light curves from ROBOSCOPE (98 exposures over three years) and the Harvard Plate Collection reveal no evidence for dwarf nova outbursts or even low optical brightness states. The system normally remains between V = 14.6 and 14.7. Ringwald et al. (1996) confirmed that its orbital period places it below the period gap. Skillman & Patterson (1993) detected permanent superhumps and found that the optical spectrum fits a power law fλ = k−α λ of index α = 2.66 ± 0.10 characteristic of a steady state accretion disk. Dhillon et al. (2000) carried out infrared spectrophotometry of BK Lyn revealing a secondary spectral type of M5V with an upper limit to the contribution of the secondary to the K-band light of 50% ± 5%. Dobrzycka & Howell (

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