Dwarf Nova V2051 Ophiuchi Research Articles

Infrared photometry of the dwarf nova V2051 Ophiuchi – II. The quiescent accretion disc and its spiral arms

ABSTRACT We report the analysis of time-series of infrared JHKs photometry of the dwarf nova V2051 Oph in quiescence with eclipse mapping techniques to investigate structures and the spectrum of its accretion disc. The light curves after removal of the ellipsoidal variations caused by the mass-donor star show a double-wave modulation signalling the presence of two asymmetric light sources in the accretion disc. Eclipse maps reveal two spiral arms on top of the disc emission, one at $R_1= 0.28\pm 0.02 \, R_\mathrm{L1}$ and the other at $R_2= 0.42\pm 0.02 \, R_\mathrm{L1}$ (where RL1 is the distance from disc centre to the inner Lagrangian point), which are seen face-on at binary phases consistent with the maxima of the double-wave modulation. The wide open angle inferred for the spiral arms (θs = 21° ± 4°) suggests the quiescent accretion disc of V2051 Oph has high viscosity. The accretion disc is hot and optically thin in its inner regions ($T_\mathrm{gas}\sim 10\!-\!12 \times 10^3\, \mathrm{ K}$ and surface densities $\sim 10^{-3}\text{ to }10^{-2}\, \mathrm{ g}\, \mathrm{ cm}^{-2}$), and becomes cool and opaque in its outer regions.

Infrared photometry of the dwarf nova V2051 Ophiuchi – I. The mass-donor star and the distance★

We report the analysis of time-series of infrared $JHK_s$ photometry of the dwarf nova V2051 Oph in quiescence. We modelled the ellipsoidal variations caused by the distorted mass-donor star to infer its $JHK_s$ fluxes. From its infrared colors we estimate a spectral type of $M(8.0\pm 1.5)$ and an equivalent blackbody temperature of $T_\mathrm{BB}=(2700\pm270)\,K$. We used the Barnes & Evans relation to infer a photometric parallax distance of $d_\mathrm{BE}=(102\pm16)$ pc to the binary. At this short distance, the corresponding accretion disc temperatures in outburst are too low to be explained by the disc-instability model for dwarf nova outbursts, underscoring a previous suggestion that the outbursts of this binary are powered by mass-transfer bursts.

Spiral structures and temperature distribution in the quiescent accretion disc of the cataclysmic binary V2051 Ophiuchi

We present the capabilities of our new code for obtaining Doppler maps implementing the maximum likelihood approach. As test data, we used observations of the dwarf nova V2051 Ophiuchi. The system was observed in quiescence at least 16 d before the onset of the next outburst. Using Doppler maps obtained for ten emission lines covering three orbital cycles, we detected spiral structures in the accretion disc of V2051 Oph. However, these structures could be biased as our data sampled the orbital period of the binary at only eight different orbital phases. Our Doppler maps show evolution from a one-arm wave structure in H$\alpha$ to two-armed waves in the other lines. The location of the two-arm structures agrees with simulations showing tidally driven spiral waves in the accretion disc. During consecutive cycles, the qualitative characteristics of the detected structures remained similar but the central absorption increased. For the first time, using the Doppler tomography method, we obtained temperature maps of the accretion disc. However, taking into account all the assumptions involved when using our method to retrieve them, the result should be treated with caution. Our maps present a relatively flat distribution of the temperature over the disc, showing no temperature increase at the location of the spiral arms. Using `ring masking', we have revealed an ionized region located close to the expected location of stream--disc interactions. We found the average temperature of the accretion disc to be 5600 K, which is below the critical limit deduced from the disc instability model.

Hubble Space TelescopeFaint Object Spectrograph Spectral Mapping of V2051 Ophiuchi in a Low State

We report a study of the spectra and structure of the accretion disk of the dwarf nova V2051 Ophiuchi while the star was in an unusual faint brightness state during 1996. The differences between the UV resonant lines and continuum disk surface brightness distributions indicate a vertically extended disk with the emission from these lines arising from the upper atmospheric layers. Distinct emission along the stream trajectory suggests the occurrence of gas stream overflow. Spatially resolved spectra show that the lines are in emission at all disk radii. The Balmer decrement becomes shallower with increasing radius. The FWHMs of the emission lines show differences with respect to that expected for a gas in Keplerian rotation, and the line intensities drop with a radial dependency of I ∝ R-1.78. The uneclipsed light contributes about 5%-10% of the total flux, and its spectrum is dominated by a Balmer jump and strong lines in emission. Broad absorption bands, possibly due to Fe II, are present in the spectra of the disk side farther away from the secondary star, suggesting it arises from absorption by a extended gas region above the disk; the differences between the spectra of the hemisphere farther from and nearer to the secondary star are interpreted in terms of chromospheric emission from a disk with a nonnegligible opening angle (limb-brightening effect). Stellar atmosphere model fits to the extracted white dwarf spectrum lead to a temperature TWD = 9500 K and a distance d = 67 pc if the inner disk is opaque or d = 92 pc if the inner disk is optically thin.

Eclipse Mapping of the Flickering Sources in the Dwarf Nova V2051 Ophiuchi

We report an eclipse-mapping analysis of an ensemble of light curves of the dwarf nova V2051 Oph, with the aim to study the spatial distribution of its steady-light and flickering sources. The data are combined to derive the orbital dependence of the steady-light and the flickering components at two different brightness levels, named the and states. The differences in brightness are caused by long-term variations in the mass transfer rate from the secondary star. The white dwarf is hardly affected by the long-term changes; its flux increases by only 10% from the faint to the bright state, whereas the disk flux raises by a factor of 2. Eclipse maps of the steady light show asymmetric brightness distributions with enhanced emission along the ballistic stream trajectory, clear evidence of gas stream overflow. Comparison between the steady-light maps of the faint and bright states suggests that the quiescent disk responds to changes in mass transfer rate in a homologous way. The ability to separate the orbital dependence of the low- and high-frequency flickering components allows us to identify the existence of two different and independent sources of flickering in V2051 Oph. The low-frequency flickering arises mainly in the overflowing gas stream and is associated with the mass transfer process. It maximum emission occurs at the position of closest approach of the gas stream to the white dwarf, and its spatial distribution changes in response to variations in the mass transfer rate. The high-frequency flickering originates in the accretion disk, showing a radial distribution similar to that of the steady-light maps and no evidence of emission from the hot spot, gas stream, or white dwarf. This disk flickering component has a relative amplitude of about 3% of the steady disk light, independent of disk radius and brightness state. If the disk flickering is caused by fluctuations in the energy dissipation rate induced by magnetohydrodynamic turbulence, its relative amplitude leads to a viscosity parameter αcool 0.1–0.2 at all radii for the quiescent disk. This value seems uncomfortably high to be accommodated by the disk-instability model.

Emission-line oscillations in the dwarf nova V2051 Ophiuchi

We have detected coherent oscillations, at multiple frequencies, in the line and continuum emission of the eclipsing dwarf nova V2051 Ophiuchi using the 10-m Keck II telescope. Our own novel data acquisition system allowed us to obtain very fast spectroscopy using a continuous readout of the CCD on the LRIS spectrograph. This is the first time that dwarf nova oscillations have been detected and resolved in the emission lines. The accretion disc is highly asymmetric with a stronger contribution from the blueshifted side of the disc during our observations. The disc extends from close to the white dwarf out to the outer regions of the primary Roche lobe. Continuum oscillations at 56.12 s and its first harmonic at 28.06 s are most likely to originate on the surface of a spinning white dwarf with the fundamental period corresponding to the spin period. Balmer and helium emission lines oscillate with a period of 29.77 s at a mean amplitude of 1.9 per cent. The line kinematics and the eclipse constraints indicate an origin in the accretion disc at a radius of 12±2Rwd. The amplitude of the emission-line oscillation modulates (0–4 per cent) at a period of 488 s, corresponding to the Kepler period at R=12Rwd. This modulation is caused by the beating between the white dwarf spin and the orbital motion in the disc. The observed emission-line oscillations cannot be explained by a truncated disc as in the intermediate polars. The observations suggest a non-axisymmetric bulge in the disc, orbiting at 12Rwd, is required. The close correspondence between the location of the oscillations and the circularization radius of the system suggests that stream overflow effects may be of relevance.

Simultaneous infrared and optical photometry of the Eclipsing Dwarf Nova V2051 Ophiuchi

AbstractA complete orbital light-curve of V2051 Oph in the IR H band is presented, together with a second eclipse in the J band. Simultaneous Rc band data were obtained. Eclipse depths in Rc, J and H are 1.8 mag, 1.0 mag and 0.8 mag respectively. No evidence for ellipsoidal variations due to the secondary was seen and constraints on the secondary are discussed.

High speed photometry of the dwarf nova V2051 Ophiuchi

High speed photometry of the dwarf nova V2051 Ophiuchi