Hydrodynamic Models of Line‐driven Accretion Disk Winds. II. Adiabatic Winds from Nonisothermal Disks (original) (raw)

A study of accretion disk wind emission

Accretion disk wind has been the most accepted scenario to explain the strong emission lines and the P Cyg profiles detected in the ultraviolet spectra of non-magnetic cataclysmic variables (CV's). Due to the main characteristics of line profiles, it is commonly accepted that those winds are radiatively driven. Here, we present an alternative method to study the disk-wind that takes into account the structure of whole system (disk+wind), as well as the interface between the disk photosphere and wind. We calculated a set of 1D NLTE atmosphere wind models with a consistent velocity wind profile, and map them into a 2D model including a 3D velocity field. The synthetic spectrum is calculated by exactly solving the radiative transfer equation throughout the inner disk, interface region and wind, taking into account the Doppler shifts for each step. We found a modeled line profile behavior according with the observed one. We also found dependences between line profiles and disk parameters. When tested with UV data for two nova-like systems, the models shows a good agreement. However, we found a lack a flux for high ionization lines. It is possible that it is due to the influence of the inner disk radiation on the outer wind regions.

Hydrodynamic Models of Line‐driven Accretion Disk Winds. III. Local Ionization Equilibrium

The Astrophysical Journal, 2003

We present time-dependent numerical hydrodynamic models of line-driven accretion disk winds in cataclysmic variable systems and calculate wind mass-loss rates and terminal velocities. The models are 2.5-dimensional, include an energy balance condition with radiative heating and cooling processes, and includes local ionization equilibrium introducing time dependence and spatial dependence on the line radiation force parameters. The radiation field is assumed to originate in an optically thick accretion disk. Wind ion populations are calculated under the assumption that local ionization equilibrium is determined by photoionization and radiative recombination, similar to a photoionized nebula. We find a steady wind flowing from the accretion disk. Radiative heating tends to maintain the temperature in the higher density wind regions near the disk surface, rather than cooling adiabatically. For a disk luminosity L disk = L ⊙ , white dwarf mass M wd = 0.6M ⊙ , and white dwarf radii R wd = 0.01R ⊙ , we obtain a wind mass-loss rate ofṀ wind = 4 × 10 −12 M ⊙ yr −1 , and a terminal velocity of ∼ 3000 km s −1. These results confirm the general velocity and density structures found in our earlier constant ionization equilibrium adiabatic CV wind models. Further we establish here 2.5D numerical models that can be extended to QSO/AGN winds where the local ionization equilibrium will play a crucial role in the overall dynamics.

The impact of accretion disc winds on the optical spectra of cataclysmic variables

Monthly Notices of the Royal Astronomical Society

Many high-state non-magnetic cataclysmic variables (CVs) exhibit blueshifted absorption or P-Cygni profiles associated with ultraviolet (UV) resonance lines. These features imply the existence of powerful accretion disc winds in CVs. Here, we use our Monte Carlo ionization and radiative transfer code to investigate whether disc wind models that produce realistic UV line profiles are also likely to generate observationally significant recombination line and continuum emission in the optical waveband. We also test whether outflows may be responsible for the single-peaked emission line profiles often seen in high-state CVs and for the weakness of the Balmer absorption edge (relative to simple models of optically thick accretion discs). We find that a standard disc wind model that is successful in reproducing the UV spectra of CVs also leaves a noticeable imprint on the optical spectrum, particularly for systems viewed at high inclination. The strongest optical wind-formed recombination lines are H α and He II λ4686. We demonstrate that a higher density outflow model produces all the expected H and He lines and produces a recombination continuum that can fill in the Balmer jump at high inclinations. This model displays reasonable verisimilitude with the optical spectrum of RW Trianguli. No single-peaked emission is seen, although we observe a narrowing of the double-peaked emission lines from the base of the wind. Finally, we show that even denser models can produce a single-peaked H α line. On the basis of our results, we suggest that winds can modify, and perhaps even dominate, the line and continuum emission from CVs.

The Impact of Accretion Disk Winds on the Optical Spectra of Cataclysmic Variables

2015

Many high-state non-magnetic cataclysmic variables (CVs) exhibit blue-shifted absorption or P-Cygni profiles associated with ultraviolet (UV) resonance lines. These features imply the existence of powerful accretion disk winds in CVs. Here, we use our Monte Carlo ionization and radiative transfer code to investigate whether disk wind models that produce realistic UV line profiles are also likely to generate observationally significant recombination line and continuum emission in the optical waveband. We also test whether outflows may be responsible for the single-peaked emission line profiles often seen in high-state CVs and for the weakness of the Balmer absorption edge (relative to simple models of optically thick accretion disks). We find that a standard disk wind model that is successful in reproducing the UV spectra of CVs also leaves a noticeable imprint on the optical spectrum, particularly for systems viewed at high inclination. The strongest optical wind-formed recombinatio...

A method for the study of accretion disk emission in cataclysmic variables

s.d.t, 2011

We have developed a spectrum synthesis method for modeling the ultraviolet (UV) emission from the accretion disk from cataclysmic variables (CVs). The disk is separated into concentric rings, with an internal structure from the Wade & Hubeny disk-atmosphere models. For each ring, a wind atmosphere is calculated in the comoving frame with a vertical velocity structure obtained from a solution of the Euler equation. Using simple assumptions, regarding rotation and the wind streamlines, these one-dimensional models are combined into a single 2.5-dimensional model for which we compute synthetic spectra. We find that the resulting line and continuum behavior as a function of the orbital inclination is consistent with the observations, and verify that the accretion rate affects the wind temperature, leading to corresponding trends in the intensity of UV lines. In general, we also find that the primary mass has a strong effect on the P Cygni absorption profiles, the synthetic emission line profiles are strongly sensitive to the wind temperature structure, and an increase in the mass-loss rate enhances the resonance line intensities. Synthetic spectra were compared with UV data for two high orbital inclination nova-like CVs-RW Tri and V347 Pup. We needed to include disk regions with arbitrary enhanced mass loss to reproduce reasonably well widths and line profiles. This fact and a lack of flux in some high ionization lines may be the signature of the presence of density-enhanced regions in the wind, or alternatively, may result from inadequacies in some of our simplifying assumptions.

A Method for the Study of Accretion Disk Emission in Cataclysmic Variables. I. The Model

Astrophysical Journal, 2011

We have developed a spectrum synthesis method for modeling the UV emission from the accretion disk from cataclysmic variables (CVs). The disk is separated into concentric rings, with an internal structure from the Wade & Hubeny disk-atmosphere models. For each ring, a wind atmosphere is calculated in the co-moving frame with a vertical velocity structure obtained from a solution of the Euler equation. Using simple assumptions, regarding rotation and the wind streamlines, these 1D models are combined into a single 2.5D model for which we compute synthetic spectra. We find that the resulting line and continuum behavior as a function of the orbital inclination is consistent with the observations, and verify that the accretion rate affects the wind temperature, leading to corresponding trends in the intensity of UV lines. In general, we also find that the primary mass has a strong effect on the P-Cygni absorption profiles, the synthetic emission line profiles are strongly sensitive to the wind temperature structure, and an increase in the mass loss rate enhances the resonance line intensities. Synthetic spectra were compared with UV data for two high orbital inclination nova-like CVs - RW Tri and V347 Pup. We needed to include disk regions with arbitrary enhanced mass loss to reproduce reasonably well widths and line profiles. This fact and a lack of flux in some high ionization lines may be the signature of the presence of density enhanced regions in the wind, or alternatively, may result from inadequacies in some of our simplifying assumptions.

Disc accretion onto white dwarfs

Disc accretion onto white dwarfs In non-magnetic cataclysmic variables (CVs) a white dwarf accretes matter from a mainsequence secondary star via an accretion disc. The dynamical behaviour of the accretion disc determines the accretion rate onto the white dwarf. Thermal instabilities in the accretion disc associated with the ionisation of hydrogen can lead to a limit-cycle behaviour in which the disc switches quasi-periodically between high and low accretion states. This thermal limit-cycle model is the generally accepted explanation for dwarf nova outbursts observed in many CVs. The process of disc accretion in non-magnetic CVs is subject to a number of external conditions, namely mass transfer variations of the secondary star, stream overflow and irradiation by the white dwarf. In this thesis I develop a model for time-dependent disc accretion onto white dwarfs and analyse the influence of these external conditions on the accretion process. I examine the effects of mass transfer variations by deriving real mass transfer variations from light curve monitoring of the disc-less CV AM Her. These mass transfer variations I include in simulations of disc accretion onto white dwarfs in non-magnetic systems and find that the mass accretion rate of the disc relaxes to an equilibrium with the prevailing mass transfer rate on a rather short timescale. I conclude that the observed changes in outburst duration and outburst magnitude are caused by nearly simultaneous variations of the mass loss rate from the secondary. I also present a new model for the stripping of the stream by the accretion disc, and find that stream overflow can have subtle effects on the evolution of the accretion disc only if the amount of overflowing stream material exceeds 25% of the mass transfer rate. I conclude that solely very large stream overflow fractions can change the outbursts of dwarf novae. For realistic amounts of stream overflow the overall outburst behaviour is marginally changed. The accretion disc is mostly influenced by the white dwarf irradiation. I present a selfconsistent model for irradiated accretion discs and find that efficient irradiation in dwarf nova systems causes small "echo" outbursts following the larger ones immediately. This result contrasts with the observations of dwarf nova outbursts. As an explanation for this discrepancy I suggest that the reprocessing efficiency of disc irradiation is rather small. This is in agreement with results I obtain from detailed simulations of irradiated discs in post novae. These systems are excellent laboratories for studying the effects of disc irradiation because the white dwarf heated during the nova eruption provides a much stronger irradiation of the disc than in normal dwarf novae. I derive time-limits for the occurrence of dwarf nova outbursts in post novae and present detailed simulations of the evolution of irradiated discs in post novae. In addition to the developed theory of irradiated discs around white dwarfs, I show preliminary results of an intensive observing campaign on the post nova system V446 Her. Finally, discussing the influence of disc irradiation on the post nova evolution in the light of the current working hypothesis leads me to put forward a new scenario.

The Physics of Wind-Fed Accretion

We provide a brief review of the physical processes behind the radiative driving of the winds of OB stars and the Bondi-Hoyle-Lyttleton capture and accretion of a fraction of the stellar wind by a compact object, typically a neutron star, in detached high-mass X-ray binaries (HMXBs). In addition, we describe a program to develop global models of the radiatively-driven photoionized winds and accretion flows of HMXBs, with particular attention to the prototypical system Vela X-l. The models combine XSTAR photoionization calculations, HULLAC emission models appropriate to X-ray photoionized plasmas, improved models of the radiative driving of photoionized winds, FLASH time-dependent adaptive-mesh hydrodynamics calculations, and Monte Carlo radiation transport. We present two- and three-dimensional maps of the density, temperature, velocity, ionization parameter, and emissivity distributions of representative X-ray emission lines, as well as synthetic global Monte Carlo X-ray spectra. Such models help to better constrain the properties of the winds of HMXBs, which bear on such fundamental questions as the long-term evolution of these binaries and the chemical enrichment of the interstellar medium.

Structure and properties of transition fronts in accretion discs

Monthly Notices of The Royal Astronomical Society, 1999

We use high-resolution time-dependent numerical simulations of accretion discs around white dwarfs to study the structure and properties of transition fronts in the context of the thermal-viscous disc instability model. The thermal structure of cooling and heating fronts is dominated by radiative cooling and viscous heating, respectively, except in a very narrow precursor region in heating fronts where advection and radial transport of energy dominate. Cooling fronts are much broader than heating fronts, but the widths of both types of fronts scale with the local vertical scale height of the disc. We confirm that during a fair fraction of the propagation time of a cooling front, the structure of the inner disc is close to self-similar. The speed of heating fronts is ~ a few km/s, while the speed of cooling fronts is ~ a fraction of a km/s. We show that direct measurements of the speed of transition fronts probably cannot discriminate between various prescriptions proposed for the viscosity parameter alpha. A natural prediction of the disc instability model is that fronts decelerate as they propagate in the disc, independent of the prescription for alpha. Observation of this effect would confirm that dwarf nova outbursts are driven by the thermal-viscous instability. Most of our results also apply to low mass X-ray binaries in which the accreting object is a neutron star or a black hole.