arXiv subjects

Sergei Dyda

Publications and source records attributed to Sergei Dyda.

26 records · Page 2Linked to original sources

arXiv2018

Non-Axisymmetric Line Driven Disc Winds I - Disc Perturbations

We study mass outflows driven from accretion discs by radiation pressure due to spectral lines. To investigate non-axisymmetric effects, we use the Athena++ code and develop a new module to account for radiation pressure driving. In 2D, our new simulations are consistent with previous 2D axisymmetric solutions by Proga et al. who used the Zeus 2D code. Specifically, we find that the disc winds are time dependent, characterized by a dense stream confined to $\sim 45^{\circ}$ relative to the disc midplane and bounded on the polar side by a less dense, fast stream. Introducing a vertical, $ϕ$-dependent, subsonic velocity perturbation in the disc midplane does not change the overall character of the solution but global outflow properties such as the mass, momentum and kinetic energy fluxes are altered by up to 100%. Non-axisymmetric density structures develop and persist mainly at the base of the wind. They are relatively small, and their densities can be a few times higher that the azimuthal average. The structure of the non-axisymmetric and axisymmetric solutions differ also in other ways. Perhaps most importantly from the observational point of view are the differences in the so called clumping factors, that serve as a proxy for emissivity due to two body processes. In particular, the spatially averaged clumping factor over the entire fast stream, while it is of a comparable value in both solutions, it varies about 10 times faster in the non-axisymmetric case.

astro-ph.HE↗

arXiv2018

Non-Axisymmetric Line Driven Disc Winds II - Full Velocity Gradient

We study non-axisymetric features of 3D line driven winds in the Sobolev approximation, where the optical depth is calculated using the full velocity gradient. We find that non-axisymmetric density features, so called clumps, form primarily at the base of the wind on super-Sobolev length scales. The density of clumps differs by a factor of $\sim 3$ from the azimuthal average, the magnitude of their velocity dispersion is comparable to the flow velocity and they produce $\sim 20\%$ variations in the column density. Clumps may be observable because differences in density produce enhancements in emission and absorption profiles or through their velocity dispersion which enhances line broadening.

astro-ph.HE↗

arXiv2017

Irradiation of Astrophysical Objects - SED and Flux Effects on Thermally Driven Winds

We develop a general method for the self consistent calculation of the hydrodynamics of an astrophysical object irradiated by a radiation field with an arbitrary strength and spectral energy distribution (SED). Using the XSTAR photoionization code, we calculate heating and cooling rates as a function of gas photoionization parameter and temperature for several examples of SEDs: bremsstrahlung, blackbody, hard and soft state XRBs, Type 1 and Type 2 AGN. As an application of our method we study the hydrodynamics of 1-dimensional spherical winds heated by a uniform radiation field using the code Athena++. We find that in all cases explored a wind settles into a transonic, steady state. The wind evolves along the radiative heating equilibrium curve until adiabatic cooling effects become important and the flow departs from radiative equilibrium. If the flow is heated very rapidly, for example as in a thermally unstable regime, the corresponding column density of gas is low. Perhaps one of the most intriguing results of our work is the two stage acceleration of the wind that happens when there are two thermally unstable regions and the flux is relatively high. The efficiency with which the radiation field transfers energy to the wind is dependent on the SED of the external source, particularly the relative flux of soft X-rays. These results suggest that detailed photoionization calculations are essential not only to predict spectra but also to properly capture the flow dynamics.

astro-ph.HE↗

arXiv2015

Asymmetric MHD Outflows/Jets from Accreting T Tauri Stars

Observations of jets from young stellar objects reveal the asymmetric outflows from some sources. A large set of $2.5$D MHD simulations has been carried out for axisymmetric viscous/diffusive disc accretion to rotating magnetized stars for the purpose of assessing the conditions where the outflows or jets are asymmetric relative to the equatorial plane. We consider initial magnetic fields that are symmetric about the equatorial plane and consist of a radially distributed field threading the disc (disc-field) and a stellar dipole field.({\bf 1}). For pure disc-fields the symmetry or asymmetry of the outflows is affected by the midplane plasma $β$ of the disc (where $β$ is the ratio of the plasma pressure to the magnetic pressure). For the low density discs with small plasma $β$ values, outflows are observed to be symmetric to within $10\%$ over timescales of hundreds of inner disc orbits. For the denser higher $β$ discs, the coupling of the upper and lower coronal plasmas is broken, and quasi-periodic field motion in the two hemispheres becomes different. This asymmetry leads to asymmetric episodic outflows. ({\bf 2.}) Accreting stars with a stellar dipole field and no disc-field exhibit episodic, two component outflows - a magnetospheric wind and an inner disc wind from somewhat larger radial distances. Both are characterized by similar velocity profiles but the magnetospheric wind has densities $\gtrsim 10$ times that of the disc wind. ({\bf 3}.)Adding a disc-field parallel to the stellar dipole field acts to enhance the magnetospheric winds but suppress the disc wind. ({\bf 4}.) In contrast, adding a disc-field which is anti-parallel to the stellar dipole field in the disc acts to suppress the magnetospheric and disc winds. Our simulations reproduce some key features of observations of asymmetric outflows of T Tauri stars.

astro-ph.HE↗

arXiv2014

Kelvin-Helmholtz Instability of Counter-Rotating Discs

Observations of galaxies and models of accreting systems point to the occurrence of counter-rotating discs where the inner part of the disc ($r<r_0$) is co-rotating and the outer part is counter-rotating. This work analyzes the linear stability of radially separated co- and counter-rotating thin discs. The strong instability found is the supersonic Kelvin-Helmholtz instability. The growth rates are of the order of or larger than the angular rotation rate at the interface. The instability is absent if there is no vertical dependence of the perturbation. That is, the instability is essentially three-dimensional. The nonlinear evolution of the instability is predicted to lead to a mixing of the two components, strong heating of the mixed gas, and vertical expansion of the gas, and annihilation of the angular momenta of the two components. As a result the heated gas will free-fall towards the disc's center over the surface of the inner disc.

astro-ph.GA↗

arXiv2014

Counter-Rotating Accretion Discs

Counter-rotating discs can arise from the accretion of a counter-rotating gas cloud onto the surface of an existing co-rotating disc or from the counter-rotating gas moving radially inward to the outer edge of an existing disc. At the interface, the two components mix to produce gas or plasma with zero net angular momentum which tends to free-fall towards the disc center. We discuss high-resolution axisymmetric hydrodynamic simulations of a viscous counter-rotating disc for cases where the two components are vertically separated and radially separated. The viscosity is described by an isotropic $α-$viscosity including all terms in the viscous stress tensor. For the vertically separated components a shear layer forms between them. The middle of this layer free-falls to the disk center. The accretion rates are increased by factors $\sim 10^2-10^4$ over that of a conventional disc rotating in one direction with the same viscosity. The vertical width of the shear layer and the accretion rate are strongly dependent on the viscosity and the mass fraction of the counter-rotating gas. In the case of radially separated components where the inner disc co-rotates and the outer disc rotates in the opposite direction, a gap between the two components opens and closes quasi-periodically. The accretion rates are $\gtrsim 25$ times larger than those for a disc rotating in one direction with the same viscosity.

astro-ph.GA↗

arXiv2012

Vacuum Instability in Chern-Simons Gravity

We explore perturbations about a Friedmann-Robertson-Walker background in Chern-Simons gravity. At large momenta one of the two circularly polarized tensor modes becomes ghostlike. We argue that nevertheless the theory does not exhibit classical runaway solutions, except possibly in the relativistic nonlinear regime. However, the ghost modes cause the vacuum state to be quantum mechanically unstable, with a decay rate that is naively infinite. The decay rate can be made finite only if one interprets the theory as an effective quantum field theory valid up to some momentum cutoff, which violates Lorentz invariance. By demanding that the energy density in photons created by vacuum decay over the lifetime of the Universe not violate observational bounds, we derive strong constraints on the two dimensional parameter space of the theory, consisting of the cutoff and the Chern-Simons mass.

gr-qc↗

arXiv2007

Cosmic Strings and Weak Gravitational Lensing

We study the deflection of light in the background of a "wiggly" cosmic string, and investigate whether it is possible to detect cosmic strings by means of weak gravitational lensing. For straight strings without small-scale structure there are no signals. In the case of strings with small-scale structure leading to a local gravitational attractive force towards the string, there is a small signal, namely a preferential elliptical distortion of the shape of background galaxies in the direction corresponding to the projection of the string onto the sky. The signal can be statistically distinguished from the signal produced by a linear distribution of black holes by employing an ellipticity axis distribution statistic.

astro-ph↗