arXiv subjects

Miljenko Čemeljić

Publications and source records attributed to Miljenko Čemeljić.

27 records · Page 2Linked to original sources

arXiv2020

Backflow in Accretion Disk

Analytical solution for a thin hydrodynamic accretion disk shows that for some values of the viscosity parameter part of the accretion flow in the disk is not towards the star, but in the opposite direction. We study thin disks by performing hydrodynamic simulations and compare the numerical with analytical results. We confirm that for viscosity coefficient smaller than a critical value, there is a backflow in simulations near the disk mid-plane. The distance from the star to the starting point of backflow is increasing with viscosity, as predicted by the analytical solution. When the viscosity coefficient is larger than critical, there is no backflow in the disk.

astro-ph.SR↗

arXiv2020

Transport of dust grain particles in the accretion disk

Entrainment of dust particles in the flow inside and outside of the proto-planetary disk has implications for the disk evolution and composition of planets. Using quasi-stationary solutions in our star-disk simulations as a background, we add dust particles of different radii in post-processing of the results, using our Python tool DUSTER. The distribution and motion of particles in the disk is followed in the cases with and without the backflow in the disk. We also compare the results with and without the radiation pressure included in the computation.

astro-ph.SR↗

arXiv2020

Towards pseudo-Newtonian black hole jets: comparison of forces

We perform resistive and viscous MHD simulations of axial jet launching from a magnetized thin accretion disk around a supermassive black hole. We compare the forces in our pseudo-Newtonian simulations for the Paczyński-Wiita and Kluźniak-Lee potentials. The results will help to find magnetic field configurations and physical parameters conducive to jet launching.

astro-ph.HE↗

arXiv2018

Analytical solution for magnetized thin accretion disk in comparison with numerical simulations

We obtained equations for a thin magnetic accretion disk, using the method of asymptotic approximation. They cannot be solved analytically-without solutions for a magnetic field in the magnetosphere between the star and the disk, only a set of general conditions on the solutions can be derived. To compare the analytical results with numerical solutions, we find expressions for physical quantities in the disk, using our results from resistive and viscous star-disk magnetospheric interaction simulations.

astro-ph.SR↗

arXiv2018

Light curve variation caused by accretion column switching stellar hemispheres

I use two-dimensional axisymmetric numerical simulations of star-disk magnetospheric interaction to construct a three-dimensional model of hot spots on the star created by infalling accretion columns. The intensity of emitted radiation is computed with minimal assumptions, as seen by observers at infinity from the different angles. In the illustrative examples, shown is a change in the intensity curve with changes in the geometry of the model, and also a change in results with modification in the physical parameters in the simulation.

astro-ph.SR↗

arXiv2018

"Atlas" of numerical solutions for star-disk magnetospheric interaction

Aims: I report results in numerical simulations of star-disk magnetospheric interaction. A thin accretion disk with corona above a rotating stellar surface is simulated in a parameter study, to find trends in the angular momentum flux. The results are presented in the case of Young Stellar Objects, but they can be rescaled to other objects with similar geometry. Methods: In the performed resistive and viscous magneto-hydrodynamic simulations, a quasi-stationary state is reached in the cases with different parameters. Angular momentum fluxes in the different components of the flow are computed, to compare the results. Results: Results in the simulations are presented with the matter density distribution and magnetic field geometry displayed in an `Atlas' of solutions. The torque exerted on the star is computed, together with the angular momentum flux loaded into outflow, in the cases when a conical outflow is formed. In the studied part of the parameter space, I find trends in both components of the flow.

astro-ph.SR↗

arXiv2014

Large resistivity in numerical simulations of radially self-similar outflows

We investigate the differences between an outflow in a highly-resistive accretion disk corona, and the results with smaller or vanishing resistivity. For the first time, we determine conditions at the base of a two-dimensional radially self-similar outflow in the regime of very large resistivity. We performed simulations using the {\sc pluto} magnetohydrodynamics code, and found three modes of solutions. The first mode, with small resistivity, is similar to the ideal-MHD solutions. In the second mode, with larger resistivity, the geometry of the magnetic field changes, with a "bulge" above the super-fast critical surface. At even larger resistivities, the third mode of solutions sets in, in which the magnetic field is no longer collimated, but is pressed towards the disk. This third mode is also the final one: it does not change with further increase of resistivity. These modes describe topological change in a magnetic field above the accretion disk because of the uniform, constant Ohmic resistivity.

astro-ph.SR↗

arXiv2014

Magnetic reconnection in a comparison of topology and helicities in two and three dimensional resistive MHD simulations

Through a direct comparison between numerical simulations in two and three dimensions, we investigate topological effects in reconnection. A simple estimate on increase in reconnection rate in three dimensions by a factor of $\sqrt{2}$, when compared with a two-dimensional case, is confirmed in our simulations. We also show that both the reconnection rate and the fraction of magnetic energy in the simulations depend linearly on the height of the reconnection region. The degree of structural complexity of a magnetic field and the underlying flow is measured by current helicity and cross-helicity. We compare results in simulations with different computational box heights.

astro-ph.SR↗

arXiv2013

Magnetospheric Accretion and Ejection of Matter in Resistive Magnetohydrodynamic Simulations

The ejection of matter in the close vicinity of a young stellar object is investigated, treating the accretion disk as a gravitationally bound reservoir of matter. By solving the resistive MHD equations in 2D axisymmetry using our version of the Zeus-3D code with newly implemented resistivity, we study the effect of magnetic diffusivity in the magnetospheric accretion-ejection mechanism. Physical resistivity was included in the whole computational domain so that reconnection is enabled by the physical as well as the numerical resistivity. We show, for the first time, that quasi-stationary fast ejecta of matter, which we call {\em micro-ejections}, of small mass and angular momentum fluxes, can be launched from a purely resistive magnetosphere. They are produced by a combination of pressure gradient and magnetic forces, in presence of ongoing magnetic reconnection along the boundary layer between the star and the disk, where a current sheet is formed. Mass flux of micro-ejection increases with increasing magnetic field strength and stellar rotation rate, and is not dependent on the disk to corona density ratio and amount of resistivity.

astro-ph.SR↗