MHD simulations of disk-star interaction

We discuss a number of topics relevant to disk-magnetosphere interaction and how numerical simulations illuminate them. The topics include: (1) disk-magnetosphere interaction and the problem of disk-locking; (2) the wind problem; (3) structure of the magnetospheric flow, hot spots at the star's surface, and the inner disk region; (4) modeling of spectra from 3D funnel streams; (5) accretion to a star with a complex magnetic field; (6) accretion through 3D instabilities; (7) magnetospheric gap and survival of protoplanets. Results of both 2D and 3D simulations are discussed.Comment: 12 pages, 10 figures, Star-Disk Interaction in Young Stars, Proceedings of the International Astronomical Union, IAU Symposium, Volume 243. See animations at http://astro.cornell.edu/~romanova/projects.htm and at http://astro.cornell.edu/us-rus

Quark-nova remnants IV: Application to radio emitting AXP transients

(Abridged) XTE J1810-197 and 1E 1547.0-5408 are two transient AXPs exhibiting radio emission with unusual properties. In addition, their spin down rates during outburst show opposite trends, which so far has no explanation. Here, we extend our quark-nova model for AXPs to include transient AXPs, in which the outbursts are caused by transient accretion events from a Keplerian (iron-rich) degenerate ring. For a ring with inner and outer radii of 23.5 km and 26.5 km, respectively, our model gives a good fit to the observed X-ray outburst from XTE J1810-197 and the behavior of temperature, luminosity, and area of the two X-ray blackbodies with time. The two blackbodies in our model are related to a heat front (i.e. Bohm diffusion front) propagating along the ring's surface and an accretion hot spot on the quark star surface. Radio pulsations in our model are caused by dissipation at the light cylinder of magnetic bubbles, produced near the ring during the X-ray outburst. The delay between X-ray peak emission and radio emission in our model is related to the propagation time of these bubbles to the light cylinder. We predict a ~1 year and ~1 month delay for XTE J1810-197 and 1E 1547.0-5408, respectively. The observed flat spectrum, erratic pulse profile, and the pulse duration are all explained in our model as a result of X-point reconnection events induced by the dissipation of the bubbles at the light cylinder. The spin down rate of the central quark star can either increase or decrease depending on how the radial drift velocity of the magnetic islands changes with distance from the central star. We suggest an evolutionary connection between transient AXPs and typical AXPs in our model.Comment: 16 journal pages, 4 figures and 1 table [Version accepted for publication in A&A

Launching of Conical Winds and Axial Jets from the Disk-Magnetosphere Boundary: Axisymmetric and 3D Simulations

We investigate the launching of outflows from the disk-magnetosphere boundary of slowly and rapidly rotating magnetized stars using axisymmetric and exploratory 3D magnetohydrodynamic (MHD) simulations. We find long-lasting outflows in both cases. (1) In the case of slowly rotating stars, a new type of outflow, a conical wind, is found and studied in simulations. The conical winds appear in cases where the magnetic flux of the star is bunched up by the disk into an X-type configuration. The winds have the shape of a thin conical shell with a half-opening angle 30-40 degrees. The conical winds may be responsible for episodic as well as long-lasting outflows in different types of stars. (2) In the case of rapidly rotating stars (the "propeller regime"), a two-component outflow is observed. One component is similar to the conical winds. A significant fraction of the disk matter may be ejected into the winds. A second component is a high-velocity, low-density magnetically dominated axial jet where matter flows along the opened polar field lines of the star. The jet has a mass flux about 10% that of the conical wind, but its energy flux (dominantly magnetic) can be larger than the energy flux of the conical wind. The jet's angular momentum flux (also dominantly magnetic) causes the star to spin-down rapidly. Propeller-driven outflows may be responsible for the jets in protostars and for their rapid spin-down. The jet is collimated by the magnetic force while the conical winds are only weakly collimated in the simulation region.Comment: 29 pages and 29 figures. This version has a major expansion after comments by a referee. The 1-st version is correct but mainly describes the conical wind. This version describes in greater detail both the conical winds and the propeller regime. Accepted to the MNRA

Nonlinear monotonization of the Babenko scheme

The goal of the paper is to present and test the nonlinear monotonization of the Babenko scheme for solving 2D linear advection equation with alternating‐sign velocities. The numerical method of monotonization is based on the idea of limited artificial diffusion. There are some approaches for constructing quasi‐monotonic second order approximation schemes for solving hyperbolic systems and equations of gas dynamics: flux correction methods, the Godunov method, TVD methods and others. In particular, many authors developed the idea of TVD method. We try to use this idea to get a new quasi‐monotonic high order accuracy scheme based on the well‐known non‐monotonic Babenko scheme. The algorithm is presented for 1D problem. For testing 2D problem we use the splitting algorithm. The proposed monotonized scheme has shown the best results among all considered in the paper schemes especially for non‐smooth initial profile. Babenko schemos ("kvadrato") netiesinė monotonizacija Santrauka. Straipsnio tikslas yra Babenko schemos dvimačiam tiesiniam advekcijos uždaviniui su ženklą keičiančiais greičiais netiesinės monotonizacijos metodo pateikimas ir testavimas. Skaitinis monotonizacijos metodas remiasi dirbtinės difuzijos įvedimo idėja. Egzistuoja keli kvazimonotoniniu antros aproksimacijos eilės schemų hiperbolinems sistemoms ir dujų dinamikos lygtims konstravimo būdai: srautų korekcijos metodas, Godunovo metodas, TVD ir kiti metodai. Mes naudojame TVD idėją naujos kvazimonotoninės aukštos tikslumo eilės schemos gavimui remiantis plačiai žinoma monotonine baigtinių skirtumų Babenko schema. Skaitinis algoritmas pateiktas vienmačio uždavinio atveju. Dvimačio uždavinio sprendimui taikomas faktorizacijos algoritmas. Pasiūlytos monotonizuotos schemos pagalba gauti rezultatai yra geriausi, lyginant su kitu straipsnyje naudojamų schemų skaičiavimų rezultatais. Ypatingai gerai tai matoma neglodaus pradinio profilio atveju. First Published Online: 14 Oct 201

The Solution of the Nonlinear Monotonized K.I.Babenko ('square') Difference Scheme.

Abstract: The paper is dedicated to construction of the solution of the nonlinear monotonized K.I.Babenko difference scheme for 1D advection equation with sign-alternative velocity. The analysis has shown that the solution exists and is unique. The formula of the exact solution has been obtained.Note: Research direction:Mathematical problems and theory of numerical method

Numerical simulations of the interaction of accretion disks with young star magnetospheres

Aims.We present the results of the numerical simulations of the interaction between a magnetized star and an imperfectly conducting accretion disk. The star is rotating with constant angular velocity. The differentially rotating Keplerian disk is treated as a boundary condition. We are interested in the magnetic field topology dependence on the electrical conductivity of the disk. Methods.To analyze the “star–disk” interaction we numerically investigate the MHD equations using Godunov-type high resolution numerical methods. Results.It was found that in our model the “star–disk” interaction occurs with a quasi-periodic reconnection of the magnetic field coronal loops and plasmoid ejections. In the case of the perfect disk conductivity, the evolution of the coronal magnetic field leads to the periodic outflow of angular momentum from the disk. In the case of an imperfectly conducting disk, the configuration of the magnetic field is formed such that the disk angular momentum carried by the magnetic field is balanced by angular momentum carried by matter. It should be noted that we used the ideal MHD equation to obtain the solutions. The reconnection process in the disk corona depends on the numerical diffusivity that exists in our numerical code. Our simulations treat reconnection as occurring in current sheets. The thickness of the current sheet is broadened by numerical resistivity. Nevertheless, we suppose that the reconnection and plasmoid ejection takes place as well for real magnetic diffusivity. To verify the method and results we also used several more detailed grids to estimate the numerical diffusivity of the scheme. It is turned out that the setup model presented in the paper quite reasonable satisfies the goal of this paper, i.e., to investigate the regime of interaction between the magnetized star and the disk

Effects of Flaring Activity on Dynamics of Accretion Disks in YSOs

We investigate the effects of strong flares on the accretion phenomena in YSOs. Among all classical assumptions, the model accounts magnetic-field oriented thermal conduction. We study the global dynamics of the system for two positions of the heating release triggering the flare

MHD Modeling of Accretion Processes in Young Stars with the PLUTO Code

As shown by observations, many young stars (age<5-10 Myr) harbor a circumstellar disk and accrete material from it through the star-disk magnetosphere. Despite the large amount of observational data in the infrared, optical and X-ray bands, different issues regarding star-disk interactions are still yet open. Many of these issues need detailed physical models of the star-disk system for a better insight. To this end, we are developing a model describing the interaction between the accreting material and the star atmosphere, using the 3D Magneto-HydroDynamical (MHD) code PLUTO developed at the University of Torino. We plan to perform a set of demanding simulations on the PI2S2 Grid infrastructure. Here, we describe the model, the main features on the most recent PLUTO version which we ported on the Grid and the first runs of the PLUTO code performed on the PI2S2 Grid

High Performance Computing on the COMETA Grid Infrastructure

We present the High Performance Computing (HPC) projects jointly developed at the INAF - Osservatorio Astronomico di Palermo and at the DSFA - Universita` di Palermo which benefits of the Grid infrastructure of COMETA. We have contributed to setup the infrastructure in order to run HPC applications on the Grid. We report on our experience regarding to porting HPC applications to the Grid and to the first HPC simulations performed. The most demanding simulations describe the interaction of a magnetized supernova shock wave with an interstellar gas cloud. We discuss the resources required for the simulations, the performance and the scalability of our code on the Grid, and present first results