Magnetospheric launching in resistive MHD simulations

Proceedings of Magnetic Fields in the Universe: From Laboratory and Stars to Primordial Structures arXiv:1110.5699v1 [astro-ph.SR] 26 Oct 2011 Aug. 21st – 27th 2011, Zakopane, Poland Eds. M. Soida, K. Otmianowska-Mazur, E.M. de Gouveia Dal Pino & A. Lazarian Magnetospheric launching in resistive MHD simulations Miljenko Čemeljić and Hsien Shang Academia Sinica, Institute of Astronomy and Astrophysics and Theoretical Institute for Advanced Research in Astrophysics, P.O. Box 23-141, Taipei 106, Taiwan Abstract We perform numerical simulations in the close vicinity of a slowly rotating young stellar object. Using our own resistive MHD Zeus347 code in 2D axisymmetry, magnetospheric interaction experiences four robust stages in evolution. Quasi-stationary axial and conical streams of outflowing matter can last many orbital periods as results of the resistivity-facilitated magnetic reconnections. The shape of the magnetic field depends on resistivity in the magnetosphere. 1 Setup and Results In simulations with our resistive version of the Zeus-3D code, in the axisymmetry option, we set innermost region of a star-disk system (see Fig. 1). The computational domain is R × Z = (90 × 90) grid cells = (0.2 × 0.2) AU. Disk is in a slightly sub-Keplerian rotation, with a rotating, hydrostatic corona and purely dipole magnetic field centered at the origin. Resistivity in the disk is constant, and in corona it is modeled by density, with η ∼ ρ1/3 , following Fendt & Čemeljić(2002). All simulations of star-disk interaction in our setup go Figure 1: In logarithmic color grading is shown the density in our initial conditions, solid lines show the poloidal magnetic field of a stellar dipole, and vectors depict the velocity. Star is set as a rotating, absorbing boundary condition inside the computational box, enclosing the origin. In the disk gap, matter is allowed to flow through the disk mid-plane. through four stages: 1) relaxation with pinching of magnetic field inwards, 2) reconnection with opening of the stellar dipole, 3) narrowing of the disk gap with formation of transient funnel flow onto the stellar surface, 4) final stage of equilibrium of magnetic and disk ram 1 2 M. Čemeljić, H. Shang pressure, with two outflows, one axial and another conical. The final stage is shown in Fig. 2. It is similar to the result reported in Romanova et al.(2009), but with the difference that our simulation is in the regime with magnetic Prandtl number smaller than unity. Physical resistivity, together with reconnection, helps launching of outflows. Figure 2: Final stage in our simulations with stellar magnetic field of the order of 100 G. In logarithmic color grading is shown the poloidal mass flux, the solid lines show poloidal magnetic field lines, and arrows show poloidal velocity vectors. Conical and axial outflows are ejected from the close vicinity of the star. Figure 3: Poloidal magnetic field lines for the different timesteps in the same simulation. Reshaping of the initial dipolar magnetic field is enabled by reconnection. After the pinching of the field in plasmoid ejected during the relaxation (left panel), reconnection helps the change of field topology (middle panel) into the stellar and disk field components (right panel). 2 Conclusions In purely resistive MHD numerical simulations, we obtain solutions with two streams of matter flowing out from the innermost magnetosphere around a young stellar object. This is the first simulation in which a conical outflow is launched with magnetic Prandtl number less than unity, that is, with resistivity larger than viscosity, by magnetic reconnection. The configurations of the field are modified during the reconnection to channel flowing out from the innermost region of the disks. References Fendt, Ch., Čemeljić, M., 2002, A&A, 395, 1045 Romanova, M.M., Ustyugova, G.V., Koldoba, A.V., Lovelace, R.V.E., 2009, MNRAS, 399, 1802