One-sided Outflows/Jets from Rotating Stars with Complex Magnetic Fields

Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 30 October 2018 (MN LATEX style file v2.2) One-sided Outflows/Jets from Rotating Stars with Complex Magnetic Fields R. V. E. Lovelace1⋆, M. M. Romanova2†, G. V. Ustyugova3‡, A. V. Koldoba4§ 1 Departments of Astronomy and Applied and Eng. Phys. Cornell University, Ithaca, NY 14853, USA 2 Department of Astronomy, Cornell University, Ithaca, NY 14853, USA 3 Keldysh Institute of Applied Mathematics, Russian Academy of Sciences, Moscow, Russia 4 Institute for Mathematical Modeling, Russian Academy of Sciences, Moscow, Russia 30 October 2018 ABSTRACT We investigate the generation of intrinsically asymmetric or one-sided outflows or jets from disk accretion onto rotating stars with complex magnetic fields using ax- isymmetric (2.5D) magnetohydrodynamic simulations. The intrinsic magnetic field of the star is assumed to consist of a superposition of an aligned dipole and an aligned quadrupole in different proportions. The star is assumed to be rapidly rotating in the sense that the star’s magnetosphere is in the propeller regime where strong outflows occur. Our simulations show that for conditions where there is a significant quadrupole component in addition to the dipole component, then a dominantly one-sided conical wind tends to form on the side of the equatorial plane with the larger value of the intrinsic axial magnetic field at a given distance. For cases where the quadrupole com- ponent is absent or very small, we find that dominantly one-sided outflows also form, but the direction of the flow “flip-flops” between upward and downward on a time- scale of ∼30 days for a protostar. The average outflow will thus be symmetrical. In the case of a pure quadrupole field we find symmetric outflows in the upward and downward directions. Key words: accretion, accretion discs; MHD; stars: magnetic fields 1 INTRODUCTION There is clear evidence, mainly from Hubble Space Telescope (HST) observations, of the asymmetry between the approach- ing and receding jets from a number of young stars. The ob- jects include the jets in HH 30 (Bacciotti et al. 1999), RW Aur (Woitas et al. 2002), TH 28 (Coffey et al. 2004), and LkHα 233 (Pererin & Graham 2007). Specifically, the radial speed of the approaching jet may differ by a factor of two from that of the receding jet. For example, for RW Aur the radial red- shifted speed is ∼100 km/s whereas the blueshifted radial speed is ∼175 km/s. The mass and momentum fluxes are also significantly different for the approaching and receding jets in a number of cases. Of course, it is possible that the ob- served asymmetry of the jets could be due to say differences in the gas densities on the two sides of the source. Here, we investigate the case of intrinsic asymmetry where the asym- metry of outflows is connected with asymmetry of the star’s magnetic field. ⋆E-mail: RVL1@cornell.edu † E-mail: romanova@astro.cornell.edu ‡ E-mail: ustyugg@rambler.ru § E-mail: koldoba@rambler.ru There is substantial observational evidence that young stars often have complex magnetic fields consisting of dipole, quadrupole, and higher order poles possibly misaligned with respect to each other and the rotation axis (Donati et al. 2007a, b; 2008; Jardine et al. 2002). Analysis of mater flow around stars with realistic fields have shown that a fraction of the star’s magnetic field lines are open and may carry outflows (e.g., Gregory et al. 2006). A number of global 3D MHD simulations have been done of disk accretion onto rotating stars with complex mag- netic fields. The star’s intrinsic field may be a superposi- tion of aligned or misaligned dipole and quadrupole fields (Long, Romanova, & Lovelace 2007, 2008), or a superposition of dipole and octupole field components (Long, Romanova, Lamb 2009; Romanova et al. 2009a; Long et al. 2010). These simulations were focused on accretion processes. To study the outflows from these systems requires a much lower coronal density than assumed in these works. Intermittent outflows from the disk-magnetosphere boundary have been found in axisymmetric simulations in cases where the star has a dipole magnetic field and where symmetry about the equatorial plane was assumed (e.g., Goodson, Winglee, & B¨ohm 1997; Goodson, B¨ohm, & Winglee 1999). c⃝0000 RAS arXiv:1004.0385v1 [astro-ph.HE] 2 Apr 2010 2 R. V. E. Lovelace et al. Figure 1. The magnetic field lines Ψ(r, z) = const and constant mag- netic pressure lines for the case of an aligned dipole and quadrupole field where the flux function is Ψ = µdr2/R3 + (3/4)µqzr2/R5, where R2 = r2 + z2 and µd is the dipole moment and µq is the quadrupole moment. Roughly, µq/µd is the distance at which dipole and quadrupole fields are equal. The funnel flow (ff) and the wind in this figure are suggested. The dashed lines are constant values of B2. In long-time axisymmetric (2.5D) simulations, long- lasting outflows were obtained first in the propeller regime where the star spins rapidly (Romanova et al. 2005; Ustyu- gova et al. 2006),

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