Magnetic Towers and Binary-Formed Disks: New Results for PN Evolution

We present new results of 3-D AMR MHD simulations focusing on two distinct aspects of PPN evolution. We first report new simulations of collimated outflows driven entirely by magnetic fields. These Poynting flux dominated "magnetic towers" hold promise for explaining key properties of PPN flows. Our simulations address magnetic tower evolution and stability. We also present results of a campaign of simulations to explore the development of accretion disks formed via wind capture. Our result focus on the limits of disk formation and the range of disk properties.Comment: 4 pages, 2 figures, to appear in the proceedings of the IAU Symposium 283 "Planetary Nebulae, an Eye to the Future", Puerto de la Cruz, Tenerife, Spai

Size of discs formed by wind accretion in binaries can be underestimated if the role of wind-driving force is ignored

Binary systems consisting of a secondary accreting form a wind-emitting primary are ubiquitous in astrophysics. The phenomenology of such Bondi-Hoyle-Lyttleton (BHL) accretors is particularly rich when an accretion disc forms around the secondary. The outer radius of such discs is commonly estimated from the net angular momentum produced by a density variation of material across the BHL or Bondi accretion cylinder, as the latter is tilted with respect to the direction to the primary due to orbital motion. But this approach has ignored the fact that the wind experiences an outward driving force that the secondary does not. In actuality, the accretion stream falls toward a retarded point in the secondary's orbit as the secondary is pulled toward the primary relative to the stream. The result is a finite separation or "accretion stream impact parameter" (ASIP) separating the secondary and stream. When the orbital radius a_o exceeds the BHL radius r_b, the ratio of outer disc radius estimated as the ASIP to the conventional estimate a_o^{1/2}/r_b^{1/2}>1. We therefore predict that discs will form at larger radii from the secondary than traditional estimates. This agrees with the importance of the ASIP emphasized by Huarte-Espinosa et al. (2013) and the practical consequence that resolving the initial outer radius of such an accretion disc in numerical simulations can be less demanding than what earlier estimates would suggest.Comment: 8 pages, 2 figures, accepted by MNRAS; in pres