Magnetic Spin-Up of Line-Driven Stellar Winds

Abstract. We summarize recent 2D MHD simulations of line-driven stellar winds from rotating hot-stars with a dipole magnetic eld aligned to the star's rotation axis. For moderate to strong elds, much wind out ow is initially along closed magnetic loops that nearly corotate as a solid body with the underlying star, thus providing a torque that results in an eective angular momentum spin-up of the out owing material. But instead of forming the \magnetically torqued disk " (MTD) postulated in previous phenemenological analyses, the dynamical simulations here show that material trapped near the tops of such closed loops tends either to fall back or break out, depending on whether it is below or above the Keplerian corotation radius. Overall the results raise serious questions about whether magnetic torquing of a wind out ow could naturally result in a Keplerian circumstellar disk. However, for very strong elds, it does still seem possible to form a centrifugally supported, \magnetically rigid disk " (MRD), in which the eld not only forces material to maintain a rigid-body rotation, but for some extended period also holds it down against the outward centrifugal force at the loop tops. We argue that such rigid-body disks seem ill-suited to explain the disk emission from Be stars, but could provide a quite attractive paradigm for circumstellar emission from the magnetically strong Bp and Ap stars. 1. Background and context Within the broad theme of this conference on \Magnetic Fields in O, B, and A Stars", a key issue is the role of magnetic elds in channeling the line-driven stellar wind out ows from such hot, luminous stars. Initial results of full MHD simulations for models without stellar rotation (ud-Doula & Owocki 2002) are reviewed in these proceedings through the write-up for the talk presented by A. ud-Doula. Here we summarize further simulation results for rotating hot-stars with a magnetic dipole aligned to the stellar rotation axis. These simula