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Magnetic Channeling of Radiatively Driven Hot-Star Winds
Owocki, Stan; Townsend, Rich; and ud-Doula, Asif

Abstract

Massive, hot, luminous stars have strong stellar winds driven by line-scattering of the star's continuum radiation. Spectropolarimetric observations have detected substantial large-scale dipole magnetic fields for several hot stars. This paper discusses our recent efforts to carry out MHD simulations of the effect of magnetic fields in channeling and confining the wind outflow, with particular emphasis on the "Magnetically Confined Wind Shock" (MCWS) paradigm for explaining the relatively hard X-ray emission observed by the Chandra X-ray observatory for magnetic hot stars like Theta 1 Ori C. We also examine the effect of magnetic fields on the wind from a rotating star, showing that this can spin-up the outflowing material, but does not readily form a Keplerian "Magnetically Torqued Disk". We further describe a new "Rigidly Rotating Magnetosphere"(RRM) model that has proven highly successful in reproducing the rotational modulated Balmer emission seen in magnetic Bp stars like sigma Ori C. We conclude with an outlook for the general role of magnetic fields in structuring hot-star mass loss and circumstellar matter.


Summary

Mostly, this paper appears to deal with chemically-peculiar hot stars, such as &sigma Ori E, a helium-strong B2p star. The authors do acknoledge the presence of weaker magnetic fields in other stars, including Be emission-line stars, slowly-pulsating B stars, and of course massive O stars such as &theta1 Ori C. In fact, the authors hypothesize that all hot stars may harbor magnetic fields, though many may not be presently detectable.

The simulations here are on dipolar surface fields only. The simulations are 2-dimensional and axisymmetric and concern only the magnetic field lines and how they interact with the line- driven hot-star wind. The authors define a critical "magnetic confinement parameter" &eta *, which is defined as follows:

&eta* &equiv (B*R*)2/[(dM/dt)v&infin]
where B* is the strength of the magnetic field at the magnetic equator at the stellar surface radius R*. (dM/dt)v&infin is the wind terminal momentum. &eta* must take on values greater than 1; for &theta1 Ori C, the authors assume &eta* &asymp 10.

These simulations, unlike the previous ones carried out by Owocki and ud-Doula, take into account the radiative cooling of shock-heated material. The authors perform a simulation of &theta1 Ori C and find the balance between gravitational pull and magnetic tension on the wind-shock regions highly unstable, leading to complex fall-back patterns. They claim that, when averaged over time (or over longitude), these fall-back patterns will yeild simple, symmetric X-ray emissions.

Finally, the authors consider centrifugal breakout for X-ray flaring. Originally, such flaring had been attributed to unseen companion stars, but this was found not to be the case. The authors propose that material buildup in the rigidly-rotating magnetosphere (RRM) cannot build up forever, and that at some point the balance between gravitational, magnetic, and (in some cases that they considered) centrifugal forces must be broken. During such a breakout, the plasma would stretch out the magnetic field lines at the equator so greatly that the lines would snap, leading to a large ejection of material from the magnetosphere. When the magnetic field lines reconnect, there would be a large energy release, believed to be a plausible source for the type of hard X-rays emitted from such stars.


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