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The Magnetic Field and Wind Confinement of &theta1 Orionis C
Donati, J-F; Babel, J; Harries, T J; Howarth, I D; Petit, P; and Semel, M

Abstract

We report the detection, through spectropolarimetric observations, of a strong dipolar magnetic field of presumably fossil origin at the surface of the very young O star &theta1 Ori C. The Stokes V signatures we detect are variable with time, the variations being consistent with rotational modulation. A detailed modelling of our observations indicates that this dipole field has an intensity of 1.1 +/- 0.1 kG and is inclined at 42o +/- 6o with respect to the rotation axis (assumed to be inclined at 45o to the line of sight). We find, in particular, that the positive magnetic pole comes closest to the observer when the variable H&alpha emission component observed on this star reaches maximum strength. This discovery represents the first definite detection of a magnetic field in an O star, as well as the first detection of a fossil field in a very young star.

We also investigate in this paper the magnetic confinement of the radiatively driven wind of &theta1 Ori C in the context of the magnetically confined wind-shock model of Babel & Montmerle. In the case of &theta1 Ori C, this model predicts the formation of a large magnetosphere (extending as far as 2­3R*), consisting of a very hot post-shock region (with temperatures in excess of 10 MK and densities of about 1010 ­ 1011 cm-3) generated by the strong collision of the wind streams from both stellar magnetic hemispheres, as well as a dense cooling disc forming in the magnetospheric equator. We find that this model includes most of the physics required to obtain a satisfactory level of agreement with the extensive data sets available for &theta1 Ori C in the literature (and, in particular, with the recent X-ray data and the phase-resolved spectroscopic observations of ultraviolet and optical wind lines) provided that the mass-loss rate of &theta1 Ori C is at least 5 times smaller than that predicted by radiatively driven wind models. We finally show how new observations with the XMM or Chandra spacecraft could help us constrain this model much more tightly and thus obtain a clear picture of how magnetic fields can influence the winds of hot stars.


Summary

Aside from the authors' determination of the strength of the magnetic field of &theta1 Ori C, this paper mostly summarizes what is already know in previous papers. However, it does serve as an excellent source of physical paramters for &theta1 Ori C. This data is summarized below.

ParameterValue
Mass44 +/- 5 Msun
Age< 0.6 Myr
Effective Temperature45000 +/- 1000 K
Radius8.2 +/- 0.1 Rsun
Rotation velocity20 km/s
Rotation period15.426 days
Inclination angle (i)45o +/- 20o
Mass-loss rate3x10-7 Msun/yr
Wind terminal velocity2500 km/s
Magnetic field strength (equitorial)1.1 +/- 0.1 kG
Inclination of magnetic axis (&beta)42o *
Plasma temp in post-shock region> 10 MK
Plasma density in post-shock region1010-1011 cm-3
Cooling disk column density1022-1023 cm-2
Cooling disk velocity gradient~100 km/s
*Assumes i=45o

One other important thing that the authors found regards the H&alpha emission component. Most of the emission was found to originate in the cooling disk, which of course is strong evidence for the existence of a cooling disk in the first place. The velocity gradient above is necessary to reproduce both the "large width and strong rotational modulation" of the H&alpha profile, which results in a mass-loss rate roughly 5 times smaller than the theoretical prediction.


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