No 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 θ1 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 42°±6° with respect to the rotation axis (assumed to be inclined at 45° to the line of sight). We find, in particular, that the positive magnetic pole comes closest to the observer when the variable Hα 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 θ1 Ori C in the context of the magnetically confined wind‐shock model of Babel & Montmerle. In the case of θ1 Ori C, this model predicts the formation of a large magnetosphere (extending as far as 2–3R∗), 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 θ1 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 θ1 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.

In this paper, we use the very recent spectropolarimetric observations of β Cep collected by Henrichs et al. and propose for this star a consistent model of the large‐scale magnetic field and of the associated magnetically confined wind and circumstellar environment. A re‐examination of the fundamental parameters of β Cep in the light of the Hipparcos parallax indicates that this star is most likely a 12‐M⊙ star with a radius of 7 R⊙, effective temperature of 26 000 K and age of 12 Myr, viewed with an inclination of the rotation axis of about 60°. Using two different modelling strategies, we obtain that the magnetic field of β Cep can be approximately described as a dipole with a polar strength of , the axis of symmetry of which is tilted with respect to the rotation axis by about . Although one of the weakest detected to date, this magnetic field is strong enough to magnetically confine the wind of β Cep up to a distance of about 8 to 9 R∗. We find that both the X‐ray luminosity and variability of β Cep can be explained within the framework of the magnetically confined wind‐shock model of Babel & Montmerle, in which the stellar‐wind streams from both magnetic hemispheres collide with each other in the magnetic equatorial plane, producing a strong shock, an extended post‐shock region and a high‐density cooling disc. By studying the stability of the cooling disc, we obtain that field lines can support the increasing disc weight for no more than a month before they become significantly elongated in an effort to equilibrate the gravitational plus centrifugal force, thereby generating strong field gradients across the disc. The associated current sheet eventually tears, forcing the field to reconnect through resistive diffusion and the disc plasma to collapse towards the star. We propose that this collapse is the cause for the recurrent Be episodes of β Cep, and finally discuss the applicability of this model to He peculiar, classical Be and normal non‐supergiant B stars.

Abstract. We present new orbits for sixteen Ap spectroscopic binaries, four of which might in fact be Am stars, and give their orbital elements. Four of them are SB2 systems: HD 5550, HD 22128, HD 56495 and HD 98088. The twelve other stars are: HD 9996, HD 12288, HD 40711, HD 54908, HD 65339, HD 73709, HD 105680, HD 138426, HD 184471, HD 188854, HD 200405 and HD 216533. Rough estimates of the individual masses of the components of HD 65339 (53 Cam) are given, combining our radial velocities with the results of speckle interferometry and with Hipparcos parallaxes. Considering the mass functions of 74 spectroscopic binaries from this work and from the literature, we conclude that the distribution of the mass ratio is the same for cool Ap stars and for normal G dwarfs. Therefore, the only differences between binaries with normal stars and those hosting an Ap star lie in the period distribution: except for the case of HD 200405, all orbital periods are longer than (or equal to) 3 days. A consequence of this peculiar distribution is a deficit of null eccentricities. There is no indication that the secondary has a special nature, like e.g. a white dwarf.

In this paper, we use the very recent spectropolarimetric observations of b Cep collected by Henrichs et al. and propose for this star a consistent model of the large-scale magnetic field and of the associated magnetically confined wind and circumstellar environment. A reexamination of the fundamental parameters of b Cep in the light of the Hipparcos parallax indicates that this star is most likely a 12-M ( star with a radius of 7 R ( , effective temperature of 26 000 K and age of 12 Myr, viewed with an inclination of the rotation axis of about 608. Using two different modelling strategies, we obtain that the magnetic field of b Cep can be approximately described as a dipole with a polar strength of 360^30 G, the axis of symmetry of which is tilted with respect to the rotation axis by about 858^108.Although one of the weakest detected to date, this magnetic field is strong enough to magnetically confine the wind of b Cep up to a distance of about 8 to 9 R * . We find that both the X-ray luminosity and variability of b Cep can be explained within the framework of the magnetically confined wind-shock model of Babel & Montmerle, in which the stellar-wind streams from both magnetic hemispheres collide with each other in the magnetic equatorial plane, producing a strong shock, an extended post-shock region and a high-density cooling disc.By studying the stability of the cooling disc, we obtain that field lines can support the increasing disc weight for no more than a month before they become significantly elongated in an effort to equilibrate the gravitational plus centrifugal force, thereby generating strong field gradients across the disc. The associated current sheet eventually tears, forcing the field to reconnect through resistive diffusion and the disc plasma to collapse towards the star. We propose that this collapse is the cause for the recurrent Be episodes of b Cep, and finally discuss the applicability of this model to He peculiar, classical Be and normal non-supergiant B stars.