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Title: Modeling the magnetospheres of luminous stars: Interactions between supersonic radiation-driven winds and stellar magnetic fields

Abstract

Hot, luminous stars (spectral types O and B) lack the hydrogen recombination convection zones that drive magnetic dynamo generation in the sun and other cool stars. Nonetheless, observed rotational modulation of spectral lines formed in the strong, radiatively driven winds of hot stars suggests magnetic perturbations analogous to those that induce 'co-rotating interaction regions' in the solar wind. Indeed, recent advances in spectropolarimetric techniques have now led to direct detection of moderate to strong (100-10 000 G), tilted dipole magnetic fields in several hot stars. Using a combination of analytic and numerical magnetohydrodynamic models, this paper focuses on the role of such magnetic fields in channeling, and sometimes confining, the radiatively driven mass outflows from such stars. The results show how 'magnetically confined wind shocks' can explain the moderately hard x-ray emission seen from the O7V star Theta-1 Ori C, and how the trapping of material in a 'rigidly rotating magnetosphere' can explain the periodically modulated Balmer line emission seen from the magnetic B2pV star Sigma Ori E. In addition, magnetic reconnection heating from episodic centrifugal breakout events might explain the occasional very hard x-ray flares seen from the latter star. The paper concludes with a brief discussion on themore » generation of hot-star fields and the broader relationship to other types of magnetospheres.« less

Authors:
; ;  [1]
  1. Bartol Research Institute, University of Delaware, Newark, Delaware 19716 (United States)
Publication Date:
OSTI Identifier:
20975075
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2472340; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; BALMER LINES; DIPOLES; EMISSION; HARD X RADIATION; INTERACTIONS; MAGNETIC FIELDS; MAGNETIC RECONNECTION; MAGNETOHYDRODYNAMICS; PERIODICITY; SIMULATION; SOLAR WIND; STELLAR MAGNETOSPHERES; STELLAR RADIATION; SUN; X-RAY SOURCES

Citation Formats

Owocki, Stan, Townsend, Rich, and Ud-Doula, Asif. Modeling the magnetospheres of luminous stars: Interactions between supersonic radiation-driven winds and stellar magnetic fields. United States: N. p., 2007. Web. doi:10.1063/1.2472340.
Owocki, Stan, Townsend, Rich, & Ud-Doula, Asif. Modeling the magnetospheres of luminous stars: Interactions between supersonic radiation-driven winds and stellar magnetic fields. United States. doi:10.1063/1.2472340.
Owocki, Stan, Townsend, Rich, and Ud-Doula, Asif. Tue . "Modeling the magnetospheres of luminous stars: Interactions between supersonic radiation-driven winds and stellar magnetic fields". United States. doi:10.1063/1.2472340.
@article{osti_20975075,
title = {Modeling the magnetospheres of luminous stars: Interactions between supersonic radiation-driven winds and stellar magnetic fields},
author = {Owocki, Stan and Townsend, Rich and Ud-Doula, Asif},
abstractNote = {Hot, luminous stars (spectral types O and B) lack the hydrogen recombination convection zones that drive magnetic dynamo generation in the sun and other cool stars. Nonetheless, observed rotational modulation of spectral lines formed in the strong, radiatively driven winds of hot stars suggests magnetic perturbations analogous to those that induce 'co-rotating interaction regions' in the solar wind. Indeed, recent advances in spectropolarimetric techniques have now led to direct detection of moderate to strong (100-10 000 G), tilted dipole magnetic fields in several hot stars. Using a combination of analytic and numerical magnetohydrodynamic models, this paper focuses on the role of such magnetic fields in channeling, and sometimes confining, the radiatively driven mass outflows from such stars. The results show how 'magnetically confined wind shocks' can explain the moderately hard x-ray emission seen from the O7V star Theta-1 Ori C, and how the trapping of material in a 'rigidly rotating magnetosphere' can explain the periodically modulated Balmer line emission seen from the magnetic B2pV star Sigma Ori E. In addition, magnetic reconnection heating from episodic centrifugal breakout events might explain the occasional very hard x-ray flares seen from the latter star. The paper concludes with a brief discussion on the generation of hot-star fields and the broader relationship to other types of magnetospheres.},
doi = {10.1063/1.2472340},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • Progress made in the determination of mass-loss rates, terminal velocities, and other parameters in the winds of the OB stars and the Wolf-Rayet (W-R) objects is considered. It is thought best to base mass-loss rates on the observations of the free-free emission of electrons in the winds, taking into account, for example, a detection in the radio wavelengths on the Very Large Array. It is found that mass-loss rates in hot stars depend on luminosity to some power. All of the studies, both observational and theoretical, have ignored the issues of variability. Questions regarding variability are examined, and requirements formore » its observation are discussed. 10 references.« less
  • The early-type stars whose spectra show Balmer emission lines are considered, taking into account the division of these stars into two groups, including the OB supergiants and the Be stars. In the case of the Be stars, recent observations at H-alpha, H-beta, and H-gamma with best S/N approximately 500 and resolution approximately 0.1 A have revealed a previously unsuspected winebottle, or two-tier, structure to the symmetric emission profile in several Be stars. For many Be stars, the circumstellar material has an apparent two-component structure. Attention is given to H-alpha variations in four typical Be stars, linear polarization, and magnetic fields.more » 23 references.« less
  • This note discusses the consequences of the interaction of radiatively driven mass loss with magnetic fields in upper-main-sequence helium-peculiar stars. The field constrains mass loss to occur in narrow cones above the magnetic poles. These jets can explain the observed behavior of UV resonance lines in the helium-peculiar stars, especially the sn and helium-rich stars. Suppression of mass outflow at the magnetic equator produces a region in which radiatively driven diffusive mass transport can levitate helium in the hottest stars, producing helium enrichment of the atmosphere. In the cooler stars, the surface becomes progressively more helium poor. 39 references.
  • The effect of a stellar wind on the evolution of stars in the mass range 7--60 M/sub sun/ has been investigated for stellar models in which Carson's opacities have been employed. Several cases of mass loss have been considered. It is found that the assumption of heavy mass loss from both blue and red supergiants can account well for the relevant observations of OBN stars, WN stars, and very luminous supergiants of all spectral types. But no amount of mass loss can account adequately for the properties of the B supergiants of lowest luminosity. A critical comparison is made betweenmore » the present results and some earlier results based on the adoption of Cox-Stewart opacities.« less