Impact of non-uniform surface magnetic fields on stellar winds

Abstract

Observations of active stars reveal highly non-uniform surface distributions of magnetic flux. Theoretical models considering magnetised stellar winds however often presume uniform surface magnetic fields, characterised by a single magnetic field strength. The present work investigates the impact of non-uniform surface magnetic field distributions on the stellar mass and angular momentum loss rates. The approach of Weber & Davis (1967) is extended to non-equatorial latitudes to quantify the impact of latitude-dependent magnetic field distributions over a large range of stellar rotation rates and thermal wind properties. The analytically prescribed field patterns are dominated by magnetic flux concentrations at intermediate and high latitudes. The global stellar mass loss rates are found to be rather insensitive to non-uniformities of the surface magnetic field. Depending on the non-uniformity of the field distribution, the angular momentum loss rates deviate in contrast at all rotation rates between -60% and 10% from the Weber & Davis-values, and the effective Alfvenic radii up to about +/-25%. These large variations albeit equal amounts of total magnetic flux indicate that a classification of stellar surface magnetic fields through a single field strength is insufficient, and that their non-uniformity has to be taken into account. The consequences for applications involving magnetised stellar winds are discussed in view of the rotational evolution of solar-like stars and of the observational determination of their mass loss rates using the terminal velocity and ram pressure of the wind. For rapidly rotating stars the latitudinal variation of the wind ram pressure is found to exceed, depending on the actual field distribution on the stellar surface, over two orders of magnitude.

13 pages, 9 figures, accepted by A&A

References in corpus (6)

• Measured Mass Loss Rates of Solar-like Stars as a Function of Age and Activity
• The global magnetic topology of AB Doradus
• Systematic construction of exact MHD models for astrophysical winds and jets
• Stellar winds, dead zones, and coronal mass ejections
• Nonradial and nonpolytropic astrophysical outflows. V. Acceleration and collimation of self-similar winds
• An analytical MHD wind model with latitudinal dependences obtained using separation of the variables

Cited by in corpus (10)

• The Effect of Combined Magnetic Geometries on Thermally Driven Winds II: Dipolar, Quadrupolar and Octupolar Topologies
• Theoretical mass loss rates of cool main-sequence stars
• Simulations of Winds of Weak-Lined T Tauri Stars. II.: The Effects of a Tilted Magnetosphere and Planetary Interactions
• The dependence of stellar mass and angular momentum losses on latitude and on active region and dipolar magnetic fields
• Simulations of Winds of Weak-Lined T Tauri Stars: The Magnetic Field Geometry and The Influence of the Wind on Giant Planet Migration
• On the Effect of Magnetic Spots on Stellar Winds and Angular Momentum Loss
• The winds of young Solar-type stars in the Hyades
• A further 'degree of freedom' in the rotational evolution of stars
• Doppler imaging of the young late-type star LO Pegasi (BD +22 4409) in September 2003
• Magnetic braking in young late-type stars: the effect of polar spots