Issue |
A&A
Volume 440, Number 2, September III 2005
|
|
---|---|---|
Page(s) | 411 - 423 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361:20053110 | |
Published online | 01 September 2005 |
Impact of non-uniform surface magnetic fields on stellar winds
School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, Scotland, UK e-mail: vrh1@st-andrews.ac.uk
Received:
22
March
2005
Accepted:
26
May
2005
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 as well as on the effective Alfvénic
radius of the wind.
Assuming an axial symmetric and polytropic magnetised wind, the
approach of Weber & Davis (1967, ApJ, 148, 217) 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.
Motivated by recent observational results, 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
and
from the Weber & Davis-values,
and the effective Alfvénic radii up to about
.
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.
The assumption of a spherical symmetric wind geometry may therefore
lead to a significant over- or underestimation of the stellar mass
loss rate.
Key words: stars: winds, outflows / stars: mass-loss / stars: magnetic fields / stars: starspots / stars: rotation / magnetohydrodynamics (MHD)
© ESO, 2005
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