Issue |
A&A
Volume 444, Number 3, December IV 2005
|
|
---|---|---|
Page(s) | 661 - 671 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361:20053111 | |
Published online | 05 December 2005 |
A further “degree of freedom” in the rotational evolution of stars
School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, Scotland, UK e-mail: vrh1@st-and.ac.uk
Received:
22
March
2005
Accepted:
22
August
2005
Observational and theoretical investigations provide evidence for
non-uniform spot and magnetic flux distributions on rapidly rotating
stars, which have a significant impact on their angular momentum loss
rate through magnetised winds.
Supplementing the formalism of MacGregor & Brenner (1991) with a
latitude-dependent magnetised wind model, we analyse the effect of
analytically prescribed surface distributions of open magnetic flux
with different shapes and degrees of non-uniformity on the rotational
evolution of a solar-like star.
The angular momentum redistribution inside the star is treated in a
qualitative way, assuming an angular momentum transfer between the
rigidly-rotating radiative and convective zones on a constant coupling
timescale of ; for the sake of simplicity we disregard
interactions with circumstellar disks.
We find that non-uniform flux distributions entail rotational histories
which differ significantly from those of classical approaches, with
differences cumulating up to
during the main sequence phase.
Their impact is able to mimic deviations of the dynamo efficiency from
linearity of up to
and nominal dynamo saturation limits at about
35 times the solar rotation rate.
Concentrations of open magnetic flux at high latitudes thus assist in
the formation of very rapidly rotating stars in young open clusters,
and ease the necessity for a dynamo saturation at small rotation
rates.
However, since our results show that even minor amounts of open flux at
intermediate latitudes, as observed with Zeeman-Doppler imaging
techniques, are sufficient to moderate this reduction of the AM loss
rate, we suggest that non-uniform flux distributions are a
complementary rather than an alternative explanation for very rapid
stellar rotation.
Key words: stars: rotation / stars: winds, outflows / stars: magnetic fields / stars: mass-loss / stars: evolution / magnetohydrodynamics (MHD)
© ESO, 2005
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