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Recent Stellar Astrophysics Colloquia
We review the history of ideas concerning the formation of binary stars and the modern objections attached to many of these ideas.
We argue that the conventional attitude toward the related mechanisms of fission and fragmentation may require fundamental
modification when the role of magnetic fields is given a proper account. We then show why gravitational instabilities will almost
inevitably introduce non-axisymmetric structures in flattened molecular cloud cores, either during their condensation stages or
during the subsequent collapse. Competition between aligned modes of instability (analogous to those that lead to the bifuraction
of Maclaurin spheroids to Jacobi or Dedekind ellipsoids, except that they occur here in compressible, magnetized, and
differentially-rotating configurations) and spiral modes of instability (analogous to those responsible for the inward transport of
mass and the outward transport of angular momentum in spiral galaxies) may determine whether any given star-forming region
produces multiple stars or a single star plus a planetary system. We speculate, however, that gravitational fragmentation into
multiple pieces is unlikely in realistic cases unless collapse into elongated structures is accompanied by a rapid loss of magnetic
flux. This decoupling hypothesis for the production of binary and multiple star-systems bears some resemblance to how
gravitational instabilities produce large-scale structure in the universe after the recombination era.
Among the stars in the middle of the main sequence (with masses between about 2 and 10 solar masses), a
small fraction have remarkable large-scale kilogauss magnetic fields of roughly dipolar structure. Recently a
number of new methods for measuring these magnetic fields have been developed, and we are on the
threshhold of greatly improved knowledge of the field structures found in these stars. Some of these data
have already revealed a surprising relationship between the field structure and stellar angular momentum, but
we are still far from understanding the relevant physics or all the evolutionary implications of this result.
Most of chemically peculiar (CP) stars of the upper main sequence are characterised by the presence of a
magnetic field, which can be detected through the analysis of the Zeeman effect on Stokes profiles. The
shape and strength of Stokes profiles appear to change periodically, with the same period as the stellar
rotation (as deduced from photometric measurements). This observed time variation is interpreted in terms
of a magnetic field `frozen' across the stellar surface, not symmetric around the rotation axis, so that the
observer sees a magnetic configuration which changes as the star rotates. Magnetic field is not always
sufficient to account for the observed changes of the line strengths, but abundance inhomogeneities must also
be invoked to explain some large variations of the line equivalent widths. The magnetic topology of CP stars is
thought to be organised at large-scale: solar-like magnetic phenomena would hardly be detected in a
spectrum which results from integration over the stellar disk. Accordingly, it makes sense to attempt a
description of the magnetic fields of CP stars with a relatively small number of free parameters. A particularly
convenient parametrisation is obtained by considering the superposition of a dipole plus (non linear)
quadrupole field. I will describe a modelling technique to recover the magnetic morphology of CP stars, and
present some applications to the observational data. Abundance anomalies and the presence of a magnetic
field are correlated phenomena. Recent results clearly show that the magnetic topologies of CP stars usually
exhibit complex structures. Further studies can be of great help to discover meaningful relationships between
the magnetic field morphology and the distribution of the elements over the stellar surface, yielding
observational constraints for the diffusion theory.
Upcoming Stellar Discussion Groups/G2000/Colloquia
None scheduled.
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Last updated February 18 2000