Cool Stars 14 - Submitted Abstract # 253
This version created on 05 October 2006

Photospheric Magnetic Field Measurements on Late-Type Stars and Brown
Dwarfs


Christopher Johns-Krull, Rice University

Magnetic fields produce activity on the Sun and cool stars.  Activity
itself involves both the dynamo production of magnetic fields and the
magnetic heating of the outer atmosphere of stars, and in some cases
brown dwarfs, which produces the radiative signatures of activity.  In
the past several years, measurements of the photospheric magnetic
field strength have become available for a number of low mass stars
and brown dwarfs, allowing us to separate the study of field
production from the study of non-radiative heating.  Much of the
recent observational effort has centered on T Tauri stars, due to the
importance of strong stellar fields in explaining this stage of star
formation, and on very low mass stars and brown dwarfs where
measurements of traditional activity indicators suggest sharp
differences in field production and/or heating efficiency compared to
what is observed in slightly higer mass stars.  Somewhat surprisingly,
measurements on these two classes of fully convective objects reveals
mean magnetic field strengths of 2-3 kG in many cases.  Here, I will
review the techniques for measuring the photospheric magnetic field by
looking for effects caused by Zeeman broadening of spectral lines.  I
will discuss the implications of the latest field measurements for
dynamo theories.  Finally, I will suggest some avenues likely to be
fruitful for future research in the field.

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