Cool Stars 14 - Submitted Abstract # 31
This version created on 05 October 2006

Direct measurements of magnetic fields in late M-dwarfs


Ansgar Reiners, University of Hamburg
Gibor Basri, University of California at Berkeley

The high incidence of H-alpha emission in cooler M-dwarfs, along with
the general decline in activity at the latest M spectral types, has
raised questions about the behavior of magnetic fields in the
ultracool photospheres of fully convective, very low-mass stars and
brown dwarfs.  Atomic lines that can be used for Zeeman diagnostics in
warmer stars are not available for the direct measurement of magnetic
fields in ultracool atmospheres.  Based on sunspot data, FeH has been
proposed as a tracer for magnetic flux in late M- and L-dwarfs.
Unfortunately, sufficient molecular data is not currently available to
theoretically predict Zeeman splitting in synthetic spectra.  We
present a method of measuring magnetic fields by interpolating between
FeH spectra of an ultracool star with no magnetic flux and a similar
star with strong magnetic flux.  We have obtained high resolution FeH
spectra of 24 mid- to late M-dwarfs with HIRES/Keck.  Employing two
earlier M-stars with good measurements of magnetic fields from atomic
lines as template stars, we detect magnetic fields in half of our
sample.  All the stars in our sample later than M5.5 display H-alpha
emission, and all are found to have significant magnetic flux (Bf >
1kG), some of them as much as Bf = 4kG.  For a given temperature, we
find a close correlation between magnetic field strength and H-alpha
activity.  This proves that emission in ultracool stars is clearly of
magnetic origin, and that fully convective stars can host a very
efficient type of magnetic dynamo.

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