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Session 103 - Cool Stars.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

[103.06] Detection of Strong Magnetic Fields on M Dwarfs

C. M. Johns-Krull (McDonald Obs., U. Texas), J. A. Valenti (JILA, U. Colorado)

We have detected Zeeman splitting of the Fe I line (\geff=2.5) at 8468.40 Å\ in the M4.5Ve flare stars Gliese 729 and Gliese 873 (EV Lac). High resolution (R=120,000), low noise (\sim0.5%) spectra clearly show resolved \sigma components from which we infer field strengths of 3 to 4 kG, independent of uncertainties in model atmospheres. Similar observations of a sequence (M0V--M5V) of low activity M dwarfs demonstrate that the wing components in the 8468.4 Å\ line are not due to the ubiquitous TiO lines in the vicinity. By fitting the ratio of active to inactive line profiles with simple one--component models, we estimate that 55\pm8% of EV Lac is covered by 3.60\pm0.40 kG magnetic fields, while 56\pm8% of Gliese 729 is covered by 2.50\pm0.35 kG fields. This confirms earlier reports (Saar amp; Linsky 1985, Saar 1994) of large magnetic fields on M dwarf flare stars. We see evidence for a distribution of magnetic field strength, spatially across the surface and/or with depth. The factor of 2--3 increase over field strengths seen in G and K dwarfs is in keeping with flux tube equilibrium arguments, but magnetic measurements for inactive stars are needed to confirm this hypothesis, since the field may instead simply grow with activity.

Program listing for Thursday