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Session 78 - Stars - Young and Old, Large and Small.
Display session, Wednesday, January 15
Metropolitan Ballroom,

[78.04] Phoenix (IR) and Optical Observations of Magnetic Fields on M Dwarfs

J. Valenti, C. M. Johns--Krull (JILA), S. Saar (CfA), K. Hinkle, D. Joyce (NOAO)

We compare high quality IR and optical spectra of EV Lac (M3.5 Ve) and Gliese 725B (M3.5 V) to obtain new constraints on magnetic regions on EV Lac. While EV Lac is very active (L_X/L_bol=-2.6), Gliese 725B is rather inactive (L_X/L_bol=-5.2). Nonetheless, TiO line strengths are similar in the two stars, making Gliese 725B an excellent inactive reference.

Using the Zeeman--broadened \sc Fe\thinspace\small I 8468.40 Å\ line and a temperature sequence of inactive standards, Johns--Krull amp; Valenti (1996) detected 3.8\pm0.5 kG magnetic fields covering 50\pm13% of EV Lac. Using this same line, we find similar field strengths in several other active M dwarfs. We have also extended the EV Lac analysis to include several other atomic lines. We generate synthetic spectra using the latest Allard amp; Hauschildt (1996) model atmospheres and extensive atomic and TiO line lists. A generalized Feautrier technique is used to solve the radiative transfer equation for all four Stokes components.

By comparing model and observed spectra, we show that current TiO line lists have wavelength errors large enough to hinder detailed analysis. For certain bands, we determined empirical wavelength corrections from our data. Alternatively, dividing the EV Lac spectrum by Gliese 725B empirically removes most TiO features, leaving only the differences in Zeeman sensitive atomic lines. We succcessfully determine magnetic parameters by fitting these ratio spectra.

Unlike M dwarfs, active G and K dwarfs typically have field strengths \le 2 kG covering <10% of their surface. Detecting these weaker fields in the optical has proven difficult, but they are measurable in the IR because Zeeman splitting grows more quickly than line broadening (\Delta\lambda^2 vs. \Delta\lambda). In preparation for an IR survey of magnetic properties in late-type stars, we obtained 2.2 \mum Phoenix spectra (S/N=100, R=60,000) of EV Lac and Gliese 725B, containing several \sc Ti\thinspace\small I lines that are potentially useful as magnetic diagnostics. Quite surprisingly, we see broad absorption lines in both stars, but lines in Gliese 725B also have a narrow component with no apparent analog in EV Lac. We will discuss various interpretations (both magnetic and nonmagnetic) of these data and assess the use of these lines to measure magnetic fields.

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Program listing for Wednesday