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Session 32 - Curriculum in Crisis: Reinventing Stellar Astrophysics for Today's Graduate Student.
Oral session, Tuesday, June 11

[32.06] M--Dwarfs: Molecules and Magnetic Fields

J. A. Valenti (JILA), C. M. Johns--Krull (UT), N. Piskunov (Uppsala)

At McDonald Observatory, we recently obtained echelle spectra (R=120,000; S/N>100) of several M0V to M5V stars. Interpreting these spectra in detail has been challenging, but also quite fascinating. From an analysis standpoint, the largest perturbation from a solar analogy arises because of the prevalence of molecules in the atmosphere. Fortunately, Allard amp; Hauschildt (1995) have constructed and continue to refine theoretical atmospheres, which include the effects of molecules. Meanwhile, J\/orgensen (1994) has recently compiled extensive lists of molecular line data. These important contributions, available in an electronic format, allowed us to synthesize spectra using standard LTE radiative transfer techniques. By comparing these synthetic spectra with selected wavelength regions in our observed spectra, we were able to correct the electronic oscillator strength for the \epsilon band of TiO, and also to refine the molecular constants for the \gamma band. In addition, we hope to assess how well the theoretical atmospheres for various effective temperatures are able to reproduce observed depths of TiO lines.

Active M dwarfs have relatively large flare luminosities and X--ray surface fluxes, implying magnetic fields and/or filling factors well above solar. Saar amp; Linsky (1985) provided the first direct evidence for 4 kG fields covering the surface of an M dwarf. We recently confirmed this result by detecting Zeeman split \sigma--components in the \sc Fe\,I line at 8468.40 Å\ (Johns--Krull amp; Valenti 1996). This demonstration was primarily empirical: the extended wings seen in the \sc Fe\,I line are not seen in spectra of inactive M dwarfs, whereas the neighboring TiO lines are all well matched, ruling out most nonmagnetic explanations. We also determined magnetic field strength and filling factor by modeling the ratio of active and inactive spectra, which is less sensitive to model uncertainties and molecular blends. We are implementing a Feautrier scheme to accurately synthesize spectra when both molecules and magnetic fields are important. We hope to have available for discussion initial results from a new analysis of active M dwarfs.

Program listing for Tuesday