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We have detected [Fe II] 17.94 um and 24.52 um emission from a sample of Msupergiants using TEXES on the IRTF. These low opacity emission lines areresolved at R = 50, 000 and provide new diagnostics of the dynamics andthermodynamics of the stellar wind acceleration zone. The [Fe II] lines, fromthe first excited term, are sensitive to the warm plasma where energy isdeposited into the extended atmosphere to form the chromosphere and windoutflow. These diagnostics complement previous KAO and ISO observations whichwere sensitive to the cooler and more extended circumstellar envelopes. Theturbulent velocities, Vturb is about 12 to 13 km/s, observed in the [Fe II]forbidden lines are found to be a common property of our sample, and are lessthan that derived from the hotter chromospheric C II] 2325 Angstrom linesobserved in alpha Ori, where Vturb is about 17 to 19 km/s. For the first time,we have dynamically resolved the motions of the dominant cool atmosphericcomponent discovered in alpha Ori from multi-wavelength radio interferometry byLim et al. (1998). Surprisingly, the emission centroids are quite Gaussian andat rest with respect to the M supergiants. These constraints combined withmodel calculations of the infrared emission line fluxes for alpha Ori implythat the warm material has a low outflow velocity and is located close to thestar. We have also detected narrow [Fe I] 24.04 um emission that confirms thatFe II is the dominant ionization state in alpha Ori's extended atmosphere.

TEXES Observations of M Supergiants: Dynamics and Thermodynamics of Wind Acceleration