2009ApJ...701.1464H

We have detected [Fe II] 17.94 µm and 24.52 µm emission from a sample of M supergiants (µ Cep, α Sco, α Ori, CE Tau, AD Per, and α Her) using the Texas Echelon Cross Echelle Spectrograph on NASA's Infrared Telescope Facility. These low opacity emission lines are resolved at R ≃ 50, 000 and provide new diagnostics of the dynamics and thermodynamics of the stellar wind acceleration zone. The [Fe II] lines, from the first excited term (a ⁴ F), are sensitive to the warm plasma where energy is deposited into the extended atmosphere to form the chromosphere and wind outflow. These diagnostics complement previous Kuiper Airborne Observatory and Infrared Space Observatory observations which were sensitive to the cooler and more extended circumstellar envelopes. The turbulent velocities of V _turb≃ 12-13 km/s observed in the [Fe II] a ⁴ F forbidden lines are found to be a common property of our sample, and are less than that derived from the hotter chromospheric C II] 2325 Å lines observed in α Ori, where V_turb≃ 17-19 km/s. For the first time, we have dynamically resolved the motions of the dominant cool atmospheric component discovered in α Ori from multiwavelength radio interferometry by Lim et al. Surprisingly, the emission centroids are quite Gaussian and at rest with respect to the M supergiants. These constraints combined with model calculations of the infrared emission line fluxes for α Ori imply that the warm material has a low outflow velocity and is located close to the star. We have also detected narrow [Fe I] 24.04 µm emission that confirms Fe II is the dominant ionization state in α Ori's extended atmosphere.