SOFIA-EXES Mid-IR Observations of $\left[FeII\right]$ Emission from the Extended Atmosphere of Betelgeuse

We present a NASA-DLR SOFIA-Echelon Cross Echelle Spectrograph (EXES) and NASA Infrared Telescope Facility-Texas Echelon Cross Echelle Spectrograph (TEXES) mid-IR $R\simeq 50,000$ spectral study of forbidden Fe ii transitions in the early-type M supergiants, Betelgeuse (α Ori: M2 Iab) and Antares (α Sco: M1 Iab + B3 V). With EXES, we spectrally resolve the ground term [Fe ii] 25.99 μm ($a{}^6{D}_{J=7/2\text{--}9/2}$: $E_{up}=540$ K) emission from Betelgeuse. We find a small centroid blueshift of 1.9 ± 0.4 $km{s}^{-1}$ that is a significant fraction (20%) of the current epoch wind speed, with a FWHM of 14.3 ± 0.1 $km{s}^{-1}$. The TEXES observations of [Fe ii] 17.94 μm ($a{}^4{F}_{J=7/2-9/2}$: $E_{up}=3400$ K) show a broader FWHM of 19.1 ± 0.2 $km{s}^{-1}$, consistent with previous observations, and a small redshift of 1.6 ± 0.6 $km{s}^{-1}$ with respect to the adopted stellar center-of-mass velocity of $V_{CoM}=20.9\pm 0.3$ $km{s}^{-1}$. To produce [Fe ii] 25.99 μm blueshifts of 20% wind speed requires that the emission arises closer to the star than existing thermal models for α Ori's circumstellar envelope predict. This implies a more rapid wind cooling to below 500 K within $10R_{\ast }$ (${\theta }_{\ast }=44$ mas, dist = 200 pc) of the star, where the wind has also reached a significant fraction of the maximum wind speed. The line width is consistent with the turbulence in the outflow being close to the hydrogen sound speed. EXES observations of [Fe ii] 22.90 μm ($a{}^4{D}_{J=5/2\text{--}7/2}$: $E_{up}=11,700$ K) reveal no emission from either star. These findings confirm the dominance of cool plasma in the mixed region where hot chromospheric plasma emits copiously in the UV, and they also constrain the wind heating produced by the poorly understood mechanisms that drive stellar outflows from these low variability and weak-dust signature stars.