SOFIA EXES Observations of Herschel’s Garnet Star and the two α Red Supergiants

We report EXES Cycle 4 and 5 observations of M supergiants, made at high spectral resolution (R > 50,000), and obtained from NASA-DLR SOFIA.Emission profiles from Herschel’s Garnet Star (μ Cephei: M2 Ia) show distinct structure and asymmetry in the [S I] 25.25 micron and [Fe II] 25.99 micron lines. These profiles are unlike the remarkably symmetric [Fe II] profile observed from Betelgeuse (α Orionis: M2 Iab) at R=65,000. These diagnostics both arise from upper energy levels with E_up ≈ 550 K and they probe the inner circumstellar envelope. The flux ratios of [S I]/[Fe II] are very different for μ Cep, α Ori, and Antares (α Scorpii: M1.5 Iab), suggesting that the intrinsic far ultraviolet photoionization field of μ Cep is weaker than that of α Ori, and that α Sco’s hot companion may contribute to ionizing neutral sulfur. We find that the mid-IR emission from μ Cep is centered close to the optical radial velocity of V(LSR)=33.0 km s^-1 [V(Helio)=19.3 km s^-1] reported in the General Catalogue of Stellar Radial Velocities (Wilson 1953), and which is substantially different to that inferred from low-J CO radio emission, i.e. V(LSR)=25 km s^-1. It appears that Herschel's Garnet Star has undergone discrete events of asymmetric mass loss.Acknowledgements: This work has made extensive use of the SAO/NASA ADS and VizieR search tools, and the ASTRAL and SIMBAD databases. Financial support has been provided by NASA SOFIA Cycle 4 and 5 grants SOF 04-0136 and 05-0073 issued by USRA.