Asymptotic Giant Branch and Supergiant Stellar Diameters in the Mid‐Infrared

The size of the continuum photospheres of α Ori, α Her, R Leo, and χ Cyg have been measured at 11 μm using heterodyne interferometry to accuracies as high as 1%. An assortment of narrow wavelength bands near 11 μm (each having a width ~0.17 cm-1) were used to avoid spectral lines. The resulting apparent diameters for α Ori and α Her are ~30% larger than measured near-infrared sizes, whereas the Mira variables, R Leo and χ Cyg, have 11 μm apparent diameters roughly twice their reported near-infrared sizes. The complexities of interpretation are discussed.