Supergiant temperatures and linear radii from near-infrared interferometry

We present angular diameters for 42 Luminosity Class (LC) I stars and 32 LC II stars that have been interferometrically determined with the Palomar Testbed Interferometer. Derived values of radius and effective temperature are established for these objects, and an empirical calibration of these parameters for supergiants will be presented as a function of spectral type and colours. For the effective temperature versus (V−K)₀ colour, we find an empirical calibration with a median deviation of ΔT= 70 K in the range of 0.7 < (V−K)₀ < 5.1 for LC I stars; for LC II, the median deviation is ΔT= 120 K from 0.4 < (V−K)₀ < 4.3. Effective temperature as a function of spectral type is also calibrated from these data, but shows significantly more scatter than the T_EFF versus (V−K)₀ relationship. No deviation of T_EFF versus spectral type is seen for these high-luminosity objects relative to LC II giants. Directly determined diameters range up to 400 R_⊙, though are limited by poor distance determinations, which dominate the error estimates. These temperature and radii measures reflect a direct calibration of these parameters for supergiants from empirical means.