An Observational Test of the Spherical Model Atmospheres for the M Class Giants: The Case of δ[TSUP]2[/TSUP] Lyrae

Abstract

We have obtained interferometric data for δ2 Lyrae, spectral type M4 II, at 1.25 and 1.65 μm at the Navy Prototype Optical Interferometer. This star has also been observed at the Mark III Optical Interferometer at optical wavelengths and at the Infrared-Optical Telescope Array at 2.20 μm. These observations have resulted in a total of seven uniform-disk diameter (UDD) estimates for δ2 Lyrae at seven wavelengths. We compare the UDD estimates for δ2 Lyrae to the most recent, static, M class giant model atmospheres. The most appropriate model that we can construct for δ2 Lyrae, matching the star in mass, radius, and luminosity, cannot account for the observed variations in the UDDs with wavelength. Among 18 alternate models, the model that provides the best fit to the data has almost the same Teff as δ2 Lyrae but a lower mass and a more extended atmosphere. None of the models investigated, however, appear to provide a satisfactory description of the atmosphere of δ2 Lyrae. We discuss several possible explanations for these results and the implications of the relative success of the lower mass model.