2004A&A...421.1149O

We present a possible interpretation for the increase of the angular diameter of the supergiants alpha Ori (M1-2 Ia-Ibe) and alpha Her (M5 Ib-II) from the K band to the 11µm region and the high-resolution 11µm spectra without any salient spectral features revealed by Weiner et al. (2003ApJ...589..976W). The angular diameters as well as the high-resolution spectra of α Ori and α Her obtained in the 11µm region can be reproduced by a warm water vapor envelope, whose presence in α Ori was revealed by Tsuji (2000ApJ...538..801T) based on the reanalysis of the near-infrared data obtained with the Stratoscope II. While prominent absorption due to H₂O can be expected from such a dense, warm water vapor envelope, the absorption lines can be filled in by emission from the extended part of the envelope. This effect leads to a significant weakening of the H₂O lines in the 11µm region, and makes the observed spectra appear to be rather featureless and continuum-like. However, the emission due to H₂O lines from the extended envelope leads to an increase of the apparent size in this spectral region. The observed angular diameter and the high resolution spectra of α Ori and α Her in the 11µm region can be best interpreted by the water vapor envelope extending to 1.4-1.5R_*, with a temperature of ∼2000K and a column density of H₂O of the order of 10²⁰cm^–2.