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We present a possible interpretation for the increase of the angular diameterof the supergiants alf Ori (M1-2 Ia-Ibe) and alf Her (M5 Ib-II) from the K bandto the 11 micron region and the high-resolution 11 micron spectra without anysalient spectral features revealed by Weiner et al. (2003). The angulardiameters as well as the high-resolution spectra of alf Ori and alf Herobtained in the 11 micron region can be reproduced by a warm water vaporenvelope, whose presence in alf Ori was revealed by Tsuji (2000) based on thereanalysis of the near-infrared data obtained with the Stratoscope II. Whileprominent absorption due to H2O can be expected from such a dense, warm watervapor envelope, the absorption lines can be filled in by emission from theextended part of the envelope. This effect leads to a significant weakening ofthe H2O lines in the 11 micron region, and makes the observed spectra appear tobe rather featureless and continuum-like. However, the emission due to H2Olines from the extended envelope leads to an increase of the apparent size inthis spectral region. The observed angular diameter and the high resolutionspectra of alf Ori and alf Her in the 11 micron region can be best interpretedby the water vapor envelope extending to 1.4--1.5 Rstar, with a temperature ofabout 2000 K and a column density of H2O of the order of 10^20 cm^-2.Comment: 10 pages, 9 figures, accepted for publication in A&

astronomy and astrophysics

Warm water vapor envelope in the supergiants <i>α</i> Ori and <i>α</i> Her and its effects on the apparent size from the near-infrared to the mid-infrared

Warm water vapor envelope in the supergiants α Ori and α Her and its effects on the apparent size from the near-infrared to the mid-infrared