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A&A 421, 1149-1158 (2004)
DOI: 10.1051/0004-6361:20035668

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

K. Ohnaka

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

(Received 12 November 2003 / Accepted 31 March 2004 )

Abstract
We present a possible interpretation for the increase of the angular diameter of the supergiants $\alpha$ Ori (M 1-2 Ia-Ibe) and $\alpha$ Her (M 5 Ib-II) from the K band to the 11  $\mu$m region and the high-resolution 11  $\mu$m spectra without any salient spectral features revealed by Weiner et al. (2003b). The angular diameters as well as the high-resolution spectra of  $\alpha$ Ori and  $\alpha$ Her obtained in the 11  $\mu$m region can be reproduced by a warm water vapor envelope, whose presence in $\alpha$ Ori was revealed by Tsuji (2000a) based on the reanalysis of the near-infrared data obtained with the Stratoscope II. While prominent absorption due to H 2O 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 H 2O lines in the 11  $\mu$m region, and makes the observed spectra appear to be rather featureless and continuum-like. However, the emission due to H 2O 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  $\alpha$ Ori and $\alpha$ Her in the 11  $\mu$m region can be best interpreted by the water vapor envelope extending to 1.4-1.5  $R_{\star}$, with a temperature of ~ 2000 K and a column density of H 2O of the order of  1020 cm -2.


Key words: infrared: stars -- molecular processes -- techniques: interferometric -- stars: late-type -- stars: supergiants -- stars: individual:  $\alpha$ Ori,  $\alpha$ Her

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