Publications
of the
MPIfR Optical & Infrared Interferometry Group

Ohnaka, K.

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

Astronomy and Astrophysics, 421, 1149(2004)

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 micrometer region and the high- resolution 11 micrometer 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 micrometer 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 micrometer 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 Ori and Her in the 11 micrometer region can be best interpreted by the water vapor envelope extending to 1.4-1.5 R*, with a temperature of ~ 2000 K and a column density of H 2O of the order of 10 20 cm -2.

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