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

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

Corresponding author: kohnaka@mpifr-bonn.mpg.de

Received: 12 November 2003
Accepted: 31 March 2004

Abstract

We present a possible interpretation for the increase of the angular diameter of the supergiants α Ori  (M 1-2 Ia-Ibe) and α Her (M 5 Ib-II) from the K band to the 11 μm region and the high-resolution 11 μm spectra without any salient spectral features revealed by Weiner et al. ([CITE]). The angular diameters as well as the high-resolution spectra of α Ori and α Her obtained in the 11 μm region can be reproduced by a warm water vapor envelope, whose presence in α Ori was revealed by Tsuji ([CITE]) 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 11 μm 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 11 μm region can be best interpreted by the water vapor envelope extending to 1.4-1.5 , with a temperature of ~2000 K and a column density of H₂O of the order of 10²⁰ cm^-2.