Issue |
A&A
Volume 424, Number 3, September IV 2004
|
|
---|---|---|
Page(s) | 1011 - 1024 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361:20041207 | |
Published online | 06 September 2004 |
Warm water vapor envelope in Mira variables 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 e-mail: kohnaka@mpifr-bonn.mpg.de
Received:
30
April
2004
Accepted:
4
June
2004
We present a possible interpretation for the increase of the
angular diameter of the Mira variables o Cet, R Leo, and χ Cyg
from the K band to the 11 μm region revealed by the recent
interferometric observations using narrow bandpasses where no salient
spectral feature is present
(Weiner et al. [CITE],b).
A simple two-layer model consisting of hot and cool H2O layers
for the warm water vapor envelope, whose
presence in Mira variables was revealed by previous spectroscopic
observations, can reproduce the angular diameters observed with
Infrared Spatial Interferometer
as well as the high-resolution TEXES spectra obtained in the 11 μm
region. The warm water vapor layers are optically thick
in the lines, and therefore, strong absorption due to H2O can be
expected from such a dense water vapor envelope.
However, the absorption lines are filled in by emission from the
extended part of the envelope, and this results in the
high-resolution 11 μm spectra which exhibit only weak,
fine spectral features, masking the spectroscopic evidences of the dense,
warm water vapor envelope. On the other hand, the presence of the warm
water vapor envelope manifests itself as the larger angular diameters
in the 11 μm region as compared to those measured in the
near-infrared.
Furthermore, comparison of the visibilities predicted in the near-infrared
(1.5–3.8 μm) with observational results available in the
literature demonstrates that
our two-layer model for the warm water vapor envelope can also reproduce
the observed near-infrared visibilities and angular diameters, and suggests
that the wavelength dependence of the angular size of Mira variables
in the infrared largely reflects the H2O opacity.
The radii of the hot H2O layers in the three Mira variables are
derived to be 1.5–1.7 with temperatures of 1800–2000 K
and H2O column densities of
1021 cm-2,
while the radii of the cool H2O layers are derived to be
2.2–2.5
with temperatures of 1200–1400 K and H2O column densities of
1021 cm-2.
Key words: infrared: stars / molecular processes / techniques: interferometric / stars: late-type / stars: AGB and post-AGB / stars: individual: o Cet, R Leo, χ Cyg
© ESO, 2004
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