Comparison of dynamical model atmospheres of Mira variables with mid-infrared interferometric and spectroscopic observations

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: kohnaka@mpifr-bonn.mpg.de
² Institut für Theoretische Astrophysik der Universität Heidelberg, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
³ School of Physics, University of Sydney, Sydney NSW 2006, Australia
⁴ Research School for Astronomy and Astrophysics, Australian National University, Canberra ACT 2600, Australia

Received: 5 August 2005
Accepted: 23 September 2005

Abstract

We present a comparison of dynamical model atmospheres with mid-infrared (~11 μm) interferometric and spectroscopic observations of the Mira variable o Cet. The dynamical model atmospheres of Mira variables pulsating in the fundamental mode can fairly explain, without assuming ad-hoc components, the seemingly contradictory mid-infrared spectroscopic and interferometric observations of o Cet: the 11 μm  sizes measured in the bandpass without any salient spectral features are about twice as large as those measured in the near-infrared. Our calculations of synthetic spectra show that the strong absorption due to a number of optically thick H₂O lines is filled in by the emission of these H₂O lines originating in the geometrically extended layers, providing a possible physical explanation for the picture proposed by Ohnaka (2004a) based on a semi-empirical modeling. This filling-in effect results in rather featureless, continuum-like spectra in rough agreement with the observed high-resolution 11 μm spectra, although the models still predict the H₂O lines to be more pronounced than the observations. The inverse P-Cyg profiles of some strong H₂O lines observed in the 11 μm spectra can also be reasonably reproduced by our dynamical model atmospheres. The presence of the extended H₂O layers manifests itself as mid-infrared angular diameters much larger than the continuum diameter. The 11 μm uniform-disk diameters predicted by our dynamical model atmospheres are in fair agreement with those observed with the Infrared Spatial Interferometer (ISI), but still somewhat smaller than the observed diameters. We discuss possible reasons for this discrepancy and problems with the current dynamical model atmospheres of Mira variables.