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

摘要

We present a comparison of dynamical model atmospheres with mid-infrared (~11micron) interferometric and spectroscopic observations of the Mira variable oCet. The dynamical model atmospheres of Mira variables pulsating in thefundamental mode can fairly explain, without assuming ad-hoc components, theseemingly contradictory mid-infrared spectroscopic and interferometricobservations of o Cet: the 11 micron sizes measured in the bandpass without anysalient spectral features are about twice as large as those measured in thenear-infrared. Our calculations of synthetic spectra show that the strongabsorption due to a number of optically thick H2O lines is filled in by theemission 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 inrather featureless, continuum-like spectra in rough agreement with the observedhigh-resolution 11 micron spectra, although the models still predict the H2Olines to be more pronounced than the observations. The inverse P-Cyg profilesof some strong H2O lines observed in the 11 micron spectra can also bereasonably reproduced by our dynamical model atmospheres. The presence of theextended H2O layers manifests itself as mid-infrared angular diameters muchlarger than the continuum diameter. The 11 micron uniform-disk diameterspredicted by our dynamical model atmospheres are in fair agreement with thoseobserved with the Infrared Spatial Interferometer (ISI), but still somewhatsmaller than the observed diameters. We discuss possible reasons for thisdiscrepancy and problems with the current dynamical model atmospheres of Miravariables.