Modelling near - IR spectra and mid - IR dust emission ofMira variables at di erent

We model the spectral changes of late oxygen-rich Miras observed in diierent pulsation phases. From a combination of variable near-IR spectra and UKIRT spectro-photometry of the 9:7 m silicate dust emission feature in diierent phases we study the innuence of the changing atmospherical circumstances on the conditions in the circumstellar dust shell. From a detailed modelling of TiO and VO bands in the near-IR spectra we determine changes of the eeective temperature and the eeective atmospheric acceleration of the central star. The corresponding model spectral energy distribution is redistributed through the dust shell by means of radiative transfer calculations in order to perform a detailed modelling of shape changes observed in the silicate feature. We show that the latter are mainly caused by changes in the ux distribution of the incident radiation eld with stellar pulsation, whereas intensity changes of the dust emission result from stellar luminosity changes as they are en-shrouded by very optically thin dust shells. In the case of Mira (o Cet) we compute that the eeective temperature increases from T ee =2400 K in the minimum phase to 3000 K (100 K) around maximum phase. The ampliied momentum transfer around maximum light enhances the acceleration of the dust outtow near the dust condensation radius of 6 R. This produces variations of the terminal dust outtow velocity with phase ((v 1 '5 km s ?1) at larger distance from the star. The corresponding small changes in ux mean opacity and gas mass-loss rates (from 2.8 to 3.2 10 ?7 M y ?1) are suucient to model the shape changes observed in the dust emission feature. A comparison with the modelling results for another long period Mira U Ori is also provided.