On the analysis of the IR spectra of cool giants: model atmospheres and time-dependent behaviour

Details On the analysis of the IR spectra of cool giants: model atmospheres and time-dependent behaviour On the analysis of the IR spectra of cool giants: model atmospheres and time-dependent behaviour

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003phdt..........v&link_type=abstract

PhD thesis

Computer Science

10

Agb Stars, T Cep, Model Atmospheres, Ir Spectroscopy

Scientific paper

We investigated the Short Wavelength Spectrometer (ISO-SWS)spectra of mostly cool giants. We confronted these observed spectra with synthetic spectra generated with MARCS model atmospheres. For cool, static giants, this comparison enabled us to detect some remaining instrumental artifacts in the 12-27 microns spectra, like residual fringes and baseline ripples. We also noted that the description of the structure variables in the outer layers of the model atmospheres is not completely consistent with the observations. A possible explanation is given by small-scale dynamical phenomena in the atmospheres of these stars.
In the case of dynamical cool giants - the pulsating AGB stars - a hydrostatic description for their highly extended atmospheres is not adequate. In a first step within our study of the time behaviour of the eight 2.38-27 microns SWS spectra of the O-rich AGB star T Cep, we nevertheless used hydrostatic MARCS model atmospheres to separate the photospheric contribution from the circumstellar component in the spectra. Then, we mimicked the extended atmosphere by putting a thermal water slab in front of the hydrostatic model atmosphere. Subsequently, this latter adopted model configuration was used to subtract dust emission spectra from the observations. Within this approach, we found that the effective temperature of T Cep varies in lock step with the visual lightcurve, while the stellar radius shows a phase lag with regard to the visual lightcurve. We also established that the density and luminosity variation of the region traced by the water layer drive the variation of most!
molecular features in the spectra of T Cep. Shocks are likely to have occurred during the observed time interval.
The circumstellar dust shell around T Cep consists of small, hot dust particles of amorphous silicates, amorphous alumina, and MgFeO. Furthermore, a 13 microns feature arises, for which we confirm the doubts about the identification of its carrier to be spinel. The variation of the dust shell is not luminosity driven, but is linked to the stellar temperature variation or to dust condensation/evaporation. Finally, we found indications of a varying composition of alumina during the observed time interval.

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