Interferometry of carbon rich AGB stars

This thesis deals with the comparison of interferometric data of Asymptotic Giant Branch (AGB) stars with hydrostatic and dynamic model atmospheres. The AGB is the late evolutionary stage of stars with masses below 8 Sun Masses. These stars are characterised by a C-O degenerated core and 2 shell with ongoing nuclear reactions (He and H shells), a convective envelope and a very extended atmosphere with molecules and dust formation. As a star evolves along the AGB, it becomes subject to photometric variability and mass loss. As it gets older and more luminous the mass loss becomes significant, enriching the interstellar medium with the products of stellar nucleosynthesis. If the AGB star has enough mass, the convective envelope will extend into the region with nuclear reactions, thereby, bringing processed material to the surface (dredge-up). The 'third' dredge-up is responsible for the existence of Carbon Stars. The spectrum of these stars is characterised by features of carbon-bearing molecules (C2, C2H2, C3, CN, HCN). Dust is mostly present in the outflows as amorphous carbon influencing the spectral energy distribution. Studying AGB stellar atmospheres is essential for a better comprehension of the late stage of stellar evolution, to understand the complicate influence of pulsation on the stellar atmosphere, the dynamic process of dust formation and mass loss. Due to their extended atmospheres and brightness in the red and infrared range, AGB stars are perfect candidates for interferometric investigations. For pulsating, mass-losing carbon stars available atmospheric models are more advanced than for oxygen-rich stars because the dust formation is better understood. Nevertheless, in literature very few interferometric studies were dedicated to this class of objects so far. Therefore, this work is concentrated on carbon stars. The first part of this thesis is devoted to the interferometric analysis of synthetic intensity profiles and visibility1 profiles using Johnson JHK broad-band filters, and some narrow filters defined ad hoc to sample features of the infrared spectra of C-stars. These profiles are computed for a specified set of hydrodynamic models for stars with different mass-loss rates. Most of the previous studies concerning interferometry of red giant stars used simple approximations for interpreting visibility profiles. Following the same approach used in literature as for M-stars, the computed visibility profiles are fitted with analytical functions (uniform discs) to investigate their dependence on wavelength, pulsation phase and stellar parameters. It was found that the radius predicted by the models increases with wavelength, and the dependence on pulsation phase is not strictly sinusoidal. The L-band turned out to be crucial region for parameter determinations. In the second part of this work newly obtained spectroscopic and interferometric data are presented for a sample of five objects with very limited dynamic effects. The observations are compared with models and the full set of stellar parameters (Teff , C/O, mass and log(g)) could be derived. The parameters determined in this way are then compared with evolutionary tracks. The distance determination remains a crucial problem. Nevertheless, very accurate effective temperature determinations can be obtained from L-band spectroscopy, and interferometry is the only tool that can give access to the mass of the object. In the third and fourth part of the work the radial structure of the atmosphere of the C-rich semiregular variable R Scl and the carbon-Mira R For are investigated with spectrointerferometric observations in the mid-infrared. The N-band variability, the stratification of the atmosphere, and the geometry of the circumstellar envelope are presented. The observations are compared with dynamic model atmospheres confirming the interferometric technique as a very powerful tool to constrain our knowledge of dynamic processes (i.e. dust formation and mass-loss process). (author)