Atmospheres of evolved C-rich stars - from observations to models, and back
Creators: name: Rau, Gioia
Publisher: name: Zenodo
Year: 2016
Subjects: interferometry
high angular resolution
circumstellar matter
stellar atmospheres
AGB and post-AGB stars
fundamental parameters
mass loss
dust
VLTI
carbon stars
evolved stars

description: A short but intriguing stage in the evolution of low to intermediate ( 0.8 to 8.0 M_sun) mass stars, the Asymptotic Giant Branch (AGB) is populated by stars with extended, complex, shell-like structured atmospheres.

Modelling giant star atmospheres remains a challenging task in contemporary astronomy. Carbon-rich AGB star atmospheres have been widely studied by several authors, who compared observations with hydrostatic and dynamic atmospheric models but rarely in a systematic way with multi-facility observations. The methodical testing of models with multi-technique observations has in fact emerged as the only adequate way to provide strong constraints on models.

The study of C-enriched AGB stars atmospheres, is the main purpose of this thesis. Advanced state-of-the-art dynamic and hydrostatic model atmospheres have been compared with new and archive mid-infrared interferometric MIDI observations and literature spectro-photometric data. In total eight C-rich stars have been investigated: RU Vir, R Lep, R Vol, Y Pav, AQ Sgr, U Hya, X TrA and R For.

The first part of this monograph consists of three introductory chapters, which present general background on AGB stars, on the types of models used to interpret the data, and on the main observational technique used in this work: optical interferometry.

Chapter 4 focuses on the study of the atmosphere of RU Vir. Hydrostatic, geometric and dynamic models have been fit to the observations. The dynamic model Spectral Energy Distributions (SED) were first fitted to the literature data, obtaining one best-fitting model; then, for all the stellar pulsational phases of this specific model, the interferometric intensity profiles and visibilities were computed. This method was adopted for the whole thesis. Showing an SED which agrees well with the models only between 2 and 14 µm, this star turned out to be a peculiar, puzzling Mira.

This research was extended to a set of carbon-enriched Mira, Semi-regular and Irregular variables, in Chapter 5. Characteristics of the atmospheres of these stars, such as their extension, dust properties and mass loss, together with further constraints on the models, are given. Stellar parameters are derived and the stars are located in in Hertzsprung-Russell diagram, with results showing a good accord between the main parameters obtained from the modelling and evolutionary computations of the thermally pulsating AGB.

A substantial amount of effort was devoted to the data-reduction, quality checks, and analysis of Very Large Telescope Interferometer/MIDI data, part of a ESO Large Program of observations. The scientific results of this program are given in Chapter 6.

Even though encouraging results regarding the dynamic models are obtained in this thesis, some differences between SEDs and interferometric studies persist. Chapter 7 investigates, from an observational standpoint, if removing the assumption of the small particle limit (SPL) could improve the agreement between models and observations. A test-case star, R Lep, is used, and its measurements are compared to the newly produced non-SPL model from the Uppsala group. Achieved improvements and remaining differences are described.

In Chapter 8 the preliminary dynamic modelling of the spectro-photometric and interferometric MIDI data of R For is presented. Finally, Chapter 9 summarizes conclusions and future prospects on this study.