The adventure of carbon stars

Observations and modeling of a set of C-rich AGB stars^⋆

¹ University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
e-mail: gioia.rau@univie.ac.at
² Institut d’Astronomie et d’Astrophysique, Université libre de Bruxelles, Boulevard du Triomphe CP 226, 1050 Bruxelles, Belgium
³ Astronomical Observatory of Padova – INAF, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Department of Physics and Astronomy, Division of Astronomy and Space Physics, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁵ Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50397 Köln, Germany

Received: 18 July 2016
Accepted: 16 January 2017

Abstract

Context. Modeling stellar atmospheres is a complex and intriguing task in modern astronomy. A systematic comparison of models with multi-technique observations is the only efficient way to constrain the models.

Aims. We intend to perform self-consistent modeling of the atmospheres of six carbon-rich AGB stars (R Lep, R Vol, Y Pav, AQ Sgr, U Hya, and X TrA) with the aim of enlarging the knowledge of the dynamic processes occurring in their atmospheres.

Methods. We used VLTI/MIDI interferometric observations, in combination with spectro-photometric data, and compared them with self-consistent, dynamic model atmospheres.

Results. We found that the models can reproduce spectral energy distribution (SED) data well at wavelengths longer than 1 μm, and the interferometric observations between 8 μm and 10 μm. Discrepancies observed at wavelengths shorter than 1 μm in the SED, and longer than 10 μm in the visibilities, could be due to a combination of data- and model-related effects. The models best fitting the Miras are significantly extended, and have a prominent shell-like structure. On the contrary, the models best fitting the non-Miras are more compact, showing lower average mass loss. The mass loss is of episodic or multi-periodic nature but causes the visual amplitudes to be notably larger than the observed ones. A number of stellar parameters were derived from the model fitting: T_Ross, L_Ross, M, C/O, and Ṁ. Our findings agree well with literature values within the uncertainties. T_Ross, and L_Ross are also in good agreement with the temperature derived from the angular diameter T(θ_(V−K)) and the bolometric luminosity from the SED fitting L_bol, except for AQ Sgr. The possible reasons are discussed in the text. Finally, θ_Ross and θ_(V−K) agree with one another better for the Miras than for the non-Miras targets, which is probably connected to the episodic nature of the latter models. We also located the stars in the H-R diagram, comparing them with evolutionary tracks. We found that the main derived properties (L, T_eff, C/O ratios and stellar masses) from the model fitting are in good agreement with TP-AGB evolutionary calculations for carbon stars carried out with the COLIBRI code.