Determination of the stellar parameters of C-rich hydrostatic stars from spectro-interferometric observations⋆
Department of Astronomy, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
2 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
3 INAF-OAPD, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
4 Joint Astronomy Centre, 660 North A’ohōkū Place, University Park, Hilo, Hawaii 96720, USA
Received: 18 January 2011
Accepted: 20 May 2011
Context. Giant stars, and especially C-rich giants, contribute significantly to the chemical enrichment of galaxies. The determination of precise parameters for these stars is a necessary prerequisite for a proper implementation of this evolutionary phase in the models of galaxies. Infrared interferometry opened new horizons in the study of the stellar parameters of giant stars, and provided new important constraints for the atmospheric and evolutionary models.
Aims. We aim to determine which stellar parameters can be constrained by using infrared interferometry and spectroscopy; for C-stars in particular we aim to determine the precision that can be achieved as well as its limitations.
Methods. For this purpose we obtained new infrared spectra and combined them with unpublished interferometric measurements for five mildly variable carbon-rich asymptotic giant branch stars. The observations were compared with a large grid of hydrostatic model atmospheres and with new isochrones that include the predictions of the thermally pulsing phase.
Results. For the very first time we are able to reproduce spectra in the range between 0.9 and 4 μm, and K broad band interferometry with hydrostatic model atmospheres. Temperature, mass, log(g), C/O and a reasonable range for the distance were derived for all objects of our study. All our targets have at least one combination of best-fitting parameters that lies in the region of the HR-diagram where C-stars are predicted.
Conclusions. We confirm that low-resolution spectroscopy is not sensitive to the mass and log(g) determination. For hydrostatic objects the 3 μm feature is very sensitive to temperature variations, therefore it is a very powerful tool for accurate temperature determinations. Interferometry can constrain mass, radius, and log(g), but a distance has to be assumed. The large uncertainty in the distance measurements available for C-rich stars remains a major problem.
Key words: stars: AGB and post-AGB / stars: atmospheres / stars: carbon / stars: fundamental parameters / techniques: spectroscopic / techniques: high angular resolution
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2011