Catching the fish - Constraining stellar parameters for TX Psc using spectro-interferometric observations

Abstract

Stellar parameter determination is a challenging task when dealing with galactic giant stars. The combination of different investigation techniques has proven to be a promising approach. We analyse archive spectra obtained with the Short-Wavelength-Spectrometer (SWS) onboard of ISO, and new interferometric observations from the Very Large Telescope MID-infrared Interferometric instrument (VLTI/MIDI) of a very well studied carbon-rich giant: TX Psc. The aim of this work is to determine stellar parameters using spectroscopy and interferometry. The observations are used to constrain the model atmosphere, and eventually the stellar evolutionary model in the region where the tracks map the beginning of the carbon star sequence. Two different approaches are used to determine stellar parameters: (i) the 'classic' interferometric approach where the effective temperature is fixed by using the angular diameter in the N-band (from interferometry) and the apparent bolometric magnitude; (ii) parameters are obtained by fitting a grid of state-of-the-art hydrostatic models to spectroscopic and interferometric observations. We find a good agreement between the parameters of the two methods. The effective temperature and luminosity clearly place TX Psc in the carbon-rich AGB star domain in the H-R-diagram. Current evolutionary tracks suggest that TX Psc became a C-star just recently, which means that the star is still in a 'quiet' phase compared to the subsequent strong-wind regime. This is in agreement with the C/O ratio being only slightly larger than 1.

11 pages, 9 figures, 5 tables

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• Evolution of Thermally Pulsing Asymptotic Giant Branch Stars II. Dust production at varying metallicity
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• Properties of carbon stars in the Solar neighbourhood based on Gaia DR2 astrometry
• The VLTI/MIDI view on the inner mass loss of evolved stars from the Herschel MESS sample
• An HI Imaging Survey of Asymptotic Giant Branch Stars
• Modelling the atmosphere of the carbon-rich Mira RU Vir
• The adventure of carbon stars - Observations and modelling of a set of C-rich AGB stars
• ALMA observations of the "fresh" carbon-rich AGB star TX Piscium. The discovery of an elliptical detached shell
• Constraining stellar parameters and atmospheric dynamics of the carbon AGB star V Oph
• The complex environment of the bright carbon star TX Psc as probed by spectro-astrometry
• From the atmosphere to the circumstellar environment in cool evolved stars