Master Thesis defense by Joris Vos – Niels Bohr Institute - University of Copenhagen

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Master Thesis defense by Joris Vos

The solar type eclipsing binary EF Aqr

Abstract
The effect of activity on astrophysical stellar parameters, especially in solar type or less massive stars, is not yet fully understood, and not incorporated in most evolutionary models. In order to get a better understanding of these effects, a larger observational sample is necessary. The goal of this project is to fully analyse the solar type eclipsing binary EF Aqr, and to test it's astrophysical properties to modern theoretical stellar evolution models.

uvbyβ standard photometry was obtained with the Strömgren Automatic Telescope at ESO, La Silla. High resolution spectra were obtained with the HERMES spectrograph at the Mercator telescope on Roque de los Muchachos, La Palma. State of the art methods were used in the analysis of the photometric and spectroscopic observations. The broadening function formalism was used to obtain the radial velocities from the spectra, which were analysed with PHOEBE in combination with the light curves. The photometry and radial velocities were analysed separately with JKTEBOP and SBOP as well. However, the analysis showed that PHOEBE was not capable of giving trustworthy results. Abundances and temperatures were derived from the spectra using the VWA package of Bruntt.

Masses and radii were determined with precisions of the order of 1%. The primary was found to be slightly heavier and larger than the Sun with a mass of 1.244 Msun and a radius of 1.338 Rsun. The 0.946 Msun secondary is slightly less massive and smaller than the Sun with a radius of 0.956 Rsun. CaII - H and K emission was found in the spectra of both components, but the emission of the secondary was much more profound than that of the primary. The calculated Rossby numbers gave the same results, indicating that the primary is likely slightly active while the secondary is certain to exhibit a high degree of activity. Indications for starspots have been found in the lightcurves, but could not be fitted with the Wilson Devinney code. Possible spots are proposed as the reason for the periodic radial velocity variations visible in the residuals of the radial velocity curves. A rather large temperature difference was found for the system with an effective temperature of 6150 K for the primary and 5185 K for the secondary component. The metalicity of the system was found to be solar.

Corresponding solar-scaled stellar models, fx Yonsei-Yale, Victoria-Regina or BaSTI isochrones and evolutionary tracks fail at reproducing EF Aqr. The models differ in age for about 4 Gyr with the primary the youngest component, and predict higher temperatures for both components. At the age of the primary, the secondary is found to be about 10% smaller than observed. Stellar evolution models which adopt significantly lower mixing length parameters, as fx the Granada code, removed the discrepancies for both components. The primary is fitted with l/Hp=1.3 while for the secondary l/Hp=1.1 is found. This leads to a mean age of 1.75 Gyr for the system.

EF Aqr supports the suggestion that chromospheric activity and its influence on convective energy transport is likely to affect the radius and temperature of stars. The discrepancies between models and observations that arise in this way can be removed by adjusting the mixing length parameter downwards. However, a larger sample of low mass (0.5-1.2 Msun) binaries is necessary to develop a full description of convective energy transport in terms of astrophysical parameters and activity level.
Supervisor: Jens Viggo Clausen
Co-supervisor: Roy Østensen