The Araucaria Project: High-precision orbital parallax and masses of the eclipsing binary TZ Fornacis

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Context: Independent distance estimates are particularly useful to check the precision of other distance indicators, while accurate and precise masses are necessary to constrain evolution models. Aim: The goal is to measure the masses and distance of the detached eclipsing-binary TZ For with a precision level lower than 1% using a fully geometrical and empirical method. Method: We obtained the first interferometric observations of TZ For with the VLTI/PIONIER combiner, which we combined with new and precise radial velocity measurements to derive its three-dimensional orbit, masses, and distance. Results: The system is well resolved by PIONIER at each observing epoch, which allowed a combined fit with eleven astrometric positions. Our derived values are in a good agreement with previous work, but with an improved precision. We measured the mass of both components to be M₁ = 2.057 ± 0.001M and M₂ = 1.958 ± 0.001M. The comparison with stellar evolution models gives an age of the system of 1.20 ± 0.10Gyr. We also derived the distance to the system with a precision level of 1.1%: d = 185.9 ± 1.9pc. Such precise and accurate geometrical distances to eclipsing binaries provide a unique opportunity to test the absolute calibration of the surface brightness-colour relation for late-type stars, and will also provide the best opportunity to check on the future Gaia measurements for possible systematic errors.