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

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. Aims. 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. Methods. 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 ± 0.001 M⊙ and M2 = 1.958 ± 0.001 M⊙. The comparison with stellar evolution models gives an age of the system of 1.20 ± 0.10 Gyr. We also derived the distance to the system with a precision level of 1.1%: d = 185.9 ± 1.9 pc. 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