High spatial resolution observations of the T Tau system - I. Astrometry in the near-infrared

T Tauri is an example how our knowledge about an object improves with spatial resolution: About 60 years ago, it was considered to be the prototypical T Tauri star. 25 years ago, it was discovered to be a close binary with an infrared companion at 0.7arcsec distance. Ten years ago, the infrared companion was itself resolved into a binary with just 50 mas separation. We collected astrometric data from the literature and from archived VLT observations. Additionally, we present a new measurement obtained on February 1, 2008. Fits of orbit models to the data yield estimates for the orbital elements of the close binary. In most other binaries, we can observe only the relative motion of both components around each other, which can be used to determine the total mass of the pair. The presence of the third component in the T Tauri system gives us the rare opportunity to observe also the motion of both components of the close binary around their common center of mass, allowing to determine dynamical masses of the individual stars. We find that the period of the best model for the orbit of T Tau Sa and Sb is 28+12-4 years. The mass of Sa is 2.28 ± 0.22 Msun and the mass of Sb is 0.41 ±0.19Msun.