VLBI observations of T Tauri South

¹ Institut für Astronomie, ETH-Zentrum, 8092 Zürich, Switzerland e-mail: benz@astro.phys.ethz.ch
² Paul Scherrer Institut, Würenlingen und Villigen, 5232 Villigen PSI, Switzerland e-mail: guedel@astro.phys.ethz.ch
³ Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: kester@mpifr-bonn.mpg.de
⁴ Onsala Space Observatory, 43992 Onsala, Sweden e-mail: michele@oso.chalmers.se;jconway@oso.chalmers.se

Corresponding author: K. Smith, kester@mpifr-bonn.mpg.de

Received: 17 March 2003
Accepted: 20 May 2003

Abstract

We report observations of the T Tauri system at 8.4 GHz with a VLBI array comprising the VLBA, VLA and Effelsberg 100 m telescopes. We detected a compact source offset approximately 40 mas from the best infrared position of the T Tau Sb component. This source was unresolved, and constrained to be less than 0.5 mas in size, corresponding to 0.07 AU or 15  at a distance of 140 pc. The other system components (T Tau Sa, T Tau N) were not detected in the VLBI data. The separate VLA map contains extended flux not accounted for by the compact VLBI source, indicating the presence of extended emission on arcsecond scales. The compact source shows rapid variability, which together with circular polarization and its compact nature indicate that the observed flux arises from a magnetically-dominated region. Brightness temperatures in the MK range point to gyrosynchrotron as the emission mechanism for the steady component. The rapid variations are accompanied by dramatic changes in polarization, and we record an at times 100% polarized component during outbursts. This strongly suggests a coherent emission process, most probably an electron cyclotron maser. With this assumption it is possible to estimate the strength of the local magnetic field to be 1.5-3 kilogauss.