Direct VLBI detection of the magnetosphere surrounding the young star S1 in Rho Ophiuchi

VLBI 6-mm data are presently used to investigate the circularly polarized radio core previously identified around the young B3 star S1 in Rho Ophiuchi. The measured angular diameter and brightness temperature are found to be consistent with gyrosynchrotrom radiation emission from mildly relativistic electrons. A simple model based on a pole-on dipolar magnetic field of about 2 kG at the stellar surface suggests itself as consistent with the main observed features of the S1 magnetosphere; an important feature of the model is its taking the influence of the X-ray-emitting plasma into account. S1 may represent a new type of young stellar object, characterized by very extended magnetic fields.