2008A&A...480..489M

The young binary system V773 Tau A exhibits a persistent radio flaring activity that gradually increases from a level of a few mJy at apoastron to more than 100 mJy at periastron. Interbinary collisions between very large (>15R_*) magnetic structures anchored on the two rotating stars of the system have been proposed to be the origin of these periodic radio flares. Magnetic structures extended over tens of stellar radii, that can also account for the observed fast decay of the radio flares, seem to correspond to the typical solar semi-open quite extended magnetic configurations called helmet streamers. We aim to find direct observational evidence for the postulated, solar-like, coronal topologies. We performed seven-consecutive-day VLBI observations at 8.4GHz using an array consisting of the VLBA and the 100-m Effelsberg telescope. V773 Tau A was phase-referenced to QSO B0400+258. Two distinctive structures appear in the radio images here presented. They happen to be associated with the primary and secondary stars of the V773 Tau A system. In one image (Fig. 2B) the two features are extended up to 18R_* each and are nearly parallel revealing the presence of two interacting helmet streamers. One image (Fig. 2E) taken a few hours after a flare monitored by the 100-m Effelsberg telescope shows one elongated fading structure substantially rotated with respect to those seen in the B run. The same decay scenario is seen in Fig. 2G for the helmet streamer associated with the other star. This is the very first direct evidence revealing that even if the flare origin is magnetic reconnection due to interbinary collision, both stars independently emit in the radio range with structures of their own. These structures are helmet streamers, observed for the first time in stars other than the Sun. The complete extent of each helmet streamer above the stellar surface is about 24R_* which implies that they can practically interact throughout the whole orbit, even rather close to apoastron where the stellar separation is 52R_*. However, the radio flares become stronger when the stars approach. Around periastron the stellar separation is only 30R_*, nearly covered by a single streamer: the two streamers overlap producing the observed giant flares.