THE ENIGMATIC T-TAURI RADIO-SOURCE

We have analyzed eight high angular resolution images of the prototype low mass pre-main-sequence star T Tauri obtained with the Very Large Array between 1987 and 1990. Our objectives were to confirm a recent report that the radio emission is both variable and circularly polarized, to determine whether this behavior originates in the optical star or in the infrared source lying 0.6'' to its south, and to identify possible emission mechanisms. No variability or circular polarization was detected in the weak (almost-equal-to 1 mJy) radio emission associated with the visible star, down to levels of almost-equal-to 10%. The observed flux can be accounted for by free-free emission from an ionized wind with a mass-loss rate M = 3.7 x 10(-8) M. yr-1, but a more accurate determination of the spectral index is needed to test the validity of spherical wind models. In sharp contrast, the 3.6 cm emission of the infrared source is variable (almost-equal-to 4-7 mJy) on time scales less-than-or-equal-to 3 days and circularly polarized at low levels of almost-equal-to 3%-5%. The polarization was left circular when detected during periods of low radio activity but changed to right circular during a radio outburst, similar to the reversals that have been seen in some RS CVn binary systems. The spectral index is negative during ''quiescence,'' and we argue that the emission is nonthermal gyrosynchrotron radiation. The detection of magnetic radio activity in this optically invisible infrared source sparks new interest in clarifying its evolutionary status. Sensitive 3.6 cm images reveal weak emission extending almost-equal-to 1'' west of the T Tau system that is probably associated with high-velocity shock-excited gas flowing toward HH-1555. We find no radio evidence for a putative third component north of the visible star.