The Variable Radio Source T Tauri

Centimeter-wavelength radio observations of the T Tauri system between 1983 and 2001 confirm that the radio emission from the optical star T Tau (T Tau N) has a spectral index of 0.6 and may be attributed to a stellar wind and nonthermal flares that are masked by the thermal emission. Right-circular polarization of 26% is detected in the 2 cm T Tau N emission. The infrared source (T Tau S) is confirmed to be nonthermal and variable in intensity, with a radio spectral index less than or equal to0. T Tau S displays weak extended emission and strong, compact left-circular polarization at wavelengths of 3.7 and 2 cm; the percentage of left-circularly polarized emission is larger at the shorter wavelengths. The 2 cm radio and optical positions of T Tau N are found to be coincident to 13 mas. Relative motion between the two radio sources (T Tau N and S) does not agree in detail with that measured at a wavelength of similar to2 mum. The difference in relative position is as large as 100 mas (15 AU) in declination in the 1980s. The recent IR detection of T Tau S as a binary star with a separation of 50 mas (7 AU) in 1997 December may explain this discrepancy. We suggest a scenario in which the radio emission detected toward T Tau S in our most recent observations (epoch 2001) is located at the position of the less luminous member of this binary system, T Tau Sb. A preliminary estimate of the orbital parameters of this system is an eccentricity of 0.7, semimajor axis 113 mas, inclination 69degrees, period 38.8 yr, and total system mass 5.3 M.. The circular polarization of the centimeter radio emission of T Tau N and T Tau S indicates that the emission from both components is gyrosynchrotron in nature. The opposite sense of circular polarization may indicate that it is dipole in nature.