CIRCULARLY POLARIZED RADIO-EMISSION FROM THE T-TAURI STAR HUBBLE-4

I present results of multiwavelength VLA observations of the weak-lined T Tauri star (WTTS) Hubble 4, which is located in the nearby (approximately 140 pc) Taurus-Auriga star-forming region. These observations include a near-simultaneous detection at 3.6, 6, and 20 cm, which provides a rare glimpse at the radio-energy distribution of a WTTS. The emission is characterized by a nearly flat spectral energy distribution with a spectral index alpha almost-equal-to 0 (S(nu) infinity nu(alpha)). Right circular polarization was detected at both 3.6 and 6 cm at levels of 19(+/-5%) and 18(+/-3%), respectively, with an upper limit of less-than-or-equal-to 15% (3 sigma) at 20 cm. The star was in a state of low radio activity ('' quiescence ''), with flux densities of 1.3-1.7 mJy and a radio luminosity L(r) almost-equal-to 10(16.)5 ergs Hz-1 s-1. The presence of circular polarization and previous VLBI source size measurements argue strongly for nonthermal emission. The unambiguous detection of circularly polarized radio emission from Hubble 4 provides direct evidence for the existence of ordered magnetic fields in this WTTS. The polarized emission is most likely optically thin gyrosynchrotron radiation from mildly relativistic electrons with characteristic total energies almost-equal-to 2.5 MeV (Lorentz factors gamma almost-equal-to 5). At an assumed view angle of 45-degrees, the observed degree of circular polarization implies a magnetic field strength of B almost-equal-to 25 G in the region responsible for the 3.6 cm emission, and a field strength about half this large in the 6 cm source region. The field strength at the stellar surface is not known, but values as large as a few kilogauss are not ruled out. The radio properties of Hubble 4 are in many respects similar to those of RS CVn binary systems during periods of low activity. This adds credibility to the hypothesis that the same physical mechanism is responsible for low-level radio emission in RS CVns and WTTS. Furthermore, the failure to detect a change in either the helicity or degree of circular polarization in two 3.6 cm observations almost-equal-to 7 months apart suggests that the magnetic field structure of Hubble 4 may remain stable for relatively long periods. Although better sampling in the time domain is needed, the new radio data provide encouragement that the magnetosphere models which have been proposed to explain low-level RS CVn emission may also apply qualitatively to WTTS.