Radio continuum studies of weak-lined T Tauri (WTT) stars have revealed widespread, nonthermal gigahertz radio activity at luminosities of 1016 to 1017 ergs s-1 Hz-1. We report here simultaneous very long baseline interferometry (VLBI) and Very Large Array (VLA) observations of six radio-bright WTT stars, directly measuring the sizes of the emitting regions for the first time. In the Taurus-Auriga cloud, active stars included HD 283447, HDE 283572, Hubble 4, and HP Tau/G2. V410 Tau, observed at the same time, was not detected to a VLBI/VLA limit of 0.5 limit. In the ρ Ophiuchi cloud, DoAr 21 was found to be active at levels comparable to the detected Taurus-Auriga stars. VLBI measurements establish that essentially all the radio emission from these pre-main-sequence (PMS) stars originates in regions 15 stellar radii or less in size. Corresponding brightness temperatures (Tb) ranged from 107.5 to > 109 K, ruling out a thermal process such as free-free bremsstrahlung radiation from a circumstellar wind. The radio luminosity and structure of several stars changed significantly between measurements separated by 1 day. HD 283447 showed intraday radio variability on time scales as short as 1 hr. Corresponding VLBI measurements show a new, unresolved component appearing after an increase in flux density, possibly reflecting that the driving agent for larger radio flares originates close to the star. Three days later, HD 283447 showed large (∼15 R*), stable radio structure during a period of constant flux density. For no stars did the VLBI measurements provide evidence for asymmetric or demonstrably complex emission regions. The high confirmation rate of nonthermal radio emission from our initial sample of radio-bright WTT stars show that these solar-type pre-main-sequence stars alter their immediate environments via magnetic processes to an extent comparable to that shown by RS CVn or Algol close binaries. High-resolution radio studies of the millijansky-level emission from WTT stars offer a new tool for studying the magnetic behavior of solartype pre-main-sequence stars at age ∼ 106 yr, and may eventually have application toward theories of classical T Tauri stars and conditions in the solar nebula during planetary formation.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science