SODIUM LINES IN T-TAURI STARS - DIAGNOSTICS OF PRE-MAIN-SEQUENCE WINDS

We report model calculations of the sodium ionization and excitation conditions in winds from T Tauri stars. We compute line profiles for the sodium D doublet resonance lines and compare them to the observations. We find that, in order to reproduce the observed features, the line optical depth, which is proportional to the neutral sodium column density in the wind, must be restricted to a narrow range (0.3 ≲ τ ≲ 3 for the blue component of the doublet). τ is a strong function of the mass-loss rate M, but it is rather insensitive to the wind temperature Tg. By using both the values of τ inferred from the observed Na line profiles for a given object and the observed luminosity of hydrogen recombination lines, it is possible to obtain with good accuracy both Tg and Ṁ. We discuss the possible influence of different wind acceleration laws, of fast turbulent motions, of different stellar radiation fields (including photospheric lines), and of circumstellar disks. The method proposed to measure Tg and Ṁ turns out to be rather robust and insensitive to a variety of effects. The application to a restricted sample of T Tauri stars indicates Tg between 6000 K and 6500 K and mass-loss rates between 3 × 10-8 and 3 × 10-7 M⊙ yr-1. The winds are mostly neutral, and the mass-loss rates are in excellent agreement with those required to drive the associated CO outflows with a momentum-conserving shock.