EINSTEIN Observations of T Tauri Stars in Taurus-Auriga. II: Relationships between X-ray Emission and Pre-Main-Sequence Activity

On the basis of the full set of reanalyzed Einstein IPC X-ray data on Tau-Aur pre-main-sequence stars, presented in a companion paper, we have studied the relationships between the X-ray emission and typical diagnostics of pre-main-sequence activity, to obtain indications on the origin of the X-ray emission and on the structure of the outer atmospheres in these stars. We argue that, in the case of classical T Tauri stars, scaling the H luminosity to the infrared (disk) emission is much more meaningful physically than scaling it to the stellar continuum (by considering the line equivalent width). We nd an inverse relation between the X-ray luminosity and this infrared-scaled H emission. We nd also an anticorrelation between X-ray luminosity and rotational period, in qualitative agreement with earlier works; this is interpreted in terms of a magnetic origin for the X-ray emission. This relationship, however, holds limitedly to stars younger than 2 10 6 yr, while for the older sample stars saturation of X-ray surface ux appears to take place. We interpret the relation found between X-ray and infrared-scaled H emission as a competition between closed and open magnetic structures on the surface of these stars. In the stars without indications of winds and accretion disks (WTTS), instead, a correlation between X-ray-and un-scaled H luminosity is found, providing further evidence for the chromospheric origin of the line emission in these latter stars.