EINSTEIN OBSERVATIONS OF T-TAURI STARS IN TAURUS-AURIGA .2. 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 Taurus-Auriga pre-main-sequence stars, presented in a companion paper, we have studied the relationships between the X-ray emission and typi cal diagnostics of pre-main-sequence activity to obtain indications of the origin of the X-ray emission and of the structure of the outer at mospheres in these stars. We argue that, in the case of classical T Ta uri stars, scaling the H alpha luminosity to the infrared (disk) emiss ion is much more meaningful physically than scaling it to the stellar continuum (by considering the line equivalent width). We find an inver se relation between the X-ray luminosity and this infrared-scaled H al pha emission. We also find an anticorrelation between X-ray luminosity and rotational period, in qualitative agreement with earlier works; t his is interpreted in terms of a magnetic origin for the X-ray emissio n. This relationship, however, is limited to stars younger than 2 x 10 (6) yr, while for the older sample stars, saturation of X-ray surface flux appears to take place. We interpret the relation found between X- ray and infrared-scaled H alpha emission as a competition between clos ed and open magnetic structures on the surface of these stars. In the stars without Indications of winds and accretion disks (weak-line T Ta uri stars), a correlation between X-ray luminosity and unscaled H alph a luminosity is found instead, providing further evidence for the chro mospheric origin of the line emission in these latter stars.