X--ray Activity and PMS Stars.

The advent of X--ray astronomy greatly expanded the number of known PMS stars. The earliest X--ray observations were carried out in the mid--1970s. X--ray luminosities of order L erg/sec were detected in the 2-11 keV range in the direction of the Orion star forming region. Imaging observations were later made by the EINSTEIN X--Ray observatory (Ku & Chanan 1979). These observations showed that the O and B stars were the brightest X--ray sources, and that TTs were also associated with emission. Feigelson and Kriss (1981) showed that some of the X--ray sources were associated with PMS stars which had not been previously detected which H surveys. Gahm (1980) suggested that the X--ray emission originated in flare-like events. Walter & Kuhi (1981, 1984) showed that about 1/3 of TTs were detected by EINSTEIN and contended that since the derived temperatures of these stars were similar to those of RS CVn type systems, the source of the emission was probably coronal. They argued that the reason only the T Tauri stars with the least veiling were observed was further evidence that the X--rays must come from close to the stellar surface.