Chromospheres and coronae in the T Tauri stars

The Tauri stars exhibit strong far ultraviolet emission lines of C II, III, IV; OI; Si II, III; IV; and sometimes N V and He II. Surface fluxes of the lines for several T Tauri stars were computed, drawing upon both International Ultraviolet Explorer observations and published spectra. The surface fluxes are quite high. The FUV lines together account for 0.1% of the stellar luminosity. These results indicate the presence of active relatively dense chromospheres. The Tauri stars showing very strong visual emission spectra exhibit weakened high temperature FUV lines of C IV and N V, as well as He II which may be produced by X-rays. In the same stars, no X-ray emission was detected with Einstein. Among all the T Tauri stars the X-ray luminosities are deficient by factors of 100 to 1000 compared to the FUV lines. It is argued that the X-ray flux has not been heavily absorbed by circumstellar gas, as has been previously suggested, but that the X-rays are truly underluminous, perhaps due to the stellar wind. It is suggested that the birth and development of the chromosphere and corona may be occurring during the T Tauri stage of protostellar evolution.