The chromospheres of pre-main sequence flare stars.

Optical spectra of pre-main sequence flare stars in Orion are presented and analyzed by the use of stellar chromospheric models. The stars observed divide into three groups on the basis of their quiescent spectra. The majority of the sample are similar to the dMe stars, with prominent Ca II and Balmer line emission. A second group possesses very strong emission spectra, including He I lines. Stars in third group, however, show little or no emission spectrum. Among the emission line stars observed progressively brighter stars exhibit increased Ca II and Balmer line surface fluxes relative to the fainter stars. This trend is also exhibited as a rise in emission line surface fluxes with increasing effective temperature. To analyze this trend a set of non-LTE stellar models were calculated. Using this analysis the trend in line fluxes between stars of different brightness and temperature is interpreted as a spread in the level of chromospheric heating. Furthermore, this trend is interpreted as indicating a spread in the effectiveness of the stellar magnetic dynamos within these stars, perhaps arising from shallowing in their convective layers as they contract to the main sequence.