X-Ray Emission from Pre-Main Sequence Stars

The discovery, observations and implications of X-ray emission from low mass pre-main sequence (PMS) stars are reviewed. Studies of eight nearby star formation regions with the Einstein X-ray Observatory show that X-ray emission ≡10³ times higher than main sequence levels is characteristic of PMS stars. The X-ray surveys reveal a new population of PMS stars not previously identified in optical or infrared surveys. The X-ray emission is not closely related to winds or circumstellar material, but appears to be caused by giant solar-type flares. Implications of such enhanced surface activity on the star formation process, molecular clouds and the early solar system are discussed.