The physics of young stellar objects: X-rays, magnetism, and high-energy phenomena

The association between star-forming regions and X-ray emission was discovered as soon as X-ray satellites became available, over 30 years ago. However, understanding of the nature of this association really started when X-ray images and the resulting source identification could be obtained. We now know that essentially all young stellar objects, T Tauri stars and protostars, are X-ray emitters, although the case of the youngest, Class 0 protostars, is less clear. The paper briefly reviews X-ray emission and absorption mechanisms, and emphasizes the "solar paradigm" for the interpretation of the X-ray emission, originating in the magnetic activity of young stellar objects. The possibility of star-disk magnetic interactions in protostars is also examined. In addition, feedback effects on the circumstellar disks are presented: ionization, and the associated coupling with magnetic fields; particle irradiation, and its consequences for the origin of "extinct radioactivities" in meteorites