High Energy Processes in Young Stellar Objects

In this paper, I present background knowledge for high energy processes in Young Stellar Objects(YSO), including observation results, theoretical model. In addition, I also summarize the recent development in the field of high energy processes in YSO, especially the relationship between X-ray luminosity and rotational period. 1. Young Stellar Objects(YSO) Pierre-Simon Laplace in his ExpositionduSystemeduMonde(1796) for the first time gave us an idea of star forming as a gravitationally collapsing cloud. In 1945, Alfred Joy found a class of unusual emission line from variable stars near dark clouds, which later were confirmed as classical T Tauri stars. T Tauri stars have convective stellar interiors with monotonically decreasing luminosity but nearly constant surface temperatures, powered principally by gravitational contraction rather than nuclear reactions. There were some observation breakthroughs made in the past century. Outflows from YSOs were observed with P Cygni-type profiles (Herbig 1962) and they are common in YSOs. It believed that they are powered by magnetic field induced by intervention between young star and circumstellar disk(Uchida & Shibata 1984). Circumstellar disks were also directly imaged by millimeter interferometer(Dutrey et al 1994) and Hubble Space Telescope(McCaughrean &O’dell 1996). There are several evolutionary phases as YSOs evolves. Figure 1 illustrates the phases in subsequence: protostars, CTT stars, and weak-lined T Tauri(WTT) stars. The evolutionary phase is generally determined by SED in infrared and millimeter band.