X-ray Study of Low-mass Young Stellar Objects in the ρ Ophiuchi ...

2.1. EVOLUTION OF LOW-MASS STARS 5which may be produced by a shock of the jet from a putative protostar on the interstellar medium(e.g., Rodriguez et al., 1989, and references theirin). About 10 5 yr later, most of the materials haveaccreted onto the central source and the emission from the accretion disk reprocessing incidentstarlight starts to be observed from the NIR to mid-infrared (MIR) bands, which is called class Isources (late phase of protostars). They still show outflow activity. Class II sources are at the earlyphase of TTSs (classical T Tauri stars = CTTSs). Although the emission from the central sourcebecomes dominant, they show excess emissions from K (2.2 µm) to MIR bands and strong opticalHα lines, indicating the presence of an accretion disk. Soon the disk disappears and the SEDsbecome a single temperature blackbody. We call them class III sources, or weak-lined T Tauri stars(WTTSs).If sources have a mass below ≈0.08 M ⊙ , dynamical contraction stops before the hydrogenburning starts, hence they never arrive at the MS stage, which is called “brown dwarfs (BDs)”(Basri, 2000). They emit faint optical–IR photons using gravitational energy and consequently arerelatively bright at a younger phase (Figure 2.1). Hereafter, we call those BDs still in the masscontraction phase “young BDs” (i.e., on the Hayashi and Henyey tracks, Figure 2.1).The evolution of high- and intermediate-mass stars (>1.5 M ⊙ ) would be the same as low-massones; they also evolve along the Hayashi and Henyey tracks. However, there are very few samples ofhigh/intermediate-mass YSOs (e.g., Waters & Waelkens, 1998; Howard, Pipher, & Forrest, 1994),because they quickly start hydrogen burning then reach the MSs during the mass contraction phase.Detailed understanding of these classes therefore has not been obtained. In this thesis, we basicallydo not discuss them. For the results of X-ray observations of high/intermediate-mass YSOs, seeHamaguchi et al. (2000); Hamaguchi (2001); Kohno et al. (2002); Takagi et al. (2002); Nakajimaet al. (2003).