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

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4 CHAPTER 2. REVIEW OF LOW-MASS YOUNG STELLAR OBJECTST Tauri StarsProtostarsYoungBrown DwarfsFig. 2.1.— The H-R diagram (D’Antona & Mazzitelli, 1997). The horizontal and vertical axes arethe surface (effective) temperature and bolometric luminosity normalized by the solar value (1 L ⊙= 3.8×10 33 ergs s −1 ). Thin solid and dashed lines represent the evolutional tracks (Hayashi andHenyey tracks) with the mass of 0.02–3.0 M ⊙ and isochrones with the ages of 10 6 and 10 7 yrs, whilethick solid and dashed lines are the main-sequence track and the birth line, respectively. Ellipsesshow the approximate positions of protostars, T Tauri stars, and young brown dwarfs.Low-mass YSOs are classified into four types based on their spectral energy distributions(SEDs) from the near-infrared (NIR) to sub-millimeter (sub-mm) wavelengths (Figure 2.2; Lada1991). Class 0 sources (André, Ward-Thompson, & Barsony, 1993; Barsony, 1994) are at the earlyphase of protostars (t ∼10 4 yr). Although the central source is hidden by the dense material, we canobserve emission from cold (∼30 K) circumstellar envelope at the far-infrared (FIR) to millimeter(mm) bands. In the class 0 phase, much angular momentum is transferred into the central sourcethrough dynamical mass accretion. Part of them escape outward via bipolar outflows, which isconfirmed by some observational evidence such as molecular (CO, H 2 , etc.) and atomic (Herbig-Haro object) line emissions with a scale of 10 4 AU 2 and centimeter (cm) continuum emissions2 The average distance from the earth to the sun (astronomical unit). 1 AU = 1.5×10 13 cm.
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).
Page 3: Contents1 Introduction 12 Review of
Page 9 and 10: List of Figures2.1 The H-R diagram
Page 11 and 12: LIST OF FIGURESix6.17 Light curves
Page 13 and 14: List of Tables3.1 Multiwavelength s
Page 15 and 16: Chapter 1IntroductionStar formation
Page 17: Chapter 2Review of Low-mass Young S
Page 21 and 22: 2.2. MOLECULAR CLOUDS 72.2 Molecula
Page 23 and 24: 2.3. X-RAY OBSERVATIONS OF LOW-MASS
Page 25 and 26: 2.3. X-RAY OBSERVATIONS OF LOW-MASS
Page 27: 2.3. X-RAY OBSERVATIONS OF LOW-MASS
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54 CHAPTER 5. CHANDRA OBSERVATIONS
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56 CHAPTER 5. CHANDRA OBSERVATIONS
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58 CHAPTER 6. INDIVIDUAL SOURCEShig
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60 CHAPTER 6. INDIVIDUAL SOURCESres
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62 CHAPTER 6. INDIVIDUAL SOURCES6.2
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64 CHAPTER 6. INDIVIDUAL SOURCESfla
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66 CHAPTER 6. INDIVIDUAL SOURCESTab
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68 CHAPTER 6. INDIVIDUAL SOURCESUsi
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70 CHAPTER 6. INDIVIDUAL SOURCESlos
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90 CHAPTER 6. INDIVIDUAL SOURCESThe
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92 CHAPTER 6. INDIVIDUAL SOURCESFig
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94 CHAPTER 6. INDIVIDUAL SOURCESPra
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96 CHAPTER 7. OVERALL FEATURE OF X-
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98 CHAPTER 7. OVERALL FEATURE OF X-
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100 CHAPTER 7. OVERALL FEATURE OF X
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Chapter 8Systematic Study of YSO Fl
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8.2. CORRELATION BETWEEN THE FLARE
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8.3. MAGNETIC RECONNECTION MODEL 11
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8.5. EFFECT OF THE QUIESCENT X-RAYS
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8.6. EVOLUTION OF YSOS AND THEIR FL
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Chapter 9ConclusionWe summarize the
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Appendix AFlare Light CurvesFig. A.
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Fig.A.2 (Continued)121
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Fig. A.4.— Same as Figure A.1, bu
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126 APPENDIX B. PHYSICAL PARAMETERS
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128 APPENDIX B. PHYSICAL PARAMETERS
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Appendix CModeling of the FlareIn t
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C.2. PREDICTED CORRELATIONS BETWEEN
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BibliographyAgeorges, N., Eckart, A
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BIBLIOGRAPHY 137Feigelson, E. D., &
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BIBLIOGRAPHY 139Johnstone, D., Wils
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BIBLIOGRAPHY 141Rutledge, R. E., Ba
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BIBLIOGRAPHY 143Yokoyama, T. & Shib
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X-ray Study of Low-mass Young Stellar Objects in the ρ Ophiuchi ...

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