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

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10 CHAPTER 2. REVIEW OF LOW-MASS YOUNG STELLAR OBJECTSFig. 2.7.— Schematic view of the α-ω dynamomechanism (Parker, 1970). Solid lineand ribbon arrows represent a magnetic lineof force and fluid motion, respectively.CoronaThe sun is covered by hot corona, which also emits X-rays. However, its X-ray properties are quitedifferent from the solar flare X-rays; steady emission from relatively low temperature (∼0.1 keV) anddensity (∼10 9 cm −3 ) plasma. Although the heating mechanism of the corona is still controversial,it is widely accepted that the magnetic field also plays a key role.2.3.2 X-rays from low-mass young stellar objectsX-rays from T Tauri starsIn 1980’s, the Einstein and ROSAT satellites discovered X-ray emission from many TTSs (Feigelson& DeCampli, 1981; Feigelson & Kriss, 1981; Montmerle et al., 1983; Feigelson & Montmerle, 1999)using the imaging capability at the soft X-ray band (
2.3. X-RAY OBSERVATIONS OF LOW-MASS STARS 11X-rays from class I protostarsFig. 2.8.— (left) Schematic view of the star-disk arcade model (Montmerle et al., 2000). Numerals(1–5), solid lines, and thick gray line indicate time sequence, magnetic field, and flare loop aftera reconnection. An arrow shows a reconnection point. (right) Quasi-periodic flares from a class IYLW15A in the ρ Ophiuchi cloud obtained with ASCA (Tsuboi et al., 2000).Protostars are deeply embedded in molecular cloud cores and shrouded by dense circumstellargas and dust, hence we can not observe them even with the soft X-ray band due to large extinction.The Japanese X-ray satellites Temma and Ginga, however, discovered hard X-ray emission (>2keV) in the direction of star-forming regions (Koyama, 1987; Koyama et al., 1992).Althoughthey have no imaging capability, X-ray characteristics of flare-like light curves and highly absorbedspectra (N H ∼ 10 22 cm −2 ) indicate that X-rays are coming from discrete sources inside (or behind)the dense cloud cores.The forth Japanese X-ray satellite ASCA (Advanced Satellite for Cosmology and Astrophysics:Tanaka, Inoue, & Holt, 1994) is the first X-ray mission to have the X-ray telescopes (XRTs: Serlemitsoset al., 1995) with the imaging capability for the hard X-ray photons (2–10 keV), whichare less absorbed as the extinction cross-section decreases as E −2.5X(E X : X-ray energy). Koyamaet al. (1994) first revealed that the hard X-rays from the ρ Ophiuchi dark cloud regions wouldcome from class I sources (WL6 and WL15 = Elias29) with ASCA/GIS. Using ASCA/SIS, whichhas better spatial resolution than GIS, Kamata et al. (1997) confirmed the X-ray emission fromthese class I sources. Independently, Casanova et al. (1995) obtained the hints of X-ray detectionfrom class I in the same star-forming area with ROSAT/PSPC. Then the successive observationswith ASCA further revealed that many class I protostars are X-ray emitters (Koyama et al., 1996;Ozawa et al., 1999; Tsuboi et al., 2000) thanks to its higher sensitivity at the hard X-ray band.ROSAT observations also detected X-rays from class I (Grosso et al., 1997; Neuhäuser & Preibisch,1997; Grosso, 2001). They share the same characteristics of thermal emission with the plasma
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 and 18: Chapter 2Review of Low-mass Young S
Page 19 and 20: 2.1. EVOLUTION OF LOW-MASS STARS 5w
Page 21 and 22: 2.2. MOLECULAR CLOUDS 72.2 Molecula
Page 23: 2.3. X-RAY OBSERVATIONS OF LOW-MASS
Page 27: 2.3. X-RAY OBSERVATIONS OF LOW-MASS
Page 30 and 31: 16 CHAPTER 3. REVIEW OF THE ρ OPHI
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Page 46 and 47: 32 CHAPTER 5. CHANDRA OBSERVATIONS
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62 CHAPTER 6. INDIVIDUAL SOURCES6.2
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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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