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

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116 CHAPTER 8. SYSTEMATIC STUDY OF YSO FLARESDonati et al. (1990) estimated B of RS CVns as ≈1000 G, which is as large as the maximum valuefor YSOs (Figures 8.2 and 8.3).8.7 Comment on the Giant Flares and Quiescent X-raysFigure 8.3 indicates that the giant flares ( 10 55 cm −3 ) previously detected with ASCAsuch as V773 Tau (class III: Tsuboi et al. (1998)) and ROXs31 (class III: §6.2), as well as BF-64= YLW16A (class I; ∼10 55 cm −3 ), should have either large L or B, if the sustainedheating is negligible. Since the flares of ROXs31 and YLW16A show relatively large rise timescale(∼10 4 s), they may have large L values (10 11 –10 12 cm). The rise time of V773 Tau, on the otherhand, is among the shortest (∼10 3 s), then the large L X would be primarily due to the large Bvalue. In fact, the detailed flare decay time analysis for V773 Tau predicted extremely large Bvalue of 1000 G (Favata et al., 2001). We suspect that the binary nature of V773 Tau (Welty,1995) causes such an exceptionally large B, which would be the same case as the main-sequenceRS CVn systems.Although the mechanism of making the quiescent X-ray emission from YSOs is still controversial,one possible idea is the composition of multiple unresolved faint flares. If we assume thatthe magnetic field with the same strength as those of the flares makes quiescent X-rays, unresolvedflares are expected to have smaller loop length, considering the systematically lower < kT >and < L X > ( ∼ = ) values (Figures 7.2 and 7.4), which causes shorter flare timescales(Eq.C.8 and Eq.C.11). Such flares are hard to be detected because of the limited statistics. Futureobservations with much higher sensitivity would reveal what makes quiescent X-rays.
Chapter 9ConclusionWe summarize the main results of the systematic analysis of two Chandra observations of ρ Ophwith ≈100 ks exposure as follows;1. From the central ∼1 pc × 2 pc region of ρ Oph, we detect 195 X-ray sources. About 60 % ofthem (110) are identified with optical and/or infrared counterpart, including class I, II, andIII sources and some young brown dwarfs. The X-ray detection rate is surprisingly high of∼60 %, hence we suspect that virtually all YSOs in ρ Oph may emit X-rays.2. A total of 71 flares are detected. Most of the flares show the typical profile of the solar andstellar flares, while some bright flares have unusually long rise timescale.3. All the X-ray spectra are well fitted by a single temperature thin plasma model of 0.3 solarabundances except for some sources. The plasma temperature is 0.6–5 keV in the quiescentphase, then becomes systematically higher (1–10 keV) in the flare phase. Class I–II sourcesshow higher flare temperature than class III sources.4. We find positive correlations of N H vs A V , < L X > vs L bol , < L X > vs mass, and < L X >vs L R .5. There is a positive correlation between τ r and τ d , which is well explained by the standardmagnetic reconnection model. Shorter τ r and τ d are due to the smaller L and/or larger Bvalues.6. We find a negative correlation between < kT > and τ r , which indicates the same order of theflare loop length regardless of their classes. Larger < kT > values for class I and II sourcesare due to larger B values.117
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Contents1 Introduction 12 Review of
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List of Figures2.1 The H-R diagram
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LIST OF FIGURESix6.17 Light curves
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List of Tables3.1 Multiwavelength s
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Chapter 1IntroductionStar formation
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Chapter 2Review of Low-mass Young S
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2.1. EVOLUTION OF LOW-MASS STARS 5w
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2.2. MOLECULAR CLOUDS 72.2 Molecula
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2.3. X-RAY OBSERVATIONS OF LOW-MASS
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16 CHAPTER 3. REVIEW OF THE ρ OPHI
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22 CHAPTER 4. INSTRUMENTATIONrespec
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24 CHAPTER 4. INSTRUMENTATIONangle
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26 CHAPTER 4. INSTRUMENTATIONThe AC
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30 CHAPTER 4. INSTRUMENTATIONTable
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32 CHAPTER 5. CHANDRA OBSERVATIONS
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62 CHAPTER 6. INDIVIDUAL SOURCES6.2
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Page 121 and 122: Chapter 8Systematic Study of YSO Fl
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Page 133 and 134: Appendix AFlare Light CurvesFig. A.
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Page 145 and 146: Appendix CModeling of the FlareIn t
Page 147 and 148: C.2. PREDICTED CORRELATIONS BETWEEN
Page 149 and 150: BibliographyAgeorges, N., Eckart, A
Page 151 and 152: BIBLIOGRAPHY 137Feigelson, E. D., &
Page 153 and 154: BIBLIOGRAPHY 139Johnstone, D., Wils
Page 155 and 156: BIBLIOGRAPHY 141Rutledge, R. E., Ba
Page 157: BIBLIOGRAPHY 143Yokoyama, T. & Shib
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