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

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74 CHAPTER 6. INDIVIDUAL SOURCESdespite of large positional uncertainty. S2 therefore has the large variation of < L X > = 10 30 –10 31 ergs s −1 together with that of the abundance = 0.2–0.7 solar, although its mechanism is anopen question.Interestingly, both members of a candidate wide binary system, GY21 and S2 (Haisch et al.2002; see also Figure 6.18) show overabundance. This may indicate anomaly of global chemicalcomposition of this region, although this interpretation is insufficient to explain the abundancevariations of GY21 and S2.6.3.4 A-34 – GY51The offset between A-34 and the nearest class II source GY51 is ∼0. ′′ 72, which is slightly larger thanthe positional uncertainty of the Chandra observation (∼0. ′′ 5) at its small off-axis angle (∼1. ′ 1, seeFigure 5.3). However, the N H /A V and < L X >/L bol ratios between A-34 and GY51 are respectively∼1.7×10 21 cm −2 mag −1 and ∼10 −3 , which are typical of YSOs (Figure 7.6 and 7.7). We henceregard that A-34 is surely associated with GY51. A detection of a solar-like flare (Figure A.1) isalso consistent with out idea.6.3.5 A-41 – S1A class III S1 is among the brightest and hottest sources in ρ Oph (L bol ∼ 1000 L ⊙ , T eff ∼20000 K), which is classified as “Herbig Ae/Be stars” (HAeBes = intermediate-mass YSOs: Andréet al., 1988). André et al. (1988) detected polarized non-thermal radio emission, which probablycomes from gyro-synchrotron particles. This indicates that S1 has relatively strong magnetic field.In obs-A, we detect significant X-rays (A-41) with a small-amplitude flare near the end of theobservation (Figure A.3). S1 was also detected at the ACIS-S2 chip in obs-BF (Hamaguchi et al.,2002). The light curve of S1 in obs-BF shows no apparent flare, although it seems to decreasethroughout the observation with the e-folding time of ∼40 ks.Figure 6.13 shows time-averaged spectra of S1 in (left) obs-A and (right) obs-BF. Except for amarginal hump at ∼6.5 keV in obs-BF, clear emission lines can not be seen, indicating either lowmetal abundance or non-thermal nature. Also, there is possible edge absorption feature around4 keV in obs-A. We hence fit them by the power-law model, as well as by the MEKAL model withabundance to be free. For obs-A, we separately make and fit the spectra in the flare and quiescentphases. Table 6.4 summarizes the fitting results. Both the thermal and non-thermal models wellreproduce the spectra. The derived abundance of the thermal model is relatively low of 0–0.2 solar.For obs-A, we additionally apply the fitting with an edge absorption, which slightly reduces the
6.3. OTHER T TAURI STARS 75χ 2 values. This reduction is significant with ∼96 (thermal) and ∼99 (non-thermal) % confidencelevels for the F -test (Bevington & Robinson, 1992).Fig. 6.13.— Time-averaged spectra of S1 in (left) obs-A and (right) obs-BF. The upper panelsshow data points (crosses) and the best-fit MEKAL model (solid line), while the lower panelsshow residuals from the best-fit model. Arrows indicate (left) possible edge absorption and (right)marginal hump (see text).If X-rays are thermal in origin, < kT > (∼2–3 keV) and the < L X > /L bol ratio (∼10 −6 ) aresimilar to those of HAeBes (Hamaguchi, 2001). Flare-like time variability is also consistent. If nonthermalX-rays are the case, on the other hand, most plausible scenario is bremsstrahlung emissionfrom non-thermal electrons accelerated just after the magnetic reconnection event (§2.3.1), whichis the same mechanism as hard (>10 keV) X-rays of the sun (§2.3.1). This is further supportedby the radio observation which suggests the presence of MeV electrons around S1 (André et al.,1988). However, this requires that thermal plasma generated via magnetic reconnection shouldhave lower temperature than those of the sun, because the non-thermal X-rays are dominant overthe thermal component above ∼1 keV. To verify which is the real case, more sensitive observations(for example, with XMM-Newton) would be needed.The threshold energy of the edge absorption is ∼3.9 keV, which corresponds to neutral Ca(4.04 keV) or ionized Ar (Ar XVI: 3.95 keV) with the temperature of ∼10 6.6 K. However, theformer is less possible, because it requires the extremely large Ca abundance of ∼500 solar. Inthe case of Ar, light elements below N are fully ionized and do not affect the absorption, hencethe unusually large abundance of Ar does not needed. It is therefore probable that the warmabsorber makes Ar edge feature. Considering that the edge feature is not seen at the spectrum inobs-BF, the absorber should have much smaller size than the stellar radius and be located near thestellar surface, if we interpret the disappearance of the edge feature in obs-BF as the effect of the
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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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Page 46 and 47: 32 CHAPTER 5. CHANDRA OBSERVATIONS
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Page 121 and 122: Chapter 8Systematic Study of YSO Fl
Page 123 and 124: 8.2. CORRELATION BETWEEN THE FLARE
Page 125 and 126: 8.3. MAGNETIC RECONNECTION MODEL 11
Page 127 and 128: 8.5. EFFECT OF THE QUIESCENT X-RAYS
Page 129 and 130: 8.6. EVOLUTION OF YSOS AND THEIR FL
Page 131 and 132: Chapter 9ConclusionWe summarize the
Page 133 and 134: Appendix AFlare Light CurvesFig. A.
Page 135 and 136: Fig.A.2 (Continued)121
Page 137: 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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