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

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86 CHAPTER 6. INDIVIDUAL SOURCESthat A-31 is surely associated with KSHK-A. The chance probability to detect a background AGNin the KSHK-A region is only ∼0.2 %. Furthermore, A-31 exhibits variable X-rays typical oflow-mass YSOs. The column density of KSHK-A is ∼10 24 cm −2 (Kamazaki et al., 2001), whichis about twice of the X-ray determined N H of A-31 (∼ 5×10 23 cm −2 ). Thus A-31 would be aYSO embedded near the center of the compact core KSHK-A. KSHK-A is spatially coincidentwith a 6 cm continuum source LFAM6 (Leous et al., 1991), which may be produced by a shockof outflow from a putative protostar (e.g., Rodriguez et al.1989, and references theirin). No NIRcounterpart with K < 15 mag, its close location at the compact core center KSHK-A, the largeabsorption (∼5×10 23 cm −2 ), and the presence of the 6 cm enhancement indicate that A-31 is astrong candidate of X-ray emitting class 0.A northwest part of the CO molecular outflow from the class 0 VLA1623 (designated as E–Hin Yu, & Chernin 1997) is not aligned to the central part (B–D) but is slightly shifted to the northdirection pointing to A-29 and A-31 (Figure 6.21). We speculate that the origin of the outflowE–H is A-29 or A-31 (not from VLA1623), although there is no CO emission at their closer vicinity(Kamazaki et al., 2001).Fig. 6.21.— 12 CO (J=1–0) map aroundVLA1623 (Yu, & Chernin, 1997). Filledsquares represent the positions of A-29 andA-31.BF-36BF-36 is located in the 850 µm clump SMM J16272−2430 with about 30 ′′ separation from its center.It lies in an elongated 1.3 mm core chain with local peaks of B1-MM2, 3, and 4 (Motte et al., 1998).The source is about 30 ′′ south of the peak of B1-MM4 (Figure 6.18). The chance probability ofbackground AGN to be associated within 30 ′′ radius from the core center is ∼20 %. However,BF-36 is among the brightest unidentified sources and is very unique which exhibits a clear flare.We therefore suggest that BF-36 is also a cloud member embedded near the dense core B1-MM4.Since millimeter/sub-millimeter peak fluxes of B1-MM4/SMM J16272−2430 are about ten timesfainter than those harbor A-29 and A-31, although the clump size is comparable (6200 AU), the2–6 times smaller N H of BF-36 (9×10 22 cm −2 ) than those of A-29 and A-31 is consistent with the
6.5. NEW YSO CANDIDATES – A-29, A-31, AND BF-36 87embedded star scenario.Considering that N H of BF-36 is rather typical of those of class I sources (see Figure 7.3), itis located at rather off-center position, and a class II source GY238 (BF-33) is closer to the centerof SMM J16272−2430, BF-36 would be a more evolved star than the class 0 stage, possibly classI or II. Assuming the average ratio of < L X >/L bol to be 10 −4.0 in the quiescent phase for YSOs(§7.4.2) and using N H of 9×10 22 cm −2 , L bol and A V of BF-36 are expected to be ∼0.8 L ⊙ and∼60 mag (§7.4.1). These are above the current detection limit; some NIR-detected sources havenearly the same values (see Figure 7.1). One possibility of no NIR counterpart is that BF-36 is aYSO with very active X-rays. Assuming < L X >/L bol of ≈10 −3.5 , we then expect L bol of 0.08 L ⊙ ,which is below or near the current detection limit (Figure 7.1).6.5.5 Ionization effect to circumstellar materialsThe flare luminosity and duty ratio of A-29, A-31, BF-36 are ∼10 30 ergs s −1 and roughly one per100 ks, respectively. Due to the large absorption, a significant fraction of X-ray energy is suppliedinto circumstellar materials. Using the parameters in Table 6.6, we estimate the X-ray ionizationrate (ζ X : Lorenzani & Palla 2001); the fraction of protons to be photo-ionized per unit time, whichis defined asζ X = 1.7 < L X > ˜σ4πr 2 ∆ɛ∫Jν e −n H˜σr( νν X) −n dν∫Jν dν[s −1 ], (6.3)where σ(E) = ˜σ(E/1keV) −n is the total photoelectric cross section with n = 2.51 and ˜σ =2.16×10 −22 cm 2 , ∆E (≡ 35 eV) is the mean energy to make an ion-electron pair with cosmicabundance, n H is the proton mass, J ν is monochromatic X-ray flux (ergs s −1 Hz −1 ), and r is thedistance from the X-ray source. We assume n H of A-31 and BF-36 to be 1.5×10 8 and 1.3×10 6 cm −3 ,following Motte et al. (1998) and Kamazaki et al. (2001). Since N H of A-29 is about a half of A-31,n H is assumed to be 7.5×10 7 cm −3 for A-29. Then we obtain the “ionization radius”, in which ζ Xexceeds that of cosmic rays, to be ∼10 2.5 , 10 2.4 , and 10 3.1 AU, respectively (Figure 6.22). Thesesizes are significantly smaller than the “parent” core size of typically 10 3.8 AU, hence have no largeimpact in global structure. In a smaller scale of close vicinity of the YSOs (KSHK-A, for example),however, the X-ray ionization may still have significant effect on the circumstellar materials. Sincethe mass accretion is thought to be coupled to the magnetic field (Feigelson & Montmerle, 1999),our result may put significant influence on the growth of YSOs.
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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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¨28 CHAPTER 4. INSTRUMENTATIONspli
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30 CHAPTER 4. INSTRUMENTATIONTable
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32 CHAPTER 5. CHANDRA OBSERVATIONS
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34 CHAPTER 5. CHANDRA OBSERVATIONS
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Page 76 and 77: 62 CHAPTER 6. INDIVIDUAL SOURCES6.2
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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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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
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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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