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

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58 CHAPTER 6. INDIVIDUAL SOURCEShigher than ∼1 keV. This corresponds to the shock velocity of > 500 km s −1 (Favata et al., 2002),indicating surprisingly high jet activity. We note that such an extreme phenomenon has also beensuggested by the ASCA detection of Doppler-shifted Fe lines from a class I in the R CrA cloud(Koyama et al., 1996). To confirm this jet-like X-rays with deeper exposure observations would beimportant to understand the outflow activity at the protostellar phase.6.1.2 BF-26 – Elias29A class I BF-26 = Elias29 exhibits an X-ray flare with a timescale of ∼10 ks (Figure A.1). < kT >and < L X > during the flare are 5.8 keV and 5.3×10 30 ergs s −1 , while those in the quiescent phaseare 3.8 keV and 2.0×10 30 ergs s −1 , respectively (Table 5.2). The ASCA satellite observed Elias29twice with respective exposure times of 40 and 100 ks. In the first observation, it exhibited anX-ray flare similar to the present observation (Kamata et al., 1997). However, it became quiescentwith < L X > = 3×10 30 ergs s −1 in the second observation (Tsuboi et al., 2000). It therefore infersthat Elias29 shows the quiescent luminosity of ∼10 30 ergs s −1 and exhibits frequent X-ray flares.6.1.3 BF-50 – WL6Kamata et al. (1997) claimed the detection of hard X-rays from a class I WL6 with ASCA, althoughnearby GY256, another class II source, may contaminate the X-rays from WL6. The ASCA lightcurve shows a sinusoidal shape of about 20 hour period. Since they found no temperature variation,which is typical of normal flares, they claimed that the modulation is due to stellar rotation. Inthe present observation, we clearly detect hard X-rays not only from WL6 (BF-50) but also fromGY256 (BF-51). Two faint flares are found from GY256, with higher temperature than that in thequiescent phase (Table 5.2 and Figure A.1). However, we can see no sinusoidal light curve fromWL6 nor from GY256.6.1.4 BF-61 – YLW15AROSAT/HRI detected an extraordinary large X-ray flare from a class I YLW15A (Grosso et al.,1997). The inferred luminosity, however, depends largely on the assumed spectrum. We hence reestimatethe luminosity using our best-fit < kT > (4.2 keV) and N H (4.8×10 22 cm −2 ) of YLW15A= BF-61 at a flare (Table 5.2). The HRI count rate at the flare peak of ∼17 cts ks −1 is thenconverted to the peak luminosity of ∼10 33 ergs s −1 , which is about ten times brighter than thatof the giant flare seen in YLW16A (see §6.1.5) but may not be extraordinary, comparable to thelargest YSO flares detected with ASCA (V773 Tau: Tsuboi et al. 1998; ROXs31: Imanishi et al.
6.1. PROTOSTARS 592002a, see also §6.2). The subsequent ASCA observation discovered quasi-periodic three X-rayflares with an interval of ∼20 hours (Tsuboi et al., 2000). Montmerle et al. (2000) interpretedthese flares with a star-disk arcade conjecture in contrast to the X-ray emission from that of TTSs,which may have magnetic arcades at the stellar surface. Although Chandra confirms hard X-rayemission and finds a typical flare from YLW15A (Figure A.1), no multiple flares are found. Thusquasi-periodic flares are not always present but rather occasional phenomena.Recently, VLA (Girart et al., 2000) and Subaru/COMICS (M. Honda, private communication)resolved YLW15A into a binary system with the separation angle of ∼0. ′′ 5. The position of BF-64favors that the origin of X-rays is the radio-faint and MIR-bright source VLA2 rather than VLA1.6.1.5 BF-64 – YLW16AThe light curve of a class I BF-64 = YLW16A comprises two flares; the first is rather complex withspike-like structure and the second is a giant flare having an unusual profile (Figure 6.2 left). Weexamine spectral evolution by slicing the data in the time intervals as shown in Figure 6.2 (left). Inthe phases 7–9, we extract the spectra from the same region (a 2. ′′ 5–7. ′′ 5 radius circle) as the lightcurve data, because these phases suffer the pileup effect (see §5.5). We first fit them by the MEKALmodel allowing the abundance and N H to be free for all the phases, then find no significant variationfrom phase to phase both in abundance ( ∼ = 0.3 solar) and N H ( ∼ = 5.2×10 22 cm −2 ). We hence fixthe abundance to be 0.3 solar and fit them assuming that N H is the same in all phases. Table 6.1shows the resultant best-fit parameters for each time interval. As are seen in Figure 6.2 (left), thetemperature (kT ) and emission measure (EM) increase from phase 2 and reach their peak valuesat phase 3, then gradually decrease. The peak luminosity is estimated to be ∼1×10 31 ergs s −1 .These phenomena of the first flare are similar to flares found in other YSOs. The second flare showsunusual time profiles in the flux and emission measure (EM). The temperature profile is, however,more typical of normal flares; at phase 7 it increases rapidly and stays almost constant or showsgradual decay. Such a property is similar to the giant flare of ROXs31 detected with ASCA (§6.2)and we interpret that it is due to a large loop length (§8.3).Figure 6.2 (right) is the time-averaged spectrum during the large flare. A remarkable findingis an additional emission line near the 6.7 keV line of highly ionized (He-like) irons. The best-fitline center energy is 6.43±0.08 keV, which is attributable to neutral or low ionized irons. The mostplausible origin is fluorescence from cold irons in the circumstellar matter. Assuming the sphericaldistribution of circumstellar gas around YLW16A, the equivalent width of iron (EW ) is estimatedto beEW ≈ 10Z Fe (10 22 ) [eV], (6.1)cm−2 where Z Fe and N H are the abundance of iron and column density of the fluorescent medium,N H
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Contents1 Introduction 12 Review of
Page 9 and 10:
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
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
Page 30 and 31: 16 CHAPTER 3. REVIEW OF THE ρ OPHI
Page 36 and 37: 22 CHAPTER 4. INSTRUMENTATIONrespec
Page 38 and 39: 24 CHAPTER 4. INSTRUMENTATIONangle
Page 40 and 41: 26 CHAPTER 4. INSTRUMENTATIONThe AC
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Page 44 and 45: 30 CHAPTER 4. INSTRUMENTATIONTable
Page 46 and 47: 32 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
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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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