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

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62 CHAPTER 6. INDIVIDUAL SOURCES6.2 X-ray Brightest T Tauri Stars (DoAr21, ROXs21, and ROXs31)Imanishi et al. (2002a) showed the Chandra/ACIS and ASCA/GIS results for a series of four longtermobservations of DoAr21 (A-2), ROXs21 (BF-46) and ROXs31 (BF-96), the X-ray brightestTTSs in the ρ Ophiuchi cloud, then studied their long-term time variability and detailed structureof X-ray spectra. Here we show details.6.2.1 Previous knowledgeThe Einstein satellite first recognized these sources as bright X-ray sources (Montmerle et al.,1983). Subsequent optical observations revealed that they are K–M type stars having weak Hαemission (Bouvier & Appenzeller, 1992), which are classified as WTTSs (class III). Simon et al.(1995) observed them by using the lunar occultation in the infrared wavelengths and found thatROXs21 and ROXs31 are binary systems with separation angles of 0. ′′ 3 and 0. ′′ 48, respectively.Shevchenko & Herbst (1998) reported the rotation period of ROXs21 to be 1.39 days. Since theyshow no continuum emission at 1.3 mm (André & Montmerle, 1994), the circumstellar envelopewould have already disappeared, while DoAr21 may still have an accretion disk as suggested bythe detection of NIR polarized emission (Ageorges et al., 1997). Using the theoretical evolutionarytracks in the H-R diagram, the age of DoAr21 is estimated to be ∼10 5 yr, which is younger thanthat of ROXs21 and ROXs31 (∼10 6 yr: Nürnberger et al., 1998). Finally, DoAr21 and ROXs31show variable centimeter emission, which is probably due to gyro-synchrotron mechanism inducedby surface magnetic field (Stine et al., 1988).6.2.2 Long-term observations with Chandra and ASCAWe use the data of obs-BF for ROXs21 and ROXs31. Although DoAr21 suffers the pileup effect inobs-A, obs-BF detected X-rays from DoAr21 at the ACIS-S2 chip, which mitigates photon pileupbecause of the large off-axis angle (= large PSF size). We hence use the ACIS-S2 data for thefollowing analysis. The background region for DoAr21 is taken from a source-free 19 arcmin 2 regionon ACIS-S2, while that for ROXs21 and ROXs31 is the same as used in the previous analyses.In order to examine long-term variability and spectral evolution of these sources, we additionallyuse the previous ASCA observations. Three observations (obs-A1, A2, and A3) were carriedout with the two Gas Imaging Spectrometers (GISs: Ohashi et al., 1996) and the two Solid-stateImaging Spectrometers (SISs: Burke et al., 1991) onboard ASCA, which are at the foci of the X-raytelescopes (XRTs: Serlemitsos et al., 1995) sensitive to photons in 0.4–10 keV. However, they wereoutside of or at the edge of the SIS’s field of view in the majority of the observations, hence we use
6.2. X-RAY BRIGHTEST T TAURI STARS (DOAR21, ROXS21, AND ROXS31) 63the GIS data only. We retrieve the ASCA unscreened data from the HEASARC online service 1 ,then filtered out the data taken at a geomagnetic cutoff rigidity lower than 4 GV, at an elevationangle less than 5 ◦ from the Earth, and during passage through the South Atlantic Anomaly.Particle events were also removed using the rise-time discrimination method. The total availableexposure times were ≈38 ks (obs-A1), 93 ks (A2), and 75 ks (A3) (Table 6.2).Table 6.2: ASCA/GIS observation logObs.ID Sequence ID Observation Date (UT) ExposureStart End (ks)A1 . . . 20015010 1993 Aug 20 02:20 Aug 20 22:46 37.7A2 . . . 25020000 1997 Mar 2 07:32 Mar 4 21:21 93.1A3 . . . 96003000 1998 Aug 13 06:35 Aug 15 21:16 74.76.2.3 Time variabilityFigures 6.3 and 6.4 show the X-ray light curves (no background subtraction) of (a) DoAr21, (b)ROXs21, and (c) ROXs31 obtained with Chandra/ACIS (obs-BF) and ASCA/GIS (obs-A1, A2,and A3), respectively. The vertical axis is photon counts normalized by the effective area at 1 keVfor comparison with the different instruments and observation epochs. The time intervals of eachdata point are 10 3 s and 10 4 s for ACIS (Figure 6.3) and GIS (Figure 6.4), respectively.energy bands of the ACIS and GIS data for DoAr21 are 0.5–9.0 keV, while the GIS light curves forROXs21 and ROXs31 are limited in the soft energy band of 0.5–1.5 keV in order to avoid possiblecontamination from a nearby variable hard X-ray source YLW15A = BF-61 (Tsuboi, 1999; Tsuboiet al., 2000). To verify whether or not the contamination from YLW15A is significant in
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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
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
Page 72 and 73: 58 CHAPTER 6. INDIVIDUAL SOURCEShig
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Page 110 and 111: 96 CHAPTER 7. OVERALL FEATURE OF X-
Page 112 and 113: 98 CHAPTER 7. OVERALL FEATURE OF X-
Page 114 and 115: 100 CHAPTER 7. OVERALL FEATURE OF X
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
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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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