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

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82 CHAPTER 6. INDIVIDUAL SOURCES6.5 New YSO Candidates – A-29, A-31, and BF-366.5.1 Source selection and X-ray imagesImanishi et al. (2001a) suggested that most of unidentified sources are likely background activegalactic nuclei (AGNs; see also §6.6). However, some fractions would be IR-faint cloud members,possibly brown dwarfs and/or very young stellar objects like class 0–I protostars. To search fornew YSO candidates, we select the unidentified sources located near 850 µm clumps (Johnstone etal., 2000), then find three candidates (A-29, A-31, and BF-36: Imanishi et al., 2002b) in the densemillimeter/sub-millimeter cores of SM1/SMM J16264−2424 and SMM J16264−2423 in ρ Oph-A,and B1-MM4/SMM J16272−2430 in ρ Oph-B1 (Motte et al., 1998; Johnstone et al., 2000).SMMJ16264-2422b-24:23:30unIDGY31Dec.(J2000)24:0024:3025:00SMMJ16264-2423SMMJ16264-2424A-31SM1KSHK-AA-29VLA1623GY2916:26:32 26:30 26:28 26:26 26:24R.A.(J2000)GY21S2Fig. 6.18.— ACIS images around the mm cores (left) SM1 and (right) B1-MM4 in the 2.0–9.0 keVband. The contour is the (left) 3 mm and (right) 1.3 mm radio intensities (Motte et al., 1998;Kamazaki et al., 2001). Crosses, Large solid and dotted circles represent the center of each mmcore, size of 850µm clumps (Johnstone et al., 2000) and field of view of the 3 mm observation,respectively. A square shows the position of the class 0 source VLA 1623. Other X-ray sources areknown YSOs (GY21, S2, GY29, GY31, and GY238) and unidentified sources (designated as unID).Figure 6.18 (left) shows the ACIS image of obs-A around SM1 in the hard X-ray band (2.0–9.0 keV), which is overlaid on a 3 mm contour map (Kamazaki et al., 2001). Large circles representthe 850 µm clumps (Johnstone et al., 2000). We see two point-like sources (A-29 and A-31) nearthe center of the 850 µm clump SMM J16264−2424 and at only ∼2 ′′ away from the center of the3 mm core A (hereafter, KSHK-A). Since there is no significant soft X-ray (0.5–2.0 keV) from A-29and A-31, they are deeply embedded in the cores. Another point source located near the core,a class II source GY29 (A-27), on the other hand, is clearly visible in the soft band because of
6.5. NEW YSO CANDIDATES – A-29, A-31, AND BF-36 83relatively small absorption (N H = 2.5×10 22 cm −2 ; Table 5.2), hence may be located in front of thecore.The hard band (2.0–9.0 keV) ACIS image near ρ Oph B1 is shown in Figure 6.18 (right). Thereis a hard X-ray source (BF-36) about 30 ′′ south of the 1.3 mm peak B1-MM4. A class II sourceGY238 (BF-33) is also appeared near the center of SMM J16272−2430, which would be located inthe clumps, as supported by the large absorption of N H = 4×10 22 cm −2 (Table 5.2).6.5.2 Time variabilityFigure 6.19 shows the X-ray light curves (background is not subtracted) of A-29, A-31, and BF-36in the 2.0–9.0 keV band. We have detected solar-like flares from A-29 and BF-36 (§5.5). The lightcurve of A-31 also seems to be time variable; A-31 has only 12 photons and about 80 % of them(10/12) are concentrated in a limited time interval of ∼30 ks. We estimate the chance probabilityof such a photon clustering from a constant-flux source to be less than 1 % with the Kolmogorov-Smirnov test (Press et al., 1992). Thus the time variability of A-31 is also significant. The X-rayactive time intervals are shown with dotted vertical lines in Figure 6.19 and are referred as the flarephase hereafter. The hard band counts in the quiescent phase (time periods outside of the flarephase) for A-29, A-31, and BF-36 are 5, 2 and 81, while those of the expected background countsare 0.7, 0.5, and 1.8, respectively. This means that A-31 has no significant quiescent flux with ≥99.9 % of the confidence level (Poisson statistics; see §6.4.1).A-29FlareFlareA-31FlareBF-36Fig. 6.19.— Light curves of (left) A-29, (middle) A-31, and (right) BF-36 in the 2.0–9.0 keV band.The time bins of each data point are 2000 (A-29), 4000 (A-31), and 4000 (BF-36) s. The arrowsindicate the times of the flare phase (see text).
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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
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.
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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
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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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