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

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78 CHAPTER 6. INDIVIDUAL SOURCES6.4 Brown DwarfsImanishi et al. (2001b) did a systematic X-ray search from bona-fide and candidate brown dwarfsin ρ Oph. Here, we show details.6.4.1 X-ray detection from brown dwarfs in ρ OphIn the fields of view of obs-A and obs-BF, 18 late M dwarfs have been reported based on the watervapor absorption at λ = 2.4–2.5 µm (Wilking et al., 1999; Cushing et al., 2000). Among them,eight sources have the upper limit of the mass less than 0.08 M ⊙ , which we call “bona-fide browndwarfs” (here and after, BDs).Other seven sources have the lower limit of the mass less than0.08 M ⊙ , which is called “candidate brown dwarfs” (BD c s) 3 . Table 6.5 shows their names andspectral types.In the X-ray source catalogue (Table 5.2), we found X-ray counterparts for four BDs and BD c s(A-30 = GY37, A-38 = GY59, BF-86 = GY310, and BF-S2 = GY141). Also, a BD c GY5 maybe associated with a soft-band source A-S2, although Imanishi et al. (2001b) reported no X-rayemission from GY5 by using the NIR position of Barsony et al. (1997), which is ∼2 ′′ away fromthe 2MASS source. Since no apparent X-ray sources are found in the error radius of the other 11catalogued BDs and BD c s, we define a circle with a half radius of the PSF size around each IRposition, then manually count X-ray photons. We note that a rather small source radius is selectedso as to increase the signal-to-noise (S/N) ratio, particularly for faint X-ray sources. Nevertheless,as is demonstrated in the bright sources, the position error between IR and X-ray is always smallerthan the source radius, because both have generally similar dependence on a source off-axis angle;both have the smaller values for sources with smaller off-axis angle. Therefore, most of the X-rayphotons for the relevant BDs and BD c s, if any, may fall in the source circles. Table 6.5 shows theX-ray counts thus counted. The mean background counts are estimated from the same regions usedin §5.6. The soft band (0.5–2.0 keV) background counts (in units of 10 −2 counts arcsec −2 ) are 2.3and 2.2 for obs-BF and obs-A, respectively, while those in the hard (2.0–9.0 keV) band are aboutthree times larger, 6.8 and 6.5 for obs-BF and obs-A. The background counts in each source areaare given in Table 6.5. We then separately calculate the confidence level (CL) of X-ray detectionfor the soft, hard, and total (0.5–9.0 keV) bands. Based on the Poisson statistics, we define the CLasCL =N∑0 −1N ′ =0e −N N N ′bg bgN ′ ! , (6.2)3 Imanishi et al. (2001b) regarded GY31, GY163, and GY326 as BD cs. Now, however, the mass of these M dwarfsis regarded to be higher than the hydrogen burning limit (Wilking et al., 1999), hence we do not include them in theBD c sample.
6.4. BROWN DWARFS 79where N 0 and N bg are the detected and the background counts in the source circle, respectively(see e.g., Gehrels, 1986, and references therein). We set a detection (or an upper limit) criterionthat the CL should be larger than 0.999 (= 3.3 σ) in any of the three energy band data. In Table6.5, we show the maximum CL value for each source. For sources located in both fields of view(GY141 and GY202), we also estimate the CL for the combined data as well as the separate data.However, no higher CL value is obtained from all the sources. In addition to the seven X-raysources, a BD c GY84 shows significant X-rays, although it may be possible that these X-rays arepart of diffuse X-ray emission A-48 (see §6.6.2). We hence regard that the X-ray detection fromGY84 is marginal.Table 6.5: Bona-fide and candidate brown dwarfs in the ρ Oph cloud coresName Sp. type * ID † X-ray ‡ PSF § X-ray counts ‖ 1 − CL # L X** log(L X /L bol ) Ref ‡‡( ′′ ) Soft Hard BGD— Brown Dwarf (BD) —CRBR14 M7.5 A · · · 5.7 4 0 0.5 ND (S)
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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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Page 46 and 47: 32 CHAPTER 5. CHANDRA OBSERVATIONS
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Page 121 and 122: Chapter 8Systematic Study of YSO Fl
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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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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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