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

More documents

Recommendations

Info

80 CHAPTER 6. INDIVIDUAL SOURCES6.4.2 X-ray spectra and luminositiesFor the brightest two sources —, one BD (GY310) and one BD c (GY59) —, we could analyze theX-ray spectra (§5.6). Figure 6.16 shows their spectra. The best-fit plasma temperature (Table 5.2)is in the range of 1–4 keV.Fig. 6.16.— Time-averaged spectra of (a) GY310 and (b) GY59. The upper panels show datapoints (crosses) and the best-fit MEKAL model (solid line), while the lower panels are the dataresiduals from the best-fit model.For the faint or non-X-ray sources, we assume the spectrum to be a thin-thermal plasma of2 keV temperature (the mean value of the bright sources), while the absorption column is estimatedfrom the empirical relation, N H = 1.38×10 21 A V cm −2 (§7.4.1), where A V is the visual extinctionderived with IR photometry (Wilking et al., 1999; Cushing et al., 2000). We then calculate theX-ray luminosities or those of the 99.9 % upper limits in the soft band, which is summarized inTable 6.5.6.4.3 Time variabilityFigure 6.17 shows the light curves of (a) GY310 and (b) GY59 in 0.5–9.0 keV, where the backgroundlevels are represented by dashed lines. The light curve of GY59 is flare-like, although our criterion(Eq.5.1) rejects this as a flare. This light curve is typical of those of low-mass main-sequence andpre-main-sequence stars; fast rise and slow decay. The flare-decay timescale (∼10 4 s) is also thesame. GY310, on the other hand, shows no clear flare, but exhibits aperiodic variability of fluxchange by a factor of ∼2 within 100 ks exposure. Since we see no large intensity variation in thebackground flux, we conclude that its variability is highly significant.
6.4. BROWN DWARFS 81Fig.6.17.— Light curves of (a) GY310 and (c) GY59 in 0.5–9.0 keV. Dashed lines show thebackground levels.6.4.4 X-ray detection rateWe detect X-ray emission from two out of eight BDs and three out of seven BD c s. The X-raydetection rate is thus 25 % (BD) and 43 % (BD c ). Neuhäuser et al. (1999) claimed the X-raydetection of only one BD (GY202) with ROSAT/PSPC in the same field. Our present observationsfind no X-rays from GY202 with the upper limit of 1.2×10 28 ergs s −1 . Instead, we find a classII WL1 (A-78 = BF-16) within the PSPC error circle of GY202. This source emits X-rays withcomparable luminosity to that of GY202 reported with ROSAT/PSPC. Conversely, Neuhäuser etal. (1999) found no X-rays with an upper limit of ∼10 28 ergs s −1 from all Chandra detected BDand BD c s. They, however, estimated the X-ray luminosity assuming < kT >= 1 keV and A v = afew mag (i.e., N H = a few ×10 21 cm −2 ), which may be significantly lower than the real case. Wehence re-estimate the upper limit with the same assumption adopted in this paper (§6.4.2), thenfind that the ROSAT upper limit could be nearly one or two order magnitude higher than those ofthe original paper. If these newly estimated upper limits are adopted, we find no significant fluxchange from the ROSAT to the Chandra era.In Table 6.5, we display the ratio of the X-ray (< L X >) and bolometric (L bol ) luminosities.The < L X >/L bol value lies between 10 −3 –10 −5 , which is similar to low-mass YSOs (Figure 7.7)and dMe stars (Giampapa et al., 1996), but is significantly higher than that of the quiescentsolar corona; ∼5×10 −7 (Vaiana & Rosner, 1978). We should also note that most of the upperlimits of L X /L bol of X-ray non-detected BDs and BD c s are scattered around 10 −3 –10 −5 , whichare comparable with or slightly lower than those of X-ray detected BDs and BD c s. We thereforesuspect that these X-ray non-detected BDs and BD c s may emit X-rays below (but possibly near)the current sensitivity limit of Chandra.
Page 3:
Contents1 Introduction 12 Review of
Page 9 and 10:
List of Figures2.1 The H-R diagram
Page 11 and 12:
LIST OF FIGURESix6.17 Light curves
Page 13 and 14:
List of Tables3.1 Multiwavelength s
Page 15 and 16:
Chapter 1IntroductionStar formation
Page 17 and 18:
Chapter 2Review of Low-mass Young S
Page 19 and 20:
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:
Page 27:
Page 30 and 31:
16 CHAPTER 3. REVIEW OF THE ρ OPHI
Page 32 and 33:
Page 34 and 35:
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
Page 42 and 43:
¨28 CHAPTER 4. INSTRUMENTATIONspli
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
Page 74 and 75: 60 CHAPTER 6. INDIVIDUAL SOURCESres
Page 76 and 77: 62 CHAPTER 6. INDIVIDUAL SOURCES6.2
Page 78 and 79: 64 CHAPTER 6. INDIVIDUAL SOURCESfla
Page 80 and 81: 66 CHAPTER 6. INDIVIDUAL SOURCESTab
Page 82 and 83: 68 CHAPTER 6. INDIVIDUAL SOURCESUsi
Page 84 and 85: 70 CHAPTER 6. INDIVIDUAL SOURCESlos
Page 88 and 89: 74 CHAPTER 6. INDIVIDUAL SOURCESdes
Page 90 and 91: 76 CHAPTER 6. INDIVIDUAL SOURCESocc
Page 100 and 101: 86 CHAPTER 6. INDIVIDUAL SOURCEStha
Page 102 and 103: 88 CHAPTER 6. INDIVIDUAL SOURCES1ke
Page 104 and 105: 90 CHAPTER 6. INDIVIDUAL SOURCESThe
Page 106 and 107: 92 CHAPTER 6. INDIVIDUAL SOURCESFig
Page 108 and 109: 94 CHAPTER 6. INDIVIDUAL SOURCESPra
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
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
Page 140 and 141: 126 APPENDIX B. PHYSICAL PARAMETERS
Page 142 and 143: 128 APPENDIX B. PHYSICAL PARAMETERS
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
Page 151 and 152:
BIBLIOGRAPHY 137Feigelson, E. D., &
Page 153 and 154:
BIBLIOGRAPHY 139Johnstone, D., Wils
Page 155 and 156:
BIBLIOGRAPHY 141Rutledge, R. E., Ba
Page 157:
BIBLIOGRAPHY 143Yokoyama, T. & Shib
show all

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

Create successful ePaper yourself

Delete template?

Save as template?