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X-Ray Study of Stellar Winds with Suzaku

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X-Ray Study of Stellar Winds with Suzaku X-ray Study of Stellar Winds with Suzaku Yoshiaki Hyodo Department of Physics, Graduate School of Science, Kyoto University Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan [email protected] This thesis was submitted to the Department of Physics, Graduate School of Science, Kyoto University on January 5 2009 in partial fulfillment of the requirements for the degree of Doctor of Philosophy in physics. Abstract Stellar winds from massive stars play an important role for dynamical and chemical evolution of galaxies. To evaluate their effects in a galaxy scale, both the total number and the properties of massive stars should be understood. Most massive stars have soft X-ray emission, while some have additional hard emission. Although it is indicated that the binarity is important, the condition for hard X-ray production has not yet been elucidated. Also, some massive star clusters exhibit diffuse (pc scale) X-ray emission, where stellar winds may be depositing energy to the interstellar space. However, previous studies were not able to clarify the origin of the diffuse emission. It is even unclear whether the emission is physically related to the star cluster or not. The X-ray Imaging Spectrometer (XIS) on board Suzaku has a good energy resolution, a low and stable background, and a large effective area, which are suited for narrow-band surveys and spectroscopy of both point-like and extended X-ray emission. Using the XIS, we performed a very deep survey in the direction of the central region of our Galaxy and pointing observations of three well-known star forming regions in our Galaxy (M 17, the Carina Nebula, and the Arches cluster). A bright point-like source (CXOGC J174645.3{281546) was found in the 6.7 keV in- tensity map of the Galactic center region. The temperature and luminosity are similar to those of wind-collision binary. The spectral energy distribution in the infrared is very similar to those of eponymous Quintuplet cluster members, indicating that the source is a WC star+OB star binary. We also present the results of X-ray spectroscopy of nine massive stars in M 17, the Carina nebula, and the Arches cluster. Many sources show hard X-ray emission. We obtained important clues for hard X-ray production mechanism in massive star binaries. WR 25 (WN6+O4f) in the Carina region shows hard X-ray emission with a flux variation correlated with the binary orbital phase, while the close binary HD 93205 (O3+O8) lacks hot component and is stable. These two facts indicate that the binary separation is a key parameter for hard X-ray emission. We speculate that the other two massive stars OI 352 in M 17, and HD 93250 in the Carina Nebula with hard X-ray emission are also colliding-wind binaries. A variety of diffuse emission is apparently associated with some massive star clusters. We argue that the diffuse source in M 17 and in the west to η Carinae are prototypical stellar wind bubbles; the absorption columns typical to these regions indicate that the diffuse < emission is physically associated with the star clusters and the O/Fe abundance ratio (∼ 1 solar) is too low for supernova remnants caused by core-collapse type supernovae. On the other hand, the diffuse emission in the south side of η Carinae is unlikely to have physical relation with the Carina Nebula, because the absorption column is very low (NH < 6 × 3 1020 cm−2). A possible nature is a foreground type Ia supernova remnant. The absorption column of diffuse emission in the Arches cluster is consistent with that of the Galactic center, indicating a physical relation to the Arches cluster. Although massive stars in the Arches cluster are the sources of strong stellar wind, the diffuse emission is unlikely to be hot plasma created by the stellar wind. It shows a strong line from neutral iron. The origin is likely due to X-ray and/or electron fluorescent, but the flux of the X-ray photons or electrons are extremely insufficient to produce the diffuse X-rays. Contents 1 Introduction 15 2 Review 17 2.1 Evolution of Massive Stars . 17 2.2 Wolf-Rayet Stars . 21 2.3 Diffuse X-ray Emission from Massive Stars . 24 2.4 Point-like X-ray Emission from Massive Stars . 27 3 Instruments 31 3.1 Overview of Suzaku . 32 3.2 XRT . 34 3.2.1 Overview of the XRT . 34 3.2.2 Onborad Performance . 35 3.3 XIS . 38 3.3.1 Overview of the XIS . 38 3.3.2 Onboard Performance . 40 3.4 HXD . 44 3.4.1 Overview of the HXD . 44 3.4.2 Onboard Performance . 46 4 Observations & Results 49 5 6 CONTENTS 4.1 A New Wolf-Rayet Star in the Galactic Center . 51 4.1.1 Objectives . 51 4.1.2 Observations & Reduction . 52 4.1.3 Band-limited Images & Source list . 54 4.1.4 A Discrete source . 55 4.2 Carina Nebula . 59 4.2.1 Objectives . 59 4.2.2 Observations & Reduction . 61 4.2.3 Band-limited Images & Source list . 61 4.2.4 Discrete Sources . 62 4.2.5 Diffuse Sources . 70 4.3 M 17 . 73 4.3.1 Objectives . 73 4.3.2 Observations & Reduction . 76 4.3.3 Band-limited Images & Source list . 76 4.3.4 Discrete sources . 80 4.3.5 Diffuse sources . 87 4.3.6 HXD-PIN Analysis . 90 4.4 Arches Cluster . 92 4.4.1 Objectives . 92 4.4.2 Observations & Reduction . 94 4.4.3 Band-limited Images & Source list . 94 4.4.4 Spectral Analysis . 95 5 Discussion 101 5.1 A New Wolf-Rayet Star in the Galactic Center . 102 5.1.1 Chandra and XMM-Newton Counterparts . 102 CONTENTS 7 5.1.2 Longer Wavelength Data . 105 5.1.3 Location of Extinction Matter . 106 5.1.4 Nature of the Source . 107 5.2 Carina Nebula . 110 5.2.1 Discrete Sources . 110 5.2.2 Diffuse Sources . 112 5.2.3 The Plasma Properties . 112 5.3 M 17 . 113 5.3.1 Discrete Sources . 113 5.3.2 Diffuse Sources . 117 5.3.3 HXD signals . 122 5.4 Arches Cluster . 124 5.4.1 Discrete Sources . 124 5.4.2 Origin of Three Spectral Components . 130 5.4.3 Diffuse Medium . 131 5.4.4 Cause of Line and Power-law Emission . 131 6 Conclusions 135 6.1 A New Wolf-Rayet Star in the Galactic Center . 135 6.1.1 Summary of the Results . 135 6.1.2 Conclusion | Contribution to the Galactic Center Diffuse Emission . 136 6.2 Discrete Sources . 137 6.2.1 Summary of the Results . 137 6.2.2 Conclusion | Mechanism for the Hard X-rays from Massive Stars . 138 6.3 Diffuse Sources . 140 6.3.1 Summary of the Results . 140 8 CONTENTS 6.3.2 Conclusion | Various origins of Diffuse Emission in Star-forming Regions . 141 List of Figures 2.1 Diagram of CNO cycle. 18 2.2 Energy production rate of the pp chain and CNO cycle. 19 2.3 HST image of M1-67 around WR 124 . 21 2.4 WR158 spectrum in the NIR bands. 22 2.5 WR star surface number density as a function of Galactocentrinc distance . 23 2.6 Velocity structure of line-driven stellar winds. 24 2.7 Temperature and density structure of a stellar wind bubble. 25 2.8 Color composite image of a stellar wind bubble around WR 136 . 26 2.9 Bolometric luminosity and X-ray luminosity in the 0.2{4.0 keV band. 27 2.10 Structure of stellar wind. 28 2.11 X-ray luminosity history of WR 140. 29 2.12 Temperature map of θ1 Ori C. 30 3.1 Schematic view of the Suzaku satellite in orbit. 32 3.2 Configuration of the detectors in the Suzaku satellite. 33 3.3 Picture of the XRT-I1 module. 34 3.4 Schematic cross-section of Wolter-I type X-ray mirror. 35 3.5 Total effective area and vignetting of the XRT . 36 3.6 Image of SS Cyg at the optical axis taken with XRT-I2. 36 3.7 PSF and EEF of the XRT. 37 9 10 LIST OF FIGURES 3.8 Picture and cross-section of XIS sensor . 38 3.9 Difference of front illumination and back illumination . 39 3.10 Schematic view of the XIS CCD. 40 3.11 QE of the XIS . 41 3.12 Long-term trend of energy resolution. 41 3.13 Long-term trend of on-axis contaminant column density . 42 3.14 XIS spectra obtained during night earth observations. 43 3.15 History of the observing condition of the XIS. 43 3.16 Picture of the HXD . 44 3.17 Effective areas of PIN and GSO . 45 3.18 Schematic view of a well-counter unit . 45 3.19 Angular response of PIN . ..
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