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The Pennsylvania State University The Graduate School Department of Astronomy and Astrophysics AN X-RAY STUDY OF MASSIVE STAR FORMING REGIONS WITH CHANDRA A Thesis in Astronomy and Astrophysics by Junfeng Wang c 2007 Junfeng Wang Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 2007 The thesis of Junfeng Wang was read and approved∗ by the following: Eric D. Feigelson Professor of Astronomy and Astrophysics Thesis Co-adviser Chair of Committee Leisa K. Townsley Senior Scientist of Astronomy and Astrophysics Thesis Co-adviser Special Member Kevin Luhman Assistant Professor of Astronomy and Astrophysics Hiroshi Ohmoto Professor of Geochemistry Donald Schneider Professor of Astronomy and Astrophysics Richard Wade Associate Professor of Astronomy and Astrophysics Lawrence Ramsey Professor of Astronomy and Astrophysics Head of the Department of Astronomy and Astrophysics ∗Signatures on file in the Graduate School. iii Abstract Massive stars are characterized by powerful stellar winds, strong ultraviolet (UV) radiation, and consequently devastating supernovae explosions, which have a profound influence on their natal clouds and galaxy evolution. However, the formation and evo- lution of massive stars themselves and how their low-mass siblings are affected in the wind-swept and UV-radiation-dominated environment are not well understood. Much of the stellar populations inside of the massive star forming regions (MSFRs) are poorly studied in the optical and IR wavelengths because of observational challenges caused by large distance, high extinction, and heavy contamination from unrelated sources. Al- though it has long been recognized that X-rays open a new window to sample the young stellar populations residing in the MSFRs, the low angular resolution of previous gen- eration X-ray telescopes has limited the outcome from such studies. The sensitive high spatial resolution X-ray observations enabled by the Chandra X-ray Observatory and the Advanced CCD Imaging Spectrometer (ACIS) have significantly improved our ability to study the X-ray-emitting populations in the MSFRs in the last few years. In this thesis, I analyzed seven high spatial resolution Chandra/ACIS images of two massive star forming complexes, namely the NGC 6357 region hosting the 1 Myr old Pismis 24 cluster (Chapter 3) and the Rosette Complex including the 2 Myr old NGC 2244 cluster immersed in the Rosette Nebula (Chapter 4), embedded clusters in the Rosette Molecular Cloud (RMC; Chapter 5), and a triggered cluster NGC 2237 (Chapter 6). The X-ray sampled stars were studied in great details. The unique power of X-ray selection of young stellar cluster members yielded new knowledge in the stellar populations, the cluster structures, and the star formation histories. The census of cluster members is greatly improved in each region. A large fraction of the X-ray detections have optical or near-infrared (NIR) stellar counterparts (from 2MASS, SIRIUS and FLAMINGOS JHK images), most of which are previously uncat- alogued young cluster members. This provides a reliable probe of the rich intermediate- mass and low-mass young stellar populations accompanying the massive OB stars in each region. For example, In the poorly-studied NGC 6357 region, our study increased the number of known members from optical study by a factor of 40. As a result, ∼ normal initial mass functions (IMFs) for NGC 6357 and NGC 2244 were found, inconsis- tent with the top-heavy IMFs suspected in previous optical studies. The observed X-ray luminosity functions (XLFs) in NGC 6357 and NGC 2244 are compared to the Orion Nebula Cluster XLF, yielding the first estimate of NGC 6357’s total cluster population, a few times the known Orion population. For NGC 2244, a total population of 2000 ∼ X-ray-emitting stars is derived, consistent with previous estimate from IR studies. The morphologies and spatial structures of the clusters are investigated with absorption-stratified stellar surface density maps. Small-scale substructures superposed on the spherical clusters are found in NGC 6357 and NGC 2244. Both of their radial stellar density profiles show a power-law cusp around the density peak surrounded by an isothermal sphere. In NGC 2244, the spatial distribution of X-ray stars is strongly concentrated around the central O5 star, HD 46150. The other O4 star HD 46223 has iv few companions. The X-ray sources in the RMC show three distinctive structures and substructures within them, which include previously known embedded IR clusters and a new unobscured cluster (RMC A). We do not find clear evidence of sequentially triggered formation. The concentration of X-ray identified young stars implies that .35% of stars could be in a distributed population throughout the RMC region and clustered star formation is the dominant mode in this cloud. The NGC 2237 cluster, similar to RMC A, may have formed from collapse of pre-existing massive molecular clumps accompanying the formation of the NGC 2244 cluster. The spatial distribution of the NIR counterparts to X-ray stars in the optical dark region northwest of NGC 2237 show little evidence of triggered star formation in the pillar objects. The observed inner disk fraction in the MSFRs as indicated by K-band excess appears lower than the IR-excess disk fractions found in the nearby low-mass star for- mation regions of similar age. An overall K-excess disk frequency of 6% for X-ray ∼ selected stars in the intermediate- to high-mass range in the NGC 6357 region (Chapter 3), and 10% for stars with mass M & 2M⊙ in NGC 2244 (Chapter 4) are derived, which ∼ indicates that the inner disks around higher-mass stars evolve more rapidly. The X-ray stars in these regions provide an important new sample for studies of intermediate-mass PMS stars that are not accreting, in addition to the accreting HAeBe stars. The low K-excess disk frequency for X-ray selected stars in the solar mass range in NGC 2244 is intriguing, which may be attributed to different sensitivities to disk materials, selec- tion effects between X-ray samples and IR samples and/or faster disk dissipation due to photoevaporation in the MSFRs. X-ray properties of stars across the mass spectrum are presented. Diversities in the X-ray spectra of O stars are seen, both soft X-ray emission consistent with the micro- shocks in stellar winds and hard X-ray components signifying magnetically confined winds or close binarity. X-ray luminosities for a sample of stars earlier than B4 in NGC 6357, NGC 2244, and M 17 confirm the long-standing log(L /L ) 7 relation, x bol ∼− although larger scatter is seen among the Lx/Lbol ratios of B-type stars. Low-mass PMS stars frequently show X-ray flaring, including intense flares with luminosities above 32 1 L 10 ergs s− . Diffuse X-ray emission is present in the NGC 6357 region and in the x ≥ NGC 2244 cluster. The derived luminosity of diffuse emission in NGC 6357 is consistent with the integrated emission from the unresolved PMS stars. The NGC 2244 diffuse emission is likely originated from the wind termination shocks, and hence is truly diffuse in nature. In summary, Chandra X-ray observations offer multifaceted approaches to study the young stellar clusters in MSFRs in depth. Future perspectives with the Spitzer Space Telescope mid-IR observations for a systematic measurement of disk frequencies in X-ray sampled massive clusters and X-ray observations of the earliest phases of massive star formation are discussed. v Table of Contents List of Tables ...................................... viii List of Figures ..................................... x Acknowledgments ................................... xii Chapter 1. Introduction ................................ 1 1.1 Observational Overview of Star Forming Regions . .... 2 1.1.1 Low-mass Star Forming Regions . 2 1.1.2 Massive Star Forming Regions . 4 1.1.3 X-ray Observations of Massive Star Forming Regions . ... 5 1.2 MotivationsforthisThesis. 7 1.2.1 Advantages of Chandra Observations of the MSFRs . 7 1.2.2 Some Outstanding Issues to Be Addressed in Understanding MSFRs in the Chandra Era................... 8 1.3 OverviewofthisThesis ......................... 10 1.3.1 Chapter 3: An X-ray Census of Young Stars in the Massive Southern Star-Forming Complex NGC 6357 . 11 1.3.2 Chapter 4: A Chandra Study of the Young Open Cluster NGC 2244intheRosetteNebula . 11 1.3.3 Chapter 5: A Chandra Study of the Stellar Populations in the Rosette Molecular Cloud . 12 1.3.4 Chapter 6: A Chandra Study of the Triggered Cluster NGC 2237................................ 13 Chapter 2. Methods .................................. 16 2.1 TelescopeandInstrument . 16 2.2 BasicACISDataReduction . 17 2.3 DataAnalysis............................... 18 2.3.1 ACISExtract........................... 18 2.3.2 SourceFinding .......................... 18 2.3.3 Source Variability and Spectral Fitting . 20 2.3.4 Simulation of Extragalactic Contamination . 21 2.3.5 Simulation of Stellar Contamination in the Galactic Disk . 22 2.3.6 NIR Color-Color and Color-Magnitude Diagrams . 23 2.3.7 Stellar Surface Density Map . 23 Chapter 3. An X-ray Census of Young Stars in the Massive Southern Star-Forming Complex NGC 6357 ........................... 26 3.1 Introduction................................ 26 3.2 Observational Overview of NGC 6357 and Pismis 24 . 27 vi 3.3 Observations and Data Reduction . 29 3.3.1 Chandra Observation and Data Selection . 29 3.3.2 Image Reconstruction and Source Finding . 29 3.3.3 Photon Extraction and Limiting Sensitivity . 30 3.3.4 Source Variability and Spectral Fitting . 32 3.3.5 SIRIUSNIRObservation . 33 3.4 Identification of the Chandra Sources . 33 3.4.1 X-ray Sources with Stellar Counterparts . 33 3.4.2 Extragalactic Contamination . 34 3.4.3 Field Star Contamination . 35 3.4.4 Likely New Stellar Members . 36 3.4.5 Classification of the X-ray Sample . 36 3.4.6 EGGsandProtostars .....................
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