The Hα Spectroscopy of Classical B-Emission Stars
Total Page：16
File Type：pdf, Size：1020Kb
Western University Scholarship@Western Electronic Thesis and Dissertation Repository 12-2-2014 12:00 AM The Hα Spectroscopy of Classical B-emission Stars Jessie M. Silaj The University of Western Ontario Supervisor C.E. Jones The University of Western Ontario Graduate Program in Physics A thesis submitted in partial fulfillment of the equirr ements for the degree in Doctor of Philosophy © Jessie M. Silaj 2014 Follow this and additional works at: https://ir.lib.uwo.ca/etd Part of the Astrophysics and Astronomy Commons Recommended Citation Silaj, Jessie M., "The Hα Spectroscopy of Classical B-emission Stars" (2014). Electronic Thesis and Dissertation Repository. 2630. https://ir.lib.uwo.ca/etd/2630 This Dissertation/Thesis is brought to you for free and open access by Scholarship@Western. It has been accepted for inclusion in Electronic Thesis and Dissertation Repository by an authorized administrator of Scholarship@Western. For more information, please contact [email protected]. THE Hα SPECTROSCOPY OF CLASSICAL B-EMISSION STARS (Thesis format: Integrated Article) by Jessie Silaj Graduate Program in Physics and Astronomy A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy The School of Graduate and Postdoctoral Studies The University of Western Ontario London, Ontario, Canada © Jessie M. Silaj 2015 Abstract Classical B-emission (Be) stars are rapidly-rotating, massive stars that possess a dense, equa- torial, gaseous disk. The presence of a disk was first inferred from the Balmer series emission that these stars exhibit, and Hα emission lines remain both a hallmark observational feature and one of the key diagnostics in determining the physical conditions within the disk. In the first chapter of this thesis, we investigate the possible role of line-driven winds in disk formation. To test if line-driven winds could supply enough material to account for the equatorial disk, we check for the presence of Hα emission in the models that result from dif- ferent combinations of line-force parameters. We find that certain combinations of line-force parameters can indeed produce significant Hα emission, and that line-driven winds may be an important mechanism in removing material from the central star. The next chapter of this thesis employs the code Beray to perform a detailed study of the Hα emission profiles of Be shell stars. Modelling the peak height and separation of these profiles gives an indication of the average density structure of the disk, although in some cases we found that more than one density structure could adequately reproduce these features. This finding indicates that, ideally, other observables should be simultaneously considered in order to constrain the models. It was found, however, that the absorption depth of the synthetic Hα profiles was relatively insensitive to the choice of density structure, depending instead largely on the inclination angle of the model. Thus, the inclination angles of these stars are determined to within a few degrees in our models, and we find that Be shell stars may not be oriented as close to 90° as originally thought. The final chapter of this thesis investigates the density structure of an asymmetric disk surrounding the well-known Be shell star 48 Librae. We begin our study with a thorough investigation of the central star’s spectral type (as it is not well-determined and has a significant impact on all other aspects of the model) by modelling the star’s spectral energy distribution (SED). Next, the SED and polarization levels are used to determine the inclination angle of the system. We then employ the HDUST code to model an observed Hα emission profile. We consider each peak separately, and then average the initial densities required to reproduce each peak to obtain the average initial disk density. We find that 48 Librae is best represented by a B3V central star surrounded by a very dense disk whose average initial density is 1.1 × 10 3 10− g cm− , and that the system is oriented at 85°. Keywords: circumstellar matter — hydrodynamics — line: formation — line: profiles — stars: emission-line, Be ii Co-Authorship Statement All articles presented in this thesis were co-authored with my supervisor, Dr. Carol Jones. Two of the three projects presented here involve the use of the Bedisk code, which she co- authored with Dr. Aaron Sigut, and she was consulted regarding the general form and content of all three projects. The roles of the other authors listed on each project are discussed below. Chapter 2: This chapter investigates whether line-driven winds can supply sufficient material to a circumstellar disk to produce Hα emission. Michel Cur´eis largely responsible for initiat- ing this work, as it was his suggestion that we combine the output of his Hydwind code with our Bedisk code. He supplied his Hydwind code for my use, and provided several instructional sessions. I then performed all of the Hydwind simulations in this work, and transformed the output such that it could be read into the Bedisk code. This also involved a small modification to Bedisk, since the radial grid employed by Hydwind was not compatible with the preexisting radial grids in Bedisk. AllBedisk simulations were also performed by me. I analyzed the re- sults and wrote the manuscript in its entirety. Chapter 3: This chapter uses a new code, Beray, to model the Hα profiles of eight Be shell stars. The project arose as a natural extension of earlier work by the same authors that had used a more simplistic line profile simulation that was inadequate for very large inclination angles. Dr. Aaron Sigut wrote Beray, and provided guidance as to its usage. Dr. Chris Tycner provided the observations of the Hα emission lines. I performed all of the simulations, and wrote a code to compare the synthetic profiles to the observed ones and compute the χ2 of the fit. I performed the analysis and wrote the manuscript in its entirety. Chapter 4: This chapter investigates modelling the peak heights of the Hα profile of 48 Librae to obtain an average disk density. Dr. Alex Carciofi suggested the use of his HDUST code for the modelling, which he supplied for my use, and Dr. Cyril Escolano provided instruction on its use. Dr. Carciofi also supplied the polarization measurements used in this work, while the Hα observation was supplied by Dr. Chris Tycner. I collected the other data presented in the paper, and performed all of the simulations and the analysis. Finally, I composed the full manuscript. iii Acknowledgements iv First and foremost, I would like to thank my supervisor, Carol Jones, who gave me tremendous freedom to do as I saw best in each project that we worked on, but was available week in and week out to discuss them, and to offer guidance and suggestions. She also orchestrated several international collaborations for which I am very grateful; the collaborations furnished me with many learning opportunities and fruitful scientific undertakings, and have enriched and broad- ened my knowledge of stellar astronomy. I would also like to thank John Landstreet, who has been my constant mentor. He has supported and encouraged me unfailingly in my endeavours. His helpful discussions on my projects and his sage advice have been invaluable. To all of my collaborators — I would like to thank each of you for the unique insights you have provided, and for generously sharing your specialized knowledge. In particular, I would like to acknowledge Michel Cur´e, who was always available to discuss our project, was always cheerful and encouraging, and who graciously shared his expertise on line-driven winds. Alex Carciofi also deserves special mention for his helpful, thorough, and patient discussions on using his HDUST code, and on the subtleties of modelling Be stars. v Contents Abstract ii Co-Authorship Statement iii Acknowledgements iv List of Figures ix List of Tables xi List of Symbols xii List of Abbreviations xv 1 Introduction 1 1.1 ThePhysicsofStellarSpectra . .... 2 1.1.1 StellarRadiation ............................. 2 1.1.2 TheFormationofaSpectrum. 5 1.1.3 SpectralLineProfiles . 6 1.2 DefiningtheClassicalBeStar . ... 7 1.3 MainObservationalPropertiesofBeStars . ........ 8 1.3.1 EmissionintheOpticalPortionoftheSpectrum . ....... 8 1.3.2 InfraredandRadioRegime . 12 1.3.3 UltravioletandX-RayObservations . .... 14 1.3.4 LinearPolarization . 16 1.4 CircumstellarEnvirons . ... 17 1.4.1 DiskGeometry .............................. 19 1.4.2 DensityandTemperatureStructureoftheDisk . ...... 20 1.4.3 DiskKinematics ............................. 21 1.4.4 AngularMomentum ........................... 22 1.5 Variability..................................... 23 vi 1.5.1 Short-termVariations . 23 1.5.2 Long-termVariations . 25 1.6 CurrentResearchAreas. .. 27 1.6.1 RotationRates .............................. 27 1.6.2 Binarity .................................. 29 1.6.3 Magnetism ................................ 30 1.6.4 EvolutionaryStatus . 30 1.7 SummaryandClosingRemarks. .. 31 References........................................ 33 2 Line-Driven Winds and Be Stars 40 2.1 Introduction .................................... 40 2.1.1 ClassicalBeStars............................. 41 2.1.2 OverviewofPreviousWork . 42 2.2 Theory....................................... 44 2.3 Results....................................... 47 2.4 SummaryandDiscussion . 54 2.5 FutureWork.................................... 57 References........................................ 58 3 The Hα Profiles of Be Shell Stars 61 3.1 Introduction ...................................