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Asteroseismology of Β Cephei and Be Type Stars

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Asteroseismology of Β Cephei and Be Type Stars Asteroseismology of β Cephei and Be type stars by Kym Jeanette Frances Goss A thesis submitted to The University of Birmingham for the degree of Doctor of Philosophy School of Physics and Astronomy The University of Birmingham May 2012 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Abstract The thesis focuses on the asteroseismology of main sequence B-type stars, particularly β Cephei and Be stars. Photometric observations of these stars were analysed in order to detect stellar oscillations. The photometric data analysed in this thesis were collected using the Solar Mass Ejection Imager (SMEI). The analysis initially focussed on known β Cephei stars, to refine information on previously detected frequencies and detect further oscillations. A survey was then conducted using data from the SMEI instrument to search for further β Cephei stars. The results from this survey were analysed, and individual stars were examined in more detail which may possibly be β Cephei like. Due to the long duration of light curves obtained with the SMEI instrument, stellar oscillations can be analysed for evidence of amplitude and phase change. This type of analysis was completed on two Be stars, Achernar and ζ Oph, and significant amplitude changes in the oscillations of both of these stars were detected. There is evidence that the amplitude variations may be linked with the outburst events that occur on Be stars, and therefore this analysis may be used to help solve the puzzle of the Be phenomenon. For Mum, Dad and Jon Without your love and support this would not have been possible i Acknowledgments Many thanks go to my supervisors, Prof Yvonne Elsworth and Prof Bill Chaplin, for their much appreciated support, encouragement and advice during my PhD. Particular thanks go to Dr Ian Stevens, with whom I have worked with over the last 3 years, also Dr Christoffer Karoff and Dr Andrea Miglio for their collaborative work. I would also like to thank the rest of the members in the HiROS research group for their help and support with my work and company and fun over the last 3 years: Neil Tarrant, Guy Davies, Elizabeth George, Anne-Marie Broomhall, Saskia Hekker, Graham Verner, Brek Miller, Steven Hale, Ian Barnes and Barry Jackson. And last, but definitely not least, I would like to say a big thank you to my family. My parents have always been dedicated to my education and supportive with whatever decisions I have made. My fianc´e, Jon, has kept me sane (with regards to work) and provided me with continuous encouragement. Their love and support throughout has made this possible. ii The work presented in this thesis, unless stated otherwise, is the original work of the author, which has been prepared under the supervision of Prof Y. Elsworth and Prof W. Chaplin. Particular credit should be made to the following people: Dr Steven Spreckley for his work in developing the SMEI pipeline. • Dr Ian Stevens for his collaborative work on the analysis of the confirmed β Cephei • stars. Dr Christoffer Karoff for providing the basics of a computer program to analyse • photometric light curves for pulsations. Dr Andrea Miglio for his help in modelling the star HIP 30122. • Dr Derek Buzasi for creating the time frequency analysis diagram on ζ Oph. • Dr Petr Harmanec for providing spectroscopic data on ζ Oph. • Dr Ian Howarth for analysing the spectroscopic data on ζ Oph. • iii Contents 1 An Introduction to asteroseismology 1 1.1 Oscillations . 3 1.2 A little on asteroseismology . 8 1.3 A little stellar evolution . 12 1.4 β Cephei,BeandSPBoverview . 21 1.5 Excitation mechanism - The κ-mechanism ................ 22 1.6 Observations................................. 26 1.7 Thesissynopsis ............................... 29 2 Instrumentation 31 2.1 The Solar Mass Ejection Imager - SMEI ................. 31 2.1.1 Introduction............................. 31 2.1.2 The SMEI Instrument ....................... 32 2.1.3 DataAnalysis............................ 38 2.2 Microvariability and Oscillations of STars - MOST ............ 42 2.3 Wide-field InfraRed Explorer - WIRE ................... 43 3 β Cephei stars 46 3.1 An Introduction to β Cepheistars..................... 47 3.1.1 Properties of β Cepheistars .................... 48 3.1.2 modelling β Cepheistars...................... 51 3.2 Confirmed β CepheistarswithSMEI . 53 iv 3.2.1 Analysis using Period04 ..................... 54 3.2.2 Results of confirmed β Cepheistars................ 57 3.2.3 Conclusion.............................. 70 3.3 Survey to search for β Cepheistars .................... 71 3.3.1 Analysis............................... 71 3.3.2 SurveyCharacteristics . 77 3.3.3 Results on individual survey stars . 90 3.3.4 Discussion.............................. 105 3.3.5 Conclusions ............................. 106 3.4 Conclusion.................................. 107 4 Be stars 109 4.1 IntroductiontoBestars . .. .. 109 4.1.1 FastrotationofBestars . 111 4.1.2 Circumstellar Disc . 113 4.1.3 TheBephenomenon ........................ 113 4.2 Achernar................................... 116 4.2.1 Analysis of light curve . 116 4.2.2 Splitting of the light curve . 119 4.2.3 Results: Change in amplitude . 120 4.2.4 Results: Change in frequency and phase . 124 4.2.5 Discussion.............................. 126 4.2.6 Conclusion.............................. 128 4.3 ζ Oph .................................... 129 4.3.1 Analysis of light curve . 130 4.3.2 Splitting of the light curve . 135 4.3.3 Results: Change in amplitude . 136 4.3.4 Results: Change in frequency and phase . 140 4.3.5 Discussion.............................. 145 v 4.3.6 Conclusion.............................. 147 5 Conclusion 150 5.1 β Cepheistars................................ 152 5.2 Bestars ................................... 153 5.3 Futureprospects .............................. 153 A Published results 160 vi List of Figures 1.1 P-mode oscillations in the stellar interior. 5 1.2 Surface displacements in different modes of oscillation. 6 1.3 Cross-sectionofstarshowingradialorder. .. 7 1.4 BiSON powerspectrum........................... 9 1.5 LargeandsmallseparationsontheSun. 10 1.6 AsteroseismicDiagram ........................... 11 1.7 Hertzsprung-Russelldiagram. 13 1.8 Structure of nuclear burning in evolved stars . 14 1.9 Oscillation characteristics on the HR diagram . 16 1.10 Convective and radiative zones along the main sequence . ... 18 1.11 κ mechanism inside SPB and β Cepheistars ............... 24 1.12 β CepheiandSPBinstabilitystrips . 26 2.1 SMEI: An illustration of the SMEI instrument onboard the Coriolis satellite. ................................... 33 2.2 SMEI: Asingleframetakenfromoneofthecameras. 34 2.3 SMEI: Allsky image produced from 1 orbit of the instrument. 34 2.4 SMEI:Rawlightcurve........................... 39 2.5 SMEI:Smoothedlightcurve........................ 40 2.6 SMEI: 30 day sample section of the light curve of Achernar. 41 2.7 AmplitudespectrumofHIP66657. 42 vii 2.8 20 day sample sections of ζ Oph light curve observed with SMEI, WIRE and MOST. ................................. 45 3.1 β Cep: Stellar mass distribution of confirmed β Cephei stars. 49 3.2 β Cep: Pulsation period distribution of confirmed β Cephei stars . 49 3.3 β Cep: Comparison of v sini and pulsation amplitude of confirmed β Cepheistars. ............................... 50 3.4 Spectrum of α Vir,Spica(HD116658,HIP65474). 58 3.5 Spectrum of ǫ Cen(HD118716,HIP66657) . 59 3.6 Spectrum of β Cen(HD122451,HIP68702) . 60 3.7 Spectrum of τ Lup(HD126341,HIP70574) . 61 3.8 Spectrum of α Lup(HD129056,HIP71860) . 62 3.9 Spectrum of δ Lup(HD136298,HIP75141) . 63 3.10 Spectrum of ω Sco(HD144470,HIP78933) . 64 3.11 Spectrum of λ Sco(HD158926,HIP85927) . 65 3.12 Spectrum of λ Sco(HD158926,HIP85927) . 66 3.13 Spectrum ofV2051Oph (HD 163472,HIP 87812) . 67 3.14 Spectrum of β Cep(HD205021,HIP106032) . 68 3.15 SpectrumofDDLac(HD214993,HIP112031) . 69 3.16 β Cep survey: A graph to determine the 1 day alias cutoff point. 75 3.17 β Cep survey: Normalised cumulative frequency distributions for KS tests comparing different frequency groups . 91 3.18 β Cep survey: HR-diagram of stars in the survey which showed at least one significant frequency greater than 3 d−1 ............... 92 3.19 AmplitudespectrumofHIP18724. 95 3.20 AmplitudespectrumofHIP30122. 97 3.21 Frequencies of pulsation modes as a function of log Teff for main-sequence models of a 8 M⊙ star with Z=0.02.................... 98 3.22 AmplitudespectrumofHIP33558. 100 viii 3.23 AmplitudespectrumofHIP60710. 101 3.24 AmplitudespectrumofHIP93808. 102 3.25 AmplitudespectrumofHIP111544 . 104 4.1 Bestars:Equatorialbulge . 113 4.2 Achernar:lightcurve............................ 117 4.3 Achernar: Amplitudespectrum . 117 4.4 Achernar: Changesinamplitude. 121 4.5 Achernar: Split SMEI spectra ....................... 123 4.6 Achernar: Amplitude, frequency and phase variations . 125 4.7 ζ Oph:Spitzerimage............................ 129 4.8 ζ Oph:Lightcurves ............................ 131 4.9 ζ Oph:Amplitudespectrum
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