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Stellar and Wind Parameters of Galactic OB Stars

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Stellar and Wind Parameters of Galactic OB Stars BULGARIAN ACADEMY OF SCIENCES INSTITUTE OF ASTRONOMY Stellar and wind parameters of Galactic OB stars A habilitation thesis submitted for the degree of Doctor of Sciences presented by Nevena Stefanova Markova Sofia, 2009 2 Contents 1 Introduction 1 1.1 MassiveStarsandtheUniverse. .......... 1 1.2 Stellarwindsfromhotmassivestars . ............ 4 1.3 Standardwindtheory–historicaloverview . .............. 6 1.4 TheCastor-Abbott-and-Kleintheory . ............. 7 1.4.1 Theradiativeforce . .... ... .... .... .... ... .... .. ..... 7 1.4.2 PredictionsfromtheCAKtheory . ....... 9 1.5 Winddiagnostics ................................. ...... 11 1.5.1 Spectrallinediagnostics . ........ 11 1.5.2 Stellarcontinuumdiagnostics . ......... 13 1.6 Studiesunderlyingthethesis . ........... 14 1.6.1 Maintopicsandgoals ... ... .... .... .... ... .... ... .... 14 1.6.2 Approachesandmethodsused . ...... 17 2 “Blanketed” properties of OB stars 21 2.1 Approximate method to determine O-star wind parameters ................. 21 2.1.1 Samplestarsandobservationalmaterial . ............ 21 2.1.2 Inputparametersand mass-loss determinations . .............. 24 2.1.3 Erroranalysis. ................................ .... 28 2.1.4 Verifyingtheapproximateapproach . .......... 29 2.1.5 Enlargingthesample . ..... 33 2.1.6 Summary....................................... 35 2.2 CompletespectralanalysisofBsupergiants . ............... 37 2.2.1 Samplestarsandobservationalmaterial . ............ 39 2.2.2 Determinationofstellarandwindparameters . ............. 41 2.2.3 The Teff scaleforB-SGs............................... 48 2.2.4 Metallicity effects................................... 51 2.2.5 Wind momentum - luminosity relationship. Comparison with results from similar studies ........................................ 53 2.2.6 Summary....................................... 55 3 Wind structure and variability in OB stars 57 3.1 Long-term monitoring campaigns of α Cam......................... 58 3.1.1 Observationalmaterial . ....... 58 3.1.2 Absorptionline-profilevariability . ............ 58 3.1.3 Wind variability as traced by Hα ........................... 61 3.1.4 Evidenceofa“photosphericconnection” . ........... 62 3.1.5 Towards a possible interpretation of Hα variability ................. 63 3.1.6 Summary....................................... 64 3.2 Long-termmonitoringcampaignofHD199478 . ............ 66 3.2.1 Observationalmaterial . ....... 66 3.2.2 Photosphericvariability . ........ 67 3.2.3 Wind variability as traced by Hα ........................... 70 i ii CONTENTS 3.2.4 Summary....................................... 73 3.3 WindstructureinlateB-supergiants . ............. 75 3.3.1 Pulsations.................................... ... 75 3.3.2 Magneticfields ................................. ... 76 3.4 Statistical approach to study wind variability . .................. 79 3.4.1 Observationalmaterial . ....... 79 3.4.2 Methodologyandmeasurements . ....... 80 3.4.3 Hα line-profile variability as a function of stellar and wind parameters . 86 3.4.4 Empirical properties of the TVS as a function of stellarandwindparameters . 88 3.4.5 Simulations of LPV in Hα .............................. 91 3.4.6 Spectral variability and the Wind Momentum LuminosityRelationship . 101 3.4.7 Summary....................................... 102 4 Wind structure and variability in P Cygni 105 4.1 Observationaldata ............................... ....... 106 4.1.1 Photometricandspectroscopicobservations . .............. 106 4.1.2 Hα equivalentwidthmeasurements . 107 4.2 Emissionlinespectrumintheoptical. ............. 108 4.2.1 Permittedlines ................................ .... 108 4.2.2 Forbiddenlines................................ .... 113 4.3 Photometricandline-profilevariability . ................ 114 4.3.1 Photometricvariability . ........ 114 4.3.2 Line-profilevariabilityintheoptical . ............. 115 4.3.3 Comparison between photometric and spectroscopic variabilities. 122 4.4 PCygniinashortSDphase . .... .... ... .... .... .... ... ...... 124 4.4.1 Photometricevidences . ...... 124 4.4.2 Spectroscopicevidences . ....... 126 4.4.3 Hα variability due to changes in M˙ ......................... 127 4.4.4 Possibleinterpretation . ........ 128 4.5 Summary ......................................... 128 5 Clumping in O-star winds 131 5.1 AcombinedHα ,IRandradioanalysis. 133 5.1.1 Variabilityofthediagnosticsused . ........... 135 5.1.2 Observationsanddataprocessing . ......... 136 5.1.3 De-reddeningandstellarradii . ......... 139 5.1.4 Simulations................................... .. 142 5.1.5 Two prototypical test cases: ζ PupandHD209975 . 147 5.1.6 Clumpingpropertiesofthecompletesample . ........... 151 5.1.7 Errorsinthederivedclumpingfactors . ........... 155 5.1.8 Clumpingpropertiesasafunctionofwinddensity . ............. 156 5.1.9 Wind-momentum–luminosityrelation . .......... 158 5.1.10 Summary ...................................... 159 6 Theory and observations 165 6.1 MonteCarlopredictions . ........ 165 6.2 Thebi-stabilityjump . .... .... .... ... .... .... .... ........ 167 6.3 TheWind-momentumLuminosityRelationship . ............. 169 6.4 Windefficiency ........................................ 176 6.5 Wind clumping. Stratification of the clumping factor . ................. 177 6.6 Summary ......................................... 180 Concluding remarks and future perspectives .......................... 182 Main results and achievements .................................. 185 List of publications ......................................... 190 References ............................................. 192 Chapter 1 Introduction The evolution of the Universe, since the time when the first stars were formed, is a central topic of present- day astrophysical research. The main engines of this evolution are stars with masses in excess of 10 M , ⊙ referred to as “massive stars” . These stars occupy the upper part of the Hertzsprung-Russell (HR) diagram with luminosities between twenty thousands and one million solar luminosities. On the main sequence, massive stars are spectroscopically identified as types O and early-B with temperatures between 25 and ∼ ∼ 50 kK while later evolutionary stages can encompass, depending on the initial mass, blue/red supergiants, Wolf-Rayet (WR) stars, Luminous Blue Variables (LBV), and supernovas (SN). 1.1 Massive Stars and the Universe Observations in the solar neighbourhood indicate that in the present cosmos Nature seems to favour the formation of lower mass stars over the formation of the massive stars. However, despite of their rarity, massive stars play very important role in the present, but also in the early Universe. Massive stars are critical agents in galactic evolution. In the present cosmos, they provide most of the metals and energy. In the distant Universe, they dominate the UV light from the young galaxies. The very first generation of these objects, the so-called Population III, might have influenced the formation and evolution of the first building blocks of galaxies. These very massive first stars might have also been responsible for the re-ionisation of the early Universe. At the endpoint of their evolution, massive stars suffer a gravitational collapse and explode as a supernova. Eventually, a Gamma-Ray-Burst emerges, the most energetic cosmic flash. Evolution of the cosmic matter Due to their short lifetime – 107 years agents one Gyr for stars like our ∼ Sun – and powerful stellar winds, massive stars deposit a large amount of momentum and processes material (helium, oxygen and other heavy elements1) into the IterStellar Medium (ISM) influencing its composition 1Carbon and nitrogen are mainly produced by low and intermediate-mass stars (Renzini & Voli 1981; Wheeler et al. 1989). 1 2 CHAPTER 1. INTRODUCTION Figure 1.1: What appears to be the hole of an elongated smoke ring in this National Radio Astronomy Observatory image, really is an enormous, nearly empty, bubble blown into the dusty, gas disk of our Milky Way Galaxy. Located at a distance of 30,000 light years from the Earth, this bubble measures 1,100 by 520 light years. and energetics, and subsequently contributing to the chemical and dynamical evolution of the host galaxies. Thus, massive stars, and in particular BA supergiants (SGs), can serve as tracers of abundance gradients in galaxies, and in stellar atmospheres. Studies of large samples of hot massive stars (HMS) in different galactic environments can also provide observational constraints on the chemical evolution of the galaxies (see Przybilla et al. 2007 and references therein). HII regions, Wind bubbles and Star formation HMS emit the bulk of their radiation in the UV. This radiation ionises and heats a very large volume of the ambient gas which then starts to expand (due to a pressure difference with the cool ISM) leading to the formation of the so-called Stromgren spheres, or S- type H-II regions. Due to the absorption of UV photospheric radiation by thousants of millions of metal lines, continuous mass loss from the stellar surface is also initiated and maintained. Flowing away from the star at supersonic velocities, the gas particles create circumstellar shells and wind-bubbles. Since massive stars are mostly seen grouped in young clusters, wind-blown bubbles frequently interact with each other forming super-bubbles, as the one shown in Fig 1.1. These super-bubbles are presently thought to be places for new stars formation (Oey & Massey 1995). Perhaps, they are also responsible for the phenomenon of galactic energetic outflows in starbursts
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