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On the Winds of Carbon Stars and the Origin of Carbon

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On the Winds of Carbon Stars and the Origin of Carbon !"#$" # #% &'' '&&' (""") ISBN 45666446 7 6 8 9 :89;: ; < 8 = 6 6 6 > 7 ? 6 ? 6 = 6 @ 6 6 6 6 = 6 :A6; 6 9 6 A6 6 > 6C = 8 @ 6 8 6 8 D8 ? E 8 @ 89 ! "# !9vvvs6 hqThpr Quvp7$ %VhyhVvr vT@&$ ! ! F =G6 =9G45666446 6: HH 7H J K 6; Oh freddled gruntbuggly, Thy micturations are to me As plurdled gabbleblotchits On a lurgid bee. Groop, I implore thee, my foonting turlingdromes And hooptiosly drangle me with crinkly bindlewurdles, Or I will rend thee in the gobblewarts with my blurglecruncheon See if I don’t. List of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I L. Mattsson,R.Wahlin&S.Höfner,"Dust Driven Mass Loss from Carbon Stars as Function of Stellar Parameters I. - A Grid of Solar-metallicity Wind Models", submitted to Astronomy & Astrophysics (2009) II L. Mattsson, R. Wahlin, S. Höfner & K. Eriksson, "Intense mass loss from C-rich AGB stars at low metallicity?", published in Astronomy & Astrophysics, 484, L5 (2008) III L. Mattsson &S.Höfner,"Dust Driven Mass Loss from Carbon Stars as Function of Stellar Parameters II. - Effects of Relaxing the Small Particle Approximation", preprint (2009) IV L. Mattsson,S.Höfner&F.Herwig,"Mass Loss Evolution and the For- mation of Detached Shells around TP-AGB Stars", published in Astron- omy & Astrophysics, 470, 339 (2007) V L. Mattsson, F. Herwig, S. Höfner, R. Wahlin, M. Lederer, & B. Paxton, "Effects of Carbon-excess Dependent Mass Loss and Molecular Opaci- ties on Models of C-star Evolution", preprint (2009) VI L. Mattsson,"The Origin of Carbon: Low-mass Stars and an Evolving IMF?", preprint (2009) Reprints were made with permission from the publishers. Contents 1 Introduction ............................................. 9 1.1 SettingtheStage....................................... 9 1.2 EvolutionofLIMStars.................................. 10 1.3 WindsofCarbonStars.................................. 11 1.4 TheCosmicMatterCycle:theOriginofCarbon............. 12 1.5 Carbon-basedLifeintheGalaxy.......................... 13 2 Modelling of Dynamic Atmospheres and Dust Driven Winds . 15 2.1 The Success of Numerical Modelling . ..................... 15 2.2 TheFullSetofEquations................................ 16 2.2.1 RadiationHydrodynamics........................... 16 2.2.2 Selfgravity........................................ 18 2.2.3 Equation of State and other aPrioriAssumptions . 18 2.2.4 RadiativeTransfer.................................. 19 2.2.5 DustFormationandOpacity......................... 21 2.3 MethodofComputation................................ 23 2.3.1 BoundaryConditions............................... 23 2.3.2 InitialConditions.................................. 24 2.3.3 The Modelling Procedure . ..................... 25 3 CarbonStarMassLossasFunctionofStellarParameters......... 27 3.1 TheGridofWindModels................................ 27 3.1.1 GeneralProperties................................. 27 3.1.2 Mass-LossThresholdsandtheCarbonExcess .......... 28 3.2 Metallicity Dependence? . ......................... 32 4 RelaxingtheSmallParticleApproximation..................... 35 4.1 SizeableGrains........................................ 35 4.2 HowBadistheSPA?.................................... 36 4.3 SizeMatters!.......................................... 37 5 StellarStructureandEvolution .............................. 41 5.1 BasicIngredients ...................................... 41 5.1.1 Hydrostatic Equilibrium and Energy Generation ........ 41 5.1.2 EnergyTransport.................................. 42 5.1.3 ConvectionintheMLTPicture....................... 43 5.2 RadiativeTransferandtheRôleofGasOpacities ............ 44 5.3 StellarEvolutioninBrief................................ 45 5.4 Nucleosynthesis....................................... 47 5.4.1 TheTriple-alphaProcess............................ 47 5.4.2 TheCN-cycle ..................................... 47 5.5 EffectsofMassLossandMixing.......................... 49 5.5.1 Carbon Production: Dredge-up and Thermal Pulses . 50 5.5.2 Carbon Destruction: The on-set of HBB and AGB life times 50 6 Mass-LossEvolutionandtheOriginofDetachedShells.......... 51 6.1 AConsequenceofHeliumShellFlashes?................... 51 6.2 Two-windInteraction................................... 52 6.3 PulsationsareCritical .................................. 53 6.4 SoHowDoesDetachedShellsForm?...................... 55 7 ModelsofCarbonStarEvolution............................. 57 7.1 Mass-lossEffectsonC-starEvolution...................... 57 7.2 TheOriginoftheSuperwind............................. 58 7.3 LimitedNucleosynthesis................................ 59 8 GalacticChemicalEvolutionModels.......................... 61 8.1 TheMilkyWayGalaxy.................................. 61 8.1.1 TheGalacticDisc.................................. 61 8.1.2 TheStellarHaloandthe"ThickDisc" ................. 62 8.2 OriginoftheElements.................................. 62 8.3 ModelIngredientsandEquations......................... 63 8.3.1 GalaxyFormation.................................. 64 8.3.2 GalacticDynamics................................. 65 8.3.3 StarFormation.................................... 66 8.3.4 TheInitialMassFunction........................... 66 8.3.5 EvolutionoftheISMandElementalRatios............. 67 8.3.6 SupernovaetypeIa................................. 68 8.4 StellarYields.......................................... 69 9 AnExtensionofthePadoan-NordlundTheoryoftheIMF ........ 71 9.1 AUniversalIMF?....................................... 71 9.2 TurbulentFragmentation ............................... 71 9.3 EnvironmentalVariationsoftheIMF...................... 75 9.4 AlternativeMechanisms ................................ 77 10TheOriginofCarbon ...................................... 79 10.1TheMakingofCarbon.................................. 79 10.2ATop-heavyIMF?...................................... 80 10.3LowMassStarsandanEvolvingIMF? ..................... 82 11ConcludingRemarksandFutureProspects .................... 85 11.1FutureWork .......................................... 85 11.1.1 Subsolar Metallicity . ............................. 85 11.1.2DriftandGrainSizes ............................... 85 11.1.3DetachedShells ................................... 86 11.1.4GalacticChemicalEvolution......................... 86 11.2LifefromaWiderPerspective............................ 86 11.2.1AGalacticHabitableZone?.......................... 86 11.2.2CarbonStarsandCarbon-basedLife.................. 88 ContributionstoIncludedPapers............................... 91 Sammanfattning............................................. 93 Acknowledgements .......................................... 97 Bibliography ................................................ 99 1. Introduction "We are all in the gutter, but some of us are looking at the stars." Oscar Wilde This is a thesis about stars, and to some extent galaxies. More precisely, it is a thesis about the gas, dust and stars that we find in our Galaxy and how it all is evolving. The content may at first glance seem as if it consists of sep- arate parts, but that is not the case. This introductory chapter presents the context of this thesis in an attempt to explain how models of the build-up of heavy elements in the Universe actually connect with models of dynamic stellar atmospheres. 1.1 Setting the Stage There exist many fundamental, unanswered questions in the natural sci- ences, not the least in astrophysics. Such questions are the origin and early evolution of the universe, as well as the formation and evolution of galax- ies. Another question of the same magnitude is the origin of life in the uni- verse (or at least on Earth). Life, as we know it, requires the existence of carbon, nitrogen, oxygen and a few other elements. However, with the ex- ception of hydrogen, helium and a small amount of lithium, all the atomic nuclei in the universe are created in stars, through nuclear reactions (re- ferred to as nucleosynthesis), where light elements are combined into heav- ier elements. These elements have then been expelled from the stars, either through relatively slow mass loss (stellar winds), as in the case of low and intermediate mass stars, or explosively, as in the case of high mass stars. Thus, the interstellar medium (ISM) in a galaxy is continuously enriched in heavy elements as new generations of stars form, evolve and expel their newly synthesised elements. This is what is referred to as the chemical evo- lution of galaxies, which in the strict sense of the word has little to do with chemistry, but rather the build-up of heavy elements over time. These new elements then form molecules and dust in stars and interstellar space and are later incorporated into new generations of stars, planets and eventually life on at least one planet. In this thesis, which is part of a rather ambitious project aiming at a better understanding of the rôle of low- and intermediate-mass (LIM) stars in this "big scheme", results are presented regarding the theoretical modelling of mass-loss from carbon stars and the cosmic origin of atomic carbon
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