Mass Loss by Inhomogeneous AGB-Winds
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Mass Loss by Inhomogeneous AGB-Winds Detailed Structures in Planetary Nebulae Dissertation eingereicht von Mag. rer. nat. Ch. Reimers zur Erlangung des akademischen Grades Doktor der Naturwissenschaften Fakult¨at f¨ur Geowissenschaften, Geographie und Astronomie der Universit¨at Wien Institut f¨ur Astronomie T¨urkenschanzstraße 17 A-1180 Wien, Osterreich¨ Oktober 2005 Preface On the one hand the distances in the universe as well as the dimensions of astro- physical objects like galaxies are almost unimaginable. On the other hand the time scales are either immeasurably long as compared with our human being (e.g. the life- time of a typical star like our Sun) or they are even faster than a “human thought” (e.g. the supernova explosion process, the rotation period of fast rotating pulsars or the atomic vibrational timescales). Therefore, the fascination to study astrophysical problems is the possibility to model and solve the “physical world” with the help of computer technology and specific software. This fascination was also a driving motivation for the realisation of this thesis. In order to reconstruct astrophysics here on Earth, computer simulations are in- evitable, which divide the space into small units (in the broadest sense this can be denoted by spatial resolution) and arrange the time as finite intervals, which are called time steps. This procedure one calls also discretisation, which was realised by the development of a program (hereafter RHD code) to simulate radiation hy- drodynamic problems at the Institute for Astronomy of the University of Vienna. The RHD code is already extensively tested by several calculations to various as- tronomical objects, e.g. RR Lyra stars, Cepheids, LBVs, protostellar collapse and AGB stars. Among other things the work is to be understood as an extension to this RHD code. I would like to thank my dissertation advisor, Ernst A. Dorfi, for his support and encouragement over the previous years. I benefited from his teaching of computer simulation and he led me to break through problems which certainly resulted in a timely completion of this thesis. Also many thanks to the preparatory work of the RHD code done by Susanne H¨ofner, Michael U. Feuchtinger as well as Ernst A. Dorfi. Furthermore, a big thank to Roland Ottensamer for proof-reading of this thesis. Finally, I acknowledge the dis- cussions, inspirations and patience of all the students and combatants, who worked in the same computer working room as me. Vienna, October 2005 Mag. Christian Reimers ii Abstract AGB (Asymptotic Giant Branch) stars generate a massive dust driven stellar wind at the end of their lives. Thereby they lose a large amount of mass. Ideally, this mass loss is spherical if the physical conditions are homogeneous at the stellar surface (e.g. temperature) and the stellar vicinity (e.g. density). Indeed, several physical processes induce deviations from these ideal conditions. A stellar rotation for exam- ple generates an asphericity of the luminosity or alternatively effective temperature at the stellar photosphere. This will affect the condensation of dust and therefore the mass loss rate. The dust formation process depends strongly on the temperature and density. Inhomogeneities can also caused by cool spots at the stellar surface. For some time it is known that spots are common on stars and are much often larger than spots on our Sun. These inhomogeneities of the temperature are able to emanate from a magnetic field or a huge convection cell within the stellar envelope. Both options are possible at the surface of AGB-stars. Due to the massive dust formation in their atmospheres these physical processes are difficult to observe. But several theoretical calculations and investigations are able to support such a theory. This thesis introduces a model for the investigation of the mass loss above cool spots. For that purpose a radiation hydrodynamic simulation (including a gas, a dust and a radiation component) has been used and modified for the special purposes of this problem. A flux tube geometry has been chosen which could have been produced by a magnetic field in the lower stellar atmosphere. Finally, a discussion has been carried out about the creation of dense knots in planetary nebula as a result of cool regions at the stellar surface. A large amount of those dense knots or cometary structures can be observed in many planetary nebula, like in the Helix or the Eskimo Nebula. The result supports the theory that stellar spots generate significant inhomo- geneities of the mass loss. But the formation of dense knots in planetary nebulae have to be interpreted as a combination of inhomogeneities in the mass loss together with hydrodynamical instabilities. The model investigated describes the formation of initial inhomogeneities which can be later amplified by an interaction of the slow AGB wind with the fast tenuous wind of the hot central star of the planetary neb- ula. Zusammenfassung AGB-Sterne (Asymptotic Giant Branch) produzieren am Ende ihres Lebens einen ausgepr¨agten staubgetriebenen Sternwind, bei dem sie einen Großteil ihrer H¨ullen- masse verlieren. Idealerweise ist dieser Massenverlust sph¨arisch symmetrisch, wenn die physikalischen Gr¨oßen an der Sternoberfl¨ache (z.B. Temperatur) und im umge- benden Medium (z.B. Dichte) homogen sind. Allerdings erzeugen verschiedene physikalische Prozesse Abweichungen von diesen idealen Bedingungen. Zum Beispiel bewirkt die Rotation des Sterns eine Aspherizit¨at der Sternleuchtkraft beziehungs- weise Effektivtemperatur an der Sternphotosph¨are, welche sich auf die Kondensation des Staubs und daraus folgend auf die Massenverlustrate auswirkt. Der Staubent- stehungsprozess ist stark von Temperatur und Dichte abh¨angig. Inhomogenit¨aten k¨onnen auch durch k¨uhle Flecken auf der Sternoberfl¨ache erzeugt werden. Schon seit einiger Zeit ist bekannt, dass es Sterne mit Flecken gibt, die mitunter einiges gr¨oßer sind als Sonnenflecken. Diese Temperaturinhomogenit¨aten k¨onnen von einem Magnetfeld oder aber von großr¨aumigen Konvektionszellen in einer konvektiven ¨außeren H¨ulle stammen. Beide M¨oglichkeiten sind f¨ur die Ober- fl¨ache von AGB-Sternen vorstellbar. Beobachtungen diesbez¨uglich sind wegen der hohen Staubproduktion in den AGB-Atmosph¨aren nur schwer zu machen. Ver- schiedene Modellrechnungen und theoretische Uberlegungen¨ unterst¨utzen jedoch diese Theorie. In dieser Arbeit wird ein Modell vorgestellt, das zur Untersuchung des Massenver- lustes ¨uber diskreten k¨uhlen Flecken dient. Dazu kam eine strahlungshydrodynam- ische Simulation zum Einsatz, die eine Gas-, Staub- und Strahlungs-Komponente beinhaltet, wobei der Computer-Code f¨ur die neue Applikation adaptiert werden musste. Um den komplexen Sachverhalt zu vereinfachen wurde eine Flussr¨ohren- Geometrie gew¨ahlt, die ein Magnetfeld in der unteren Sternatmosph¨are erzeugt. Eine abschließende Diskussion soll kl¨aren, ob diese k¨uhlen Regionen auf der Stern- oberfl¨ache die Existenz von dichten Knoten in Planetarischen Nebeln hervorrufen kann. In vielen Planetarischen Nebeln sind wir in der Lage eine große Anzahl dichter Knoten oder “kometenartiger” Strukturen zu beobachten (z.B. im Helix- oder im Eskimo-Nebel). Das Ergebnis unterst¨utzt die Theorie, dass Sternflecken eine signifikante Inho- mogenit¨at im Massenverlust verursachen k¨onnen. Allerdings m¨ussen die beobachte- ten dichten Knoten in Planetarischen Nebeln in Verbindung mit hydrodynamischen Instabilit¨aten entstanden sein. Das untersuchte Modell erzeugt dabei eine anf¨angliche Inhomogenit¨at im stellaren Ausfluss, welche sp¨ater durch die Wechselwirkung des langsamen AGB-Windes mit dem schnellen d¨unnen Wind des heißen Zentralsterns Planetarischer Nebel verst¨arkt werden kann. Contents I Introduction and Motivation 1 1 Evolution of Stars 3 1.1 TheCycleofMatter ........................... 3 1.1.1 Interstellar Medium . 3 1.1.2 ExchangeofMatter ....................... 4 1.2 StellarEvolution ............................. 4 1.2.1 StarFormation .......................... 4 1.2.2 Constant Light of Hydrogen Fusion - The Main Sequence . 5 1.2.3 FinalStagesofStars....................... 5 1.3 Origin and Composition of Stellar Dust . 7 1.3.1 PropertiesofAGBstars . 7 1.3.2 Detecting and Measuring Interstellar Dust Grains . ... 9 1.3.3 Dust Formation and Destruction . 10 1.4 FromAGBstarstoPNe ......................... 11 2 Planetary Nebulae 13 2.1 Morphology and Classification . 13 2.1.1 ListofProminentPNe. 14 2.2 Examples ................................. 15 2.2.1 Proto-PNe (or Young PNe) . 15 2.2.2 RoundandElliptical. 18 2.2.3 Bipolar and Quadrupolar . 25 2.3 GlobalModelstoShapeaPN. 29 2.3.1 Multiple-Winds Model . 29 2.3.2 Aspherical Mass Loss of AGB stars . 29 2.3.3 The Role of Magnetic Fields . 30 2.3.4 Interaction with the ISM . 31 2.3.5 MHDModels ........................... 31 2.4 DetailsinPNe .............................. 32 2.4.1 Halo................................ 32 2.4.2 Jets,LobesandAnsae . 33 2.4.3 Knots ............................... 33 v vi CONTENTS II Theoretical Models 35 3 Radiation Hydrodynamics Simulation 37 3.1 BasicEquations.............................. 37 3.1.1 Conservationform . .. .. .. .. .. .. .. 37 3.1.2 GasComponent ......................... 38 3.1.3 Radiation Field . 40 3.1.4 Dust................................ 41 3.2 Additional Equations and Constitutive Relations . ...... 42 3.2.1 GridEquation .......................... 42 3.2.2 MassEquation .......................... 42 3.2.3 PoissonEquation......................... 43 3.2.4 EquationofState(EOS).