Models for circumstellar nebulae around red and blue supergiants Sabina Chit¸aˇ c 2011 Sabina Mˇadˇalina Chit¸ˇa Alle rechten voorbehouden ISBN 978-90-393-5249-6 Cover image: Sher 25 nebula, as observed by Hubble Space Telescope, collated with the model of emission density from circumstellar material around a blue supergiant as presented in chapter 3 of this thesis. Design: Frans Snik Models for circumstellar nebulae around red and blue supergiants Modellen van nevels rond rode en blauwe superreuze (met een samenvatting in het Nederlands) Proefschrift ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de rec- tor magnificus, prof. dr. G. J. van der Zwaan, ingevolge het besluit van het college voor promoties in het openbaar te verdedigen op donderdag 6 oktober 2011 des middags te 2.30 uur door Sabina Madˇ alinaˇ Chit¸aˇ geboren op 26 februari 1980 te Brˇaila, Roemeni¨e Promotoren: Prof. dr. N. Langer Prof. dr. A. Achterberg pentru mami ¸si tati Contents 1 Introduction 1 Thequestforknowledge ............................ 1 Theinfluenceofstarsontheirenvironment . ... 1 Thesisachievements .............................. 2 Thelifeofstars-ashortoverview . .... 3 Formation.................................... 3 Classificationandevolution. 3 Stellarwinds .................................. 5 Thedynamicalshapingofcircumstellarnebulae . ......... 5 Themainsequencebubble .... .... .... .... ... .... .... 5 Postmainsequenceevents .... .... .... .... ... .... .... 7 Fingerprintsleftbytheevolutionofthedonorstar . ........ 7 Observing the circumstellar medium around Red and Blue Supergiants. 8 Modeling the circumstellar medium around Red Supergiants . ........ 8 Modeling the circumstellar medium around Blue Supergiants ......... 10 SpecificsoftheZEUScode. 12 Thesiscontent..................................... 14 2 The circumstellar medium of massive stars. I. Models for red supergiants 17 Introduction...................................... 18 Stellarevolutionmodels. ... 19 Numericalmethod................................... 21 1Dresults ....................................... 22 2Dresults ....................................... 26 Model8n .................................... 26 Model12n.................................... 28 Model20n.................................... 30 RSGshellinstabilities. 31 CharacteristicsoftheRSGnebulae . .. 32 Discussion....................................... 34 Modeluncertainties............................... 34 Comparisonwithobservations . 35 vii viii Contents Summaryandconclusions . 36 Additionalinformation . .. 37 3 Multiple ring nebulae around blue supergiants 45 Introduction...................................... 45 Computationalmethod ................................ 46 Results......................................... 48 Discussion....................................... 49 Additionalinformation . .. 52 4 The circumstellar medium of massive stars. II. Models for blue supergiants 57 Introduction...................................... 57 Stellarmodels..................................... 58 Numericalmethod................................... 63 1Dresults ....................................... 63 2Dresults ....................................... 69 Model10n.................................... 69 Model12n.................................... 70 Model12r.................................... 70 Model18sw................................... 71 Discussion of BSG nebulae properties and comparison with observations. 71 Nebulaefromslowlyrotatingstars . .. 71 Nebulaefromrapidlyrotatingstars . ... 73 Comparisonwithobservations . 73 SummaryandConclusions . 74 5 A new mechanism to form mid-latitude circumstellar rings 87 Introduction...................................... 87 Stellarmodelsandcomputationaltechnique . ........ 88 Results......................................... 91 ModelTest1................................... 91 ModelTest2................................... 92 ComparisontoSN1987A............................... 92 Discussion....................................... 93 6 Nederlandse Samenvatting 101 Hetlevenvanzwaresterrenvangeboortetotdood . ........101 Vingerafdrukken van de sterevolutie: circumstellair materiaal. ... .... ....102 Ditproefschrift .................................... 104 7 Rezumat in limba romanˆ aˇ 107 Viat¸a stelelor: geneza, clasificare s¸i evolut¸ie . ..............107 Amprente lˇasate de evolut¸ia stelarˇa: materia circumstelarˇa. 108 Aceastˇatezˇa...................................... 110 Contents ix Bibliography 113 Acknowledgments 117 Curriculum Vitae 119 Chapter 1 Introduction The quest for knowledge Human beings have wondered about the mysteries of the sky for centuries and struggled to understand its secrets. The study of astronomy is a genuine extension of human curiosity. Understanding the true, wonderful nature of the universe lies not only in observing stars and galaxies, but also in considering the physical processes that cause the stars to exist. Archeoastronomy, or the study of the astronomy of past cultures, relied on the alignment of ancient structures toward celestial objects. Mathematical models of Nature, first proposed by the ancient Greeks such as Pythagoras, led to the Copernican revolution. Our perception of the universe has changed dramatically since the physical causes of observable phenomena started to be considered and understood. The application of physics to astronomy has proved to be very successful in explaining a wide range of observations: strange and exotic objects and events such as pulsating stars, supernovae, black holes and quasars. The rapid development of astronomy over the past decades occurred because of the equally large improvement of the tools we use to study the universe: telescopes and com- puters. With modern telescopes, both on the ground and in space, we are able to see X-rays, ultraviolet light (UV), infrared radiation (IR) and radio signals, while computers have pro- vided us with the capability to check complicated mathematical models that use fundamental physical principles. Understanding these physical processes is the first step to reveal the secrets encrypted in the stars and thus the formation and evolution of the universe. The influence of stars on their environment Astronomical objects such as stars, stellar clusters, nebulae, galaxies, clusters of galaxies and more exotic objects like pulsating stars, black holes and quasars are all surrounded by interstellar gas. This interstellar gas consists roughly of two states: a cold state, where most of the hydrogen consists of neutral atoms (H I), and a hot phase where hydrogen is either partially or fully ionized: the so-called H II regions. The H II regions, also known as diffuse nebulae, can be observed in our galaxy as well in the nearby galaxies. H II regions play a crucial part in the star formation history and the evolution of chemical elements in our galaxy and other galaxies. Star formation starts in giant molecular clouds where the density is high, and the temperature is low. Only after the stars ignite their nuclear fusion do they providethe UV photons that ionizes the hydrogen. H II regionsindicate sites of 1 2 Chapter 1 star formation and arise at locations where a large number of stars have been created recently. Some of these young stars are the most massive and hot stars, which use their nuclear energy quickly. These can be single stars, but are often grouped in clusters that provide more photons capable of ionizing the H II regions. The hot ionized gas of the H II region,or diffuse nebula, can expand into the cold surrounding neutral gas, decreasing the density of the nebula and increasing the volume of ionized gas. During their lives, stars interact with the surrounding interstellar medium through their ionizing radiation and through the mass, momentum and energy carried by their winds. The mass lost by the star is returned into the interstellar medium. Because of the nuclear processes that occur in the interior of stars, this material is very often chemically enriched. Dynamical interaction between stellar winds and their surroundings produce wind bub- bles. The winds of massive stars release large amounts of energy. Moreover, massive stars are often grouped in stellar clusters, so the energy of their ejected winds can accumulate. Galaxies contain many of such large mass aggregates of stars. Here dynamical processes take place, such as binary formation and disruption, stellar collisions and mergers, supernova explosions and gamma-ray bursts. If a star ejects consecutive winds at different velocities and with different densities, the wind-wind interaction creates a circumstellar nebula with a complex structure. These are isolated nebulae typically indicating that the star which created them is a fairly old object. Many circumstellar nebulae are observed in the Galaxy and in nearby systems such as the Magellanic clouds. Their luminosity is smaller than that of H II regions as they are powered by a single and older star. Massive stars end their lives as supernovae. The material ejected in stars catastrophic supernova explosions, comes back to the interstellar medium, contributing to the birth of new stars. Thin layers or filaments of gas surrounding the supernova, called supernova remnants can be observed. Most of their radiation energyis poweredby the thermal energy generated in shocks when the gas filaments movearoundand collide with the ambient gas or the interstellar medium. Thesis achievements This thesis constitutes the first generic study of the circumstellar