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Supernovae Interacting with Their Circumstellar Medium

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Supernovae Interacting with Their Circumstellar Medium Supernovae Interacting with their Circumstellar Medium Bob van Veelen c 2010 Bob van Veelen Alle rechten voorbehouden ISBN 978-90-393-5436-0 Cover image: A breaking wave due to the interaction of the wave with the shore. Similar to the subject of this thesis, this process is described by hydrodynamics. Supernovae Interacting with their Circumstellar Medium De Interactie van Supernovae met hun omgeving (met een samenvatting in het Nederlands) Proefschrift ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de rector magnificus, prof.dr. J. C. Stoof, ingevolge het besluit van het college voor promoties in het openbaar te verdedigen op 19 november 2010 des ochtends te 10.30 uur door Bob van Veelen geboren op 22 mei 1983 te Nijmegen Promotor: Prof.dr. N. Langer Contents 1 Introduction 1 1.1 General . 1 1.2 Stellar evolution and winds . 1 1.3 Supernova explosions . 4 1.4 The Interaction of supernovae with their CSM . 6 1.5 This Thesis . 8 2 The hydrodynamics of the supernova remnant Cassiopeia A 11 2.1 Introduction . 12 2.2 Method and Assumptions . 13 2.3 Results . 16 2.4 Discussion and conclusions . 24 3 The Destruction of Cosmological Minihalos by Primordial Supernovae 31 3.1 Introduction . 32 3.2 Numerical Method . 34 3.3 SN Progenitor HII Regions . 39 3.4 SN Blast Wave Evolution . 43 3.5 Observational Signatures of Primordial Supernovae . 56 3.6 Chemical Enrichment . 61 3.7 Discussion and Conclusions . 62 4 Circumstellar Interaction of Electron Capture Supernovae 67 4.1 Introduction . 68 4.2 Methods . 70 4.3 Shaping the CSM . 73 4.4 Exploding in a superwind . 76 4.5 Comparison with observations . 83 4.6 Summary . 88 5 Pulsating Red Supergiants as possible progenitors for type IIn super- novae 93 5.1 Introduction . 94 5.2 Stellar evolution models . 94 5.3 Method . 96 5.4 pre-supernova CSM evolution . 98 v vi Contents 5.5 Interaction of SN ejecta with the CSM . 100 5.6 Comparison with observations . 110 5.7 Discussion and conclusions . 113 Nederlandse Samenvatting 125 Acknowledgments / Dankwoord 129 Curriculum Vitae 131 Chapter 1 Introduction 1.1 General The night sky is not as static as once thought. Even though looking to the stars gives the impression that the night sky does not change, everything in the universe is most certainly in motion. While most events in the universe take place on such long time scales that we do not see any change within a human life time, there are events, typically more dramatic, which lead to changes in a split second. Among these most dramatic events are supernovae: exploding stars that out- shine everything (even entire galaxies) for periods of days. In some cases they become bright enough to be visible with the naked eye. Historically, the appear- ance and disappearance of stars has put an end to the notion of a never changing universe. With time came knowledge and some of these ’new stars’ are now known to be the end result of stellar evolution. In this thesis we will look at the evolution of supernovae. More specifically, we will investigate how the star, prior to the explosion, affects its surrounding medium and therefore influences the evolution of the subsequent supernova explosion. Dif- ferent types of supernovae are connected to different mass ranges, and therefore different evolutionary paths. This results in specific features in terms of the explo- sion energy, the ejected mass and the composition of the ejected mass, among other things. However, the progenitor evolution prior to the explosion also proceeds dif- ferently and has to be taken into account. By combining both the pre-supernova evolution of the circumstellar medium and the subsequent interaction with the su- pernova ejecta a better understanding can be gained regarding supernovae and their remnants. 1.2 Stellar evolution and winds This thesis focuses on single massive stars, which start their lives with a mass greater than approximately 8 times the mass of the sun (M⊙). Although less mas- 1 2 Chapter 1 ­23 log(d) [g cm­3] ­24 log(T) [K] MS shell ] ­25 ­3 ­26 contact discontinuity wind termination shock ­27 log(d) [g cm ­28 ­29 ­30 0 10 20 30 40 R [pc] Figure 1.1: Example of density and temperature structure of the circumstellar medium at the end of the Main Sequence of a 9 M⊙ star. The material surrounding the star which was first evenly distributed in the ISM, is now swept up into a cold, dense shell residing at 36 pc. The density in the interior region has dropped by roughly 5 orders of magnitude. sive stars can also change their surroundings significantly, they do not explode as supernovae. The most important effect these stars have on their surroundings, prior to the supernova explosion, is through the influence of their stellar wind. Mate- rial is blown away from the stellar surface which alters the shape of the interstellar medium (ISM). A star can lose a significant portion of its mass before exploding as a super- nova. The mass loss rate and the velocity of the stellar wind are determined by the conditions at the surface of the star, which change strongly during its life. The link between the surface properties of the star and its evolutionary state couple the different phases in stellar evolution with different mass loss phases. During the first ∼ 90% of its life, a star fuses hydrogen in its core into helium, the main sequence (MS) phase. This process creates the energy which is needed to compensate for the energy loss at the surface of the star. The majority of this energy loss is caused by the emission of electromagnetic radiation. A part of this energy loss however is also due to the stellar wind. During the MS phase, the star has a supersonic wind, which shocks and sweeps Introduction 3 up the ISM. The structure of the ISM during the MS phase can be separated into four different zones (see Fig.1.1). Moving from the outside inward we first find the undisturbed ISM. In the next zone, the swept up and shocked material from the surroundings of the star is accumulated into a cold dense shell, called the MS shell. This shell of shocked circumstellar material is separated from the third zone by a contact discontinuity1, which consists of shocked stellar wind. The shocked wind material has a very low density compared to the MS shell and a very high temperature, and is therefore referred to as the hot bubble. Much closer to the star is the wind termination shock which is the shock that slows down the free streaming wind which constitutes the last zone. For a more thorough review regarding a steady stellar wind interacting with a constant density ISM we refer to Weaver et al. (1977). When all the hydrogen in the core of the star has been converted into helium the star needs another source of energy. The star makes a transition from hydrogen fusion in the core to hydrogen fusion in a shell surrounding the core. Simultane- ously it also starts fusing helium into carbon in its core. During this transition from a main sequence star to the next burning phase, the star increases its radius while keeping its luminosity. The temperature at the surface of the star therefore goes down and the star becomes red. The star has become a red supergiant (RSG). During the RSG phase, the velocity of the stellar wind becomes lower and the mass loss rate increases (compared to the MS mass loss). The material lost during this phase accumulates into another shell which is located at the region where the ram pressure of the current stellar wind is equal to the thermal pressure of the hot bubble. This shell is called the RSG shell. For stars with masses between 8 and roughly 30 M⊙, the RSG phase is the final phase during which it will explode (Schaller et al. 1992, Woosley et al. 2002). For more massive stars the RSG will eventually become a Wolf-Rayet (WR) star. These stars have smaller radii and higher surface temperatures, which causes a high mass loss rate and a high wind velocity. Because the wind during the WR phase is fast, the shell that is created by the WR wind sweeping up the material lost during the RSG phase is very thin. Due to a hydrodynamical instabilities this shell will be unstable and fragment into smaller pieces (Garc´ıa-Segura et al. 1996a). If the WR phase lasts long enough it might even destroy the previously formed RSG shell. The picture painted above of the evolution of the circumstellar medium (CSM) already shows that the CSM of stars can take on a multitude of forms. The stellar wind can create multiple shells and large cavities, which will strongly influence the supernova ejecta moving through the different regions. Nevertheless, this picture remains a rather simple one. Changes due to the rotation of the star, magnetic fields, binarity or an inhomogeneous ISM can change the stellar wind. In extreme cases the mass loss might behave more like an eruptive event rather than a quiescent wind slowly blowing off material from the surface of the star. In some cases these eruptive events might even be mistaken for supernovae. Supernova 2006jc was seen to have a massive outburst, which was first taken for a supernova explosion, 2 years prior to the supernova actually occurring (Pastorello et al. 2007b). 1A transition along which the pressure is constant but the density may change.
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