Modelling of the galactic chemical evolution of r-process elements

Are we made of star dust? This question which already guided me during my master studies could be a good motivation to justify the efforts taken in this work. The Big Bang made space itself and time itself. Also, a lot of energy was put in the newly formed space. At that time, space was only filled with high energetic photons, so there was only ”light”. After the inflation phase, parts of the light transformed to quarks and gluons which formed a plasma. After 10−6 seconds, baryons could form from the quarks and gluons. However, from that point of time until 10−4 seconds, only matter and anti-matter pairs formed, which annihilated shortly after their formation. A very tiny imbalance, an excess of only one matter constituent per 10 billion matteranti matter pairs permitted that, after 10−4 seconds, matter could become the dominant species. Since the temperature dropped, the large hadrons decayed and formed a soup of protons and neutrons. They were evenly abundant and due to the high temperature, could still transition from one to the other. In these processes also neutrinos were formed and absorbed. After one second, neutrons could not transform to protons any more and vice versa. The neutrinos decoupled and hence stopped interacting with matter. At that time, the ratio between protons and neutrons was 6:1. Deuterons (2H) could form, but were almost immediately disintegrated due to high energetic photons. When a minute had passed, the temperature had become so low, that not too much of these high energetic photons were around any more. So, deuterons could not form effectively during that time. Since (free) neutrons decay very fast, the proton to neutron ratio decreased further to 7:1. From here, most of the neutrons became bound in a 4He nucleus. Since there is 1 no stable nucleus with mass number 5 or 8, only traces of 7Li and 3He could be formed. After five minutes, the density in the Universe had dropped so far that further nuclear reactions were not possible at that time. From here on, the primordial nucleosynthesis had come to an end, resulting in a mass ratio of approx. 75% 1H, and approx. 25% 4He, and traces of 3He and 7Li. Those neutrons unable to find a reaction partner decayed spontaneously after a couple of minutes. Since only 1H, 4He, and traces of 3He and 7Li were formed, but we observe and actually consist of heavier elements as carbon and oxygen, and some heavy elements, we might ask the question: So, if they were not formed in the beginning, where were they actually formed? The answer to that is not too easy and still debated. While the situation of the formation of elements more massive than helium up to iron is relatively well-known, the situation beyond iron is still under investigation. This thesis tries to contribute a tiny piece to the big puzzle of solving the riddle where the heaviest elements were actually formed. Considering the path from the Big Bang nucleosynthesis up to iron, stars are the key ingredient. Stars can be seen as giant ”pressure cookers”: They are formed from clouds of interstellar matter which contracted gravitationally to form gas balls. Under certain conditions, the inward pull of the selfgravitating gas is high enough to trigger nuclear fusion reactions in the core of the gas ball. When this gas ball has started its reaction(s), we call this ball a ”star”. The mass of a newly born star will (among others) determine the way how it will burn and ultimately determine its faith. The lowest end of the mass spectrum of stars (low mass stars, LMS) starts with approx. 0.07 Solar masses (or the equivalent of approx. 75 Jupiter masses), since at this point, the requirements to trigger the hydrogen fusion in the core are met. Stars of this mass only process hydrogen to helium and are unable to perform further processing, extinguish their nuclear burning and just cool off. In the range approx. 0.8 Solar masses and approx. 8 Solar masses (intermediate mass stars, IMS, including our Sun) are also able to burn the produced helium to carbon and 2 Chapter 1 oxygen. After the hydrostatic hydrogen burning (the stage which our Sun is actually in), these stars start to ”pulsate”, or ignite ”flashes”, and loose their outer envelopes via stellar winds, forming the beautiful objects called ”planetary nebulae”. When the nuclear ”fuel” is consumed or densities are not sufficient any more, these stars ultimately end as ”white dwarfs”, being very hot in the beginning, but radiating their heat away and slowly cool off until they reach their surroundings temperature. Stars more massive than approx. 8 Solar masses (high mass / massive stars, HMS) go through all stellar burning stages including hydrogen-, helium-, carbon-, neon-, oxygen-, and silicon burning, then collapse under their own weight and die in giant explosions which are commonly referred to as ”supernovae”. Supernovae might also be triggered by binary systems of intermediate mass stars when the conditions are met. However, this mechanism is completely different than the mechanism of the central collapse of massive stars. What both ”types” of these supernovae have in common is that the progenitor star gets disrupted (partially leaving a neutron star or black hole behind) and blast processed elements into the interstellar medium. Stars more massive than 300 Solar masses (the actual limit is depending on many parameters and also strongly debated) collapse directly to a black hole. Neutron stars (the name is actually misleading since they are neither purely consisting of neutrons nor are they ”stars” in the sense that they ”shine” due to nuclear processes) are very dense and interesting objects which are produced during core collapse supernovae, and if they are born in a double star system, there is a possibility that they merge with their companion. These neutron star merger events are extremely violent processes where the conditions are met for the onset of mechanisms that are able to synthesize nuclei more massive than iron. However, since they require two evolved stars being gone through all their stellar burning stages and having gone supernova, there is one striking open question about the actual contribution of these systems to the nucleosynthesis of heavy elements remaining: Since we already observe stars being born at an early stage of our Galaxy’s history, but neutron star mergers require 1.1 Motivation 3 all prerequisites to be met, which takes lots of time, is the contribution of neutron star mergers possibly to late? This thesis intends to address a portion of this open question of the nucleosynthesis of the heaviest elements. In the following, the (relative or exclusive) nucleosynthesis contribution of neutron star mergers is tested as well as alternative sites for the formation of the heaviest elements, with a chemical evolution model of our Galaxy. This thesis is organized as follows. • In chapter one, a general introduction and the relevant nuclear/hydro physics, structure and observational properties of stars and the interstellar medium is presented. This chapter is loosely oriented on D. Arnett, ”Supernovae and Nucleosynthesis” (1996), T. Padmanabhan, ”Theoretical Astrophysics, Volume I: Astrophysical Processes” (2000), T. Padmanabhan, ”Theoretical Astrophysics, Volume II: Stars and Stellar Systems” (2001), F. Matteucci, ”The chemical evolution of the Galaxy” (2001), A. Weigert, H. J. Wendker, L. Wisotzki, ”Astronomie und Astrophysik” (2006), Cowan, Thielemann & Truran, ”The Nuclear Evolution of the Universe” (in prep.), and the lecture notes of nuclear astrophysics held by Thomas Rauscher (fall semester 2011 and spring semester 2012). • Chapter two consists of a detailed explanation of our chemical evolution model and an application to the formation scenario of the heaviest elements. This chapter has been previously published in Monthly Notices of the Royal Astronomical Society, Volume 452, 1970 ff., with Marco Pignatari and Friedrich-Karl Thielemann as co-authors. • Chapter three describes the application of our model to the Draco dwarf galaxies which orbits the Milky Way