Heidelberg, Universiẗat, Diss., 2002.

No description available.

Due to their initially metal-free composition, the fi rst stars in the Universe, which are termed Population III (Pop III) stars, were fundamentally different than later generations of stars. As of now, we have yet to observe a truly metal-free star although much effort has been placed on this task and that of nding the second generation of stars. Given they were the first stars, Pop III stars are expected to have made the fi rst contributions to elements heavier than those produced during the Big Bang. For decades signi cant mixing between H and He burning layers has been reported in simulations of massive Pop III stars. In this thesis I investigate this poorly understood phenomenon and I posit that interactions between hydrogen and helium-burning layers in Pop III stars may have had a profound impact on their nucleosynthetic contribution to the early universe, and second generation of stars. First, I examined a single massive Pop III star. This was done using a combination of stellar evolution and single-zone nucleosynthesis calculations. For this project I investigated whether the abundances in the most iron-poor stars observed at the time of publication, were reproducible by an interaction between H and He-burning layers. Here it was found that the i process may operate under such conditions. The neutrons are able to ll in odd elements such as Na, creating what is sometimes called the `light-element abundance signature' in observed CEMP stars. I also present the finding that it is possible to produce elements heavier than iron as a result of the i process operating in massive Pop III stars. A parameter study I conducted on H-He interactions in a grid of 22/26 MESA stellar evolution simulations is then described. I grouped these interactions into four categories based on the core-contraction phase they occur in and the convective stability of the helium-burning layer involved. I also examine in detail the hydrogen burning conditions within massive Pop III stars and the behaviour of the CN cycle during H-He interactions. The latter is compared to observed CN ratios in CEMP stars. Finally, I describe the first ever 4pi 3D hydrodynamic simulations of H-He shells in Pop III stars. I also examine the challenges in modelling such con gurations and demonstrate the contributions I have made in modelling Pop III H and He shell systems in the PPMStar hydrodynamics code. My contributions apply to other stellar modelling applications as well. / Graduate

This thesis investigates the potential for detection and identification of primordial stars, galaxies, and supernovae at high redshift. Simulations indicate that the first Population III stars should appear in minihalos of mass M = 105-106 Msol at z ≈ 10-30. To assess the detectability of these objects, theoretical models of these stars and their surrounding HII regions are used. We assess the plausibility of detection with the upcoming James Webb Space Telescope (JWST), using the gravitational lensing provided by the galaxy cluster MACSJ0717.5+3745. The conclusion is that the detection of these objects is highly improbable but not impossible. To investigate the prospects of detecting and identifying the first galaxies, the spectral synthesis code Yggdrasil is introduced. According to this code, JWST may be able to detect Population III galaxies with stellar masses as low as 105 Msol at z ≈ 10 in unlensed fields. We find that, over limited redshift intervals, it could be possible to use Hubble Space Telescope (HST) and/or JWST broadband color criteria to single out Population III galaxy candidates. The prospects of detecting gravitationally lensed Population III galaxies with JWST and HST is investigated. A lower limit to detect ≈1 Population III galaxy of ε ≈ 10-2 (HST/CLASH) and ε ≈ 10-3 (JWST using MACS J0717.5+3745 as lens) is derived, where ε is the baryon fraction converted to Population III stars in a host halo. By fitting HST/CLASH data to Yggdrasil and comparison grids, two Population III galaxy candidates are discovered. These two candidates are the first Population III galaxy candidates discovered at z &gt; 6.5. A highly-magnified and doubly lensed extremely high-redshift (z ≈ 7.8) object is also identified. Finally the prospects of detecting core-collapse (CC) supernovae (SN) from the first galaxies at z ≈ 5-12 are investigated. The prediction is that no primordial SN is detectable, but 2-3 CC SN should be discovered by the HST/CLASH. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 5: Submitted. Paper 6: Submitted. Paper 7: Manuscript.</p>

The chemical fingerprints of old, metal-poor stars can be used to unravel the events of the newborn Universe and help us understand the properties of the first stars and star clusters. The study of nearby stars to infer properties in the distant past is often referred to as Galactic archaeology. However, the chemical composition of stars cannot be observed directly, but must be inferred by means of spectroscopic modelling. Traditionally, this modelling utilises one-dimensional (1D) stellar atmospheres in hydrostatic and local thermodynamic equilibrium (LTE). Today, we know that departures from LTE (known as NLTE), and differences between 1D model atmospheres and their hydrodynamical three-dimensional (3D) counterparts, become increasingly severe at lower metallicity. The development of NLTE modelling of spectral line formation in 3D atmospheres is still in its infancy, but constitutes a remarkable step forward that has been made possible by parallelised codes and supercomputers. The central theme of this thesis is the application of NLTE analyses to metal-poor stars, to help usher the field of Galactic archaeology forward with important consequences for the nature of the first stellar generations. I present a theoretical NLTE study of aluminium, where I validate the analysis using a set of bright standard stars and provide calculated NLTE effects for a large parameter space. I perform 3D NLTE calculations for the solar spectrum to better constrain the zero-point of the cosmic abundance scale, and find excellent agreement with the meteoritic aluminium abundance. I also present NLTE analyses of metal-poor stars in the globular clusters NGC 6397 and M4. While globular cluster stars were long expected to form from a chemically homogeneous medium, star-to-star abundance variations of light elements indicate multiple epochs of star formation. Massive first-generation stars polluted the interstellar medium from which later generations formed, and I use the observed abundance variations to deduce the properties of the polluting stars. Among the heavier elements, I uncover evolutionary abundance variations that match predictions of stellar evolution models with atomic diffusion. The results indicate that the chemical abundance ratios of unevolved metal-poor stars are affected by gravitational settling, with a bias of the order 25-50 %, increasing towards lower metallicity. This atmospheric depletion mechanism is a probable explanation to why the stellar abundances of lithium fall short of the predictions from standard Big Bang nucleosynthesis. Finally, I apply a 3D NLTE abundance analysis to the red giant SMSS 0313-6708, which is the most iron-deficient star known. The chemical abundance pattern of this star indicates that it formed from gas affected only by Big Bang nucleosynthesis and a single faint supernova. Comparison of the inferred abundance pattern to theoretical predictions leads to constraints on the explosion mechanism and the mass of the metal-free progenitor star.

abstract: The formation of the firsts stars some 100-300 Myr after the Big Bang marked the end of the cosmic darks ages and created the elemental building blocks of not only rocky planets but eventually us. Understanding their formation, lifetimes, and contributions to the evolution of our universe is one of the current frontiers in astronomy and astrophysics. In this work I present an improved model for following the formation of Pop III stars, their effects on early galaxy evolution, and how we might search for them. I make use of a new subgrid model of turbulent mixing to accurately follow the time scales required to mix supernova (SN) ejecta -- enriched with heavy elements -- into the pristine gas. I implement this model within a large-scale cosmological simulation and follow the fraction of gas with metallicity below a critical value marking the boundary between Pop III and metal enriched Population II (Pop II) star formation. I demonstrate that accounting for subgrid mixing results in a Pop III stars formation rate that is 2-3 times higher than standard models with the same physical resolution. I also implement and track a new "Primordial metals" (PM) scalar that tracks the metals generated by Pop III SNe. These metals are taken up by second generation stars and likely result in a subclass of carbon-enhanced, metal-poor (CEMP) stars. By tracking both regular metals and PM, I can model, in post-processing, the elemental abundances of simulation stars. I find good agreement between observations of CEMP-no Milky Way halo stars and second generation stars within the simulation when assuming the first stars had a typical mass of 60 M☉, providing clues as to the Pop III initial mass function. / Dissertation/Thesis / Doctoral Dissertation Astrophysics 2018

La formation des premières étoiles, quelques centaines de millions d'années après le Big Bang, marque la fin de l’âge sombre. Actuellement, nous n’avons aucune observation de la formation de ces étoiles, appelée popIII, mais d’après des simulations numériques de différents groupes, il semblerait que ces étoiles primordiales étaient très massives: plusieurs centaines de masses solaires. Ces premières étoiles, ont produits aussi des sursauts gamma (GRBs). Ainsi, l’étude des GRBs produits à partir des popIII, pourraient permette d’étudier directement le stade final des étoiles primordiales. Les télescopes d'aujourd'hui ne peuvent pas regarder assez loin dans le passé cosmique pour observer la formation des premières étoiles, mais la nouvelle génération de télescopes permettra de tester des idées théoriques sur la formation des premières étoiles.Les GRBs sont liés à la mort d’étoiles massives et qu'ils sont connectés avec des supernovae. En ce sens, les GRBs sont l'une des classes de processus explosifs en physique stellaire et devraient suivre les mêmes lois physiques que l'explosion des supernovae. Ce travail tente d'aborder le problème des GRBs comme un problème d'explosion stellaire et utilise les données d’observation sur les spectres et les courbes de lumières notamment.Dans le cadre de cette thèse, des outils spécifiques ont été développés pour étudier les explosions stellaires: un code numérique pour résoudre les réactions nucléaires a été incorporé dans le code hydrodynamique existant. Ces outils ont été utilisés dans les simulations de supernovae afin d’étudier les connections avec les sursauts gamma: analyse spectrale et étude statistique en fonction du redshift. / The formation of the first stars hundreds of millions of years after the Big-Bang marks the end of the Dark Ages. Currently, we have no direct observations on how the primordial stars formed, but according to modern theory of stellar evolution these stars should be very massive (about 100 Msun) Population III stars have a potential to produce probably most energetic flashes in the Universe - gamma-ray bursts. GRBs may provide one of the most promising methods to probe directly final stage of life of primordial stars. Today's telescopes cannot look far enough into the cosmic past to observe the formation of the first stars, but the new generation of telescopes will test theoretical ideas about the formation of the first stars.Thanks to many years of observations we have good GRB's data -statistics of occurrence, spectrum, lightcurves. But there are still a lot of questions in the theory of GRBs. We know that GRBs are related to the death of stars and that they are connected with supernovae. So gamma-ray bursts are one of the classes of explosive processes in stellar physics that should have a lot of common with supernovae explosions. In that case GRBs should follow the same physical laws of explosion as supernovae. This work tries to approach the problem of GRBs as a problem of stellar explosion.Necessary instruments of studying stellar explosion were developed as a part of doctoral research: code for solving systems of nuclear reaction equations was incorporated into hydrodynamical code. These tools were applied for supernovae simulations in order to find possible connection with GRBs. Basing on analysis of supernovae simulations spectral analysis of GRBs was performed.

In this project we investigate the feasibility of detecting the signatures of Pop III stars in metal poor second generation stars and in gas clouds at high redshifts. First, the nucleosynthetic yields of Pair Instability Supernova and how they are manifested in gas clouds are presented. Next, some basic quantities of radio astronomy are explained and the requirements of SKA are shown. Then, the minimum detectable hydrogen column density of SKA for gas clouds at high redhsift is calculated and after that the basic principles of spectroscopy and the requirements of the HiReS instrument of E-ELT are demonstrated. Finally, suggestions about where the observations with HiReS should focus are made.

Through the spectral synthesis model YGGDRASIL, developed by E. Zackris- son et al. 2011, luminosities for the chemically pristine population III galaxies are retrieved and compared to the capabilities of the upcoming infrared telescopes, in particluar the Wide Field InfraRed Survey Telescope (WFIRST). In order to push the very faint galaxies into the detectable regime of the telescope, magnification by gravitational lensing needs to be introduced. The probabilities for the nec- essary magnifications at different redshifts in combination with the depth of the telescope are translated into required minimum number densities and minimum formation rates for detecting one object per survey area. Both photometric and spectroscopic detections are investigated and compared to limits predicted by theory. Three different initial mass functions (IMF) for the stars that make up the galaxy are used, two that are top-heavy, i.e centers around stellar masses of 10 − 500M⊙, and one that resembles the IMF that are used for stars of younger generations, with its distribution peak at below one solar mass. The most optimistic results comes from the model that focusses on a galaxy of very heavy stars (with a total galaxy mass of 105 − 106M⊙), that yields minimum number densities for photometric detection at z &gt; 10 that are lower or comparable to what has been predicted by theory. When the calculations are concerned with spectroscopy, the minimum number densities goes up and very massive galaxies (107M⊙) are required to reach the predicted limits at z &gt; 10 and to enable detection by WFIRST. A comparison between the upcoming infrared telescopes; WFIRST, James Webb Space Telescope (JWST) and Euclid, are performed with WFIRST as the strongest candidate for photometry and JWST as the preferred instrument for spectroscopy, with a strong dependence on the survey area.

No description available.