More than 80% of the stars in the Universe are expected to have initial masses below eight to ten times the mass of our sun. These low mass stars, including our sun, become cool red giants during one of the final evolutionary stages of their life: the Asymptotic Giant Branch (or AGB) phase. AGB stars are among the main producers of carbon and heavy (s-process) elements in the Universe. These elements are synthesized inside the star and mixed to the stellar atmosphere where stellar winds are responsible for the loss of more than 50% of the stellar mass, hence, AGB stars are strong polluters of the interstellar medium. The ejected material can clump together into dusty particles which may serve as ingredients for the birth of new stars and planets. When most of the AGB stellar envelope is lost, the AGB star stops releasing nuclear energy from interior processes and swaps its giant face for a planetary nebulae look, whereafter it fades away as a white dwarf.<p><p>The dredge-up of carbon and s-process elements into the AGB atmosphere causes an important chemical anomaly among them: initial oxygen-rich stars (M stars) are transformed into carbon-rich stars (C stars). As a consequence, a group of oxygen-rich AGB stars exists which makes the transition between M and C stars. These transition stars are classified as S.<p><p>Although AGB stars are identified as producers of heavy elements, their nucleosynthesis and mixing processes are weakly constrained due to large uncertainties on their estimated temperature, gravity and chemical composition. Stronger constraints on the atmospheric parameter space, hence interior processes, of AGB stars can be obtained by investigating the atmosphere of S stars. Since they are transition objects on the AGB, they trace the rise of the s-process. S stars are less numerous than C stars, but their optical spectra are brighter making it easier to identify atomic and molecular lines. Therefore, S stars belong to the most interesting objects along the AGB to perform this task.<p><p><p><p>From a practical point of view, the spectra of S stars are extremely difficult to study since they are dominated by different, overlapping molecular bands, and the spectral shape may vary strongly from star to star due to their transition status. Therefore, tailored model atmospheres for S stars are of utmost importance to understand the spectroscopic, and even photometric, changes in terms of variations in the atmospheric parameters. A comparison between the models and observations aims not only at constraining the atmospheric parameter space of S stars, it will also test the reliability of 1D state-of-the-art model atmospheres for such complex stars.<p><p><p><p>From an evolutionary point of view, the S-star family is contaminated with stars who gained their atmospheric enrichment in heavy elements from a companion star. Evidences were found that these binary S stars are not at all located on the AGB, hence, they are labelled as extrinsic S stars while S stars on the AGB are labelled as intrinsic. The difference in evolutionary stages between intrinsic and extrinsic S stars was already found 20 years ago, however, a separation in terms of surface temperature, gravity and chemical composition is not well-established due to the lack of S-star model atmospheres. Such a distinction in atmospheric parameters will facilitate the discovery of these intruders and even help to calibrate stellar evolutionary models of single and binary stars.<p>To achieve these goals, the first step consists in the construction of a grid of model atmospheres for S stars. The grid will be used to quantify the influence of atmospheric parameters on the model structure and emergent flux. These results will be analyzed to derive precise atmospheric parameters of observed S stars, using a set of well-defined photometric and spectroscopic indices. Once the best model atmosphere has been selected for all observed S stars, their atmospheric parameters will be discussed in view of their evolutionary stage. The best-fitting model atmosphere will also be used to derive abundances from spectral syntheses. The abundance profiles are compared with stellar evolution model prediction to constrain nucleosynthesis and mixing processes inside S stars. Derived abundances of unstable elements will be used to estimate, for the first time, the age of AGB stars. Finally, their abundance profile will be discussed as a function of their time spent on the AGB. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

The discovery of the Spite plateau in the abundances of 7Li for metal-poor stars led to the determination of an observationally deduced primordial lithium abundance. However, with the determination of the baryon density, Omega_B_h^2, from the Wilkinson Microwave Anisotropy Probe (WMAP) data, a discrepancy arose between observationally determined and theoretically determined abundances of 7Li. This is what has become known as the “lithium problem”. Of all the uncertain factors in determining a stellar Li abundance, the effective temperature is the most important. This thesis is concerned with determining an accurate effective temperature scale for metal-poor halo dwarfs, paying specific attention to eliminating any possible systematic errors. This is done by utilising the exponential term, Chi/T, of the Boltzmann equation. Two assumptions are adopted; firstly the simplifying assumptions of local thermodynamic equilibrium (LTE), and secondly the more sophisticated techniques of non-local thermodynamic equilibrium (NLTE). The temperature scales are compared to others derived using different techniques; a photometric scale, where I find comparable Teff in LTE and hotter temperatures by an average of ~ 150 K in NLTE; a scale derived using Balmer lines, for which I have comparable values in LTE and hotter Teff values, by typically 110 K – 160 K, in NLTE; and finally a scale derived using an infrared flux method (IRFM). Here I find their Teff values are hotter by ~ 250 K for LTE and ~ 190 K in NLTE. Lithium abundances are then calculated for the program stars and a mean Li abundance is derived. I find values ranging from A(Li) = 2.10 dex – 2.16 dex with the LTE scales and A(Li) = 2.19 dex – 2.21 dex for the NLTE scales. These mean Li abundances are compared to other observationally deduced abundances, for which I find comparable results in LTE and higher values in NLTE, and to the WMAP + big bang nucleosynthesis calculated Li abundance. I find that my new values are still considerably lower than the WMAP value and are therefore unable to reconcile the lithium problem. Second to this primary investigation, I use Ti as an independent test of the derived Teff values and log g’s. I find that Ti is not a useful constraint on the temperatures or, therefore, on the lithium problem. I also assess the impact of the new Teff scales on the different models of Galactic chemical evolution (GCE), comparing newly calculated abundances with GCE determined abundances. It was found that trends exist in several of the elements; however, these were not statistically relevant. Also a larger degree of scatter was found in the abundances compared to the Arnone et al. (2005). This scatter was not to the degree found in the Argast et al. (2000). Reasons for the differences have been discussed.

The principal theory concerning the origin of the elements heavier than the Fe-peak, such as Ba, strongly suggest that for old, metal-poor environments, the rapid (r-) process is the most likely path taken in their synthesis, while the slow (s-) process becomes more substantial in younger, more metal-rich stellar populations. In this work I test this theory by evaluating the isotope ratios of Ba. It is understood that Ba consists of seven stable isotopes, five of which are synthesised by the two neutron-capture processes. The two odd isotopes, 135,137Ba, as well as 138Ba are synthesised via both the r- and s-processes while two of the even isotopes, 134,136Ba are synthesised via the s-process only. The relative contribution of the r- and s-process to these isotopes can be understood via nucleosynthesis calculations and is described using the parameter fodd, where fodd = [N (135Ba) + N (137Ba)] /N (Ba). Low values of fodd (~0.11) indicate an s-process regime, while high values of fodd (~0.46) indicate an r-process regime. In the Ba II 4554 A line the even isotopes lie close to the line centre, while the odd isotopes, which are hyperfine split because of their non-zero nuclear spin, lie in the wings of the line. From an analysis of the line profile shape, one can determine whether Ba has been synthesised primarily through the r-process or s-process; a broad, asymmetric line would indicate a high r-process contribution, while a line with a deeper core and shallower wings would indicate a high s-process contribution. Using the radiative transfer code ATLAS, which assumes local thermodynamic equilibrium (LTE) and employs 1-dimensional (1D) KURUCZ06 model atmospheres, I synthesised line profiles for six metal-poor stars: HD140283, HD122563, HD88609, HD84937, BD-04 3208 and BD+26 3578 - for a range of isotope ratios. All six are of sufficiently low metallicity that Ba was expected to have an r-process origin. These were fit to high resolution (R\equiv \lamda/\Delta\lamda = 90 000 - 95 000), high signal-to-noise to the Ba II 4554 A line which has multiple components. In the first test, synthetic spectra were computed using the non local thermodynamic equilibrium (NLTE) radiative transfer code MULTI. The synthetic line profiles were fit to a number of lines in HD140283. Although this technique might have improved the fit in the line core, it was found that such a treatment did not improve upon fitting errors associated with the best fit 1D LTE synthetic profiles. The second test used a 3-dimensional (3D) radiative transfer code (LINFOR3D) that employed 3D, time-dependent atmospheres produced with CO5BOLD. The 3D synthetic pro les were fit to a selection of Fe lines and improvements over the poor fits produced by the 1D LTE synthesis were seen. It was found that the 3D synthesis could almost completely reproduce the line asymmetries seen in the observed stellar spectrum. This result suggests that further work to refine the 3D calculations and synthesis code would be valuable.

Classical nova explosions are stellar explosions that take place in close binary systems with an energy release only exceeded by gamma-ray bursts and supernova explosions. Matter from the white dwarf flows through the inner lagrangian point and spirals in towards the white dwarf for about 10^4-10^5 years, forming an accretion disk around it. Ultimately, part of this hydrogen-rich matter piles-up on top of the compact object and becomes partially degenerate due to the high densities attained. Consequently, temperature is allowed to rise, but the envelope does not experience any expansion. Actually, this is the key mechanism that controls the subsequent phases and powers a thermonuclear runaway, which is followed by an ejection of part of the accreted envelope. The ejecta are enriched with the products from the nuclear processes, presenting a final metallicity much above solar. This model, introduced in the early 70s, is a solid theory that can account for the gross scenario of nova explosions. Nevertheless, the theory relies on the fact that a mixing episode with matter from the white dwarf core has to take place at the core-envelope interface to successfully account for the high metallicities inferred from observations. During the past 40 years, theoreticians have performed many one-dimensional simulations, which can reproduce the abundances in the ejecta and other important observational properties. However, these calculations performed in spherical symmetry cannot study the mixing process, since they exclude a suite of very important multi-dimensional effects, such as convection. Therefore, multi-dimensional calculations are required to shed light into the mixing episode. In this thesis we have performed two- and three- dimensional simulations of CO novae to study the mixing mechanisms operating at the core-envelope interface, how convection sets in and how the deflagration spreads over the domain, by means of the Eulerian, parallelized, hydrodynamical FLASH code. The two-dimensional results show how convection sets in at the innermost envelope layers, after the appearance of temperature fluctuations that arise from the interface. Convection, in turn, powers the formation of kelvin-Helmholtz instabilities, which efficiently dredge-up 12C from the core and carry it into the envelope, reproducing correctly the high metallicity found in the ejecta. This result solves the controversy generated by the two existing two-dimensional calculations up-to-date. We have also realized a sensitivity study to analyze the impact of some initial parameters, such as the temperature perturbation, resolution of the simulations and the size of the computational domain. The results point out that these parameters have a negligible impact on the degree of mixing and, therefore, the calculations are not affected by numerical artifacts. Although two-dimensional calculations can quantitatively reproduce the mixing episode, they cannot describe correctly the convective pattern due to conservation of vorticity, which translates into recombination of the convective cells. Therefore, we have extended the work to three dimensions and performed the first three-dimesional model of mixing in classical novae up-to-date. These calculations can successfully reproduce the intermittency present in turbulent convection, with an energy cascade into smaller scales which clearly fulfills the Kolmogorov theory, while the thermonuclear runaway continues propagating with almost spherical symmetry. Mixing proceeds through the filamentary structure powered by robust kelvin-Helmholtz instabilitites that arise from the interface, resulting in a CNO enhancement which agrees with observations. This convective profile also generates density contrasts that could be the origin of the inhomogeneous distribution of chemical species. / Les explosions de noves tenen lloc en un sistema estel.lar binari, on un dels estels ha arribat a la fi de la seva vida convertit en una nana blanca. En sistemes binaris molt propers, l'estel acompanyant cedeix part del seu gas (material ric en hidrogen), el qual s'arremolina al voltant de la nana blanca durant prop de 10^4 - 10^5 anys. Una fracció d'aquest material acaba apilant-se a la superfície de l'objecte compacte i esdevé parcialment degenerat com a conseqüència de l'elevada densitat. Aquest fet és clau en el procés, ja que permet que la temperatura augmenti sense que es produeixi una expansió de l'embolcall, desencadenant un allau termonuclear i finalment, l'ejecció de matèria. El material ejectat està enriquit amb els isòtops processats en les reaccions nuclears, presentant una metal.licitat molt superior a la solar. Aquest model, presentat a principis dels anys 70, és una teoria sòlida que explica raonablement l'explosió de noves. No obstant, la teoria rau en el fet que s'ha de produir un procés de barreja entre el material de la nana blanca i el material de les capes més internes de l'embolcall per poder explicar l'alta metal.licitat que s'observa en el material ejectat. Durant els últims 40 anys, s'han fet molts estudis en una dimensió que aconsegueixen reproduir correctament les abundàncies del material ejectat i altres importants propietats observacionals, però que no poden explicar com es produeix el procés de barreja, ja que aquests càlculs amb simetria esfèrica exlouen tota una sèrie d'importants fenòmens multidimensionals. Per tant, per estudiar aquests aspectes de la teoria es requereixen estudis multidimensionals. En aquesta tesi hem realitzat simulacions en dues i tres dimensions de noves de CO per estudiar els mecanismes de barreja que es produeixen a la interfície del nucli de la nana blanca i l'embolcall, com s'estableix la convecció i com es propaga el front deflagratiu, mitjançant el codi hidrodinàmic FLASH, que és Eulerià i està paral.lelitzat. Els resultats en dues dimensions mostren com es genera convecció a les capes més internes de l'embolcall, després de la formació de fluctuacions de temperatura a la interfície. La convecció, al seu torn, origina inestabilitats Kelvin-Helmholtz que transporten eficientment 12C del nucli cap a l'embolcall, aconseguint reproduir correctament el grau de metal.licitat observat. Aquest resultat resol la controvèrsia generada pels dos estudis en dues dimensions realitzats fins ara. També hem realitzat un estudi per analitzar l'impacte dels paràmetres inicials tals com la perturbació inicial, la resolució de les simulacions o les dimensions del domini computacional. Els resultats indiquen que cap d'aquests paràmetres influeix en el grau de barreja final i, per tant, que els càlculs no estan condicionats per aspectes numèrics. Finalment, hem presentat el primer model tridimensional de barreja de noves fet fins ara. Aquest càlcul és necessari, ja que les simulacions bidimensionals, tot i que quantitativament reprodueixen la barreja esperada, no poden representar el patró convectiu correctament, degut a la conservació de la vorticitat, fent que les cel.les convectives esdevinguin cada cop més grans. El nostre càlcul aconsegueix reproduir el comportament intermitent de la turbulència, amb una cascada d'energia que flueix cap a escales cada cop més petites, tal i com prediu la teoria de Kolmogorov, alhora que el front convectiu avança pràcticament amb simetria esfèrica. La barreja procedeix a través de l'estructura filamentosa originada per l'aparició de potents inestabilitats Kelvin-Helmholtz a la interfície, obtenint-se una metal.licitat final a l'embolcall que concorda amb els valors observacionals. Aquest patró convectiu també genera contrastos de densitat que podrien ser l'origen de la distribució inhomogènia que presenten les espècies químiques.

Structural analyses of DNA oligonucleotides indicate the presence of bound water molecules in the major and minor grooves of DNA. These water molecules participate in DNA charge transfer by their reaction with guanosine radical cation to form 7,8-dihydro-8-oxo-guanine (8-oxoG), which when treated with a base leads to DNA strand cleavage. We probed the reaction of guanosine radical cation with water with series of alkyl substituted cytidines and thymidines by incorporating the modified nucleosides into anthraquinone linked DNA duplexes and irradiating them with UV light at 350 nm. The incorporation of these hydrophobic substituents disrupt the DNA spine of hydration, and we have observed that these modifications in the major and minor groove do not effect the trapping or long distance hopping of radical cations in DNA. The second part of the work reported herein examines the role of charged nucleosides in long range charge transfer in duplex DNA. DNA methylation is a naturally occurring process mediated by enzymes responsible for such functions in biological systems. Hypermethylation of DNA can also occur as a result of environmental alkylating agents leading to mutation of the affected cells. Methylation of the ring nitrogen of a purine base can introduce a positive charge in the ring resulting in the cleavage of the glycosidic bond of the nucleoside. To understand the role of a charged nucleoside on charge transfer in DNA, we designed and synthesized cationic nucleoside mimics, which were incorporated into anthraquinone-linked DNA strands and irradiated at 350 nm. The presence of the cationic bases on the duplexes inhibits the migrating hole from hopping along the DNA strand, and induces a prominent local structural distortion of the DNA as a result of the charged nucleobase.

Nuclear reactions induced by neutrons, protons and alpha particles on copper isotopes are being studied in an effort to understand the nucleosynthesis of elements in stars, specifically, the p-process. The p-process occurs toward the end of a star's life and produces those elements which have a high proton to neutron ratio, which are heavier than iron. Little is known about the nature of the p-process-inside stars. Isotopes of copper are studied since they are close in mass number to iron, which has the highest nuclear binding energy. Nuclear cross sections will be calculated for copper, as well as S-factors, which are important from an astrophysical point of view. These values are needed to calculate reaction rates which are the main ingredients for understanding nucleosynthesis. / Department of Physics and Astronomy

The existence of charged electroweak-scale particles in the early universe can drastically affect the evolution of elemental abundances. Through the formation of Coulombic bound states with light nuclei, these exotic relic particles (hereafter referred to as X–) act to catalyze nuclear reactions by reducing their threshold energies. This thesis examines the properties of the X– bound states, and uses primordial element observations to constrain the abundance, lifetime, and mass of this exotic particle species. If the X– is a Dirac Fermion, its abundance relative to baryons is found to be YX- ~ 0.01, with a lifetime of 1500s ≤ τX- ≤ 3000s, and a mass of order 100 GeV. Assuming that the X– is a Scalar particle that decays into gravitinos, the resulting bounds become, 5x10-4 ≤ YX- ≤ 0.07, 1600s ≤ τX- ≤ 7000s, and 60GeV ≤ mX- ≤ 1000GeV. These ranges are consistent with Dark Matter constraints.

The R Coronae Borealis (RCB) stars are an enigmatic class of hydrogen-deficient supergiant stars, which along with the companion classes of Hydrogen-deficient Carbon (HdC) stars and Extreme Helium (EHe) stars, have been touted as being a result of mergers of low mass carbon-oxygen (CO) and helium (He) white dwarfs. Such mergers of white dwarfs are expected to be the genesis of several interesting stellar objects such as Type Ia supernovae, neutron stars and AM CVn stars, amongst others. The RCBs, HdCs and EHes are mostly near-solar mass single stars, which along with having predominantly helium atmospheres that are extremely exhausted in hydrogen and rich in carbon, are also host to some extraordinary nuclear isotopic ratios. The RCBs and EHes have 12C/13C >= 100, enhancements of up to 3 orders in fluorine compared to solar and significant amounts of s-process elements. The most outstanding characteristic of RCBs is that they, along with the HdCs, have the lowest O-isotopic ratios measured in any star in the Universe viz., 16O/18O ~ 1-10. We perform nucleosynthesis calculations with conditions found in the three-dimensional hydrodynamic simulations of CO and He WD mergers and compare the nuclear yields thus obtained with those measured in the surfaces of RCB stars. We do not find an agreement between the calculated yields and the measured ones and thus conclude that RCBs are not formed immediately after the merger of the white dwarfs. This leads us to surmise that the surface chemical composition of RCBs may be due to the result of nuclear processes occuring in a longer evolutionary period following the merger. To this end, we first construct chemical compositions of the merged white dwarfs based on the results of the hydrodynamic simulations. We then impose these compositions on homogeneous, spherically symmetric, one-dimensional stellar models and evolve these models through the giant phase of RCBs. Along with convection zones that develop in the stellar envelope, we induce a continuous envelope mixing profile that is meant to represent processes related to rotation in these merged objects. We then analyse the nuclear yields from the surface of these models and compare them with those of RCBs. Our models achieve the aforementioned striking characteristics of RCBs, viz., the low O-isotopic ratios, high C-isotopic ratios, high fluorine and s-process element enhancments. Along with these, for the first time, we have reproduced simultaneously, the range in observations of almost all the other elements measured in RCBs. Moreover, our one-dimensional models also place useful constraints on so far unexplored three-dimensional processes, thus providing directives for future studies about them. / Graduate

Massive stars live fast and die young. They shine furiously for a few million years, during which time they synthesize most of the heavy elements in the universe in their cores. They end by blowing themselves up in a powerful explosion known as a supernova (SN). During this process, the core collapses to a neutron star or a black hole, while the outer layers are expelled with velocities of thousands of kilometers per second. The resulting fireworks often outshine the entire host galaxy for many weeks. The explosion energy is eventually radiated away, but powering of the newborn nebula continues by radioactive isotopes synthesized in the explosion. The ejecta are now quite transparent, and we can see the material produced in the deep interiors of the star. To interpret the observations, detailed spectral modeling is needed. This thesis aims to develop and apply state-of-the-art computational tools for interpreting and modeling SN observations in the nebular phase. This requires calculation of the physical conditions throughout the nebula, including non-thermal processes from the radioactivity, thermal and statistical equilibrium, as well as radiative transport. The inclusion of multiline radiative transfer, which we compute with a Monte Carlo technique, represents one of the major advancements presented in this thesis. On February 23 1987, the first SN observable by the naked eye since 1604 exploded, SN 1987A. Its proximity has allowed unprecedented observations, which in turn have lead to significant advancements in our understanding of SN explosions. As a first application of our model, we analyze the 44Tipowered phase (t &amp; 5 years) of SN 1987A. We find that a magnetic field is present in the nebula, trapping the positrons that provide the energy input, and resulting in strong iron lines in the spectrum. We determine the 44Ti mass to 1.5(+0.5−0.5)*10−4 M⊙. From the near-infrared spectrum at an age of 19 years, we identify strong emission lines from explosively synthesized metals such as silicon, calcium, and iron. We use integral-field spectroscopy to construct three-dimensional maps of the ejecta, showing a morphology suggesting an asymmetric explosion. The model is then applied to the close-by and well-observed Type IIP SN 2004et, analyzing its ultraviolet to mid-infrared evolution. Based on its Mg I] 4571 Å, Na I 5890, 5896 Å, [O I] 6300, 6364 Å, and [Ne II] 12.81 mm nebular emission lines, we determine its progenitor mass to be around 15 M⊙. We confirm that silicate dust, SiO, and CO have formed in the ejecta. Finally, the major optical emission lines in a sample of Type IIP SNe areanalyzed.We find that most spectral regions in Type IIP SNe are dominated by emission from the massive hydrogen envelope, which explains the relatively small variation seen in the sample. We also show that the similar line profiles seen from all elements suggest extensive mixing occurring in most hydrogenrich SNe. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Accepted.

The existence of charged electroweak-scale particles in the early universe can drastically affect the evolution of elemental abundances. Through the formation of Coulombic bound states with light nuclei, these exotic relic particles (hereafter referred to as X–) act to catalyze nuclear reactions by reducing their threshold energies. This thesis examines the properties of the X– bound states, and uses primordial element observations to constrain the abundance, lifetime, and mass of this exotic particle species. If the X– is a Dirac Fermion, its abundance relative to baryons is found to be YX- ~ 0.01, with a lifetime of 1500s ≤ τX- ≤ 3000s, and a mass of order 100 GeV. Assuming that the X– is a Scalar particle that decays into gravitinos, the resulting bounds become, 5x10-4 ≤ YX- ≤ 0.07, 1600s ≤ τX- ≤ 7000s, and 60GeV ≤ mX- ≤ 1000GeV. These ranges are consistent with Dark Matter constraints.