We study the influence of convection and non-local thermodynamic equilibrium (NLTE) on the formation of spectral lines of different chemical elements in the atmospheres of different metallicity main sequence turn-off (MSTO) and red giant branch (RGB) stars. The influence of convection is assessed by analyzing differences in the elemental abundances obtained by using 3D hydrodynamical and 1D hydrostatic stellar model atmospheres of MSTO and RGB stars, within the framework of spectral line formation under the assumption of local thermodynamic equilibrium (LTE). We find that the influence of convection on the formation of spectral lines of C, O, Si, Ti, Fe, and Ni is largest at lowest metallicities, both in the atmospheres of TO and RGB stars: the differences in the abundaces predicted by the 3D hydrodynamical and 1D hydrostatic stellar model atmospheres may reach to –1.1 and –0.8 dex in the TO and RGB stars, respectively. The analysis of 3D hydrodynamical and NLTE effects in the atmospheres of stars that belong to Galactic globular clusters 47 Tuc and NGC 6752 shows that NLTE effects have significantly larger impact on the formation of O and Na spectral lines than the effects related to convection do. Finally, we determine 3D + NLTE abundances of Li, O, Na, and Ba in the atmospheres of TO and RGB in Galactic globular clusters 47 Tuc and NGC 6752, investigate the connections between the abundances of different chemical species, and discuss the possible chemical evolution... [to full text] / Disertacijoje nagrinėjama konvekcijos ir nelokalios termodinaminės pusiausvyros (NLTP) įtaka įvairių cheminių elementų spektro linijų formavimuisi skirtingo metalingumo pagrindinės sekos posūkio taško (PSPT) bei raudonųjų milžinių sekos (RMS) žvaigždžių atmosferose. Konvekcijos įtaka spektro linijų formavimuisi buvo tirima analizuojant cheminių elementų gausos įverčių, gaunamų taikant 3D hidrodinaminius ir 1D hidrostatinius PSPT bei RMS žvaigždžių atmosferų modelius bei lokalios termodinaminės pusiausvyros (LTP) artinį, skirtumus. Gauti rezultatai rodo, jog konvekcijos įtaka C, O, Si, Ti, Fe, and Ni spektro linijų formavimuisi yra didžiausia mažiausio metalingumo PSPT ir RMS žvaigždžių atmosferose, o gausų skirtumai tarp 3D hidrodinaminių ir 1D hidrostatinių modelių prognozuojamų cheminių elementų gausų gali siekti atitinkamai iki –1.1 ir –0.8 dex. 3D hidrodinaminių ir NLTP reiškinių analizė Galaktikos kamuoliniams spiečiams 47 Tuc ir NGC 6752 priklausančių žvaigždžių atmosferose parodė, jog NLTP efektų įtaka deguonies ir natrio spektro linijų formavimuisi šių žvaigždžių atmosferose yra žymiai didesnė už konvekcijos įtaką. Disertacijoje taip pat gauti lengvųjų cheminių elementų (Li, O ir Na) bei bario gausų įverčiai Galaktikos kamuolinių spiečių 47 Tuc ir NGC 6752 žvaigždžių atmosferose, ištirtos cheminių elementų gausų tarpusavo sąsajos, bei aptarti galimi spiečių cheminės raidos scenarijai.

A parametric model of granulation employing a small number of parameters was developed. Synthetic spectra calculated using this model closely match observed spectra and, in particular, reproduce the asymmetries observed in spectral lines. Both the microturbulent motions and the large-scale flow velocity decrease exponentially with a scale height of 368 km as the height within the photosphere increases. The model agrees with observations of the solar granulation (from which it was derived). The horizontal motions associated with granulation were found and used to calculate spectra emergent away from disk centre. These calculated spectra were compared to observed spectra, with the agreement supporting the accuracy of the granular model. Also in the course of this work, the Brueckner-O'Mara damping theory was found to predict damping constants accurately. The photospheric abundances of a number of elements were determined. The abundance obtained for iron agrees with the meteoric iron abundance. Astrophysical f-values for some lines were also determined.

The Galactic disk is home of many billion stars, one of which isour Sun. The stellar population of which the Sun is a member residesin the vertically thin spiral structure of the disk. There is a seconddisk population, the so-called thick disk, that has somewhat dierentspatial, kinematic and chemical properties as compared to the thindisk. It may be systematically older than the thin disk (Bernkopf et al.2001), with a star-formation hiatus separating the two. Observationsof thick-disk subgiants allow us to probe the chemical properties ofthese stars. As the subgiant evolutionary phase is short, age-datingthese stars is also possible. Are they in fact systematically older thanthe oldest thin-disk stars? This project will take rst steps towardsanswering this question based on new target selections done on DataRelease 1 of the Gaia mission. / Vintergatans disk innehåller flera miljarder stjärnor, varav en är vår Sol. Den stjärnbefolkning som Solen är medlem i ligger i den vertikalt tunna spiralstrukturen på skivan (tunna disken). Det finns en andra diskpopulation, den så kallade tjocka disken, som har något annorlunda rumsliga, kinematiska och kemiska egenskaper jämfört med den tunna disken. Den kan vara äldren än tunna disken (Bernkopf et al. 2001), med en stjärnbildningsstopp som skiljer de två. Genom att observera subjättar som är en del av den tjocka disken blir det möjligt att analysera dessa stjärnors kemiska egenskaper. Eftersom subjätte grenen är en kort evolutionär fas, åldersbestämmelse är också möjlig för dessa stjärnor. Är de faktiskt systematiskt äldre än de äldsta tunna diskens stjärnor? Detta projekt kommer att ta de första stegen mot att svara på denna fråga baserat på nya målval som gjorts på Data Release 1 från Gaia-uppdraget.

Circular polarization from the Zeeman effect is difficult to detect whenever the ratio of the magnetic field strength to the linewidth is small, as might be expected in the winds of hot stars. However, globally structured fields, such as radially swept fields, do present a characteristically complex Zeeman signature that can be distinguished from noise even when small, because of its telltale features that are antisymmetric about the line. If the emission lines are skewed blueward, a signature of photospheric occultation of the redshifted hemisphere, we find that occultation will also reduce the detectability of the Zeeman effect on the red side of the line, further limiting our ability to detect weak magnetic fields in hypersonic winds. Hence, as instrumental precision improves sufficiently, symmetric emission lines will present advantages over lines skewed blueward by occultation, when seeking a Zeeman signal.

No description available.

<p>Stellar atmospheres constitute excellent environments to study the chemical evolution of our Galaxy. The chemical composition of these atmospheres reflects the composition of the gas from where these stars were born. As the Galaxy evolves, the composition of the gas changes from being primordial (Big-Bang nucleosynthesis) to being enriched in heavy elements (stellar and interstellar nucleosynthesis). The abundances of fragile chemical elements can be affected by stellar mixing processes. Precise lithium, beryllium and oxygen abundance determinations in old stars are presented in this thesis. These determinations are based on the analysis of the observed spectra of a sample of thirteen metal-poor subgiant stars. According to stellar mixing theories, these stars are in a stellar evolutionary stage in which mixing by convection is expected. Abundances of fragile elements like lithium and beryllium are thus expected to be affected by such mixing processes. As a consequence of this, the abundances of these elements are discussed in a dilution context. Lithium and beryllium abundances are compared with the abundances of stars with similar characteristics but in a less evolved stellar phase so that mixing processes have not acted yet. As expected, our abundances seem to be depleted following reasonably well the standard predictions. Stellar abundances of oxygen should give an estimate of the oxygen contribution of core-collapse supernovae to the interstellar medium. However, there is poor agreement among the abundances determined from different atomic or molecular indicators in general. Abundances coming from three different indicators are compared in this thesis. The abundances determined from the O I infrared triplet lines at 777.1-5 nm give the poorest agreement among the three indicators. The abundances based on OH ultraviolet lines around 310 nm are lower for the subgiants in comparison with previous studies of main-sequence stars, becoming even lower than values based on the O I forbidden line at 630.03 nm. Still the most reliable indicator appears to be the O I forbidden line which suggests a plateau-like or only slowly increasing [O/Fe] towards lower [Fe/H]. In addition, the line formation of the Be II ultraviolet resonance lines at 313.0-1 nm, commonly used for abundance determinations purposes, is investigated under non-local thermodynamic equilibrium. We find that the common assumption of local thermodynamic equilibrium typically gives systematic errors of about 0.1 dex.</p>

<p>The information about the chemical compositions of stars is encoded in their spectra. Accurate determinations of these compositions are crucial for our understanding of stellar nucleosynthesis and Galactic chemical evolution. The determination of elemental abundances in stars requires models for the stellar atmospheres and the processes of line formation. Nearly all spectroscopic analyses of late-type stars carried out today are based on one-dimensional (1D), hydrostatic model atmospheres and on the assumption of local thermodynamic equilibrium (LTE). This approach can lead to large systematic errors in the predicted stellar atmospheric structures and line-strengths, and, hence, in the derived stellar abundances. In this thesis, examples of departures from LTE and from hydrostatic equilibrium are explored. The effects of background line opacities (line-blocking) due to atomic lines on the statistical equilibrium of Fe are investigated in late-type stars. Accounting for this line opacity is important at solar metallicity, where line-blocking significantly reduces the rates of radiatively induced ionizations of Fe. On the contrary, the effects of line-blocking in metal-poor stars are insignificant. In metal-poor stars, the dominant uncertainty in the statistical equilibrium of Fe is the treatment of inelastic H+Fe collisions. Substantial departures of Fe abundances from LTE are found at low metallicities: about 0.3 dex with efficient H+Fe collisions and about 0.5 dex without. The impact of three-dimensional (3D) hydrodynamical model atmospheres on line formation in red giant stars is also investigated. Inhomogeneities and correlated velocity fields in 3D models and differences between the mean 3D stratifications and corresponding 1D model atmospheres can significantly affect the predicted line strengths and derived abundances, in particular at very low metallicities. In LTE, the differences between 3D and 1D abundances of C, N, and O derived from CH, NH, and OH weak low-excitation lines are in the range -0.5 dex to -1.0 dex at [Fe/H]=-3. Large negative corrections (about -0.8 dex) are also found in LTE for weak low-excitation neutral Fe lines. We also investigate the impact of 3D hydrodynamical model stellar atmospheres on the determination of elemental abundances in the carbon-rich, hyper iron-poor stars HE 0107-5240 and HE 1327-2326. The lower temperatures of the line-forming regions of the 3D models compared with 1D models cause changes in the predicted spectral line strengths. In particular we find the 3D abundances of C, N, and O to be lower by about -0.8 dex (or more) than estimated from a 1D analysis. The 3D abundance of Fe is decreased but only by -0.2 dex. Departures from LTE for Fe might actually be very large for these stars and dominate over the effects due to granulation.</p>

The presented thesis describes the observations of the Galactic center Quintuplet cluster, the spectral analysis of the cluster Wolf-Rayet stars of the nitrogen sequence to determine their fundamental stellar parameters, and discusses the obtained results in a general context. The Quintuplet cluster was discovered in one of the first infrared surveys of the Galactic center region (Okuda et al. 1987, 1989) and was observed for this project with the ESO-VLT near-infrared integral field instrument SINFONI-SPIFFI. The subsequent data reduction was performed in parts with a self-written pipeline to obtain flux-calibrated spectra of all objects detected in the imaged field of view. First results of the observation were compiled and published in a spectral catalog of 160 flux-calibrated $K$-band spectra in the range of 1.95 to 2.45,$mu$m, containing 85 early-type (OB) stars, 62 late-type (KM) stars, and 13 Wolf-Rayet stars. About 100 of these stars are cataloged for the first time. The main part of the thesis project was concentrated on the analysis of the WR stars of the nitrogen sequence and one further identified emission line star (Of/WN) with tailored Potsdam Wolf-Rayet (PoWR) models for expanding atmospheres (Hamann et al. 1995) that are applied to derive the stellar parameters of these stars. For this purpose, the atomic input data of the PoWR models had to be extended by further line transitions in the near-infrared spectral range to enable adaequate model spectra to be calculated. These models were then fitted to the observed spectra, revealing typical paramters for this class of stars. A significant amount of hydrogen of up to $X_text{H} sim 0.2$ by mass fraction is still present in their stellar atmospheres. The stars are also found to be very luminous ($log{(L/L_odot)} > 6.0$) and show mass-loss rates and wind characteristics typical for radiation-driven winds. By comparison with stellar evolutionary models (Meynet & Maeder 2003a; Langer et al. 1994), the initial masses were estimated and indicate that the Quintuplet WN stars are descendants from the most massive O stars with $M_text{init} > 60 M_odot$ and their ages correspond to a cluster age of 3-5,million years. The analysis of the individual WN stars revealed an average extinction of $A_K =3.1 pm 0.5$,mag ($A_V = 27 pm 4$) towards the Quintuplet cluster. This extinction was applied to derive the stellar luminosities of the remaining early-type and late-type stars in the catalog and a Hertzsprung-Russell diagram could be compiled. Surprisingly, two stellar populations are found, a group of main sequence OB stars and a group of evolved late-type stars, i.e. red supergiants (RSG). The main sequence stars indicate a cluster age of 4 million years, which would be too young for red supergiants to be already present. A star formation event lasting for a few million years might possibly explain the Quintuplet's population and the cluster would still be considered coeval. However, the unexpected and simultaneous presence of red supergiants and Wolf-Rayet stars in the cluster points out that the details of star formation and cluster evolution are not yet well understood for the Quintuplet cluster. / Die vorgelegte Arbeit befasst sich mit der Spektralanalyse der massereichen Sterne, speziell der Wolf-Rayet Sterne der Stickstoffsequenz, des Quintuplet-Sternhaufens im Galaktischen Zentrum, welches durch Staubwolken vor visuellen Beobachtungen verborgen ist. Der Sternhaufen wurde in einer der ersten Infrarot-Durchmusterungen entdeckt (Okuda et al. 1987, 1989) und f"ur dieses Projekt mit dem Grossteleskop ESO-VLT und dem Infrarotinstrument SINFONI-SPIFFI beobachtet. Die Daten wurden aufbereitet und die flusskalibrierten Spektren in einem Katalog ver"offentlicht. Darin enthalten sind 85 Sterne fr"uhen Spektraltyps (O- und B-Sterne), 62 Sterne sp"aten Spektraltyps (K- und M-Sterne), sowie 13 Wolf-Rayet Sterne. Etwa 100 Sterne sind zum ersten mal detektiert und katalogisiert worden. Die flusskalibrierten Spektren der Wolf-Rayet Sterne der Stickstoffsequenz (WN) wurden mit den Potsdam Wolf-Rayet Modellen f"ur expandierende Sternatmosph"aren (Hamann et al. 1995) untersucht, wozu zun"achst Atomdaten der Modelle f"ur den Infrarotbereich erg"anzt werden mussten. Verschiedene Modellspektren und -energieverteilungen wurden mit den Beobachtungen verglichen, um die Sternparameter Temperatur, Radius, Leuchtkraft und die Charakteristik des Sternwinds Geschwindigkeit, chemische Zusammensetzung und Massenverlustrate zu bestimmen. Der ermittelte hohe Wasserstoffgehalt der Winde der WN-Sterne zeigt, dass sie Abk"ommlinge von massereichen O-Sternen sind, die die Hauptreihe verlassen haben. Desweiteren sind die Sterne sehr leuchtkr"aftig ($log(L/L_odot) > 6$) und zeigen Massenverlustraten, die typisch sind f"ur strahlungsgetriebenen Sternwinde. Im Vergleich mit Sternentwicklungsmodellen (Meynet & Maeder 2003a; Langer et al. 1994) ergeben sich Anfangsmassen von $M_text{init}>60,M_odot$, sowie ungef"ahre Sternalter von 3-5 Millionen Jahren f"ur die WN-Sterne, was dem angenommenen Altern des Quintuplet-Haufens entspricht. Durch die Analyse der spektralen Energieverteilungen der einzelnen WN-Sterne konnte eine mittlere interstellare Extinktion von $A_K =3.1 pm 0.5$,mag ($A_V = 27 pm 4$,mag) in der Richtung des Quintuplet-Haufens ermittelt und f"ur die Bestimmung der Leuchtkr"afte der verbleibenden Sterne des Katalog verwendet werden. Die anschliess ende vorl"aufige Analyse ergab eine Dichotomie der Sternpopulation von fr"uhen und sp"aten Sternen im Hertzsprung-Russell-Diagramm. W"ahrend die OB-Sterne entsprechend der Entwicklungstheorie auf der Hauptreihe des Haufens liegen, befinden sich die KM-Sterne im entwickelten Stadium der Roten Riesen, welches f"ur Sterne diesen Typs fr"uhestens nach 7 Millionen Jahren erwartet wird. Somit steht die zeitgleiche Entstehung aller Sterne des Sternhaufens in Frage. Sie wird im Rahmen von Haufenzugeh"origkeit und einer Phase ausgedehnter Sternentstehung diskutiert. Es bleibt anzuerkennen, dass die Sternentstehung und -entwicklung auch im speziellen Fall des Quintuplet-Haufens noch nicht hinreichend gut verstanden sind.

Stellar atmospheres constitute excellent environments to study the chemical evolution of our Galaxy. The chemical composition of these atmospheres reflects the composition of the gas from where these stars were born. As the Galaxy evolves, the composition of the gas changes from being primordial (Big-Bang nucleosynthesis) to being enriched in heavy elements (stellar and interstellar nucleosynthesis). The abundances of fragile chemical elements can be affected by stellar mixing processes. Precise lithium, beryllium and oxygen abundance determinations in old stars are presented in this thesis. These determinations are based on the analysis of the observed spectra of a sample of thirteen metal-poor subgiant stars. According to stellar mixing theories, these stars are in a stellar evolutionary stage in which mixing by convection is expected. Abundances of fragile elements like lithium and beryllium are thus expected to be affected by such mixing processes. As a consequence of this, the abundances of these elements are discussed in a dilution context. Lithium and beryllium abundances are compared with the abundances of stars with similar characteristics but in a less evolved stellar phase so that mixing processes have not acted yet. As expected, our abundances seem to be depleted following reasonably well the standard predictions. Stellar abundances of oxygen should give an estimate of the oxygen contribution of core-collapse supernovae to the interstellar medium. However, there is poor agreement among the abundances determined from different atomic or molecular indicators in general. Abundances coming from three different indicators are compared in this thesis. The abundances determined from the O I infrared triplet lines at 777.1-5 nm give the poorest agreement among the three indicators. The abundances based on OH ultraviolet lines around 310 nm are lower for the subgiants in comparison with previous studies of main-sequence stars, becoming even lower than values based on the O I forbidden line at 630.03 nm. Still the most reliable indicator appears to be the O I forbidden line which suggests a plateau-like or only slowly increasing [O/Fe] towards lower [Fe/H]. In addition, the line formation of the Be II ultraviolet resonance lines at 313.0-1 nm, commonly used for abundance determinations purposes, is investigated under non-local thermodynamic equilibrium. We find that the common assumption of local thermodynamic equilibrium typically gives systematic errors of about 0.1 dex.

The information about the chemical compositions of stars is encoded in their spectra. Accurate determinations of these compositions are crucial for our understanding of stellar nucleosynthesis and Galactic chemical evolution. The determination of elemental abundances in stars requires models for the stellar atmospheres and the processes of line formation. Nearly all spectroscopic analyses of late-type stars carried out today are based on one-dimensional (1D), hydrostatic model atmospheres and on the assumption of local thermodynamic equilibrium (LTE). This approach can lead to large systematic errors in the predicted stellar atmospheric structures and line-strengths, and, hence, in the derived stellar abundances. In this thesis, examples of departures from LTE and from hydrostatic equilibrium are explored. The effects of background line opacities (line-blocking) due to atomic lines on the statistical equilibrium of Fe are investigated in late-type stars. Accounting for this line opacity is important at solar metallicity, where line-blocking significantly reduces the rates of radiatively induced ionizations of Fe. On the contrary, the effects of line-blocking in metal-poor stars are insignificant. In metal-poor stars, the dominant uncertainty in the statistical equilibrium of Fe is the treatment of inelastic H+Fe collisions. Substantial departures of Fe abundances from LTE are found at low metallicities: about 0.3 dex with efficient H+Fe collisions and about 0.5 dex without. The impact of three-dimensional (3D) hydrodynamical model atmospheres on line formation in red giant stars is also investigated. Inhomogeneities and correlated velocity fields in 3D models and differences between the mean 3D stratifications and corresponding 1D model atmospheres can significantly affect the predicted line strengths and derived abundances, in particular at very low metallicities. In LTE, the differences between 3D and 1D abundances of C, N, and O derived from CH, NH, and OH weak low-excitation lines are in the range -0.5 dex to -1.0 dex at [Fe/H]=-3. Large negative corrections (about -0.8 dex) are also found in LTE for weak low-excitation neutral Fe lines. We also investigate the impact of 3D hydrodynamical model stellar atmospheres on the determination of elemental abundances in the carbon-rich, hyper iron-poor stars HE 0107-5240 and HE 1327-2326. The lower temperatures of the line-forming regions of the 3D models compared with 1D models cause changes in the predicted spectral line strengths. In particular we find the 3D abundances of C, N, and O to be lower by about -0.8 dex (or more) than estimated from a 1D analysis. The 3D abundance of Fe is decreased but only by -0.2 dex. Departures from LTE for Fe might actually be very large for these stars and dominate over the effects due to granulation.