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No description available.

No description available.

Stellar model atmospheres are a fundamental tool for our understanding of stars. Because the chemical composition of stars cannot be measured directly, the inferred stellar parameters are model dependent. In recent years great progress has been made in the modeling of stellar atmospheres, allowing the relaxation of simplifying assumptions made in previous models. The use of new 3D model atmospheres to infer the solar chemical composition has resulted in a decrease of the solar metallicity. This result has caused some controversy and is being challenged. The main aim of this thesis is to ascertain if the new models of stellar atmospheres are realistic and can be trusted to derive the chemical composition of stars in general, and the Sun in particular. Other objectives also include the study of line formation in the Sun at high spatial resolution, and possible implications in the modeling. The Sun is the ideal test-bench for detailed analyses of stellar atmospheres. With the solar surface being resolved in great detail and at different viewing angles, a wealth of information can be gathered that allows for very robust tests of atmosphere models. The testing detailed here addresses several fronts. On one hand, the model�s temperature structure is directly tested with the classical tests of continuum centre-to-limb variations and absolute fluxes. On the other hand, the line formation is tested at different viewing angles and high spatial and spectral resolution. Here the main focus is on oxygen lines, as oxygen has an important contribution for the total solar metallicity. However, other lines are also tested. High quality data were specifically obtained for these line formation tests, using the Swedish 1-m Solar Telescope. For the temperature structure tests we find a surprisingly good agreement between the 3D model and the observations, surpassing even semi-empirical models. The solar 1D non-LTE models have a very similar behaviour to 1D LTE models, confirming that LTE is a good approximation in the solar photosphere. The 3D theoretical model performs consistently better than its 1D counterparts. The oxygen line formation tests are carried out in great detail, with a careful wavelength calibration, revised atomic data, and allowing for departures from LTE. Again we find a reassuring agreement between the 3D model predictions and the observations, both for the centre-to-limb variation of the lines and the line formation at high spatial resolution. The observations at different viewing angles also allowed the empirical determination of the role of hydrogen collisions with oxygen, important when deriving the oxygen abundance. The tests undertaken here show that the 3D model atmospheres are indeed very realistic. Their predicted temperature structure and velocity fields compare very favourably with observations of the Sun. Together with previous tests, this indicates they can be relied upon to derive the chemical composition of the Sun and similar late-type stars.

No description available.

Our knowledge of stellar magnetic fields relies almost entirely on circular polarization observations, which has historically limited our understanding of the stellar magnetic field topologies. Recently, it has become possible to obtain phase-resolved high-resolution spectropolarimetric observations in all four Stokes parameters for early-type magnetic stars. Interpretation of such observations with the Magnetic Doppler imaging technique has uncovered a new, previously unknown, level of complexity of surface stellar magnetic fields. This new insight is critical for understanding the origin, evolution and structure of magnetic fields in early-type stars. In this study we observed the magnetic, chemically peculiar Ap stars HD 24712 (DO Eri, HR 1217) and HD 125248 (CS Vir, HR 5355) in all four Stokes parameters with the HARPSpol spectropolarimeter at the ESO 3.6-m telescope. The resulting spectra have high signal-to-noise ratio and superb resolving power, by far surpassing the quality of any existing stellar Stokes parameter observations. We studied variation of the spectrum and magnetic observables of HD 24712 as a function of rotational phase (paper I). In the subsequent magnetic Doppler imaging investigation of this star, we interpreted the phase-resolved Stokes line profile observations (paper II). This analysis showed that HD 24712, unlike more massive Ap stars studied in all four Stokes parameters, has a dominant dipolar field component with a negligible contribution of small-scale magnetic structures. Simultaneously with magnetic mapping we derived surface abundance distributions of Fe, Nd, Na, and Ca. Building upon the technique of Magnetic Doppler imaging, we developed the first three-dimensional abundance inversion code and applied it to reconstruct the abundance distributions of Fe and Ca in three dimensions in the atmosphere of HD 24712 (paper III). We also performed Magnetic Doppler imaging analysis of the spectropolarimetric observations of HD 125248 (paper IV). The reconstructed detailed maps of the surface abundance distribution and magnetic field topology of HD 125248 revealed a magnetic field with significant deviations from the canonical dipolar field geometry, and strong surface abundance inhomogeneities for Cr and several rare earth elements. We assessed our inversion results in the context of magnetic Doppler imaging studies of other magnetic, chemically peculiar Ap stars and latest theoretical research on the evolution and stability of magnetic fields in radiative stellar interiors. Our analysis suggests that old or less massive Ap stars have predominantly dipolar magnetic fields while more massive or younger stars exhibit more complicated field topologies. We also compared our three-dimensional chemical abundance maps of HD 24712 to the predictions of theoretical atomic diffusion calculations in magnetized stellar atmospheres, generally finding a lack of agreement between theory and observations.

In this dissertation, we develop new tools for the study of stellar atmospheres, pulsating stellar atmospheres and mass loss from pulsating stars. These tools provide new insights into the structure and evolution of stars and complement modern observational techniques such as optical interferometry and high resolution spectroscopy. In the first part, a new spherically symmetric version of the Atlas program is developed for modelling extended stellar atmospheres. The program is used to model interferometric observations from the literature and to study limb-darkening for stars with low gravity. It is determined that stellar limb-darkening can be used to constrain fundamental properties of stars. When this is coupled with interferometric or microlensing observations, stellar limb-darkening can predict the masses of isolated stars. The new SAtlas program is combined with the plane-parallel hydrodynamic program Hermes to develop a new spherically-symmetric radiative hydrodynamic program that models radial pulsation in the atmosphere of a star to depths including the pulsation-driving regions of the stars. Preliminary tests of this new program are discussed. In the second part, we study the recent observations of circumstellar envelopes surrounding Cepheids and develop a mass-loss hypothesis to explain their formation. The hypothesis is studied using a modified version of the Castor, Abbott, & Klein theory for radiative-driven winds to contain the effects of pulsation. In the theory, pulsation is found to be a driving mechanism that increases the mass-loss rates of Cepheids by up to four orders of magnitude. These mass-loss rates are large enough to explain the formation of the envelopes from dust forming in the wind at large distances from the surface of the star. The mass-loss rates are found to be plausible explanation for the Cepheid mass discrepancy. We also compute mass-loss rates from optical and infrared observations of Large Magellanic Cloud Cepheids from the infrared excess and find mass loss to be an important phenomena in these stars. The amount of infrared excess is found to potentially affect the structure of the infrared Leavitt law.

In this dissertation, we develop new tools for the study of stellar atmospheres, pulsating stellar atmospheres and mass loss from pulsating stars. These tools provide new insights into the structure and evolution of stars and complement modern observational techniques such as optical interferometry and high resolution spectroscopy. In the first part, a new spherically symmetric version of the Atlas program is developed for modelling extended stellar atmospheres. The program is used to model interferometric observations from the literature and to study limb-darkening for stars with low gravity. It is determined that stellar limb-darkening can be used to constrain fundamental properties of stars. When this is coupled with interferometric or microlensing observations, stellar limb-darkening can predict the masses of isolated stars. The new SAtlas program is combined with the plane-parallel hydrodynamic program Hermes to develop a new spherically-symmetric radiative hydrodynamic program that models radial pulsation in the atmosphere of a star to depths including the pulsation-driving regions of the stars. Preliminary tests of this new program are discussed. In the second part, we study the recent observations of circumstellar envelopes surrounding Cepheids and develop a mass-loss hypothesis to explain their formation. The hypothesis is studied using a modified version of the Castor, Abbott, & Klein theory for radiative-driven winds to contain the effects of pulsation. In the theory, pulsation is found to be a driving mechanism that increases the mass-loss rates of Cepheids by up to four orders of magnitude. These mass-loss rates are large enough to explain the formation of the envelopes from dust forming in the wind at large distances from the surface of the star. The mass-loss rates are found to be plausible explanation for the Cepheid mass discrepancy. We also compute mass-loss rates from optical and infrared observations of Large Magellanic Cloud Cepheids from the infrared excess and find mass loss to be an important phenomena in these stars. The amount of infrared excess is found to potentially affect the structure of the infrared Leavitt law.

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. / 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]