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.

Thesis (Ph. D.)--University of Canterbury, 2009. / Typescript (photocopy). "December 18, 2009." Includes bibliographical references (p. 251-255). Also available via the World Wide Web.

Thesis (Ph. D.)--University of Hawaii at Manoa, 2005. / Includes bibliographical references (leaves 136-146).

Thesis (M. Sc.)--University of Canterbury, 2009. / Typescript (photocopy). "August 3, 2009." Includes bibliographical references (leaves 87-90). Also available via the World Wide Web.

abstract: As the detection of planets become commonplace around our neighboring stars, scientists can now begin exploring their possible properties and habitability. Using statistical analysis I determine a true range of elemental compositions amongst local stars and how this variation could affect possible planetary systems. Through calculating and analyzing the variation in elemental abundances of nearby stars, the actual range in stellar abundances can be determined using statistical methods. This research emphasizes the diversity of stellar elemental abundances and how that could affect the environment from which planets form. An intrinsic variation has been found to exist for almost all of the elements studied by most abundance-finding groups. Specifically, this research determines abundances for a set of 458 F, G, and K stars from spectroscopic planet hunting surveys for 27 elements, including: C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, La, Ce, Nd, Eu, and Hf. Abundances of the elements in many known exosolar planet host stars are calculated for the purpose investigating new ways to visualize how stellar abundances could affect planetary systems, planetary formation, and mineralogy. I explore the Mg/Si and C/O ratios as well as place these abundances on ternary diagrams with Fe. Lastly, I emphasize the unusual stellar abundance of τ Ceti. τ Ceti is measured to have 5 planets of Super-Earth masses orbiting in near habitable zone distances. Spectroscopic analysis finds that the Mg/Si ratio is extremely high (~2) for this star, which could lead to alterations in planetary properties. τ Ceti's low metallicity and oxygen abundance account for a change in the location of the traditional habitable zone, which helps clarify a new definition of habitable planets. / Dissertation/Thesis / Ph.D. Astrophysics 2014

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) has observed similar to 600 transiting exoplanets and exoplanet candidates from Kepler (Kepler Objects of Interest, KOIs), most with >= 18 epochs. The combined multi-epoch spectra are of high signal-to-noise ratio (typically >= 100) and yield precise stellar parameters and chemical abundances. We first confirm the ability of the APOGEE abundance pipeline, ASPCAP, to derive reliable [Fe/H] and effective temperatures for FGK dwarf stars-the primary Kepler host stellar type-by comparing the ASPCAP-derived stellar parameters with those from independent high-resolution spectroscopic characterizations for 221 dwarf stars in the literature. With a sample of 282 close-in (P < 100 days) KOIs observed in the APOGEE KOI goal program, we find a correlation between orbital period and host star [Fe/H] characterized by a critical period, P-crit = 8.3(-4.1)(+0.1) days, below which small exoplanets orbit statistically more metal-enriched host stars. This effect may trace a metallicity dependence of the protoplanetary disk inner radius at the time of planet formation or may be a result of rocky planet ingestion driven by inward planetary migration. We also consider that this may trace a metallicity dependence of the dust sublimation radius, but we find no statistically significant correlation with host T-eff and orbital period to support such a claim.

The PPAK Integral Field Spectroscopy (IFS) Nearby Galaxies Survey: PINGS, a 2-dimensional spectroscopic mosaicking of 17 nearby disk galaxies in the optical wavelength range. This project represents the first attempt to obtain continuous coverage spectra of the whole surface of a galaxy in the nearby universe. The final data set comprises more than 50000 individual spectra, covering in total an observed area of nearly 80 arcmin square. The powerful capabilities of wide-field 2D spectroscopic studies areproven. The chemical composition of the whole surface of a spiral galaxy ischaracterised for the first time as a function not only of radius, but of theintrinsic morphology of the galaxy, allowing a more realistic determination oftheir physical properties. The methodology, analysis and results of thisdissertation will hopefully contribute in a significant way to understand thenature of the physical and chemical properties of the gas phase in spiralgalaxies.

Magister Scientiae (Biodiversity and Conservation Biology) / Kelp forests are dynamic and productive ecosystems which host large biodiversity of sessile fauna and flora, including diatoms. These microalgae occur at the base of coastal marine food webs and contribute substantially to the productivity of marine ecosystems. Diatoms constitute one of the most common and species-rich groups of both phytoplankton and phytobenthos. Possessing a unique silica cell wall, diatoms play a key role in the global carbon and silicon cycles. As the changes in species composition of diatom communities are a direct reaction to the combination of environmental factors prevailing in their ecosystems, diatom analysis is widely and successfully used in biomonitoring of various environmental conditions and paleoecological reconstructions.

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