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.

Šiuolaikiniai astronominiai stebėjimai leidžia įtarti, kad „standartinis“ žvaigždžių evoliucijos modelis yra nepilnas ir turi būti patikslintas, įvedant „papildomą“ maišymąsi. Disertacija yra skirta Galaktikos raudonosios sankaupos žvaigždžių cheminės sudėties tyrimui ir maišymosi procesų sukeltų cheminės sudėties pokyčių mažos masės žvaigždžių atmosferose tyrimui bei papildomo maišymosi teorinių modelių įvertinimui. Tam tikslui buvo gauti ir ištirti 62-jų Galaktikos raudonosios sankaupos žvaigždžių ir 9 padrikojo spiečiaus NGC 7789 žvaigždžių didelės skiriamosios gebos spektrai, nustatyti žvaigždžių pagrindiniai parametrai ir cheminė sudėtis. Tyrimas patvirtino hipotezę, kad Galaktikos sankaupos žvaigždės yra santykinai jauni Saulės metalingumo objektai, susiformavę per pastaruosius keletą milijardų metų. Tirtose žvaigždėse buvo įvertintos maišymosi procesų pasekmėje pasikeitusių anglies ir azoto gausų vertės. Pademonstruota, kad sankaupos žvaigždės gali būti išskirtos į skirtingas evoliucines grupes pagal 12C/13C kriterijų. Nustatyta, kad anglies izotopų santykių vertes Galaktikos sankaupos žvaigždžių atmosferose gerai aprašo šaltojo žemutinių sluoksnių papildomo maišymosi modelis, tuo tarpu termohalinio papildomo maišymosi modelis turi būti papildytas, kad sutaptų su visais stebėjimų rezultatais. Nustatyta, kad anglies izotopų santykiai spiečiaus žvaigždėse yra labiau paveikti maišymosi procesų nei numato dabartiniai teoriniai žvaigždžių evoliucijos modeliai. / Modern observations provide evidences that the ‘standard’ stellar evolution model is incomplete and should take into account ‘extra’-mixing processes. This dissertation is dedicated to an observational study of the Galactic red clump as well as to investigations of evolutionary effects in atmospheres of low-mass helium-core burning stars and evaluation of theoretical models of extra-mixing processes in interiors of stars. For this purpose, the high-resolution spectra of 62 Galactic red clump stars and 9 evolved stars of the old open cluster NGC 7789 have been analysed and the main atmospheric parameters and chemical compositions were determined. The investigation confirmed the hypothesis that clump stars of the Galaxy are relatively young objects, reflecting mainly the near–solar metallicities developed in the local disk during the last few billion years of its history. Mixing induced carbon and nitrogen abundance modifications in the Galactic clump stars were investigated. The clump stars can be divided into distinct evolutionary groups using the 12C/13C criterion. The carbon isotope ratios of the Galactic clump stars are consistent with the Cool Bottom Processing (CBP) extra-mixing model; the Thermohaline extra-mixing model needs to be complemented in order to agree with observational data. The carbon isotope ratios in the NGC 7789 stars indicate a larger extra–mixing than it is foreseen by theoretical models.

Modern observations provide evidences that the ‘standard’ stellar evolution model is incomplete and should take into account ‘extra’-mixing processes. This dissertation is dedicated to an observational study of the Galactic red clump as well as to investigations of evolutionary effects in atmospheres of low-mass helium-core burning stars and evaluation of theoretical models of extra-mixing processes in interiors of stars. For this purpose, the high-resolution spectra of 62 Galactic red clump stars and 9 evolved stars of the old open cluster NGC 7789 have been analysed and the main atmospheric parameters and chemical compositions were determined. The investigation confirmed the hypothesis that clump stars of the Galaxy are relatively young objects, reflecting mainly the near–solar metallicities developed in the local disk during the last few billion years of its history. Mixing induced carbon and nitrogen abundance modifications in the Galactic clump stars were investigated. The clump stars can be divided into distinct evolutionary groups using the 12C/13C criterion. The carbon isotope ratios of the Galactic clump stars are consistent with the Cool Bottom Processing (CBP) extra-mixing model; the Thermohaline extra-mixing model needs to be complemented in order to agree with observational data. The carbon isotope ratios in the NGC 7789 stars indicate a larger extra–mixing than it is foreseen by theoretical models. / Šiuolaikiniai astronominiai stebėjimai leidžia įtarti, kad „standartinis“ žvaigždžių evoliucijos modelis yra nepilnas ir turi būti patikslintas, įvedant „papildomą“ maišymąsi. Disertacija yra skirta Galaktikos raudonosios sankaupos žvaigždžių cheminės sudėties tyrimui ir maišymosi procesų sukeltų cheminės sudėties pokyčių mažos masės žvaigždžių atmosferose tyrimui bei papildomo maišymosi teorinių modelių įvertinimui. Tam tikslui buvo gauti ir ištirti 62-jų Galaktikos raudonosios sankaupos žvaigždžių ir 9 padrikojo spiečiaus NGC 7789 žvaigždžių didelės skiriamosios gebos spektrai, nustatyti žvaigždžių pagrindiniai parametrai ir cheminė sudėtis. Tyrimas patvirtino hipotezę, kad Galaktikos sankaupos žvaigždės yra santykinai jauni Saulės metalingumo objektai, susiformavę per pastaruosius keletą milijardų metų. Tirtose žvaigždėse buvo įvertintos maišymosi procesų pasekmėje pasikeitusių anglies ir azoto gausų vertės. Pademonstruota, kad sankaupos žvaigždės gali būti išskirtos į skirtingas evoliucines grupes pagal 12C/13C kriterijų. Nustatyta, kad anglies izotopų santykių vertes Galaktikos sankaupos žvaigždžių atmosferose gerai aprašo šaltojo žemutinių sluoksnių papildomo maišymosi modelis, tuo tarpu termohalinio papildomo maišymosi modelis turi būti papildytas, kad sutaptų su visais stebėjimų rezultatais. Nustatyta, kad anglies izotopų santykiai spiečiaus žvaigždėse yra labiau paveikti maišymosi procesų nei numato dabartiniai teoriniai žvaigždžių evoliucijos modeliai.

Through the cosmic matter cycle, the chemical evolution of the Milky Way is imprinted in the elemental abundance patterns of late-type stars (spectral types F to K). Due to their long lifetimes ( 1 Hubble time), these stars are of particular importance when it comes to studying the build-up of elements during the early times of our Galaxy. The chemical composition of the atmospheric layers of such stars is believed to resemble the gas from which they were formed. However, recent observations in globular clusters seem to contradict this assumption. The observations indicate that processes are at work that alter the surface compositions in these stars. The combined effect of processes responsible for an exchange of material between the stellar interior and atmosphere during the main sequence lifetime of the star, is referred to as atomic diffusion. Yet, the extent to which these processes alter surface abundances is still debated. By comparing abundances in unevolved and evolved stars all drawn from the same stellar population, any surface abundance anomalies can be traced. The anomalies, if found, can be compared to theoretical predictions from stellar structure models including atomic diffusion. Globular clusters provide stellar populations suitable to conduct such a comparison. In this thesis, the results of three independent analyses of two globular clusters, NGC 6752 and M4, at different metallicities are presented. The comparison between observations and models yields constraints on the models and finally a better understanding of the physical processes at work inside stars.

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