This thesis has used medium- and high-resolution spectral data to derive elemental abundances, in particular light and heavy s-process elemental abundances, for groups of giant stars in the globular clusters 47 Tuc, NGC 6388 and NGC 362. These analyses were undertaken using both curve-of-growth and spectrum synthesis techniques. The techniques were calibrated with respect to the metal-poor giant star Arcturus in order to reduce systematic errors in the analysis process. A feasibility study was undertaken that compared synthetic spectra at different resolutions throughout the colour-magnitude diagram (CMD) of a metal-rich ([Fe/H] = -0.5 dex) globular cluster. This study identified where on the CMD light and heavy s-process elemental abundances could be derived at medium resolution (R ~ 10,000). Abundance analyses could be undertaken on the giant branches down to just below the horizontal branch and then again on the main sequence below Teff ~ 4500 K. At all other places on the CMD high-resolution spectra (R ~ 30,000) are required to derive these abundances. Performance verification data at R ~ 5,000$ was obtained using the Robert Stobie Spectrograph (RSS) on the Southern African Large Telescope (SALT) and showed that there were no large scale s-process elemental abundance variations in 47 Tuc giant branch stars. The level of precision in this preliminary data was [X/Fe] ~ 0.5dex. A resolution of R ~ 10,000 should be achievable with SALT RSS in the future which will improve this limit. The AAOmega survey of 47 Tuc stars at R ~ 6,500 was more promising in certain aspects of elemental abundance determination. The observed wavelengths included the key features of CN and CH molecular bands, and light (Z < 30) and heavy (38 < Z < 63) element spectral lines. CN indices were measured and calibrated to previous results. The well-known CN bimodality was observed in the 47 Tuc stars, as well as a radial gradient in CN strength. A preliminary subset of ten of the survey stars have undergone an abundance analysis for which the abundances of Fe, Si, and Ca were found to be homogeneous within this cluster. The Na abundances had a large range in values that were observed to correlate with CN strength. The s-process elemental abundance results were inconclusive. The Zr abundances showed little to no enhancement in the sample and the Ba abundances varied considerably due to strong lines of Ba II being extremely sensitive to microturbulence. Various high-resolution studies were carried out using spectra of giant stars in 47 Tuc, NGC 6388 and NGC 362 observed on the Australian National Observatory (ANO) 2.3 m echelle spectrograph and the Ultra-Violet Echelle Spectrograph (UVES) on the Very Large Telescope (VLT). The high-resolution analysis of 47 Tuc giant star Lee 2525 found an enhanced Zr abundance in this star which resolved a discrepancy between two previous 47 Tuc elemental abundance studies (Brown & Wallerstein 1992; Wylie et al. 2006). The stars in the VLT dataset that were analysed here included five giant branch stars in 47 Tuc, two in NGC 6388 and thirteen in NGC 362. The low temperatures and gravities of these stars caused departures from local thermodynamic equilibrium in low excitation potential neutral species, particularly Fe and Zr, that needed to be taken into account before reliable stellar parameters and elemental abundances could be determined for these stars. Veiling effects due to circumstellar dust were postulated to have produced artificially low metallicities for the infra-red excess stars in this sample, particularly for the 47 Tuc stars. The element abundance analyses of 47 Tuc, NGC 6388 and NGC 362 stars found the derived metallicities to be homogeneous for each cluster (<[Fe/H]>(47Tuc) = -0.88 +/- 0.09 dex; <[Fe/H]>(NGC6388) = -0.60 +/- 0.06 dex; <[Fe/H]>(NGC362) = -1.21 +/- 0.08 dex). The 47 Tuc sample included Lee 2525 and the five VLT stars. The derived metallicities were in reasonable agreement with previously reported values. The light (ls) and heavy (hs) s-process element abundances were enhanced and homogeneous in the stars of each cluster. The abundances determined for 47 Tuc and NGC 6388 were in good agreement, reflecting the similarity in metallicity of the stars in these two clusters (<[ls/Fe]>(47Tuc) = +0.53 +/- 0.02 dex; <[hs/Fe]>(47Tuc) = +0.40 +/- 0.06 dex; <[ls/Fe]>(NGC6388) = +0.58 +/- 0.13 dex; <[hs/Fe]>(NGC6388) = +0.39 +/- 0.07 dex). The more metal-poor cluster NGC 362 was less enhanced in ls elemental abundances and slightly more enhanced in hs elemental abundances (<[ls/Fe]>(NGC362) = +0.32 +/- 0.10 dex, <[hs/Fe]>(NGC362) = +0.46 +/- 0.09 dex). The clear enhancement in the s-process elemental abundances and homogeneity in the results for each globular cluster is evidence that these stars have been enhanced extrinsically in s-process elements. Pollution events in the history of each cluster has resulted in the abundance distribution in both the light elements and the heavy elements that has been observed in the stars analysed in this thesis. The enhancements in Na, ls and hs elemental abundances favours intermediate mass AGB stars as the source of the pollution.

Photometric and spectroscopic analyses have shown that the Galactic bulge cluster Terzan 5 hosts several populations with different metallicities and ages that manifest as a double red horizontal branch (HB). A recent investigation of the massive bulge cluster NGC 6569 revealed a similar, though less extended, HB luminosity split, but little is known about the cluster's detailed chemical composition. Therefore, we have used high- resolution spectra from the Magellan-M2FS and VLT-FLAMES spectrographs to investigate the chemical compositions and radial velocity distributions of red giant branch and HB stars in NGC 6569. We found the cluster to have a mean heliocentric radial velocity of -48.8 km s(-1) (sigma = 5.3 km s(-1); 148 stars) and <[Fe/H]> = -0.87 dex (19 stars), but the cluster's 0.05 dex [Fe/H] dispersion precludes a significant metallicity spread. NGC 6569 exhibits light- and heavy-element distributions that are common among old bulge/inner Galaxy globular clusters, including clear (anti) correlations between [O/Fe], [Na/Fe], and [Al/Fe]. The light-element data suggest that NGC 6569 may be composed of at least two distinct populations, and the cluster's low <[La/Eu]> = -0.11 dex indicates significant pollution with r- process material. We confirm that both HBs contain cluster members, but metallicity and lightelement variations are largely ruled out as sources for the luminosity difference. However, He mass fraction differences as small as Delta Y similar to 0.02 cannot be ruled out and may be sufficient to reproduce the double HB.

Context. Post-extreme horizontal branch stars (post-EHB) are helium-shell burning objects evolving away from the EHB and contracting directly towards the white dwarf regime. While the stars forming the EHB have been extensively studied in the past, their hotter and more evolved progeny are not so well characterized. Aims. We perform a comprehensive spectroscopic analysis of four such bright sdO stars, namely Feige 34, Feige 67, AGK+81 degrees 266, and LS II + 18 degrees 9, among which the first three are used as standard stars for flux calibration. Our goal is to determine their atmospheric parameters, chemical properties, and evolutionary status to better understand this class of stars that are en route to become white dwarfs. Methods. We used non-local thermodynamic equilibrium model atmospheres in combination with high quality optical and UV spectra. Photometric data were also used to compute the spectroscopic distances of our stars and to characterize the companion responsible for the infrared excess of Feige 34. Results. The four bright sdO stars have very similar atmospheric parameters with T-eff between 60 000 and 63 000 K and log g (cm s(-2)) in the range 5.9 to 6.1. This places these objects right on the theoretical post-EHB evolutionary tracks. The UV spectra are dominated by strong iron and nickel lines and suggest abundances that are enriched with respect to those of the Sun by factors of 25 and 60. On the other hand, the lighter elements, C, N, O, Mg, Si, P, and S are depleted. The stars have very similar abundances, although AGK + 81 degrees 266 shows differences in its light element abundances. For instance, the helium abundance of this object is 10 times lower than that observed in the other three stars. All our stars show UV spectral lines that require additional line broadening that is consistent with a rotational velocity of about 25 km s(-1). The infrared excess of Feige 34 is well reproduced by a M0 main-sequence companion and the surface area ratio of the two stars suggests that the system is a physical binary. However, the lack of radial velocity variations points towards a low inclination and/or long orbital period. Spectroscopic and HIPPARCOS distances are in good agreement for our three brightest stars. Conclusions. We performed a spectroscopic analysis of four hot sdO stars that are very similar in terms of atmospheric parameters and chemical compositions. The rotation velocities of our stars are significantly higher than what is observed in their immediate progenitors on the EHB, suggesting that angular momentum may be conserved as the stars evolve away from the EHB.

The Apache Point Observatory Galactic Evolution Experiment (APOGEE) provides spectroscopic information of regions of the inner Milky Way, which are inaccessible to optical surveys. We present the first large study of the metallicity distribution of the innermost Galactic regions based on high-quality measurements for 7545 red giant stars within 4.5 kpc of the Galactic center, with the goal to shed light on the structure and origin of the Galactic bulge. Stellar metallicities are found, through multiple Gaussian decompositions, to be distributed in several components, which is indicative of the presence of various stellar populations such as the bar or the thin and the thick disks. Super-solar ([Fe/H] = +0.32) and solar ([Fe/H] = +0.00) metallicity components, tentatively associated with the thin disk and the Galactic bar, respectively, seem to be major contributors near the midplane. A solar-metallicity component extends outwards in the midplane but is not observed in the innermost regions. The central regions (within 3 kpc of the Galactic center) reveal, on the other hand, the presence of a significant metal-poor population ([Fe/H] = -0.46), tentatively associated with the thick disk, which becomes the dominant component far from the midplane (vertical bar Z vertical bar >= +0.75 kpc). Varying contributions from these different components produce a transition region at +0.5 kpc <= vertical bar Z vertical bar <= +1.0 kpc, characterized by a significant vertical metallicity gradient.

We find two chemically distinct populations separated relatively cleanly in the [Fe/H]-[Mg/Fe] plane, but also distinguished in other chemical planes, among metal-poor stars (primarily with metallicities [Fe/H] < -0.9) observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and analyzed for Data Release 13 (DR13) of the Sloan Digital Sky Survey. These two stellar populations show the most significant differences in their [X/Fe] ratios for the alpha-elements, C+N, Al, and Ni. In addition to these populations having differing chemistry, the low metallicity high-Mg population (which we denote "the HMg population") exhibits a significant net Galactic rotation, whereas the low-Mg population (or "the LMg population") has halo-like kinematics with little to no net rotation. Based on its properties, the origin of the LMg population is likely an accreted population of stars. The HMg population shows chemistry (and to an extent kinematics) similar to the thick disk, and is likely associated with in situ formation. The distinction between the LMg and HMg populations mimics the differences between the populations of low-and high-a halo stars found in previous studies, suggesting that these are samples of the same two populations.

Carbon isotope ratios, along with carbon and nitrogen abundances, are derived in a sample of 11 red-giant members of one of the most metal-rich clusters in the Milky Way, NGC 6791. The selected red-giants have a mean metallicity and standard deviation of [Fe/H] = +0.39 +/- 0.06 (Cunha et al. 2015). We used high-resolution H-band spectra obtained by the SDSS-IV Apache Point Observatory Galactic Evolution Experiment. The advantage of using high-resolution spectra in the H band is that lines of CO are well represented and their line profiles are sensitive to the variation of C-12/C-13. Values of the C-12/C-13 ratio were obtained from a spectrum synthesis analysis. The derived C-12/C-13 ratios varied between 6.3 and 10.6 in NGC 6791, in agreement with the final isotopic ratios from thermohaline-induced mixing models. The ratios derived here are combined with those obtained for more metal poor red-giants from the literature to examine the correlation between C-12/C-13, mass, metallicity, and evolutionary status.

We report the peculiar chemical abundance patterns of 11 atypical Milky Way (MW) field red giant stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). These atypical giants exhibit strong Al and N enhancements accompanied by C and Mg depletions, strikingly similar to those observed in the so-called second-generation (SG) stars of globular clusters (GCs). Remarkably, we find low Mg abundances ([Mg/Fe]. < 0.0) together with strong Al and N overabundances in the majority (5/7) of the metal-rich ([Fe/H] greater than or similar to-1.0) sample stars, which is at odds with actual observations of SG stars in Galactic GCs of similar metallicities. This chemical pattern is unique and unprecedented among MW stars, posing urgent questions about its origin. These atypical stars could be former SG stars of dissolved GCs formed with intrinsically lower abundances of Mg and enriched Al (subsequently self-polluted by massive AGB stars) or the result of exotic binary systems. We speculate that the stars Mg-deficiency as well as the orbital properties suggest that they could have an extragalactic origin. This discovery should guide future dedicated spectroscopic searches of atypical stellar chemical patterns in our Galaxy, a fundamental step forward to understanding the Galactic formation and evolution.

We present a near-infrared spectral sequence of the electromagnetic counterpart to the binary neutron star merger GW170817 detected by Advanced Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo. Our data set comprises seven epochs of J + H spectra taken with FLAMINGOS-2 on Gemini-South between 1.5 and 10.5 days after the merger. In the initial epoch, the spectrum is dominated by a smooth blue continuum due to a high-velocity, lanthanide-poor blue kilonova component. Starting the following night, all of the subsequent spectra instead show features that are similar to those predicted in model spectra of material with a high concentration of lanthanides, including spectral peaks near 1.07 and 1.55 mu m. Our fiducial model with 0.04 M-circle dot of ejecta, an ejection velocity of v = 0.1c, and a lanthanide concentration of X-lan = 10(-2) provides a good match to the spectra taken in the first five days, although it over-predicts the late-time fluxes. We also explore models with multiple fitting components, in each case finding that a significant abundance of lanthanide elements is necessary to match the broad spectral peaks that we observe starting at 2.5 days after the merger. These data provide direct evidence that binary neutron star mergers are significant production sites of even the heaviest r-process elements.

Les données moléculaires précises, comme les taux de collisions, sont très important pour interpréter les observations de raies moléculaires, et par conséquent, pour estimer la abondance moléculaire dans le milieu interstellaire. Nous avons utilisé différents approximations quantiques pour étudier les excitations rotationnelles de AlCN(1Σ), AlNC(1Σ), MgCN(2Σ), MgNC(2Σ), SiCN(2∏) et SiNC(2∏) à cause des collisions avec atomes de He . On a utilisé He pour simuler les collisions avec H2 en multipliant les taux par un facteur d'échelle. Nous avons aussi étudié le excitation rotationnel de HCN(1Σ) à cause des collisions avec H2. Puis, des calculs de transfert radiatif on été faite pour estimer la abondance relative du isomères dans différentes régions du milieu interstellaire. Malgré les caractéristiques spectroscopiques semblables des isomères, ce travail démontre l'importance d'effectuer des calculs des taux de collisions séparément pour chaque isomère, afin d'obtenir leur abondances. / Accurate molecular data, such collisional rate coefficients, are essential to model molecular lines and then to estimate molecular abundances in the interstellar medium (ISM). For this reason, we have used quantum approximations to study the rotational (de-)excitation of AlCN(1Σ), AlNC(1Σ), MgCN(2Σ), MgNC(2Σ), SiCN(2∏) and SiNC(2∏) molecules by collisions with He, as a model of H2. We have also considered the rotational (de-)excitation of HCN(1Σ) molecules by ortho-H2 and para-H2 molecules.Then, we have performed radiative transfer calculations in order to estimate the relative abundances of cyanide/isocyanide species in the ISM. The impact of our molecular data in the simulation of molecular emissions is discussed. Despite the similar spectroscopic characteristics of the isomers, this work demonstrates the importance of conducting separate collisional rate calculations for each isomer in order to obtain their abundances.

We describe an ultraviolet spectroscopic survey of interstellar high-velocity cloud (HVC) absorption in the strong λ1206.500 line of Si III using the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Because the Si III line is 4-5 times stronger than O VI λ1031.926, it provides a sensitive probe of ionized gas down to column densities N Si III 5 × 1011 cm-2 at Si III equivalent width 10 m. We detect high-velocity Si III over 91% 4% of the sky (53 of 58 sight lines); 59% of the HVCs show negative local standard of rest velocities. The mean HVC column density per sight line is 〈log N Si III 〉 = 13.19±0.45, while the mean for all 90 velocity components is 12.92±0.46. Lower limits due to Si III line saturation are included in this average, so the actual mean/median values are even higher. The Si III appears to trace an extensive ionized component of Galactic halo gas at temperatures 104.0-4.5 K indicative of a cooling flow. Photoionization models suggest that typical Si III absorbers with 12.5 < log N Si III < 13.5 have total hydrogen column densities N H 1018-1019 cm-2 for gas of hydrogen density n H 0.1 cm-3 and 10% solar metallicity. With typical neutral fractions N H I/N H 0.01, these HVCs may elude even long-duration 21 cm observations at Arecibo, the EVLA, and other radio facilities. However, if Si III is associated with higher density gas, n H ≥ 1 cm-3, the corresponding neutral hydrogen could be visible in deep observations. This reservoir of ionized gas may contain 10 8M and produce a mass infall rate of 1 M yr-1 to the Galactic disk.