Investigating globular cluster abundance anomalies in different environments

The origin of the star-to-star abundance variations for light elements shown in globular clusters (GCs) is not well understood, which represents a significant problem for stellar astrophysics. While the light element abundance variations are very common in globular clusters, they are comparatively rare in the galactic halo field population. However, relatively little is known regarding the occurrence of the abundance anomalies in other environments such as dwarf spheroidal galaxies and massive star clusters younger than the ancient globular clusters. Consequently, spectra obtained at Gemini-S with the GMOS spectrograph and at the AAT with the 2dF/AAOmega instrument have been used to investigate the anti-correlation and bimodality of CH and CN band strengths through the measurement of CH(lambda ~ 4300), S(3839) and S(4142) indices for 45 red giants stars in the Sculptor dwarf spheroidal galaxy. Using a calibration from five GCs (NGC 1851, NGC 288, NGC 6752, M 55 and M 30) we also have studied the CN/Na correlation in the Sculptor red giant sample. Our results indicate that variations analogous to those seen in GCs are not present in our Sculptor sample. Instead we find a positive correlation between CH and CN, and no correlation between Na and CN. Also, a deficiency of [Na/Fe] in Sculptor relative to GCs has been found that is consistent with previous work for dSph galaxies. The outcome reinforces the apparent need for a high stellar density environment to produce the light element abundance variations. We also report the discovery of a third carbon-enhanced metal-poor star in Sculptor that shows evidence of a strong s-process element enhancement. In addition, spectra obtained with the VLT/FORS and Gemini-S/GMOS instruments have been used to investigate CN and CH band strengths along with carbon and nitrogen abundances in a sample of massive star clusters in the Small Magellanic Cloud. The clusters have metallicity and luminosity comparable to GCs but are younger. The clusters investigated are Lindsay 1, Kron 3 and NGC 339, which have ages between 6 and 7.5 Gyr. Our results for Lindsay 1 and NGC 339 are not as clear-cut as they are for Kron 3, where the large spread in nitrogen (~ 1.2 dex), and a well-established bimodality of CH and CN band strengths with a CN/CH anti-correlation support the presence of multiple populations. Due to the possibility of evolutionary mixing on the red giant branch, the existence of CN-CH anti-correlation is not sufficient in itself to indicate the presence of abundance anomalies. Consequently, we have investigated the Na, CN correlation through the measurement of sodium D-line strengths and Na abundances, finding a spread in sodium of ~ 0.8 dex in Kron 3. For this cluster we therefore confirm a positive correlation between nitrogen and sodium abundances. These results add to existing photometric and spectroscopic indications of abundance anomalies in clusters of intermediate age, confirming that the mechanism responsible for the multiple populations cannot be an early cosmological effect applying only to old GGCs. This outcome is a key factor in gaining understanding on the origin of the abundance anomalies

Scl-1013644: A CEMP-s star in the Sculptor dwarf spheroidal galaxy

Recent studies of the MilkyWay and its satellites have paid special attention to the importance of carbon-enhanced metal-poor (CEMP) stars due to their involvement in Galactic formation history and their possible connection with the chemical elements originating in the first stellar generation. In an ongoing study of red giants in the Sculptor dwarf galaxy, we have discovered a star with extremely strong CN and CH molecular bands. This star, Scl-1013644, has previously been identified by Geisler et al. as a star with an enrichment in the heavy elements. Spectrum synthesis has been used to derive the carbon, nitrogen and barium abundances for Scl-1013644. Our findings are [C/Fe]=+0.8, [N/Fe]=-0.3 and [Ba/Fe]=+2.1 with the latter result consistent with the value found by Geisler et al. These results reveal Scl-1013644 as a CEMP-s star, the third such star discovered in this dwarf galaxy

An investigation of C, N, and Na abundances in red giant stars of the Sculptor dwarf spheroidal galaxy

The origin of the star-to-star abundance variations found for the light elements in Galactic globular clusters (GGCs) is not well understood, which is a significant problem for stellar astrophysics. While the light element abundance variations are very common in globular clusters, they are comparatively rare in the Galactic halo field population. However, little is known regarding the occurrence of the abundance anomalies in other environments such as that of dwarf spheroidal (dSph) galaxies. Consequently, we have investigated the anticorrelation and bimodality of CH- and CN-band strengths, which are markers of the abundance variations in GGCs, in the spectra of red giants in the Sculptor dwarf spheroidal galaxy. Using spectra at the Na D lines, informed by similar spectra for five GGCs (NGC 288, 1851, 6752, 6809, and 7099), we have also searched for any correlation between CN and Na in the Sculptor red giant sample. Our results indicate that variations analogous to those seen in GGCs are not present in our Sculptor sample. Instead, we find a weak positive correlation between CH and CN and no correlation between Na and CN. We also reveal a deficiency in [Na/Fe] for the Sculptor stars relative to the values in GGCs, a result that is consistent with the previous work for dSph galaxies. The outcomes reinforce the apparent need for a high stellar density environment to produce the light element abundance variations.This research has been supported in part by the Australian Research Council through Discovery Projects grants DP120101237, DP150103294, and FT140100554