Publ. Astron. Soc. Pac., 108, 1141-1141 (1996/December-0)
Stellar population studies in nearby galaxies. (Dissertation summary).
Abstract (from CDS):
My Thesis on "Stellar Population Studies in Nearby Galaxies" covers the development of new tools for stellar population studies, study of young Magellanic Cloud (MC) clusters and their surroundings study of old and intermediate-age populations in two dwarf spheroidal (dSph) galaxies. The new tools for population studies include the removal of extinction considering its dependence on stellar temperature, surface gravity, and metallicity (e.g., Grebel & Roberts 1995), and the determination of age, metallicity, reddening, and distance through simultaneous isochrone fitting to multi-colour photometry (Grebel et al. 1994a). The young SMC cluster NGC 330 is considered a cornerstone for understanding stellar evolution in metal-poor environments. I investigate why previous [Fe/H] determinations for this cluster differ by more than 1 dex and show that they can be largely reconciled when using the same reddening. Different star formation mechanisms seem to have been at work for clusters of similar age. Large-scale star formation processes in the field appear to have triggered the collapse of the molecular clouds that formed NGC 330 and NGC 2004 in the LMC, while the surrounding field of NGC 1818 in the LMC is older than the cluster itself (Grebel 1996). Previous studies found several bright blue stars in young MC clusters to be giants spectroscopically and to lie in the blue Hertzsprung gap suggesting that stellar evolutionary theories may need to be revised and that large age spreads exist in these clusters. We argue (Grebel et al. 1996) that these stars probably are core H burning main-sequence stars that appear as blue stragglers resulting from binary evolution and/or rapid rotation. Many are Be stars and likely rapid rotators. Blue stragglers are also present in NGC 1818, NGC 2004, and NGC 2100 and possibly in all young clusters in the MCs. This suggests that (1) previously inferred large spreads in star formation times are much smaller, (2) ages of young MC clusters may have been systematically underestimated by mistaking blue stragglers for the main-sequence turnoff, and (3) the upper slopes of the IMFs may need to be re-evaluated (Grebel et al. 1996). With an efficient photometric survey method for emission-line stars (Grebel et al. 1992, 1994b, Grebel 1996) we investigated the Be star content of young MC clusters. In NGC 330 also variable Be stars and binaries were identified. NGC 330 and the slightly older NGC 1818 are very rich in Be stars (Grebel 1996), while NGC 2004 has a lower Be star content despite similar metallicity and age. H-alpha luminosity and infrared excess are well correlated though they do not seem to depend on spectral class, while H-alpha luminosity and ultraviolet excess are only weakly correlated. For the Sculptor and Fornax dSph galaxies we established the existence of subpopulations distinct in metallicity and in age (Grebel et al. 1994a). In Sculptor old and metal-poor populations dominate. In Fornax, we find a mixture of intermediate-age field populations and an age gradient with galactocentric radius. The lack of a distinguishable old field population, the metallicity spread in Fornax's globular clusters, and their larger age indicate that they may not have formed in Fornax itself. On the other hand, the suggested 1 Gyr-old subgiants may instead represent a sparsely populated horizontal branch of the missing old field population. Fornax's intermediate-age field population formed over a period of about 10 Gyr and shows considerable enrichment, while Sculptor's populations are comparatively well-confined in ages and abundances. The impact on evolutionary scenarios of the Local Group is discussed. This work was supported by a Student Fellowship of the European Southern Observatory, by the Collaborative Visitors Program of the Space Telescope Science Institute, and by a Graduate Fellowship of the German Research Foundation (DFG-Graduiertenkolleg "Magellanic Clouds").
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