Stellar Populations in the Local Group of Galaxies

The characteristics and properties of the stellar populations and evolutionary histories of Local Group galaxies are summarized and compared to predictions of cosmological models. No clear signature of the re-ionization epoch is observed; in particular, there is no cessation of star formation activity in low-mass dwarf galaxies at the end of re-ionization. Arguments against the morphological transformation of dwarf irregular into dwarf spheroidal galaxies are derived from their pronounced evolutionary differences at early epochs as evidenced by the offset in the metallicity-luminosity relation between gas-rich and gas-poor dwarfs. While there is increasing evidence for past and ongoing accretion events the overall importance of dwarf galaxies as building blocks remains unclear considering their differences in modes of star formation and detailed chemistry.Comment: Invited talk at the first international workshop on "Stellar Astrophysics with the World's Largest Telescopes", Torun, Poland, 7-10 September 2004, 14 pages, 2 figure

Structural transitions in the 309-atom magic number Lennard-Jones cluster

The thermal behaviour of the 309-atom Lennard-Jones cluster, whose structure is a complete Mackay icosahedron, has been studied by parallel tempering Monte Carlo simulations. Surprisingly for a magic number cluster, the heat capacity shows a very pronounced peak before melting, which is attributed to several coincident structural transformation processes. The main transformation is somewhat akin to surface roughening, and involves a cooperative condensation of vacancies and adatoms that leads to the formation of pits and islands one or two layers thick on the Mackay icosahedron. The second transition in order of importance involves a whole scale transformation of the cluster structure, and leads to a diverse set of twinned structures that are assemblies of face-centred-cubic tetrahedra with 6 atoms along their edges, i.e., one atom more than the edges of the 20 tetrahedra that make up the 309-atom Mackay icosahedron. A surface reconstruction of the icosahedron from a Mackay to an anti-Mackay overlayer is also observed, but with a lower probability.Comment: 7 pages, 4 figure