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We present a detailed chemical composition analysis of 35 red giant stars in the globular cluster M 22. High resolution spectra for this study were obtained at five observatories, and analyzed in a uniform manner. We have determined abundances of representative light proton-capture, α, Fe-peak and neutron-capture element groups. Our aim is to better understand the peculiar chemical enrichment history of this cluster, in which two stellar groups are characterized by a different content in iron, neutron capture elements Y, Zr and Ba, and α element Ca

Myoblast fusion is an essential step during muscle differentiation. Previous studies in Drosophila have revealed a signaling pathway that relays the fusion signal from the plasma membrane to the actin cytoskeleton. However, the function for the actin cytoskeleton in myoblast fusion remains unclear. Here we describe the characterization of solitary (sltr), a component of the myoblast fusion signaling cascade. sltr encodes the Drosophila ortholog of the mammalian WASP-interacting protein. Sltr is recruited to sites of fusion by the fusion-competent cell-specific receptor Sns and acts as a positive regulator for actin polymerization at these sites. Electron microscopy analysis suggests that formation of F-actin-enriched foci at sites of fusion is involved in the proper targeting and coating of prefusion vesicles. These studies reveal a surprising cell-type specificity of Sltr-mediated actin polymerization in myoblast fusion, and demonstrate that targeted exocytosis of prefusion vesicles is a critical step prior to plasma membrane fusion.

We used high-resolution, high signal-to-noise ratio spectra obtained with the Very Large Telescope and the UVVisual Echelle Spectrograph to determine abundances of 17 elements in four red giants in the Sculptor (Scl ) dwarf spheroidal galaxy. Our ½Fe=H -values range from À2.10 to À0.97, confirming previous findings of a large metallicity spread. We combined our data with similar data for five Scl giants studied recently by Shetrone et al. to form one of the largest samples of high-resolution abundances yet obtained for a dwarf spheroidal galaxy, covering essentially the full known metallicity range in this galaxy. These properties allow us to establish trends of ½X=Fe with ½Fe=H for many elements X. The trends are significantly different from the trends seen in Galactic halo and globular cluster stars. This conclusion is evident for most of the elements from oxygen to manganese. We compare our Scl sample with the most similar Galactic counterparts and find substantial differences remain even with these stars. The many discrepancies in the relationships between ½X=Fe as seen in Scl compared with Galactic field stars indicate that our halo cannot be made up in bulk of stars similar to those presently seen in dwarf spheroidal galaxies like Scl, corroborating similar conclusions reached by Shetrone et al., Fulbright, and Tolstoy et al. These results have serious implications for the Searle-Zinn and hierarchical galaxy formation scenarios. We also find that the most metal-rich star in our sample is a heavy element-rich star. This star and the ½Ba=Eu trend we see indicate that asymptotic giant branch stars must have played an important role in the evolution of the s-process elements in Scl. A very high percentage of such heavy-element stars are now known in dwarf spheroidals compared with the halo, further mitigating against the formation of the halo from such objects.

We propose the concept of a "Galactic Habitable Zone" (GHZ). Analogous to the Circumstellar Habitable Zone (CHZ), the GHZ is that region in the Milky Way where an Earth-like planet can retain liquid water on its surface and provide a long-term habitat for animal-like aerobic life. In this paper we examine the dependence of the GHZ on Galactic chemical evolution. The single most important factor is likely the dependence of terrestrial planet mass on the metallicity of its birth cloud. We estimate, very approximately, that a metallicity at least half that of the Sun is required to build a habitable terrestrial planet. The mass of a terrestrial planet has important consequences for interior heat loss, volatile inventory, and loss of atmosphere. A key issue is the production of planets that sustain plate tectonics, a critical recycling process that provides feedback to stabilize atmospheric temperatures on planets with oceans and atmospheres. Due to the more recent decline from the early intense star formation activity in the Milky Way, the concentration in the interstellar medium of the geophysically important radioisotopes, 40 K, 235,238 U, 232 Th, has been declining relative to Fe, an abundant element in the Earth. Also likely important are the relative abundances of Si and Mg to Fe, which affects the mass of the core relative to the mantle in a terrestrial planet. All these elements and isotopes vary with time and location in the Milky Way; thus, planetary systems forming in other locations and times in the Milky Way with the same metallicity as the Sun will not necessarily form habitable Earth-like planets. As a result of the radial Galactic metallicity gradient, the outer limit of the GHZ is set primarily by the minimum required metallicity to build large terrestrial planets. Regions of the Milky Way least likely to contain Earth-mass planets are the halo (including globular clusters), the thick disk, and the outer thin -3 -disk. The bulge should contain Earth-mass planets, but stars in it have a mix of elements different from the Sun's. The existence of a luminosity-metallicity correlation among galaxies of all types means that many galaxies are too metal-poor to contain Earth-mass planets. Based on the observed luminosity function of nearby galaxies in the visual passband, we estimate that: 1) the Milky Way is among the 1.3% most luminous (and hence most metal-rich) galaxies, and 2) about 23% of stars in a typical ensemble of galaxies are more metal-rich than the average star in the Milky Way. The GHZ zone concept can be easily extrapolated to the universe as a whole, especially with regard to the changing star formation rate and its effect on metallicity and abundances of the long-lived radioisotopes.

Myoblast fusion is essential for the formation and regeneration of skeletal muscle. In a genetic screen for regulators of muscle development in Drosophila, we discovered a gene encoding a guanine nucleotide exchange factor, called loner, which is required for myoblast fusion. Loner localizes to subcellular sites of fusion and acts downstream of cell surface fusion receptors by recruiting the small GTPase ARF6 and stimulating guanine nucleotide exchange. Accordingly, a dominant-negative ARF6 disrupts myoblast fusion in Drosophila embryos and in mammalian myoblasts in culture, mimicking the fusion defects caused by loss of Loner. Loner and ARF6, which also control the proper membrane localization of another small GTPase, Rac, are key components of a cellular apparatus required for myoblast fusion and muscle development. In muscle cells, this fusigenic mechanism is coupled to fusion receptors; in other fusion-competent cell types it may be triggered by different upstream signals.

The results of new spectroscopic analyses of 30 stars with giant planet and/or brown dwarf companions are presented. Values for T eff and [Fe/H] are used in conjunction with Hipparcos data and Padova isochrones to derive masses, ages, and theoretical surface gravities. These new data are combined with spectroscopic and photometric metallicity estimates of other stars harboring planets and published samples of F, G, and K dwarfs to compare several subsets of planet bearing stars with similarly well-constrained control groups. The distribution of [Fe/H] values continues the trend uncovered in previous studies in that stars hosting planetary companions have a higher mean value than otherwise similar nearby stars. We also investigate the relationship between stellar mass and the presence of giant planets and find statistically marginal but suggestive evidence of a decrease in the incidence of radial velocity companions orbiting relatively less massive stars. If confirmed with larger samples, this would represent a critical constraint to both planetary formation models as well as to estimates of the distribution of planetary systems in our galaxy.