We describe the public ESO near-IR variability survey (VVV) scanning the Milky Way bulge and an adjacent section of the mid-plane where star formation activity is high. The survey will take 1929 h of observations with the 4-m VISTA telescope during 5 years (2010-2014), covering ˜109 point sources across an area of 520 deg2, including 33 known globular clusters and ˜350 open clusters. The final product will be a deep near-IR atlas in five passbands (0.9-2.5 μm) and a catalogue of more than 106 variable point sources. Unlike single-epoch surveys that, in most cases, only produce 2-D maps, the VVV variable star survey will enable the construction of a 3-D map of the surveyed region using well-understood distance indicators such as RR Lyrae stars, and Cepheids. It will yield important information on the ages of the populations. The observations will be combined with data from MACHO, OGLE, EROS, VST, Spitzer, HST, Chandra, INTEGRAL, WISE, Fermi LAT, XMM-Newton, GAIA and ALMA for a complete understanding of the variable sources in the inner Milky Way. This public survey will provide data available to the whole community and therefore will enable further studies of the history of the Milky Way, its globular cluster evolution, and the population census of the Galactic Bulge and center, as well as the investigations of the star forming regions in the disk. The combined variable star catalogues will have important implications for theoretical investigations of pulsation properties of stars

We present Hubble Space Telescope observations that show a bifurcation of colors in the middle main sequence of the globular cluster q Centauri. We see this in three different fields, observed with different cameras and filters. We also present high-precision photometry of a central Advanced Camera for Surveys field, which shows a number of main-sequence turnoffs and subgiant branches. The double main sequence, the multiple turnoffs and subgiant branches, and other population sequences discovered in the past along the red giant branch of this cluster add up to a fascinating but frustrating puzzle. We suggest various explanations, none of them very conclusive.

Having shown in a recent paper that the main sequence of ω Centauri is split into two distinct branches, we now present spectroscopic results showing that the bluer sequence is less metal-poor. We have carefully combined GIRAFFE@VLT spectra of 17 stars on each side of the split into a single spectrum for each branch, with adequate S/N to show clearly that the stars of the blue main sequence are less metal poor by 0.3 dex than those of the dominant red one. From an analysis of the individual spectra, we could not detect any abundance spread among the blue main sequence stars, while the red main sequence stars show a 0.2 dex spread in metallicity. We use stellar-structure models to show that only greatly enhanced helium can explain the color difference between the two main sequences, and we discuss ways in which this enhancement could have arisen.

Aims. In this paper we derive the structure of the Galactic stellar warp and flare. Methods. We use 2MASS red clump and red giant stars, selected at mean and fixed heliocentric distances of R 3, 7 and 17 kpc. Results. Our results can be summarized as follows: (i) a clear stellar warp signature is derived for the 3 selected rings, proving that the warp starts already within the solar circle; (ii) the derived stellar warp is consistent (both in amplitude and phase-angle) with that for the Galactic interstellar dust and neutral atomic hydrogen; (iii) the consistency and regularity of the stellar-gaseous warp is traced out to about R GC ∼ 20 kpc; (iv) the Sun seems not to fall on the line of nodes. The stellar warp phase-angle orientation (φ ∼ 15 • ) is close to the orientation angle of the Galactic bar and this, most importantly, produces an asymmetric warp for the inner R 3 and 7 kpc rings; (v) a Northern/Southern warp symmetry is observed only for the ring at R 17 kpc, at which the dependency on φ is weakened; (vi) treating a mixture of thin and thick disk stellar populations, we trace the variation with R GC of the disk thickness (flaring) and derive an almost constant scale-height (∼0.65 kpc) within R GC ∼ 15 kpc. Further out, the disk flaring increase gradually reaching a mean scale-height of ∼1.5 kpc at R GC ∼ 23 kpc; (vii) the derived outer disk warping and flaring provide further robust evidence that there is no disk radial truncation at R GC ∼ 14 kpc. Conclusions. In the particular case of the Canis Major (CMa) over-density we confirm its coincidence with the Southern stellar maximum warp occurring near l ∼ 240 • (for R 7 kpc) which brings down the Milky Way mid-plane by ∼3 • in this direction. The regularity and consistency of the stellar, gaseous and dust warp argues strongly against a recent merger scenario for Canis Major. We present evidence to conclude that all observed parameters (e.g. number density, radial velocities, proper motion etc) of CMa are consistent with it being a normal Milky Way outer-disk population, thereby leaving no justification for more complex interpretations of its origin. The present analysis or outer disk structure does not provide a conclusive test of the structure or origin of the Monoceros Ring. Nevertheless, we show that a warped flared Milky Way contributes significantly at the locations of the Monoceros Ring. Comparison of outer Milky Way H i and CO properties with those of other galaxies favors the suggestion that complex structures close to planar in outer disks are common, and are a natural aspect of warped and flaring disks.

We have obtained new spectrophotometric data for 28 H II regions in the spiral galaxy NGC 300, a member of the nearby Sculptor Group. The detection of several auroral lines, including [O III] λ4363, [S III] λ6312 and [N II] λ5755, has allowed us to measure electron temperatures and direct chemical abundances for the whole sample. We determine for the first time in this galaxy a radial gas-phase oxygen abundance gradient based solely on auroral lines, and obtain the following least-square solution: 12 + log(O/H) = 8.57 (±0.02) − 0.41 (±0.03) R/R 25 , where the galactocentric distance is expressed in terms of the isophotal radius R 25 . The characteristic oxygen abundance, measured at 0.4×R 25 , is 12 + log(O/H) = 8.41. The gradient corresponds to −0.077 ± 0.006 dex kpc −1 , and agrees very well with the galactocentric trend in metallicity obtained for 29 B and A supergiants in the same galaxy, −0.081 ± 0.011 dex kpc −1 . The intercept of the regression for the nebular data virtually coincides with the intercept obtained from the stellar data, which is 8.59 (±0.05). This allows little room for depletion of nebular oxygen onto dust grains, although in this kind of comparison we are somewhat limited by systematic uncertainties, such as those related to the atomic parameters used to derive the chemical compositions.We discuss the implications of our result with regard to strong-line abundance indicators commonly used to estimate the chemical compositions of star-forming galaxies, such as R 23 . By applying a few popular calibrations of these indices based on grids of photoionization models on the NGC 300 H II region fluxes we find metallicities that are higher by 0.3 dex (a factor of two) or more relative to our nebular (T e -based) and stellar ones.We detect Wolf-Rayet stellar emission features in ∼1/3 of our H II region spectra, and find that in one of the nebulae hosting these hot stars the ionizing field has a particularly hard spectrum, as gauged by the 'softness'We suggest that this is related to the presence of an early WN star. By considering a larger sample of extragalactic H II regions we confirm, using direct abundance measurements, previous findings of a metallicity dependence of η, in the sense that softer stellar continua are found at high metallicity.

We address the problem of the formation and evolution of elliptical galaxies (from dwarf to normal/giant systems). In particular, by means of N-body-tree-SPH simulations, incorporating cooling, star formation, energy feedback and chemical evolution, we intend to explore whether the formation of elliptical galaxies from the monolithic collapse of baryons inside non-rotating, virialized haloes of dark matter leads to results compatible with the main body of observational data on these systems. We show that this scenario can indeed reproduce the basic structural properties of observed elliptical galaxies of different mass.We study the star formation history and the chemical enrichment of the models, showing that the duration, strength and shape of the star formation rate as a function of time depend strongly on the galaxy mass and the initial density. More precisely, massive elliptical galaxies, independent of their initial density, show a single burst of star formation, whereas the low-mass ones have a varied star formation history. It is early and monolithic in the high initial density systems, and irregular and intermittent in the low-density ones.The occurrence of galactic winds is also analysed, and we conclude that in general all galaxies are able to eject part of their gas content into the intergalactic medium. However, the percentage of the ejected material increases with decreasing galaxy mass.Our models are used to interpret the fundamental plane (mass-to-light ratio versus mass relationship), the mass-to-light ratio versus central velocity dispersion, the colour-magnitude relation and the mass-radius relationships of elliptical galaxies from dwarfs to giants. For each relationship in question the agreement between observations and theory is remarkable.Based on this, we endeavour to speculate on a unified scheme for the formation of stellar aggregates going from globular clusters to normal/giant elliptical galaxies in which initial density, type of star formation (single initial episode or recurrent bursts of activity) and galactic winds are the key ingredients. Finally, we draw some conclusions concerning the long-standing question as to whether the hierarchical or the monolithic scheme for the formation of galaxies (ellipticals) ought to be preferred.

Eighteen RR Lyrae variables (RRLs) that lie in the ``12.4h clump'' identified by the Quasar Equatorial Survey Team (QUEST) have been observed spectroscopically to measure their radial velocities and metal abundances. Ten blue horizontal branch (BHB) stars identified by the Sloan Digital Sky Survey (SDSS) were added to this sample. Six of the nine stars in the densest region of the clump have a mean radial velocity in the Galactic rest frame (Vgsr) of 99.8 and σ=17.3 km s-1, which is slightly smaller than the average error of the measurements. The whole sample contains eight RRLs and five BHB stars that have values of Vgsr suggesting membership in this stream. For seven of these RRLs, the measurements of [Fe/H], which have an internal precision of 0.08 dex, yield <[Fe/H]>=-1.86 and σ=0.40. These values suggest that the stream is a tidally disrupted dwarf spheroidal galaxy of low luminosity. Photometry from the database of the SDSS indicates that this stream covers at least 106 deg2 of the sky in the constellation Virgo. The name Virgo stellar stream is suggested

A comparison is made between the age–metallicity relations obtained from four different types of studies: F and G stars in the solar neighbourhood, analysis of open clusters, galactic structure studies with the stellar population synthesis technique and chemical evolution models. Metallicities of open clusters are corrected for the effects of the radial gradient, which we find to be −0.09 dex kpc−1 and most likely constant in time. We do not correct for the vertical gradient, because its existence and value are not firmly established. Stars and clusters trace a similar age–metallicity relation, showing an excess of rather metal‐rich objects in the age range 5–9 Gyr. Galactic structure studies tend to give a more metal‐poor relation than chemical evolution models. Neither relation explains the presence of old, relatively metal‐rich stars and clusters. This might be caused by uncertainties in the ages of the local stars, or pre‐enrichment of the disc with material from the bulge, possibly as a result of a merger event in the early phases of the formation of our Galaxy.