We report on the distribution of metallicities, [Fe/H], for very metal-poor stars in the halo of the Galaxy. Although the primary information on the nature of the Metallicity Distribution Function (MDF) is obtained from the two major recent surveys for metal-poor stars, the HK survey of Beers and collaborators, and the Hamburg/ESO Survey of Christlieb and collaborators, we also discuss the MDF derived from the publicly available database of stellar spectra and photometry contained in the third data release of the Sloan Digital Sky Survey (SDSS DR-3). Even though the SDSS was not originally planned as a stellar survey, significant numbers of stars have been observed to date – DR-3 contains spectroscopy for over 70,000 stars, at least half of which are suitable for abundance determinations. There are as many very metal-poor ([Fe/H] $< -2.0$) stars in DR-3 as have been obtained from all previous survey efforts combined. We also discuss prospects for significant expansion of the list of metal-poor stars to be obtained from the recently funded extension of the SDSS, which includes the project SEGUE: Sloan Extension for Galactic Understanding and Evolution.

We describe a programme that aims to increase the known sample of massive young stellar objects (MYSOs) by an order of magnitude. About 2000 candidates colour-selected from the MSX survey are being followed up at radio, mm and IR wavelengths to identify genuine MYSOs from the UCHII regions and other contaminants. Results so far indicate that the strategy does indeed deliver a significant fraction of luminous YSOs that will provide the basis for future galaxy-wide systematic studies.

Our mid-infrared and near-infrared surveys over the last five years have helped to strengthen and clarify the relationships between water, methanol, and OH masers and the star formation process. Our surveys show that maser emission seems to be more closely associated with mid-infrared emission than cm radio continuum emission from UC HII regions. We find that masers of all molecular species surveyed trace a wide variety of phenomena and show a proclivity for linear distributions. The vast majority of these linear distributions can be explained by outflows or shocks, and in general do not appear to trace circumstellar disks as was previously thought. Some water and methanol masers that are not associated with radio continuum emission appear to trace infrared-bright hot cores, the earliest observable stage of massive stellar life before the onset of a UC HII region.

Effective temperatures of 30 turnoff stars with $-3.2 {<}$[Fe/H]${<}{-}1.5$ have been derived from the profiles of Balmer lines in high S/N, VLT/UVES spectra. While the systematic error of $T_{\rm eff}$ may be of the order of 100K, the differential values of $T_{\rm eff}$ are determined with a one-sigma precision of ${\sim}25$K. These precise $T_{\rm eff}$ values are used in a study of the slope and dispersion of the Li abundance as a function of [Fe/H]. A small, but significant cosmic dispersion in $A$(Li) appears to be present exemplified by the two very metal-poor stars G64-12 and G64-37.

We calculate evolution, explosion and nucleosynthesis of 1000$M_{\odot}$ stars. Even such massive stars may explode at the end of their lives if they rotate. We use a 2 dimensional hydrodynamical code to take aspherisity by the effect of rotation into account. Our results show that (1) abundance pattern of ejected matter by explosion is consistent with observational data of intracluster medium gas and M82 hot gas, (2) such massive stars can supply more efficient UV photons to re-ionize HI, HeI and HeII than ordinary massive stars (less than 100 solar-masses) and (3) final black hole mass is 500 solar-mass, which is consistent of the mass scale of intermediate-mass black hole (IHBH) found in M82.

This review discusses near- and mid- infrared observations of Ultracompact (UC) HII regions. The importance of ISO mid-IR fine-structure nebular lines is emphasised, since only a small fraction of UCHII regions are observed directly in the near-IR. The reliability of contemporary atmospheric models for such indirect diagnostics is discussed, whilst a revised spectral type-temperature calibration is presented for Galactic O3 to B3 dwarfs. In particular, fine-structure line derived properties of G29.96–0.02 differ from the direct near-IR spectroscopic result and represents a serious discrepancy which needs to be addressed. Mid-IR MSX and Spitzer imaging permits the identification of those UCHII regions for which far-IR IRAS fluxes are reliable, relevant to the single versus cluster nature of individual sources. High spatial resolution imaging with ground-based 8m telescopes allows more direct tests, as recently applied to G70.29+1.60. Finally, recent Spitzer mid-IR observations of Giant HII regions are briefly discussed.

IR imaging, K–band spectroscopy and X-ray data of the stellar cluster in M 17 suggests three categories of massive young objects: I) “Naked” early-type O-stars in the cluster center that exhibit X-ray emission and normal optical/IR-spectra; they do not show signs for any circumstellar material. II) Late O-type stars, that show X-ray emission and H/He absorption lines in their optical/IR-spectra. There is evidence for extended circumstellar material in the form of elongated L–band nebulae, circumstellar MIR emission from regions of about 10.000 AU in diameter and filaments of He I $1.083\,\mu$m. III) Massive stars without X-ray emission, K–band spectra with emission lines of H and He and in some cases with CO band head emission. They are heavily enshrouded and also show elongated L–band nebulae, circumstellar MIR emission from regions of about 4.000 AU and ionized He envelopes.

Obviously, M 17 harbors several generations of massive stars: Only the oldest “Category I”, located in the cluster center, is free of circumstellar material. “Category II” and “III” comprise younger dust-enshrouded objects of a second generation embedded in the interface of the HII region and the adjacent molecular cloud. Due to the presence of flattened dusty disks and perpendicular gaseous outflows, accretion seems to be important for the formation of stars up to type O8.

A population of X-ray emitting sources with($L_{\rm bol} \sim 10^4\,$L$_\odot$) shows CO band head absorption; one source was found to have variable CO absorption as known from FU Orionis stars.

New data on the 20.000 AU accretion disk in M 17 reveals a collimated H$_2$ $2.122\,\mu$m jet with three distinct emission knots that emerges from the disk center. The broad H$\alpha$ and Ca II emission lines from the bipolar nebula display blue-shifted absorption originating from accretion disk-driven outflows with velocities of $\pm 500$ km/s.

We present results from a high resolution study of six Li-depleted halo dwarfs. These stars exhibit Lithium abundances more than 0.5 dex below the Spite Plateau. The study is comprised of two parts: 1) a detailed abundance analysis covering many light, alpha, Fe-peak and neutron capture elements and 2) a study of rotational line broadening. We find that there are no common abundance anomalies, other than the Lithium depletion, that distinguish these stars from the Li-normal population. Four of the six stars however, show evidence of abnormally high projected rotation velocities. These findings support a non-standard depletion mechanism in Li-depleted halo stars, and emphasize their necessary exclusion from future studies of the Spite Plateau.

Despite its importance, the Galactic Bulge and its globular cluster (GC) system in particular, remain relatively unexplored due to high foreground extinction and severely crowded field populations. The new generation of high resolution IR spectrographs opens new and unique perspectives to obtain detailed abundances in the Bulge field and GCs. Using the NIRSPEC spectrograph at Keck II, high resolution, echelle spectra in the range 1.5–1.8 micron of bright giants in the Baade's window and other fields closer to the Galactic center as well as in a sample of 10 Bulge GCs have been obtained. We present the results of our abundance analysis of Fe, Al, 12C, 13C, O, and other α-elements, by using full spectral synthesis techniques.

We present atmospheric parameters, Li abundances, obtained from a detailed spectroscopic analysis, for a sample of stars with different evolutionary stages (turn–off, subgiant, giant stars) in the solar age cluster M67. Observations were carried out with high resolution ($R\sim47000$) at high S/N using the UVES${+}$FLAMES at VLT/UT2. From available photometry, computed synthetic spectra for the region around the Lithium line at 6707.78Å, we derived atmospheric parameters (T$_{\scriptsize\it eff}$, $\log g$, [Fe/H], $v\sin i$), A$_{Li}$ for each star, in order to better underst, the level of mixing, convective dilution of evolved stars in M67.

We have determined the CNO abundances from OH, CH and NH lines in the VLT/UVES UV spectra of HE0107-5240 and CD-38 245 together with the abundances of several metal species, including FeII, whose lines were not detected in blue spectra. The UV abundance analyses support those determined from the blue spectra, in particular the abundance from the FeII lines is in good agreement with the Fe abundance derived from the Fe I lines. It seems unlikely that the large overabundances of CNO have been produced in a medium-mass AGB star which now evolved to a white dwarf. The oxygen abundance is consistent with the predictions of Umeda & Nomoto (2003, 2005), who proposed that HE0107-5240 has formed from a gas cloud which has been enriched by the yields of a $\approx 25$ solar mass Population III star exploding as a supernova of low explosion energy E${=} 3.10^{50}$ erg with mixing and fallback. The odd-even effect predicted in the original models are not evident in the Sc and Co abundances of HE0107-5240, which are better fitted by the low density variant progenitor model.

This research forms the second part of an investigation into heavy element abundances in Asymptotic Giant Branch (AGB) stars in different stellar environments. High resolution spectroscopy was taken of seven known AGB stars in 47 Tucanae and spectrum synthesis undertaken to obtain abundances for s-process elements. Contrary to theoretical predictions, preliminary results show that these AGB stars do show s-process enhancements relative to previously studied Red Giant Branch (RGB) stars in the same cluster.

Observations of the nearest regions of massive star formation such as Orion are reviewed. Early-type stars in the local OB associations, as well as their superbubbles and supershells provide a fossil record of massive star birth in the Solar vicinity over about the last 40 Myr. This record shows that most massive stars are born from dense, high-pressure, hot cores which spawn transient clusters that dissipate into the field soon after formation. A large fraction (15 to 30%) of massive stars are high-velocity runaways moving at more than 20 km s$^{-1}$. High-mass stars have a larger companion fraction than their lower-mass siblings. The Orion star forming complex contains the nearest site of on-going massive star formation. Studies of the Orion Nebula and the dense molecular cloud core located immediately behind the HII region provide our sharpest view of massive star birth. This region has formed a hierarchy of clusters within clusters. The Trapezium, OMC-1S, and OMC-1 regions represent three closely spaced sub-clusters within the more extended Orion Nebula Cluster. The oldest of these sub-clusters, which consists of the Trapezium stars, has completely emerged from its natal core. The OMC-1S and OMC-1 regions, are still highly embedded and forming clusters of additional moderate and high mass stars. Over a dozen YSOs embedded in OMC-1S are driving jets and outflows, many of which are injecting energy and momentum into the Orion Nebula. Recent proper motion measurements indicate that the Becklin-Neugebauer object is a high-velocity star moving away from the OMC1 core with a velocity of 30 km s$^{-1}$, making it the youngest high-velocity star known. Source I may be moving in the opposite direction with a velocity of about 12 km s$^{-1}$. The projected separation between source I and BN was less than few hundred AU about 500 years ago. The spectacular bipolar molecular outflow and system of shock-excited H$_2$ fingers emerging from OMC-1 has a dynamical age of about 1100 years. It is possible that a dynamical interaction between three or more stars in OMC-1 led to the formation of this eruptive outflow.

Searching for objects in the earliest phases of star formation, e.g sources at the beginning of a gravitational collapse, are essential to our understanding of massive star formation. Today a number of precursors of ultra compact HII regions (PUCHs) have been found. Embedded in dense gas and dust, these PUCHs have a high bolometric luminosity but little or no 6 cm radio continuum emission (Molinari, et al. 2000; Beuther, et al. 2002). Evidence for Collapse was found in ultra compact (UC) HII regions and 12 water maser sources (Zhang, et al. 1998; Wu & Evans II 2003). This paper presents the identification of massive cores with no detectable infrared and radio sources. These kinds of cores usually have strong sub-mm emission. A special case is the SCUBA core JCMT 18354-0649S which has both infall and outflow motions as indicated by the profiles of high excitation molecular lines. This core is at a stage earlier than PUCHs. Blue profiles are also found in UC HII region, which indicates that material is still infalling in this phase. Our observations suggest that infall exists in different evolutionary stages for high mass star formation, similar to the low mass cases.

We present a detailed and uniform study of oxygen abundances in a large set of 155 metal-rich dwarfs. EW measurements were carried out for the [OI] 6300 Å line and the OI triplet, while spectral synthesis was performed for several OH lines. NLTE corrections were calculated and applied to the LTE abundance results derived from the triplet. Abundances from [OI], the OI triplet and near-UV OH were obtained in 103, 87 and 77 dwarfs, respectively. A good agreement between [O/H] ratios from forbidden and OH lines is found, while the NLTE triplet shows a systematically lower abundance. Nevertheless, the consistency with other indicators improves if we consider LTE triplet results. In any case, discrepancies between OH, [OI] and the OI triplet hardly exceed 0.2dex. All three indicators show that, on average, [O/Fe] decreases with [Fe/H] in the metallicity range −0.8<[Fe/H]<0.5.

The LTE abundances of sulfur (S) were explored in the sample of 15 metal-poor stars with the metallicity range of −4<[Fe/H]<−1.5, based on the equivalent widths of the S I(1) 9212 and 9237 Å lines measured on high-resolution spectra, which were observed by the Keck I HIRES. Combining our results and those of Takada-Hidai et al. (2005), we found that the behavior of [S/Fe] against [Fe/H] shows a nearly flat trend in the range of metallicity down to [Fe/H]∼−4.

The international Project “Lithium in magnetic CP stars” has been put into operation using telescopes of the Crimean Astrophysical Observatory (2.6-m reflector ZTSH), European Southern Observatory (1.4-m CAT, 1.52-m with FEROS), Mount Stromlo Observatory (74-inch), Nordic Optical Telescope Scientific Association (2.4-m with SOFIN), Special Astrophysical Observatory of the Russian Academy of Sciences (6-m BTA). Here we present an historical report of the different scientific results of this project since its beginning in 1996.

Stars form by gravoturbulent fragmentation of interstellar gas clouds. The supersonic turbulence ubiquitously observed in Galactic molecular gas generates strong density fluctuations with gravity taking over in the densest and most massive regions. Collapse sets in to build up stars and star clusters.

Turbulence plays a dual role. On global scales it provides support, while at the same time it can promote local collapse. Stellar birth is thus intimately linked to the dynamic behavior of parental gas clouds, which governs when and where protostellar cores form, and how they contract and grow in mass via accretion from the surrounding cloud material to build up stars. The equation of state plays a pivotal role in the fragmentation process. Under typical cloud conditions, massive stars form as part of dense clusters following the “normal” mass function observed, e.g. in the solar neighborhood. However, for gas with an effective polytropic index greater than unity star formation becomes biased towards isolated massive stars. This is relevant for understanding the properties of zero-metallicity stars (Population III) or stars that form under extreme environmental conditions like in the Galactic center or in luminous starbursts.

Fundamental parameters and lithium abundances, ALi, have been derived for a sample of evolved stars in the globular cluster 47 Tuc (from GIRAFFE spectra and MARCS models of atmosphere). These data sample a complete evolutionary sequence from subgiant stage to the tip of the Asymptotic Giant Branch. With this unique observational data set we have analyzed the evolution of ALi along the Red Giant Branch (RGB) using non-standard stellar evolution models, so as to explore the occurence and the efficiency of extra-mixing processes in low mass stars at the so-called Red Giant bump.

The question of how metals are produced in the Universe and where they are located is fundamental for our understanding of galaxy formation and evolution. This question can be best addressed using absorption lines seen in the spectra of remote quasars. It has been realized that the spatial distribution of metals around galaxies and more generally in the Intergalactic Medium is complex, and it is often very difficult to associate one absorption system with one galaxy. Except for possibly in the special case of DLA systems, it may be more appropriate to discuss the clustering properties of different classes of objects and to measure their correlation functions. I illustrate these issues with three examples: the distribution of metals around galaxies at intermediate redshift, the modelling of the clustering of C IV systems, and the determination of abundances in Damped Lyman-α systems (and in particular the oxygen abundance). Finally, I note that there is a mass-metallicity relation in Damped Lyman-α systems.