Most stars, including all massive stars, form in clusters. Here, I present (sub-)millimeter observations of young stellar clusters in two well kown massive star forming regions. First, I discuss relatively low mass cluster formation across the Rosette molecular cloud and the influence of the OB association, NGC 2244 (the Rosette nebula) on their properties. Second, I present SMA observations of the Trapezium cluster in Orion and the detection of emission from protoplanetary disks (proplyds) around 4 low mass stars. The implications for Solar System scale planet formation around low mass stars in high mass star forming environments are discussed.

We present preliminary results of a few observing programs conducted with the FLAMES fiber facility at VLT2 ESO telescope. These programs show the large potentiality of FLAMES for investigations of globular clusters. The programs described here concern the derivation of precise reddening and metallicity for globular clusters, and the derivation of abundances for stars on the main sequence of ω Cen. Reddenings with errors of $\Delta E(B-V)=0.005$ mag and metallicities with errors of ±0.02 dex (in a scale defined by local subdwarfs) can be obtained in very short observing time. Our results for ω Cen show that the blue main sequence is more metal-rich than the red-main sequence: this requires a large He-content for the blue main sequence.

Theoretical considerations lead to the expectation that stars should not have masses larger than about $m_{\rm max*}=60$–$120M_\odot$, while the observational evidence has been ambiguous. Only very recently has a physical stellar mass limit near $150M_\odot$ emerged thanks to modern high-resolution observations of local star-burst clusters. But this limit does not appear to depend on metallicity, in contradiction to theory. Important uncertainties remain though. It is now also emerging that star-clusters limit the masses of their constituent stars, such that a well-defined relation between the mass of the most massive star in a cluster and the cluster mass, $m_{\rm max}={\cal F}(M_{\rm ecl}) \le m_{\rm max*}\approx 150M_\odot$, exists. One rather startling finding is that the observational data strongly favour clusters being built-up by consecutively forming more-massive stars until the most massive stars terminate further star-formation. The relation also implies that composite populations, which consist of many star clusters, most of which may be dissolved, must have steeper composite IMFs than simple stellar populations such as are found in individual clusters. Thus, for example, $10^5$ Taurus–Auriga star-forming groups, each with 20 stars, will ever only sample the IMF below about $1M_\odot$. This IMF will therefore not be identical to the IMF of one cluster with $2\times 10^6$ stars. The implication is that the star-formation history of a galaxy critically determines its integrated galaxial IMF and thus the total number of supernovae per star and its chemical enrichment history. Galaxy formation and evolution models that rely on an invariant IMF would be wrong.

Young stars on their way to the ZAMS evolve in significantly different ways depending on their mass. While the theoretical and observational properties of low- and intermediate-mass stars are rather well understood and/or empirically tested, the situation for massive stars ([gsim]10–15 M$_\odot$) is, to say the least, still elusive. On theoretical grounds, the PMS evolution of these objects should be extremely short, or nonexistent at all. Observationally, despite a great deal of effort, the simple (or bold) predictions of simplified models of massive star formation/evolution have proved more difficult to be checked. After a brief review of the theoretical expectations, I will highlight some critical test on young stars of various masses.

We provide new nucleosynthesis yields depending on metallicity and energy (i.e., (normal supernovae and hypernovae), and show the evolution of heavy element abundances from C to Zn in the solar neighborhood. We then show the chemodynamical simulation of the Milky Way Galaxy and discuss the G-dwarf problem. We finally show the cosmological simulation and discuss the galaxy formation and chemical enrichment.

We present the values of CN and Mg overabundances with respect to Fe, for a large sample of elliptical galaxies in different environments. Abundances were derived by confronting observed absorption line indices with stellar population model spectra. We obtained significant differences between the [CN/Fe] and [Mg/Fe] abundance ratios as a functions of: i) the environment, and ii) the galaxy mass. This is interpreted as implying varying formation timescales for CN, Mg and Fe, combined with different star formation histories in elliptical galaxies depending on their mass and environment. Our principal conclusions are: 1) CN is sensitive to the characteristic assembly timescales of elliptical galaxies, 2) more massive elliptical galaxies are assembled on shorter timescales than less massive ones, 3) elliptical galaxies in denser environments are assembled on shorter timescales than those in lower density environments, and 4) our results strongly suggest an upper limit for the assembly timescale of ∼1 Gy, in all cases.

We have observed seven giants in the metal-poor globular cluster M15 using Subaru/HDS. We confirmed that there are significant star-to-star variations in the neutron-capture elemental abundances. This abundance variation means there were primordial chemical inhomogeneities in the proto-globular cluster cloud of M15. This result implies that there was insufficient time for complete mixing after r-process nucleosynthesis. It suggests that the main r-process occurs probably in supernovae which explode in later stages of globular cluster formation.

The importance of massive stars in astrophysics is self-evident to all of us. However the full scope of this role has only become apparent over the last few years, as we begin to understand the central role that massive star formation and evolution plays in phenomena ranging from the structure and evolution of the interstellar medium, galaxy formation and evolution, nuclear activity in galaxies, and even the reionization of the universe itself. This paper briefly reviews this broad relevance of massive star formation, and the wide range in massive star formation environments found in the local universe.

To determine ages of individual (field) stars, composition, mass and distance should be known accurately, which is usually not the case. An alternative way is to use turn-off colours of field star populations. This method led in the past to ages significantly higher than those of globular clusters of the same metallicity. We show that colour-based relative ages between the field and cluster population indicate that they have similar if not equal ages. First steps using large samples of stars from SDSS are presented.

2-D and 3-D radiation transfer models of forming stars generally produce bluer 1-10 μm colors than 1-D models of the same evolutionary state and envelope mass. Therefore, 1-D models of the shortwave radiation will generally estimate a lower envelope mass and later evolutionary state than multidimensional models. 1-D models are probably reasonable for very young sources, or longwave analysis ($\lambda >100 \mu$m). In our 3-D models of high-mass stars in clumpy molecular clouds, we find no correlation between the depth of the 10 μm silicate feature and the longwave ($> 100 \mu$m) SED (which sets the envelope mass), even when the average optical extinction of the envelope is ${> }100$ magnitudes. This is in agreement with the observations of Faison et al. (1998) of several UltraCompact HII (UCHII) regions, suggesting that many of these sources are more evolved than embedded protostars.

We have calculated a large grid of 2-D models and find substantial overlap between different evolutionary states in the mid-IR color-color diagrams. We have developed a model fitter to work in conjunction with the grid to analyze large datasets. This grid and fitter will be expanded and tested in 2005 and released to the public in 2006.

We describe the discovery of HE 1327–2326, a dwarf or subgiant with $\mbox{[Fe/H]}=-5.4$. The star was found in a sample of bright metal-poor stars selected from the Hamburg/ESO survey. Its abundance pattern is characterized by very high C and N abundances. The detection of Sr which is overabundant by a factor of 10 as compared to iron and the Sun, suggests that neutron-capture elements had already been produced in the very early Galaxy. A puzzling Li depletion is observed in this unevolved star which contradicts the value of the primordial Li derived from WMAP and other Li studies. Possible scenarios for the origin of the abundance pattern (Pop. II or Pop. III) are presented as well as an outlook on future observations.

We have determined the chemical composition of the carbon dwarf G77-61, from Keck IR and optical spectra. We present here a new analysis with the oxygen abundance measured for the first time. We show that G77-61 is extremely metal-poor ([Fe/H]$\,{=}\,{-}$4), with large overabundances of C, N and O ([C/Fe] = 3.2, [N/Fe] = 2.2, [O/Fe] = 2.2). It also shows moderate enhancements of Ca and Mg, Na, and Cr of typically 0.5 dex relative to Fe. We discuss the possible origin of these peculiarities.

Various physical processes are believed to trigger star formation on the borders of Galactic HII regions. Among these, the collect & collapse process is particularly attractive as it allows the formation of massive objects (single stars or clusters). In order to identify specific cases of this way of triggering star formation we are carrying out a multi-wavelength study of Galactic HII regions that exhibit signposts of massive-star formation at their borders. Hereby, we present two typical examples of such sources and discuss the results in the framework of the collect and collapse process, which seems to be at work as the major triggering agent in these two cases.

A deformation of the interface between convectively stable, unstable layers in a star can lead to intensified mixing of the elements. Our three-dimensional simulations show that under a condition of deformability of the interface, a coupled long-lived system of large-scale flows, penetrative convection is established. This effect can explain lithium depletion in the solar atmosphere

By means of chemical evolution models for galaxies of different morphological types (i.e. spirals and irregular/starburst galaxies) we study the nature of Damped Lyman-alpha (DLA) systems. By focusing on individual systems, we can derive important constraints on both their star formation history and their age. Our results indicate that the local conterparts of most DLAs are represented by dwarf galaxies having had low star formation rates, but some systems can also be associated to spirals. Some systems are already old, with ages of ∼1 Gyr, and some others are experiencing the very first star formation episodes.

Lithium abundances have been determined in more than 100 metal-poor halo stars both in the field and in clusters. From these data we find trends of Li with both temperature and metallicity and a real dispersion in Li abundances in the Spite Li plateau. We attribute this dispersion primarily to Li depletion (presumably due to extra mixing induced by stellar rotation) and to Galactic chemical evolution. We derive a primordial Li of 2.44$\,{\pm}\,$0.18 for A(Li)$_p$ = log N(Li/H) + 12.00. This agrees with the Li abundances predicted by the WMAP results. For stars cooler than the Li plateau we have evidence that Li depletion sets in at hotter temperatures for the higher metallicity stars than for the low-metal stars. This is the opposite sense of predictions from stellar models. The smooth transition of the Li content from the Li plateau stars to the cool stars adds weight to the inference of Li depletion in the plateau stars.

The Chandra X-ray Observatory is providing remarkable new views of massive star-forming regions, revealing all stages in the life cycle of high-mass stars and their effects on their surroundings. We present a Chandra tour of several high-mass star-forming regions, highlighting physical processes that characterize the life of a cluster of high-mass stars, from deeply-embedded cores too young to have established an HII region to superbubbles so large that they shape our views of galaxies. Along the way we see that X-ray observations reveal hundreds of stellar sources powering great HII region complexes, suffused by both hard and soft diffuse X-ray structures caused by fast O-star winds thermalized in wind-wind collisions or by termination shocks against the surrounding media. Finally, we examine the effects of the deaths of high-mass stars that remained close to their birthplaces, exploding as supernovae within the superbubbles that these clusters created. We present new X-ray results on W51 IRS2E and 30 Doradus and we introduce new data on Trumpler 14 in Carina and the W3 HII region complexes W3 Main and W3(OH).

We have observed with UVES@VLT 15 field Blue Straggler Stars for which kinematical attribution to the Halo or Thick Disk is available. We present here the first results of our detailed chemical analysis.

Stars with masses of $\gtsim 20$ $M_{odot}$ have short Kelvin times that enable them to reach the main sequence while still accreting from their natal clouds. The resulting nuclear burning produces a huge luminosity and a correspondingly large radiation pressure force on dust grains in the accreting gas. This effect may limit the upper mass of stars that can form by accretion. Indeed, simulations and analytic calculations to date have been unable to resolve the mystery of how stars of 50 $M_{odot}$ and up form. We present two new ideas to solve the radiation pressure problem. First, we use three-dimensional radiation hydrodynamic adaptive mesh refinement simulations to study the collapse of massive cores. We find that in three dimensions a configuration in which radiation holds up an infalling envelope is Rayleigh-Taylor unstable, leading radiation driven bubbles to collapse and accretion to continue. We also present Monte Carlo radiative transfer calculations showing that the cavities created by protostellar winds provides a valve that allow radiation to escape the accreting envelope, further reducing the ability of radiation pressure to inhibit accretion.

Massive protostars are generally enshrouded in dust, so that most of their radiation emerges in the far infrared. For protostars embedded in opaque, spherical cores, the spectral energy distribution (SED) is determined by two distance-independent parameters, the luminosity-to-mass ratio, $L/M_c$, and the surface density of the core, $\Sigma=M_c/(\pi R_c^2)$, where $R_c$ is the radius of the core. Chakrabarti & McKee (2005a) have derived an approximate analytic expression for the SED that agrees well with numerical results. It is generally not possible to infer the power-law of the density from the SED of a massive protostar. Masses and accretion rates are inferred for several well-studied sources.