The scope of this contribution is to review and discuss recent findings concerning the molecular environment of high-mass star forming regions. Special attention is devoted to “hot molecular cores” and their role in the formation of massive stars. After analysing the relationship between such cores and the surrounding molecular clumps, we discuss the results of interferometric observations of these objects and propose an evolutionary sequence proceeding from cold, pre-stellar cores to ultracompact HII regions.

Nine 20$\,M_\odot$ models were computed with metallicities ranging from solar, through $Z=10^{-5}$ ([Fe/H]∼−3.1) down to $Z=10^{-8}$ ([Fe/H]∼−6.1) and with initial rotational velocities between 0 and 600 km s−1 to study the impact of initial metallicity and rotational velocity (Hirschi (2005)). The very large amounts of 14N observed (∼0.03 M[odot]) are only produced at $Z=10^{-8}$ (PopII 1/2). The strong dependence of the 14N yields on rotation and other parameters like the initial mass and metallicity may explain the large scatter in the observations of 14N abundance. The metallicity trends are best reproduced by the models with $\upsilon_{ini}/\upsilon_c \sim 0.75$, which is slightly above the mean observed value for OB solar metallicity stars. Indeed, in the model with $\upsilon_{ini}$ = 600 km s−1 at $Z=10^{-8}$, the 16O yield is reduced due to strong mixing. This allows in particular to reproduce the upturn for C/O and a slightly decreasing [C/Fe], which are observed below [Fe/H]∼−3.

I describe some common themes that emerged from the meeting, ending with some thoughts on connecting our studies with those of other galaxies.

I review the formation of massive stars in the context of a forming stellar cluster. High-mass stars form in the centre of stellar clusters and thus must be understood in the context of low-mass star formation. Furthermore, they are predominantly in binary systems making further constraints on the formation mechanism. The fragmentation of a turbulent molecular cloud produces a large number of stars with initial masses close to the Jeans mass of the cloud. These stars fall together to form small-N clusters that grow through the infall of gas and stars into the cluster's potential well. Competitive accretion in clusters produces high-mass stars in the cluster centre and a full initial mass function of lower-mass stars. Massive star formation is a process that occurs in the cores of stellar clusters and commonly produces close binary systems. Accretion also forces the cluster to contract, increasing the stellar densities to the point where stellar collisions may occur. Furthermore, accretion in clusters reproduces the high binary frequency of massive stars. Systems evolve from low-mass wide binaries to high-mass close binaries due to gas accretion. This evolution can produce very tight binaries that are expected to merge to form the most massive stars. Binary mergers require stellar densities of order 106 stars pc−3, 100 times smaller than is required for single-star collisions.

We discuss the results obtained so far in our ongoing search for the $^6$Li isotope in very metal-poor halo stars through very high resolution and S/N spectroscopy with the Subaru High Dispersion Spectrograph, and the consequent implications. Besides definitively confirming the existence of $^6$Li in the star HD 84937, we achieve a tentative detection in the extremely metal-poor star G 64-12 ([Fe/H]$\,{\simeq}\,{-}$3.2). For two other stars with [Fe/H] ${\sim}\,{-}3$, only upper limits were derived. Together with the VLT/UVES results of Asplund et al., this indicates unexpectedly high $^6$Li abundances in at least some stars at very low [Fe/H]. The findings are discussed in light of different production scenarios, including the structure formation cosmic ray model and other possibilities.

This contribution retraces the scientific careers of Monique and François. It highlights the impressive contributions that they have brought to astrophysics, from the discovery of the lithium plateau in subdwarfs the second year of operation of the Canada-France-Hawaii telescope, to the exceptional contribution of Monique to the ESO VLT Large Programme “First Stars”, passing by several other findings which have marked our knowledge of the nuclear evolution of our Galaxy and of the Magellanic Clouds.

Massive stars begin their lives in cold, dense cores which are much more massive than the stars which form in them. We summarise the results of a program to find the earliest examples of massive star formation, and to examine the evolutionary sequence of events that occurs as such a star begins to form and heat its surroundings. Methanol maser emission has proved to be a particularly potent tool to locate such cores, though there are also clearly many massive cores which do not exhibit such maser emission. Our program began with a survey for 6.6 GHz methanol maser emission, but expanded to include dust continuum surveys in the mm and sub–mm, a survey for hot molecular cores associated with ‘isolated’ masers through mm-line CH3CN emission, and follow-up probing of some cores through sub-arcsecond, diffraction limited observations in the mid–IR. This program is outlined below.

HE0141-3932 ($z_{\rm em} = 1.80$) is a bright blue radio-quiet quasar which reveals an emission line spectrum with an unusually weak Lyα line. In addition, large redshift differences ($\Delta z = 0.05$) are observed between high ionization and low ionization emission lines. Absorption systems identified at $z_{\rm abs} = 1.78, 1.71$, and 1.68 show mild oversolar metallicities ($Z \approx 1-2Z_\odot$) and can be attributed to the associated gas clouds ejected from the circumnuclear region. The joint analysis of the emission and absorption lines leads to the conclusion that this quasar is seen almost pole-on. Its apparent luminosity may be Doppler boosted by ∼10 times. The absorbing gas shows high abundance of Fe, Mg, and Al ([Fe, Mg, Al/C] $\simeq 0.15\pm0.10$) along with underabundance of N ([N/C] $\leq -0.5$). This abundance pattern is at variance with current chemical evolution models of QSOs predicting [N/C] < 0 and [Fe/C] <0 at $Z \sim Z_\odot$. Full details of this work are given in Reimers et al. (2005).

We started (sub-)millimeter continuum and line studies of entire molecular cloud complexes located at intermediate distances from the Sun (1–3 kpc). Such an unbiased approach allows to identify and characterize the earliest phases of high-mass stars overlooked by IRAS or MSX. Our complete MAMBO-2 surveys of the Cygnus X and NGC 7538 complexes reveal a large population of ${\sim} 0.1$ pc-size massive young stellar objects (MYSOs) harboring high-mass infrared-quiet protostars. The determination of the nature of all the new millimeter sources is still in progress but we have already collected evidence that the infrared-quiet (or class 0-like) protostellar phase might last as long as the better-known infrared-bright protostellar phase. Besides, our complete census of MYSOs fails to discover the high-mass analogues of pre-stellar dense cores. We propose that the observed lower-density pre-stellar clumps (${>} 1$ pc) rapidly concentrate and collapse as also found in the kinematical studies of other prominent clumps. Indeed, CS and HCO$^+$ mappings in W43 and Cygnus X suggest global supersonic contraction with inward velocities of several km s−1 on parsec scales. Our work and similar studies of entire star-forming complexes will thus definitively contribute to a better knowledge of the earliest phases of high-mass star formation.

We present results of an extensive spectroscopic survey of Subgiant stars in the stellar system ω Centauri. Using infrared CaII triplet lines we derived metallicities and radial velocities for more than 250 stars belonging to different stellar populations of the system. A small age spread (<2 Gyr) among the stellar populations of ω Cen has been estimated regardless of any choice of helium abundance. These results impose severe constraints on the time-scale of the enrichment process of this stellar system, excluding the possibility of an extended star formation period. The radial velocities analysis of the entire sample demonstrates that only the metal-intermediate populations ($-1.4<[Fe/H]<-1.0$) are kinematically cooler than the others.

We present non–Local Thermodynamic Equilibrium (non–LTE) calculations for neutral carbon spectral line formation, carried out for a grid of model atmospheres covering the range of late-type stars. The results of our detailed calculations suggest that the carbon non–LTE corrections in these stars are higher than usually adopted, remaining substantial even at low metallicity. For the most metal-poor stars in the sample of Akerman et al (2004), the effects are of the order of $\Delta\log\epsilon_{\rm C} \simeq -0.35\ldots-0.45$ (when neglecting H collisions). Applying our results to those observations, the apparent [C/O] upturn seen in their LTE analysis is no longer present, thus revealing no need to invoke contributions from Pop. III stars to the carbon nucleosynthesis.

The abundance distributions of cluster and field halo populations will be considered. Abundance data for most elements of the Periodic Table are much sparser for globular clusters than for individual halo field stars, mainly due to the relative lack of high resolution spectra in the blue for the former group. But the available data do suggest that, in spite of remaining uncertainties in the observational data, that chemical composition differences in the heavier elements (Fe-peak and beyond) are small, indicating common nucleosynthetic origins for all halo populations.

The Spitzer Space Telescope is very efficient at moderate-depth infrared mapping. The Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) and other projects have now imaged most of the massive star formation regions in the inner Galaxy. In this case, quantity is quality–the large datasets allow statistical and global analysis of star forming regions that was previously difficult. Data such as the GLIMPSE survey is proving useful to study different evolutionary stages of massive star formation, from pre-protostellar clouds through feedback and triggered star formation. We discuss selected team results relevant to each of these stages: mapping infrared dark clouds and the mid-IR extinction law, studies using radiative transfer models of the entire protostellar population of star forming regions such as M16 and M17 (see also Whitney, this conference), feedback and star formation in the G305 and RCW79 giant HII regions, and GLIMPSE's comment on even older objects such as debris disks and PN. Continuing similar analysis will hopefully lead to better understanding of large-scale questions of Galactic star formation such as the total star formation rate and the relationships between star formation and Galactic structure.

I review some important questions in the field of massive star formation: What are the initial conditions for proto star clusters and how do they arise? What are the initial conditions for individual massive star formation within star clusters? How do massive protostars accumulate their mass? I compare the Turbulent Core Model (McKee & Tan 2003) to several nearby regions, including Orion KL. Here I also discuss the origin of BN's high proper motion.

6.7 and 12.2 GHz CH$_{3}$OH (methanol) and 22.2 GHz H$_{2}$O masers are believed to be good tracers of the earliest phases of high-mass star formation. Interferometric and VLBI (Very Long Baseline Interferometry) observations have shown that water masers are predominantly associated with the innermost portions of the jets/outflows emerging from (proto-)stellar objects. On the other hand, the astrophysical environment traced by the 6.7 GHz (and the associated 12.2 GHz) CH$_{3}$OH masers is still to be more precisely determined. So far, most high-resolution studies have focused either on CH$_{3}$OH or on H$_{2}$O masers and little is known on their connection, wehereas it would be important to study both types of maser emission in the same object. The goal of our long-term project is to perform interferometric and VLBI observations of H$_{2}$O and CH$_3$OH masers towards a selected sample of high-mass YSOs where both maser types have been detected. This work presents preliminary results obtained for a few objects of our sample, and discusses possible implications.

The observations of Israelian et al. (2004) show that, in an effective temperature range between 5600, 5850 K, the planet host stars present a significant lithium underabundance compared to the stars without planets. We have studied this phenomena in order to discriminate the different planetary formation scenarii.

In order to use the lithium abundance of the Spite plateau to constrain the Big Bang Nucleosynthesis, one has to determine how much of the original lithium has been destroyed by the various physical processes that are known to operate in stellar radiation zones. These are briefly reviewed, with emphasis on the mixing occurring in tachoclines and on that generated indirectly by the transport of angular momentum through internal gravity waves.

We present detailed abundance measurements of neutron-capture elements for the two very metal-poor stars HD 6268 and HD 122563, based on very high-quality, near-UV spectra (S/N >140 @3100A) using Subaru/HDS. Abundances have been obtained for a total of 26 and 19 neutron-capture elements in these two stars, respectively, including Nb, Mo, Ru, Pd, Ag, Pr, and Sm. We have confirmed that the abundance pattern of neutron-capture elements in HD 6268 agrees very well with that of previously known r-process-enhanced stars. In contrast, the elemental abundances of HD 122563 are found to steeply decrease with increasing atomic number than those of HD 6268, and are much lower than than the r-process pattern in solar-system material. This result provides a new, strong constraint on models of the nucleosynthetic process that has provided light neutron-capture elements in the very early Galaxy.

Very recent observations of the $^6$Li isotope in halo stars reveal a $^6$Li plateau about 1000 times above the predicted BBN abundance. We calculate the evolution of $^6$Li versus redshift generated from an initial burst of cosmological cosmic rays (CCRs) up to the formation of the Galaxy. We show that a pregalactic production of the $^6$Li isotope can account for the $^6$Li plateau observed in metal poor halo stars without additional over-production of $^7$Li. The derived properties of the CCRs could then be used to put constraints on the physics and history of the objects, such as Pop III stars, possibly responsible for these early cosmic rays. Consequently, we consider the evolution of $^6$Li in the Galaxy. Since $^6$Li is also produced by Galactic cosmic ray nucleosynthesis, we argue that $^6$Li can be depleted in halo stars with metallicities between [Fe/H]=−2 and −1.

Among the metal-poor dwarfs (Population II), a few are enriched in Nitrogen. Surprisingly, in spite of this peculiarity, their lithium abundance is similar to the Li abundance of the other dwarfs. Several scenarios of nitrogen enrichment are discussed, none is completely satisfactory, the most likely is a contamination by some very highly N-rich matter. But it could be speculated that these N-rich dwarfs may perhaps be stars escaped from N-rich globular clusters. An homogeneous analysis of this class of stars could be useful.

The rather low level of the lithium abundance in the old dwarfs, contrasting with the high level found in the Population I, requires a surprisingly large and rapid production of Li. Recent observations show in one Population I red giant, a very high lithium abundance. This observation, in agreement with some predictions of some theoretical models of giants and/or AGB stars is very encouraging.