Some basic observational properties of Damped Lymna α systems (DLAs) have been examined by Monte Carlo simulation based on the assumption that DLAs are disks form at the center of dark halo at redshift 3. We found that the predicted cosmic star formation rate density contributed by DLAs is consistent with the most recent observations if the star formation timescale in DLAs is assumed to be 1 ∼ 3 Gyr. By comparing the UV luminosity function between DLA host galaxies and that of Lyman Break Galaxies(LBGs), we show that the DLAs host galaxies are much fainter than LBGs, and that only few percent of DLAs can host LBGs. The discrepancy between model prediction and observation in the correlation between metallicity and HI column density for DLAs is reproduced on the basis of Kennicutt star formation formalism at high redshift. It is quite difficult to interpret this concerning the complexities of dust depletion, star formation mode as well as model limitations. Further investigations are needed.

How and when did the first generation of stars form at the end of the cosmic dark ages? Quite generically, within variants of the cold dark matter model of cosmological structure formation, the first sources of light are expected to form in $\sim 10^{6} M_{\odot}$ dark matter potential wells at redshifts $z\geq 20$. I discuss the physical processes that govern the formation of the first stars. These so-called Population III stars are predicted to be predominantly very massive, and to have contributed significantly to the early reionization of the intergalactic medium. Such an early reionization epoch is inferred from the recent measurement of the Thomson optical depth by the WMAP satellite. I address the importance of heavy elements in bringing about the transition from an early star formation mode dominated by massive stars, to the familiar mode dominated by low mass stars, at later times, and present possible observational probes. This transition could have been gradual, giving rise to an intermediate-mass population of still virtually metal-free stars (“Population II.5”). These stars could have given rise to the peculiar class of black-hole forming supernovae inferred from the abundance pattern of extremely iron-poor stars.

We present stellar parameters, Li abundances for 21 stars of the young ($\sim$50 Myears) AB Dor association, based on high-resolution spectra obtained with FEROS at the ESO 1.52m telescope, the Coudé Spectrograph at the OPD 1.6m telescope. These results are part of an ongoing project for the determination of stellar parameters, abundances for a large sample of T Tauri, post-T Tauri stars in PMS associations identified on the Search for Associations Containing Young Stars survey. The sample consists of G, K, M stars counterparts of ROSAT bright sources.

Since we expect Li depletion for the K, M stars, but small Li depletion for the G stars, we can compare the upper values of Li abundances for these stars with the one of the Galactic disk interstellar mean, at the same time provide constrains for the study of Li depletion on a chemically homogeneous high metallicity sample.

We are conducting a survey of several regions of high-mass star formation to assess their content and structure. The observations include spitzer observations, ground-based optical and near-IR imaging surveys, and optical and IR spectra of objects and locations in the molecular clouds. The goal of the survey is to gain a better understanding of the processes involved in high mass star formation by determining the characteristics of the stars detected in these regions and investigating the properties of the interstellar medium (ISM) environment in which these stars form. In this contribution, we present results on the identification and spatial analysis of young stars in three clusters, W5/AFGL 4029, S255, and S235. First we show how the IRAC data are used to roughly segregate young stars according to their mid-infrared colors, into two groups corresponding the SED class I and class II young stellar objects. Then using the IRAC data in combination with 2MASS, we show how more young stars can be identified. Finally, we examine the spatial distributions of young stars in these clusters and find a range of morphologies and of peak surface densities.

The Submillimeter Array located atop of Mauna Kea in Hawaii is a collaborative project of the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics. The high angular resolution provided by the SMA is particularly suitable for studying massive star-forming cores which often exhibit strong (sub)millimeter continuum and spectral features but mostly locate within crowded regions at large distances. We report the latest SMA status and recent results in the area of massive star formation obtained with the array.

Accurate relative abundances of light elements (C to Ca) have been obtained in a sample of mildly metal-poor stars (Decauwer et al. 2005). Combined with the results of a previous study (Jehin et al. 1999), we find different slopes in the correlations between the different α-elements. These results can be explained by postulating that the stars exhibiting lower than average α/Fe form in low mass clouds, unable to sustain the formation of very massive stars.

We present a study of the s–process nucleosynthesis (weak s-process) occurring during convective core He–burning and convective shell Carbon–burning in a massive star of 25 M$_{\odot}$. We use an updated nuclear network for the various neutron sources and for all neutron captures and β-decay rates involved. Large uncertainties affect the final yields due to the present unsatisfactory knowledge of all neutron capture cross sections involved.

I report X-ray features of young stage of intermediate and high mass stars. The giant molecular cloud Sagittarius B2 (Sgr B2) exhibits more than dozen X-ray sources, two are associated with the ultra compact (UC) HII complex, Sgr B2 Main. The sources show large absorption of $\gg$1023 Hcm−2, the largest among any known stellar X-ray sources.

The Arches cluster exhibits 3 extremely bright (a few $\times10^{33}$ ergs s−1) X-ray sources associated with the infrared (IR) massive stars. The X-ray spectra have 1-3 keV temperature in thin thermal model. Elongated diffuse 6.4 keV line emission is found.

The Monoceros R2 cloud exhibits half dozen X-ray sources associated with young high-mass IR stars. They show rapid time variability and a thin thermal spectrum of ${\sim }2$ keV temperature. Among 28 ASC A pointing on intermediate-mass pre-main-sequence stars, or Herbig Ae/Be stars (HAeBes), eleven are found to be plausible X-ray sources. The general X-ray properties of these HAeBes are; (1) the plasma temperature and time variability are higher than those of high mass main-sequence stars, and more similar to low mass stars; (2) the X-ray luminosities come to the upper end of low mass pre-main sequence stars, or in some cases exceed that; (3) the X-ray activity of HAeBe decreases at the age of about a few×106 years.

Using above observational facts, I propose a unified picture of X-ray activity of young stars in the wide mass range.

We report the determination of the age of the Galactic thin disk by means of Th/Eu nucleocosmochronology. This method is only weakly dependent on stellar evolutions models, therefore allowing an important verification of the most used dating techniques, which are the fitting of isochrones to the oldest Galactic open clusters, and the calculation of white dwarf cooling sequences. This work builds upon our previous determination (del Peloso et al. 2005a, 2005b), by including 7 new objects to the sample which was originally composed of 19 disk dwarfs/subgiants of F5 to G8 spectral types – a 37% extension. The obtained result, (8.8±1.7)Gyr, corroborates the most recent white dwarf ages determined via cooling sequence calculations, which indicate a low age ($\widetilde{<}10~\mbox{Gyr}$) for the disk.

We discuss the effects of very massive Population III stars on the chemical evolution of the Milky Way, elliptical galaxies and the intergalactic medium (IGM) at high reshift. It is shown that the effects produced by Pop III stars on the early evolution of the most common chemical abundances (C, N, O, $\alpha$-elements, Fe) are negligible if these stars formed only for a very short period of time, corresponding to the suggested threshold metallicity ($Z_{thr} \sim 10^{-4}Z_{\odot}$). For a higher threshold metallicity and therefore a longer period of time, the predicted results are at variance with observations. It is also concluded that the IGM at high redshift ($z=5.0$) cannot have been enriched only by very massive Pop III stars, but that the contribution of lower mass stars is necessary. The same conclusion holds for DLA systems at high redshift.

A stochastic model of the chemical enrichment of metal-poor systems by core-collapse supernovae is used to study the scatter in relative elemental abundances in extremely metal-poor stars. The resulting scatter in abundance ratios is demonstrated to be crucially dependent on the as yet uncertain supernovae yields. The relatively small star-to-star scatter observed in many of these abundance ratios, e.g. by Cayrel et al. (2004), is tentatively explained by the averaging of a large number of contributing supernovae and by the cosmic selection effects favoring contributions from supernovae in a certain mass range for the most metal-poor stars. “Spurs”, very narrow sequences in abundance-ratio diagrams, may disclose a single-supernova origin of the elements of the stars on the sequence and would thus be an indication of an unmixed interstellar medium (ISM). Verification of the existence of such features, called single supernova sequences (SSSs), is challenging. This will require samples of several hundred stars with abundance ratios observed to accuracies of 0.05 dex or better.

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.