The VLT-FLAMES survey of massive stars: Observations in the Galactic clusters NGC 3293, NGC 4755 and NGC 6611

We introduce a new survey of massive stars in the Galaxy and the Magellanic Clouds using the Fibre Large Array Multi- Element Spectrograph ( FLAMES) instrument at the Very Large Telescope ( VLT). Here we present observations of 269 Galactic stars with the FLAMES- Giraffe Spectrograph ( R similar or equal to 25 000), in fields centered on the open clusters NGC3293, NGC4755 and NGC6611. These data are supplemented by a further 50 targets observed with the Fibre- Fed Extended Range Optical Spectrograph ( FEROS, R = 48 000). Following a description of our scientific motivations and target selection criteria, the data reduction methods are described; of critical importance the FLAMES reduction pipeline is found to yield spectra that are in excellent agreement with less automated methods. Spectral classifications and radial velocity measurements are presented for each star, with particular attention paid to morphological peculiarities and evidence of binarity. These observations represent a significant increase in the known spectral content of NGC3293 and NGC4755, and will serve as standards against which our subsequent FLAMES observations in the Magellanic Clouds will be compared.

Effects of tip structure on the generation of metal clusters by an STM tip: a way to control the orientation of nanocrystallites?

The role of the crystalline orientation of the STM tip in the generation of metal clusters is studied by atom dynamics simulations. When a (111) facet is facing the surface, the process is accompanied by a perturbation of the surface stronger than that observed for more open tip structures. This implies a technological application: the possibility of orienting a nanocrystallite deposited on a tip according to the changes observed in the force on the tip.

Vibrational properties of nanometric AB(2) ionic clusters

A broad survey of harmonic dynamics in AB(2) clusters with up to N = 3000 atoms is performed using a simple rigid ion model, with ionic radii selected to give rutile as the ground state structure for the corresponding extended crystal. The vibrational density of states is already close to its bulk counterpart for N similar to 500, with characteristic differences due to surfaces, edges and vertices. Two methods are proposed and tested to map the cluster vibrational states onto the rutile crystal phonons. The net distinction between infrared (IR) active and Raman active modes that exists for bulk rutile becomes more and more blurred as the cluster size is reduced. It is found that, in general, the higher the IR activity of the mode, the more this is affected by the system size. IR active modes are found to spread over a wide frequency range for the finite clusters. Simple models based on either a crude confinement constraint or surface pressure arguments fail to reproduce the results of the calculations. The effects of the stoichiometry and dielectric properties of the surrounding medium on the vibrational properties of the clusters are also investigated.

Hydrogen bonding and collective proton modes in clusters and periodic layers of squaric acid: A density functional study

Hydrogen bonding in clusters and extended layers of squaric acid molecules has been investigated by density functional computations. Equilibrium geometries, harmonic vibrational frequencies, and energy barriers for proton transfer along hydrogen bonds have been determined using the Car-Parrinello method. The results provide crucial parameters for a first principles modeling of the potential energy surface, and highlight the role of collective modes in the low-energy proton dynamics. The importance of quantum effects in condensed squaric acid systems has been investigated, and shown to be negligible for the lowest-energy collective proton modes. This information provides a quantitative basis for improved atomistic models of the order-disorder and displacive transitions undergone by squaric acid crystals as a function of temperature and pressure. (C) 2001 American Institute of Physics.

Density functional calculations for polymers and clusters - progress and limitations

The applicability of density functional (DF) methods has progressed greatly since the first workshop of this series ten years ago. Applications that show both the successes and the limitations can be found in the fields of: (a) the structures of the isomers of atomic clusters. and (b) the structure of organic molecules and polymers, and their reactions with additional molecules. We shall review some of the results and the lessons to be learned from them. (C) 2001 Elsevier Science B.V. All rights reserved.

An isothermal-isobaric Langevin thermostat for simulating nanoparticles under pressure: Application to Au clusters

We present a method for simulating clusters or, molecules subjected to an external pressure, which is exerted by a pressure-transmitting medium. It is based on the canoninical Langevin thermostat, but extended in such a way that the Brownian forces are allowed to operate only from the region exterior to the cluster. We show that the frictional force of the Langevin thermostat is linked to the pressure of the reservoir in a unique way, and that this property manifests itself when the particle it acts upon is not pointlike but has finite dimensions. By choosing appropriately the strength of the random forces and the friction coefficient, both temperature and pressure can be controlled independently. We illustrate the capabilities of this new method by calculating the compressibility of small gold clusters under pressure.

On the generation of metal clusters with the electrochemical scanning tunneling microscope

In the present work we consider two aspects of the deposition of metal clusters on an electrode surface. The formation of such clusters with the tip of a scanning tunneling microscope is simulated by atom dynamics. Subsequently the stability of these clusters is investigated by Monte Carlo simulations in a grand-canonical ensemble. In particular, the following systems were considered explicitly: Pd clusters on Au(111), Cu on Au(111), Ag on Au(111), Pb on Au(111) and Cu on Ag(111). The analysis of the results obtained for the different systems leads to the conclusion that optimal systems for nanostructuring are those where the metals participating have similar cohesive energies and negative heats of alloy formation. In this respect, the system Cu-Pd(111) is predicted as a good candidate for the formation of stable clusters. (c) 2005 Elsevier B.V. All rights reserved.

The basis for the formation of stable metal clusters on an electrode surface

Metal nanoclusters can be produced cheaply and precisely in an electrochemical environment. Experimentally this method works in some systems, but not in others, and the unusual stability of the clusters has remained a mystery. We have simulated the deposition of the clusters using classical molecular dynamics and studied their stability by grand-canonical Monte Carlo simulations. We find that electrochemically stable clusters occur only in those cases where the two metals involved form stable alloys.

A combined experimental and theoretical study of the generation of palladium clusters on Au(111) with a scanning tunnelling microscope

Palladium clusters have been deposited on the surface of a Au(111) electrode with the tip of a scanning tunnelling microscope. The distance over which the tip was moved towards the surface has a decisive influence on the properties of the clusters: the larger this distance, the larger the generated clusters, and the more stable they are. These findings are supported by computer simulations, which further suggest that the larger clusters contain a sizable amount of gold, which enhances their stability. Dissolution of the clusters occurs from the edges rather than layer by layer.

Generation of palladium clusters on Au(111) electrodes: Experiments and simulations

The properties of palladium clusters, generated with the electrochemical scanning tunneling microscope, have been investigated both by experiments and by computer simulations. The clusters are found to be larger and more stable if the tip is moved further towards the electrode surface in the generation process. The simulations suggest that the larger clusters consist of a palladium - gold mixture, which is more stable than pure palladium. Dissolution of the clusters occurs from the edges rather than layer by layer

Neutral and Charged 1-Butyl-3-methylimidazolium Triflate Clusters: Equilibrium Concentration in the Vapor Phase and Thermal Properties of Nanometric Droplets

Ground state energy, structure, and harmonic vibrational modes of 1-butyl-3-methylimidazolium triflate ([bmim][Tf]) clusters have been computed using an all-atom empirical potential model. Neutral and charged species have been considered up to a size (30 [bmim][Tf] pairs) well into the nanometric range. Free energy computations and thermodynamic modeling have been used to predict the equilibrium composition of the vapor phase as a function of temperature and density. The results point to a nonnegligible concentration of very small charged species at pressures (P ~ 0.01 Pa) and temperatures (T 600 K) at the boundary of the stability range of [bmim][Tf]. Thermal properties of nanometric neutral droplets have been investigated in the 0 T 700 K range. A near-continuous transition between a liquidlike phase at high T and a solidlike phase at low T takes place at T ~ 190 K in close correspondence with the bulk glass point Tg ~ 200 K. Solidification is accompanied by a transition in the dielectric properties of the droplet, giving rise to a small permanent dipole embedded into the solid cluster. The simulation results highlight the molecular precursors of several macroscopic properties and phenomena and point to the close competition of Coulomb and dispersion forces as their common origin.

Iron abundances of B-type post-asymptotic giant branch stars in globular clusters: Barnard29 in M13 and ROA5701 in ωCen

High-resolution optical and ultraviolet (UV) spectra of two B-type post-asymptotic giant branch (post-AGB) stars in globular clusters, Barnard29 in M13 and ROA5701 in ?Cen, have been analysed using model atmosphere techniques. The optical spectra have been obtained with FEROS on the ESO 2.2-m telescope and the 2d-Coudé spectrograph on the 2.7-m McDonald telescope, while the UV observations are from the Goddard high-resolution spectrograph on the Hubble Space Telescope (HST). Abundances of light elements (C, N, O, Mg, Al and S) plus Fe have been determined from the optical spectra, while the UV data provide additional Fe abundance estimates from FeIII absorption lines in the 1875-1900 Å wavelength region. A general metal underabundance relative to young B-type stars is found for both Barnard29 and ROA5701. These results are consistent with the metallicities of the respective clusters, as well as with previous studies of the objects. The derived abundance patterns suggest that the stars have not undergone a gas-dust separation, contrary to previous suggestions, although they may have evolved from the AGB before the onset of the third dredge-up. However, the Fe abundances derived from the HST spectra are lower than those expected from the metallicities of the respective clusters, by 0.5 dex for Barnard29 and 0.8 dex for ROA5701. A similar systematic underabundance is also found for other B-type stars in environments of known metallicity, such as the Magellanic Clouds. These results indicate that the FeIII UV lines may yield abundance values which are systematically too low by typically 0.6 dex and hence such estimates should be treated with caution.