The age–metallicity relationship in the Fornax spheroidal dwarf galaxy

We produce a comprehensive field star age–metallicity relationship (AMR) from the earliest epoch until ∼1 Gyr ago for three fields in the Fornax dwarf spheroidal galaxy by using VI photometric data obtained with FORS1 at the VLT. We find that the innermost one does not contain dominant very old stars (age > 12 Gyr), whereas the relatively outer field does not account for representative star field populations younger than ∼3 Gyr. When focusing on the most prominent stellar populations, we find that the derived AMRs are engraved by the evidence of an outside-in star formation process. The studied fields show bimodal metallicity distributions peaked at [Fe/H] = (−0.95 ± 0.15) dex and (−1.15 or −1.25 ± 0.05) dex, respectively, but only during the first half of the entire galaxy lifetime. Furthermore, the more metal-rich population appears to be more numerous in the outer fields, while in the innermost Fornax field the contribution of both metallicity populations seems to be similar. We also find that the metallicity spread ∼6 Gyr ago is remarkable large, while the intrinsic metallicity dispersion at ∼1–2 Gyr results smaller than that for the relatively older generations of stars. We interpret these outcomes as a result of a possible merger of two galaxies that would have triggered a star formation bursting process that peaked between ∼6 and 9 Gyr ago, depending on the position of the field in the galaxy.Fil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; ArgentinaFil: del Pino, Andres. Instituto de Astrofısica de Canarias. Tenerife; EspañaFil: Aparicio, Antonio. Instituto de Astrofısica de Canarias. Tenerife; España. Universidad de la Laguna; EspañaFil: Hidalgo, Sebastian L. Instituto de Astrofısica de Canarias. Tenerife; Españ

Role of Rotations on Surface Diffusion of Water Trimers on Pd\{111\}

Diffusion barriers for a cluster of three water molecules on Pd(111) have been estimated from ab-initio Density Functional Theory. A model for the diffusion of the trimer based in rotations yields a simple explanation of why the cluster can diffuse faster than a single water molecule by a factor $\approx 10^{2}$. This model is based on the differences between the adsorption geometry for the three monomers forming the cluster. One member interacts strongly with the surface and sits closer to the surface (d) while the other two interact weakly and stay at a larger separation from the surface (u). The trimer rotates nearly freely around the axis determined by the d monomer. Translations of the whole trimer imply breaking the strong interaction of the d monomer with the surface. Alternatively, thermal fluctuations exchange the actual monomer sitting closer to the surface with a lower energetic cost. Rotations around different axis introduce a diffusion mechanism where a strong interaction is kept along the diffusion path between the water molecule defining the axis of rotation and the Pd underneath.Comment: water ; monomer ; trimer ; water clusters ; diffusion ; rotation assisted ; Pd\{111\} ; ab-initio ; density functional theor

Ab-initio calculation of the effect of stress on the chemical activity of graphene

Graphene layers are stable, hard, and relatively inert. We study how tensile stress affects $\sigma$ and $\pi$ bonds and the resulting change in the chemical activity. Stress affects more strongly $\pi$ bonds that can become chemically active and bind to adsorbed species more strongly. Upon stretch, single C bonds are activated in a geometry mixing $120^{o}$ and $90^{o}$; an intermediate state between $sp^{2}$ and $sp^{3}$ bonding. We use ab-initio density functional theory to study the adsorption of hydrogen on large clusters and 2D periodic models for graphene. The influence of the exchange-correlation functional on the adsorption energy is discussed

Mechanical response of a self avoiding membrane: fold collisions and the birth of conical singularities

An elastic membrane that is forced to reside in a container smaller than its natural size will deform and, upon further volume reduction, eventually crumple. The crumpled state is characterized by the localization of energy in a complex network of highly deformed crescent-like regions joined by line ridges. Previous studies have focused on the onset of the crumpled state by analyzing the mechanical response and stability of a conical dislocation, while others have simulated the highly packed regime neglecting the importance of the connectivity of the membrane. Here we show, through a combination of experiments, numerical simulations, and analytic approach, that the emergence of new regions of high stretching is a generic outcome when a self avoiding membrane is subject to a severe geometrical constraint. We demonstrate that, at moderate packing fraction, interlayer interactions produce a response equivalent to the one of a thicker membrane that has the shape of the deformed one. Evidence is found that friction plays a key role stabilizing the folded structures.Comment: 10 page

Effects of fluid inertia and turbulence on force coefficients for squeeze film dampers

The effects of fluid inertia and turbulence on the force coefficients of squeeze film dampers are investigated analytically. Both the convective and the temporal terms are included in the analysis of inertia effects. The analysis of turbulence is based on friction coefficients currently found in the literature for Poiseuille flow. The effect of fluid inertia on the magnitude of the radial direct inertia coefficient (i.e., to produce an apparent added mass at small eccentricity ratios, due to the temporal terms) is found to be completely reversed at large eccentricity ratios. The reversal is due entirely to the inclusion of the convective inertia terms in the analysis. Turbulence is found to produce a large effect on the direct damping coefficient at high eccentricity ratios. For the long or sealed squeeze film damper at high eccentricity ratios, the damping prediction with turbulence included is an order of magnitude higher than the laminar solution

Crystal structure and electronic states of tripotassium picene

The crystal structure of potassium doped picene with an exact stoichiometry (K3C22H14, K3picene from here onwards) has been theoretically determined within Density Functional Theory allowing complete variational freedom of the crystal structure parameters and the molecular atomic positions. A modified herringbone lattice is obtained in which potassium atoms are intercalated between two paired picene molecules displaying the two possible orientations in the crystal.Along the c-axis, organic molecules alternate with chains formed by three potassium atoms. The electronic structureof the doped material resembles pristine picene, except that now the bottom of the conduction band is occupied by six electrons coming from the ionized K atoms (six per unit cell). Wavefunctions remain based mainly on picene molecular orbitals getting their dispersion from intralayer edge to face CH/pi bonding, while eigenenergies have been modified by the change in the electrostatic potential. The small dispersion along the c-axis is assigned to small H-H overlap. From the calculated electronic density of states we expect metallic behavior for potassium doped picene.Comment: Published version: 8 twocolumn pages, 7 color figures, 2 structural .cif files include

Trapping of electrons near chemisorbed hydrogen on graphene

Chemical adsorption of atomic hydrogen on a negatively charged single layer graphene sheet has been analyzed with ab-initio Density Functional Theory calculations. We have simulated both finite clusters and infinite periodic systems to investigate the effect of different ingredients of the theory, e.g. exchange and correlation potentials, basis sets, etc. Hydrogen's electron affinity dominates the energetic balance in the charged systems and the extra electron is predominantly attracted to a region nearby the chemisorbed atom. The main consequences are: (i) the cancellation of the unpaired spin resulting in a singlet ground-state, and (ii) a stronger interaction between hydrogen and the graphene sheet.Comment: 11 pages, 8 figures, to be published in PR