New fundamental parameters of the Galactic open clusters Berkeley 26, Czernik 27, Melotte 72, NGC 2479 and BH 37

We have obtained CCD UBVI_{KC} photometry down to V ~ 21.0 for the open clusters Berkeley 26, Czernik 27, Melotte 72, NGC 2479 and BH 37. The latter has never been studied before. Cluster stellar density profiles were obtained from star counts in appropriate-sized boxes distributed throughout the entire observed fields. Based on different measured indices, we estimate the ages of Berkeley 26, Melotte 72 and NGC 2479. On the other hand, we indicate possible solutions for the cluster fundamental parameters by matching theoretical isochrones which reasonably reproduce the main cluster features in their CMDs. In the case of NGC 2479, the cluster E(B-V) and E(V-I) colour excesses and apparent distance modulus were estimated from the fit of the Zero-Age Main Sequence (ZAMS) to the colour-colour and colour-magnitude diagrams, respectively.Comment: MNRAS accepte

A sample of relatively unstudied star clusters in the Large Magellanic Cloud: fundamental parameters determined from Washington photometry

To enlarge our growing sample of well-studied star clusters in the Large Magellanic Cloud (LMC), we present CCD Washington CT1 photometry to T1 ~ 23 in the fields of twenty-three mostly unstudied clusters located in the inner disc and outer regions of the LMC. We estimated cluster radii from star counts. Using the cluster Washington (T1,C-T1) colour-magnitude diagrams, statistically cleaned from field star contamination, we derived cluster ages and metallicities from a comparison with theoretical isochrones of the Padova group. Whenever possible, we also derived ages using delta_T1 - the magnitude difference between the red giant clump and the main sequence turn off - and estimated metallicities from the standard giant branch procedure. We enlarged our sample by adding clusters with published ages and metallicities determined on a similar scale by applying the same methods. We examined relationships between their positions in the LMC, ages and metallicities. We find that the two methods for age and metallicity determination agree well with each other. Fourteen clusters are found to be intermediate-age clusters (1-2 Gyr), with [Fe/H] values ranging from -0.4 to -0.7. The remaining nine clusters turn out to be younger than 1 Gyr, with metallicities between 0.0 and -0.4. Our 23 clusters represent an increase of ~ 30% in the current total amount number of well-studied LMC clusters using Washington photometry. In agreement with previous studies, we find no evidence for a metallicity gradient. We also find that the younger clusters were formed closer to the LMC centre than the older ones.Comment: 11 pages, 10 figures. A&A, in pres

Proximity Eliashberg theory of electrostatic field-effect-doping in superconducting films

We calculate the effect of a static electric field on the critical temperature of a s-wave one band superconductor in the framework of proximity effect Eliashberg theory. In the weak electrostatic field limit the theory has no free parameters while, in general, the only free parameter is the thickness of the surface layer where the electric field acts. We conclude that the best situation for increasing the critical temperature is to have a very thin film of a superconducting material with a strong increase of electron-phonon (boson) constant upon charging.Comment: 9 pages, 5 figure

Spectral evolution of star clusters in the Large Magellanic Cloud: I. Blue concentrated clusters in the age range 40-300 Myr

Integrated spectroscopy of a sample of 17 blue concentrated Large Magellanic Cloud (LMC) clusters is presented and its spectral evolution studied. The spectra span the range ~3600-6800A with a resolution of ~14A FWHM, being used to determine cluster ages and, in connection with their spatial distribution, to explore the LMC structure and cluster formation history. Cluster reddening values were estimated by interpolation, using the available extinction maps. We used two methods to derive cluster ages: (i) template matching, in which line strengths and continuum distribution of the cluster spectra were compared and matched to those of template clusters with known astrophysical properties, and (ii) equivalent width (EW) method, in which new age/metallicity calibrations were used together with diagnostic diagrams involving the sum of EWs of selected spectral lines (KCaII, G band (CH), MgI, Hdelta, Hgamma and Hbeta). The derived cluster ages range from 40Myr (NGC2130 and SL237) to 300Myr (NGC1932 and SL709), a good agreement between the results of the two methods being obtained. Combining the present sample with additional ones indicates that cluster deprojected distances from the LMC center are related to age in the sense that inner clusters tend to be younger. Spectral libraries of star clusters are useful datasets for spectral classifications and extraction of parameter information for target star clusters and galaxies. The present cluster sample complements previous ones, in an effort to gather a spectral library with several clusters per age bin.Comment: 13 pages, 22 figures. Accepted for publication in A&

Characteristics of the periodic faradaic current oscillations of Zn/alkaline solution interfaces under potentiostatic conditions

Phenomenological aspects of the current oscillation at a constant potential related to the Zn/alkaline solution interfaces are described. The results are correlated to those already reported for other metal/aqueous solution interfaces.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Periodic current oscillations at the gold/acid aqueous interfaces induced by HCl additions

The periodic oscillating current recorded under a constant potential step for Au/acid solution interfaces containing different concentrations of Cl− ion is studied in the 25–50°C range. Two main electrochemical reactions coupled to two diffusion processes are responsible for the periodic effect. The electrochemical reactions occurring successively are the electrodissolution of gold as the AuCl−2 complex, a reaction which is diffusion controlled and then the oxide layer formation when the solution is depleted of Cl− ion. Subsequently, as the anodic current decreases due to the electrode passivation, the HCl diffuses backwards and dissolves the oxide film. The active part of the periodic cycle is interpreted in terms of a diffusional process involving at its initiation the simultaneous uncoverage of the electrode surface.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

The electrochemical kinetic behaviour of nickel in acid aqueous solutions containing chloride and perchlorate ions

The electrochemical behaviour of polycrystalline nickel in acid solutions containing large excess either of NaCl or NaClO4 has been investigated, at potentials either more positive or negative than the rest potential of the metal. The electrode reactions were studied over a wide range of experimental conditions (pH, nickelous ion activity, temperature), and the influence of each variable on the experimental kinetic parameters has been determinedInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Effect of heat treatment on the periodic current oscillation of polycrystalline zinc in alkaline solution

The phenomenology of the periodic oscillations of the faradaic current of zinc electrodes in alkaline solutions offers the possibility of investigating the influence of the non-uniformity of the polycrystalline metal surface on the periodic effect because the grain size of zinc can be easily controlled by the annealing temperature and time. Zinc rods (0.6 cm dia., Leico 99.999% purity) were annealed at 390°C during a period of between 2 and 6 h and cooled down for about 18 h. Each annealed rod was axially embedded in a PTFE rod to make a still zinc disc working electrode in contact with 1.5 M NaOH at 25°C. Other experimental details are as described in a previous publication. The morphology of the attack of the different electrode surfaces is shown in Fig. 1. From these pictures the surface density of grains at the electrode was calculated.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Current oscillations in austenitic stainless steel induced by the presence of chloride ions

The conditions for current oscillations in austenitic stainless steel (AISI Type 303) in 1M H2SO4, containing CI- ions are given. The periodic oscillations are produced in a close potential range determined at the active-passive transition region. It requires a non-homogeneous distribution of inclusions and carbides at the metal surface and a concentration range of CI- ion where active and passive areas coexist on the metal surface. The constraints at the surface determining these two regions are related to the CI- ion competitive adsorption and the local accumulation of corrosion products. Electrochemical data and SEM observations are correlated.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Kinetics of iron passivation and corrosion in molten alkali nitrates

The electrochemical behaviour of iron anodes in molten alkali nitrates has been investigated at temperatures ranging from ca 240–320°C. The E/I curves exhibit a potential region where the electrode passivates. Two different oxidized states of the passive iron surface exist. Within definite potential regions, the oxidation of nitrite and iron occurs, the latter being characterized by a Tafel slope of 2RT/F. At high positive potentials the breakdown of passivity is observed. Results are interpreted in terms of the stability of possible iron oxides during the electrode reaction. The kinetics of nitrite and nitrate ion oxidation on the passive surface is discussed on the basis of previous work. A comparison between iron passivation and corrosion in molten nitrites and nitrates is made.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada