arXiv subjects
Publications and source records attributed to J. Navarro.
In a previous paper, we pointed out how a dimensional analysis of the stress-energy tensor of the gravitational field allows to derive the field equation of General Relativity. In this note, we comment an analogous reasoning in presence of a 2-form, that allows to characterize the so called electromagnetic stress-energy tensor.
A finite-temperature density functional approach to describe the properties of parahydrogen in the liquid-vapor coexistence region is presented. The first proposed functional is zero-range, where the density-gradient term is adjusted so as to reproduce the surface tension of the liquid-vapor interface at low temperature. The second functional is finite-range and, while it is fitted to reproduce bulk pH2 properties only, it is shown to yield surface properties in good agreement with experiments. These functionals are used to study the surface thickness of the liquid-vapor interface, the wetting transition of parahydrogen on a planar Rb model surface, and homogeneous cavitation in bulk liquid pH2.
We use deep HST/ACS observations to calculate the star formation history (SFH) of the Cetus dwarf spheroidal (dSph) galaxy. Our photometry reaches below the oldest main sequence turn-offs, which allows us to estimate the age and duration of the main episode of star formation in Cetus. This is well approximated by a single episode that peaked roughly 12+/-0.5 Gyr ago and lasted no longer than about 1.9+/-0.5 Gyr (FWHM). Our solution also suggests that essentially no stars formed in Cetus during the past 8 Gyrs. This makes Cetus' SFH comparable to that of the oldest Milky Way dSphs. Given the current isolation of Cetus in the outer fringes of the Local Group, this implies that Cetus is a clear outlier in the morphology-Galactocentric distance relation that holds for the majority of Milky Way dwarf satellites. Our results also show that Cetus continued forming stars through z ~ 1, long after the Universe was reionized, and that there is no clear signature of the epoch of reionization in Cetus' SFH. We discuss briefly the implications of these results for dwarf galaxy evolution models. Finally, we present a comprehensive account of the data reduction and analysis strategy adopted for all galaxies targeted by the LCID (Local Cosmology from Isolated Dwarfs) project. We employ two different photometry codes (DAOPHOT/ALLFRAME and DOLPHOT), three different SFH reconstruction codes (IAC-pop/MinnIAC, MATCH, COLE), and two stellar evolution libraries (BaSTI and Padova/Girardi), allowing for a detailed assessment of the modeling and observational uncertainties.
We have implemented a simple model to identify the likely sites of the first stars and galaxies in the high-resolution simulations of the formation of galactic dark matter halos of the Aquarius Project. The first star in a galaxy like the Milky Way formed around redshift z=35; by z=10, the young galaxy contained up to 30000 dark matter haloes capable of forming stars by molecular hydrogen cooling. These minihaloes were strongly clustered and feedback may have severely limited the actual number of Population III stars that formed. By the present day, the remnants of the first stars would be strongly concentrated to the centre of the main halo. If a second generation of long-lived stars formed near the first (the first star relics), we would expect to find half of them within 30kpc/h of the galactic centre and a significant fraction in satellites where they may be singled out by their anomalous metallicity patterns. The first halo in which gas could cool by atomic hydrogen line radiation formed at z=25; by z=10, the number of such first galaxies had increased to ~300. Feedback might have decreased the number of first galaxies at the time they undergo significant star formation, but not the number that survive to the present because near neighbours merge. Half of all the "first galaxies" that form before z=10 merge with the main halo before z~3 and most lose a significant fraction of their mass. However, today there should still be more than 20 remnants orbiting within the central ~30kpc/h of the Milky Way. These satellites have circular velocities of a few kilometers per second or more, comparable to those of known Milky Way dwarfs. They are a promising hunting ground for the remnants of the earliest epoch of star formation.
The structure of small mixed helium clusters doped with one calcium atom has been determined within the diffusion Monte Carlo framework. The results show that the calcium atom sits at the $^4$He-$^3$He interface. This is in agreement with previous studies, both experimental and theoretical, performed for large clusters. A comparison between the results obtained for the largest cluster we have considered for each isotope shows a clear tendency of the Ca atom to reside in a deep dimple at the surface of the cluster for $^4$He clusters, and to become fully solvated for $^3$He clusters. We have calculated the absorption spectrum of Ca around the $4s4p \leftarrow 4s^2$ transition and have found that it is blue-shifted from that of the free-atom transition by an amount that depends on the size and composition of the cluster.
Landau's Fermi Liquid parameters are calculated for non-superfluid pure neutron matter in the presence of a strong magnetic field at zero temperature. The particle-hole interactions in the system, where a net magnetization may be present, are characterized by these parameters in the framework of a multipolar formalism. We use either zero- or finite-range effective nuclear forces to describe the nuclear interaction. Using the obtained Fermi Liquid parameters, the effect of a strong magnetic field on some bulk magnitudes such as isothermal compressibility and spin susceptibility is also investigated.
We construct the moduli space of r-jets at a point of Riemannian metrics on a smooth manifold. The construction is closely related to the problem of classification of jet metrics via differential invariants. The moduli space is proved to be a differentiable space which admits a finite canonical stratification into smooth manifolds. A complete study on the stratification of moduli spaces is carried out for metrics in dimension n=2.
We use a continued fraction approximation to calculate the RPA response function of nuclear matter. The convergence of the approximation is assessed by comparing with the numerically exact response function obtained with a typical effective finite-range interaction used in nuclear physics. It is shown that just the first order term of the expansion can give reliable results at densities up to the saturation density value.
The excitation energies of parahydrogen clusters have been systematically calculated by the diffusion Monte Carlo technique in steps of one molecule from 3 to 40 molecules. These clusters possess a very rich spectra, with angular momentum excitations arriving up to L=13 for the heavier ones. No regular pattern can be guessed in terms of the angular momenta and the size of the cluster. Clusters with N=13 and 36 are characterized by a peak in the chemical potential and a large energy gap of the first excited level, which indicate the magical character of these clusters. From the calculated excitation energies the partition function has been obtained, thus allowing for an estimate of thermal effects. An enhanced production is predicted for cluster sizes N=13, 31 and 36, in agreement with experiment.
Ground state energies and chemical potentials of parahydrogen clusters are calculated from 3 to 40 molecules using the diffusion Monte Carlo technique with two different pH2-pH2 interactions. This calculation improves a previous one by the inclusion of three-body correlations in the importance sampling, by the time step adjustement and by a better estimation of the statistical errors. Apart from the cluster with 13 molecules, no other magic clusters are predicted, in contrast with path integral Monte Carlo results.
We present preliminary results concerning the search for short-period variable stars in Tucana and LGS3 based on very deep HST/ACS imaging. In the fraction of the observed field we studied in each galaxy, a total of 133 and 30 variables were found, respectively. For Tucana, we identified 76 of them as RR Lyrae (RRL) stars pulsating in the fundamental mode (RRab) and 32 in the first-overtone mode (RRc), as well as 2 anomalous Cepheids (AC). The mean period of the RRab and RRc is 0.59 and 0.35 days, respectively. In the case of LGS3, we found 24 RRab and 4 RRc, with mean periods 0.61and 0.39 days, respectively, plus two candidate ACs. These values place both galaxies in the Oosterhoff gap.
Binding energies and density profiles of two-dimensional systems of liquid He-4 with different geometries are studied by means of a zero-range density functional adjusted to reproduce the line tension obtained in a previous diffusion Monte Carlo calculation (lambda_{DMC}=0.121 K/A). It is shown that this density functional provides accurate results for the binding energy of large clusters with a reasonable computational effort.
The question of nuclear response functions in a homogeneous medium is examined. A general method for calculating response functions in the random phase approximation (RPA) with exchange is presented. The method is applicable for finite-range nuclear interactions. Examples are shown in the case of symmetric nuclear matter described by a Gogny interaction. It is found that the convergence of the results with respect to the multipole truncation is quite fast. Various approximation schemes such as the Landau approximation, or the Landau approximation for the exchange terms only, are discussed in comparison with the exact results.
The diffusion Monte Carlo technique is used to calculate and analyze the excitation spectrum of $^3$He atoms bound to a cluster of $^4$He atoms, by using a previously determined optimum filling of single-fermion orbits with well defined orbital angular momentum $L$, spin $S$ and parity quantum numbers. The study concentrates on the energies and shapes of the three kinds of states for which the fermionic part of the wave function is a single Slater determinant: maximum $L$ or maximum $S$ states within a given orbit, and fully polarized clusters. The picture that emerges is that of systems with strong shell effects whose binding and excitation energies are essentially determined over configuration at fixed number of particles and spin, i.e., by the monopole properties of an effective Hamiltonian.
The minimal number of 3He atoms required to form a bound cluster has been estimated by means of a Diffusion Monte Carlo procedure within the fixed-node approximation. Several importance sampling wave functions have been employed in order to consider different shell-model configurations. The resulting upper bound for the minimal number is 32 atoms.
The binding energies of two-dimensional clusters (puddles) of $^4$He are calculated in the framework of the diffusion Monte Carlo method. The results are very well fitted by a mass formula in powers of $x=N^{-1/2}$, where $N$ is the number of particles. The analysis of the mass formula allows for the extraction of the line tension, which turns out to be 0.121 K/A. Sizes and density profiles of the puddles are also reported.
The diffusion Monte Carlo technique is used to calculate and analyze the excitation spectrum of a single 3He atom bound to a cluster with N 4He atoms, with the aim of establishing the most adequate filling ordering of single-fermion orbits to the mixed clusters with a large number of 3He atoms. The resulting ordering looks like the rotational spectrum of a diatomic molecule, being classified only by the angular momentum of the level, although vibrational-like excitations appear at higher energies for sufficiently large N.
95, 97Mo NMR experiments have been performed on a series of Sr2FeMoO6 and electron-doped Sr2-xLaxFeMoO6 ceramics. Detailed analysis of the NMR spectra from pristine Sr2FeMoO6 conclusively shows that the Mo hyperfine field is mainly due to atomic Mo magnetic moments. No contribution of transferred hyperfine field has been observed, confirming the absence of s-electrons in the conduction band. Upon La doping, the NMR frequency (hyperfine field) gradually increases proving that the concentration of spin polarized electrons at Mo ion is enhanced by the La substitution. A simple linear correlation between magnetic moment at Mo sites and the Curie temperature of the system has been found. Implications for understanding the electronic structure and the ferromagnetic coupling in these systems are underlined.