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Context. Recent studies have suggested that moving groups have a dynamic or "resonant" origin. Under this hypothesis, these kinematic structures become a powerful tool for studying the large-scale structure and dynamics of the Milky Way. Aims. We aim to characterize these structures in the U-V-age-[Fe/H] space and establish observational constraints that will allow us to study their origin and evolution. Methods. We apply multiscale techniques -wavelet denoising (WD)- to an extensive compendium of more than 24000 stars in the solar neighbourhood with the best available astrometric, photometric and spectroscopic data. Results. We confirm that the dominant structures in the U-V plane are the branches of Sirius, Coma Berenices, Hyades-Pleiades and Hercules, which are nearly equidistant in this kinematic plane and show a negative slope. The abrupt drops in the velocity distribution are characterized. We find a certain dependence of these kinematic structures on Galactic position with a significant change of contrast among substructures inside the branches. A large spread of ages is observed for all branches. The Hercules branch is detected in all subsamples with ages older than ~ 2 Gyr and the set of the other three branches is well established for stars > 400 Myr. The age-metallicity relation of each branch is examined and the relation between kinematics and metallicity is studied. Conclusions. Not all of these observational constraints are successfully explained by the recent models proposed for the formation of such kinematic structures. Simulations incorporating stellar ages and metallicities are essential for future studies. The comparison of the observed and simulated distributions obtained by WD will provide a physical interpretation of the existence of the branches in terms of local or large-scale dynamics. [Abridged], Comment: 18 pages, 17 figures, 4 tables, accepted for publication in A&A

The critical behavior of long straight rigid rods of length $k$ ($k$-mers) on square and triangular lattices at intermediate density has been studied. A nematic phase, characterized by a big domain of parallel $k$-mers, was found. This ordered phase is separated from the isotropic state by a continuous transition occurring at a intermediate density $\theta_c$. Two analytical techniques were combined with Monte Carlo simulations to predict the dependence of $\theta_c$ on $k$, being $\theta_c(k) \propto k^{-1}$. The first involves simple geometrical arguments, while the second is based on entropy considerations. Our analysis allowed us also to determine the minimum value of $k$ ($k_{min}=7$), which allows the formation of a nematic phase on a triangular lattice., Comment: 23 pages, 5 figures, to appear in The Journal of Chemical Physics

Context: Over the last decade, several groups of young (mainly low-mass) stars have been discovered in the solar neighbourhood (closer than ~100 pc), thanks to cross-correlation between X-ray, optical spectroscopy and kinematic data. These young local associations -- including an important fraction whose members are Hipparcos stars -- offer insights into the star formation process in low-density environments, shed light on the substellar domain, and could have played an important role in the recent history of the local interstellar medium. Aims: To study the kinematic evolution of young local associations and their relation to other young stellar groups and structures in the local interstellar medium, thus casting new light on recent star formation processes in the solar neighbourhood. Methods: We compiled the data published in the literature for young local associations. Using a realistic Galactic potential we integrated the orbits for these associations and the Sco-Cen complex back in time. Results: Combining these data with the spatial structure of the Local Bubble and the spiral structure of the Galaxy, we propose a recent history of star formation in the solar neighbourhood. We suggest that both the Sco-Cen complex and young local associations originated as a result of the impact of the inner spiral arm shock wave against a giant molecular cloud. The core of the giant molecular cloud formed the Sco-Cen complex, and some small cloudlets in a halo around the giant molecular cloud formed young local associations several million years later. We also propose a supernova in young local associations a few million years ago as the most likely candidate to have reheated the Local Bubble to its present temperature., Comment: 27 pages, 13 figures, 14 tables. Accepted for publication in Astronomy & Astrophysics

Monte Carlo simulations and finite-size scaling analysis have been carried out to study the critical behavior and universality for the isotropic-nematic phase transition in a system of long straight rigid rods of length $k$ ($k$-mers) on two-dimensional lattices. The nematic phase, characterized by a big domain of parallel $k$-mers, is separated from the isotropic state by a continuous transition occurring at a finite density. The determination of the critical exponents, along with the behavior of Binder cumulants, indicate that the transition belongs to the 2D Ising universality class for square lattices and the three-state Potts universality class for triangular lattices., Comment: 7 pages, 8 figures, uses epl2.cls, to appear in Europhysics Letters

In this work we propose a scenario for the history of the recent star formation (during the last 20-30 Myr) in the nearest solar neighbourhood (~150 pc), from the study of the spatial and kinematic properties of the members of the so-called young local associations, the Sco-Cen complex and the Local Bubble, the most important structure observed in the local interstellar medium (ISM)., Comment: 6 pages, 1 figure, 1 table. To appear in "Highlights of Spanish Astrophysics IV". Proceedings of the VII Scientific Meeting of the Spanish Astronomical Society (SEA), Barcelona, September, 2006, eds. F. Figueras, J.M. Girart, M. Hernanz and C. Jordi (Springer)

The young star velocity field is analysed by means of a galactic model which takes into account solar motion, differential galactic rotation and spiral arm kinematics. We use two samples of Hipparcos data, one containing O- and B-type stars and another one composed of Cepheid variable stars. The robustness of our method is tested through careful kinematic simulations. Our results show a galactic rotation curve with a classical value of $A$ Oort constant for the O and B star sample ($A^{\mathrm{OB}} =$ 13.7-13.8 km s$^{-1}$ kpc $^{-1}$) and a higher value for Cepheids ($A^{\mathrm{Cep}} =$ 14.9-16.9 km s$^{-1}$ kpc $^{-1}$, depending on the cosmic distance scale chosen). The second-order term is found to be small, compatible with a zero value. The study of the residuals shows the need for a $K$-term up to a heliocentric distance of 4 kpc, obtaining a value $K = -$(1-3) km s$^{-1}$ kpc$^{-1}$. The results obtained for the spiral structure from O and B stars and Cepheids show good agreement. The Sun is located relatively near the minimum of the spiral perturbation potential ($\psi_\odot =$ 284-20$\degr$) and very near the corotation circle. The angular rotation velocity of the spiral pattern was found to be $\Omega_{\mathrm{p}} \approx 30$ km s$^{-1}$ kpc $^{-1}$., Comment: 18 pages, 6 figures, LaTeX, uses aa.cls. Accepted for publication in A&A

Two samples of O- and B-type stars and Cepheids with Hipparcos data have been used to characterize galactic rotation and spiral arm kinematics in the solar neighbourhood. An extensive set of simulations has been performed in order to assess the capabilities of the method and its sensitivity to sample errors and biases in the adopted parameters., Comment: 4 pages, no figures, LaTex, uses crckapb.sty. To appear in the Proceedings of the IV Scientific Meeting of the Sociedad Espanola de Astronomia (SEA)

A sample of O- and B-type stars with Hipparcos astrometric data, ages computed from Str\"omgren photometry and radial velocities, has been used to characterize the structure, age and kinematics of the Gould Belt system. The local spiral structure of our galaxy is determined from this sample, and also from a sample of Hipparcos Cepheid stars. The Gould Belt, with an orientation with respect to the galactic plane of $i_{\mathrm{G}} = 16$-$22^\circ$ and $\Omega_{\mathrm{G}} = 275$-$295^\circ$, extends up to a distance of 600 pc from the Sun. Roughly the 60-65% of the O and B stars younger than 60 Myr in the solar neighbourhood belong to this structure. Our results indicate that the kinematical behaviour of this system is complex, with an expansion motion in the solar neighbourhood ($R <$ 300 pc). In the frame of the Lin's theory, and analysing the O and B stars further than 600 pc and the Cepheids, we found a galactic spiral structure characterized by a 4-arm spiral pattern with the Sun located at $\psi_\odot = 350$-$355 \pm 30^\circ$ -- near the Sagittarius-Carina arm -- and outside the corotation circle. The angular rotation speed of the spiral pattern was found to be $\Omega_{\mathrm{p}} = 31$-$32 \pm 4$ km s$^{-1}$ kpc$^{-1}$., Comment: 4 pages, no figures, LaTex, uses kluwer.cls. To appear in Ap&SS, in the proceedings of "Astrophysical Dynamics - Conference to commemorate the Work of Franz D.Kahn, FRS (1925-1998)", eds. D. Berry, et al

The local velocity field of young stars is dominated by the galactic rotation, the kinematics of the Gould Belt and the nearest OB associations and open clusters, and the kinematics of the spiral structure. We re-examined here this local velocity field by using a large sample of nearby O and B stars from the Hipparcos Catalogue. The high quality astrometric data are complemented with a careful compilation of radial velocities and Str\"omgren photometry, which allows individual photometric distances and ages to be derived. The Gould Belt extends up to 600 pc from the Sun with an inclination with respect to the galactic plane of $i_{\mathrm{G}} = 16$-$22\degr$ and the ascending node placed at $\Omega_{\mathrm{G}} = 275$-$295\degr$. Approximately 60% of the stars younger than 60 Myr belong to this structure. The values found for the Oort constants when different samples selected by age or distance were used allowed us to interpret the systematic trends observed as signatures induced by the kinematic behaviour of the Gould Belt. The contribution of Sco-Cen and Ori OB1 complexes in the characterization of the expansion of the Gould Belt system is also discussed. We found that a positive $K$-term remains when these aggregates are excluded. From the kinematic behaviour of the stars and their spatial distribution we derive an age for the Gould Belt system in the interval 30-60 Myr., Comment: 23 pages, 18 figures, LaTex, uses aa.cls. Accepted for publication in A&A

We propose and analyze in detail a method to measure the in-air spatial spread parameter of clinical electron beams. Measurements are performed at the center of the beam and below the adjustable collimators sited in asymmetrical configuration in order to avoid the distortions due to the presence of the applicator. The main advantage of our procedure lies in the fact that the dose profiles are fitted by means of a function which includes, additionally to the Gaussian step usually considered, a background which takes care of the dose produced by different mechanisms that the Gaussian model does not account for. As a result, the spatial spread is obtained directly from the fitting procedure and the accuracy permits a good determination of the angular spread. The way the analysis is done is alternative to that followed by the usual methods based on the evaluation of the penumbra width. Besides, the spatial spread found shows the quadratic-cubic dependence with the distance to the source predicted by the Fermi-Eyges theory. However, the corresponding values obtained for the scattering power are differing from those quoted by ICRU nr. 35 by a factor ~2 or larger, what requires of a more detailed investigation., Comment: 11 pages, 5 Postscript figures, to be published in Medical Physics