The Post-Quasistatic Approximation as a test bed for Numerical Relativity

It is shown that observers in the standard ADM 3+1 treatment of matter are the same as the observers used in the matter treatment of Bondi: they are comoving and local Minkowskian. Bondi's observers are the basis of the post--quasitatic approximation (PQSA) to study a contracting distribution of matter. This correspondence suggests the possibility of using the PQSA as a test bed for Numerical Relativity. The treatment of matter by the PQSA and its connection with the ADM 3+1 treatment are presented, for its practical use as a calibration tool and as a test bed for numerical relativistic hydrodynamic codes.Comment: 4 pages; to appear as a Brief Report in Physical Review

Statistical GIS-based analysis of energy consumption for residential buildings in Turin (IT)

Greenhouse gas emission is an important issue and the largest source of it is from human activities and from building sectors. Therefore, the building stocks play a key role in the reduction of GHG emissions through the analysis of the energy performance of buildings, in order to understand their behavior and to identify effective models that will allow expanding investigations in vast areas as districts or cities. This work analyses space heating energy performance of buildings with a multi-scale approach using the main energy related variables at building, block of buildings and district scale. The purpose of this study is to identify a simple regression model in order to evaluate the space heating energy consumption of a large part of residential buildings in Turin (IT). A cluster analysis was applied in order to find groups of buildings with similar energy consumptions and to identify the main energy-related characteristics of each group. The analysis was developed with the support of a GIS tool to evaluate the buildings characteristics and a statistical software to identify a stable model at urban scale. The identified models evidenced that the space heating energy consumption not only depends on the characteristics of the building itself, but also on the urban characteristics. At urban scale, the most influential variables were: the heating degree days, positively correlated with the space heating consumption, and the albedo that was negatively correlated. Also, socio-economic variables were utilized: the percentage of working people with a positive correlation and the percentage of young inhabitants with a negative correlation. The statistical GIS-based methodology proposed in this study is simple and then replicable to other urban contexts. This kind of analysis can be useful for policy makers in defining specific energy efficiency measures for each group of buildings to identify new more effective energy performance variables and benchmarks for the different groups of buildings and then to improve the energy performance of a city reducing energy consumptions and the relative GHG emissions

U B V R I Photometry of Stellar Structures throughout the Disk of the Barred Galaxy NGC 3367

We report new detailed surface U, B, V, R, and I photometry of 81 stellar structures in the disk of the barred galaxy NGC 3367. The images show many different structures indicating that star formation is going on in the most part of the disk. NGC 3367 is known to have a very high concentration of molecular gas distribution in the central regions of the galaxy and bipolar synchrotron emission from the nucleus with two lobes (at 6 kpc) forming a triple structure similar to a radio galaxy. We have determined the U, B, V, R, and I magnitudes and U - B, B - V, U - V, and V - I colors for the central region (nucleus), a region which includes supernovae 2003 AA, and 79 star associations throughout NGC 3367. Estimation of ages of star associations is very difficult due to several factors, among them: filling factor, metallicity, spatial distribution of each structure and the fact that we estimated the magnitudes with a circular aperture of 16 pixels in diameter, equivalent to $6''.8\sim1.4$ kpc. However, if the colors derived for NGC 3367 were similar to the colors expected of star clusters with theoretical evolutionary star tracks developed for the LMC and had a similar metallicity, NGC 3367 show 51 percent of the observed structures with age type SWB I (few tens of Myrs), with seven sources outside the bright surface brightness visible disk of NGC 3367.Comment: Accepted for publication (abr 2007) in The Astronomical Journal (July 2007 issue

Equation of state and transport processes in self--similar spheres

We study the effect of transport processes (diffusion and free--streaming) on a collapsing spherically symmetric distribution of matter in a self--similar space--time. A very simple solution shows interesting features when it is matched with the Vaidya exterior solution. In the mixed case (diffusion and free--streaming), we find a barotropic equation of state in the stationary regime. In the diffusion approximation the gravitational potential at the surface is always constant; if we perturb the stationary state, the system is very stable, recovering the barotropic equation of state as time progresses. In the free--streaming case the self--similar evolution is stationary but with a non--barotropic equation of state.Comment: 9 pages, 2 figure

Successor features for transfer in reinforcement learning

Transfer in reinforcement learning refers to the notion that generalization should occur not only within a task but also across tasks. Our focus is on transfer where the reward functions vary across tasks while the environment's dynamics remain the same. The method we propose rests on two key ideas: "successor features," a value function representation that decouples the dynamics of the environment from the rewards, and "generalized policy improvement," a generalization of dynamic programming's policy improvement step that considers a set of policies rather than a single one. Put together, the two ideas lead to an approach that integrates seamlessly within the reinforcement learning framework and allows transfer to take place between tasks without any restriction. The proposed method also provides performance guarantees for the transferred policy even before any learning has taken place. We derive two theorems that set our approach in firm theoretical ground and present experiments that show that it successfully promotes transfer in practice

Self-similar and charged spheres in the diffusion approximation

We study spherical, charged and self--similar distributions of matter in the diffusion approximation. We propose a simple, dynamic but physically meaningful solution. For such a solution we obtain a model in which the distribution becomes static and changes to dust. The collapse is halted with damped mass oscillations about the absolute value of the total charge.Comment: 15 pages, 7 figure

HI aperture synthesis and optical observations of the pair of galaxies NGC 6907 and 6908

NGC 6908, a S0 galaxy situated in direction of NGC 6907, was only recently recognized as a distinct galaxy, instead of only a part of NGC 6907. We present 21 cm radio synthesis observations obtained with the GMRT and optical images and spectroscopy obtained with the Gemini North telescope of this pair of interacting galaxies. From the radio observations we obtained the velocity field and the HI column density map of the whole region containing the NGC 6907/8 pair, and by means of the Gemini multi-object spectroscopy we obtained high quality photometric images and $5 {\AA}$ resolution spectra sampling the two galaxies. By comparing the rotation curve of NGC 6907 obtained from the two opposite sides around the main kinematic axis, we were able to distinguish the normal rotational velocity field from the velocity components produced by the interaction between the two galaxies. Taking into account the rotational velocity of NGC 6907 and the velocity derived from the absorption lines for NGC 6908, we verified that the relative velocity between these systems is lower than 60 km s$^{-1}$. The emission lines observed in the direction of NGC 6908, not typical of S0 galaxies, have the same velocity expected for the NGC 6907 rotation curve. Some of them, superimposed on the absorption profiles, which reinforces the idea that they were not formed in NGC 6908. Finally, the HI profile exhibits details of the interaction, showing three components: one for NGC 6908, another for the excited gas in the NGC 6907 disk and a last one for the gas with higher relative velocities left behind NGC 6908 by dynamical friction, used to estimate the time when the interaction started in $(3.4 \pm 0.6)\times10^7$ years ago.Comment: 11 pages, 5 tables, 13 figures. Corrected typos. Accepted for publication in MNRAS. The definitive version will be available at http://www.blackwell-synergy.co

Fabry Perot Halpha Observations of the Barred Spiral NGC 3367

We report the gross properties of the velocity field of the barred spiral galaxy NGC 3367. The following values were found: inclination with respect to the plane of the sky, i=30 deg; position angle (PA) of receding semi major axis PA=51 and systemic velocity V(sys)=3032 km/s. Large velocity dispersion are observed of upt o 120 km/s in the nuclear region, of up to 70 km/s near the eastern bright sources just beyond the edge of the stellar bar where three spiral arms seem to start and in the western bright sources at about 10 kpc. Deviations from normal circular velocities are observed from all the disk but mainly from the semi circle formed by the string of south western Halpha sources. An estimate of the dynamical mass is M(dyn)=2x10^11 Msolar.Comment: Accepted to be published in May 2001 issue in the A.J. 19 pages, 7 figure