Driven low density granular mixtures

We study the steady state properties of a 2D granular mixture in the presence of energy driving by employing simple analytical estimates and Direct Simulation Monte Carlo. We adopt two different driving mechanisms: a) a homogeneous heat bath with friction and b) a vibrating boundary (thermal or harmonic) in the presence of gravity. The main findings are: the appearance of two different granular temperatures, one for each species; the existence of overpopulated tails in the velocity distribution functions and of non trivial spatial correlations indicating the spontaneous formation of cluster aggregates. In the case of a fluid subject to gravity and to a vibrating boundary, both densities and temperatures display non uniform profiles along the direction normal to the wall, in particular the temperature profiles are different for the two species while the temperature ratio is almost constant with the height. Finally, we obtained the velocity distributions at different heights and verified the non gaussianity of the resulting distributions.Comment: 19 pages, 12 figures, submitted for publicatio

Thermal convection in mono-disperse and bi-disperse granular gases: A simulation study

We present results of a simulation study of inelastic hard-disks vibrated in a vertical container. An Event-Driven Molecular Dynamics method is developed for studying the onset of convection. Varying the relevant parameters (inelasticity, number of layers at rest, intensity of the gravity) we are able to obtain a qualitative agreement of our results with recent hydrodynamical predictions. Increasing the inelasticity, a first continuous transition from the absence of convection to one convective roll is observed, followed by a discontinuous transition to two convective rolls, with hysteretic behavior. At fixed inelasticity and increasing gravity, a transition from no convection to one roll can be evidenced. If the gravity is further increased, the roll is eventually suppressed. Increasing the number of monolayers the system eventually localizes mostly at the bottom of the box: in this case multiple convective rolls as well as surface waves appear. We analyze the density and temperature fields and study the existence of symmetry breaking in these fields in the direction perpendicular to the injection of energy. We also study a binary mixture of grains with different properties (inelasticity or diameters). The effect of changing the properties of one of the components is analyzed, together with density, temperature and temperature ratio fields. Finally, the presence of a low-fraction of quasi-elastic impurities is shown to determine a sharp transition between convective and non-convective steady states.Comment: 11 pages, 12 figures, accepted for publication on Physical Review

NGC 2849 and NGC 6134: two more BOCCE open clusters

We present CCD photometry of two southern open clusters. As part of the Bologna Open Cluster Chemical Evolution project we obtained BVI and UBVI imaging for NGC 2849 and NGC 6134, respectively. By means of the synthetic colour-magnitude diagram method and using various evolutionary sets of stellar evolution tracks with various metallicities, we determined at the same time age, distance and reddening. We also determined an approximate metallicity for NGC 2849, for which the information is not available from sounder methods like high-resolution spectroscopy. NGC 2849 turned to be 0.85-1.0 Gyr old with a solar metallicity. The foreground reddening is E(B - V) = 0.28 - 0.32, and the true distance modulus (m - M) 0 = 13.8-13.9. For NGC 6134 we did not obtain fully consistent answers from the V, B - V and V, V - I photometry, an unexpected problem, since both the metallicity and the reddening are known (from high-resolution spectroscopy and the U - B, B - V two colours diagram, respectively). This may either indicate a difficulty of current models (evolutionary tracks and/or models of atmosphere) to accurately reproduce colours, or be related to differences in the metal mixture assumed by the models and those of the clusters. Assuming the spectroscopic abundance and the colour excess [E(B - V) = 0.35] from the U - B, B - V plot, we derived a best age between 0.82 and 0.95 Gyr and a distance modulus 10.5. In agreement with previous studies, the NGC 6134 colour-magnitude diagram shows also a clear main sequence gap at V ˜ 15 and B-V ˜ 0.9-1.0 that is unexplained by canonical stellar evolution models.Fil: Ahumada, Andrea Veronica. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba. Departamento de Astrofisica Estelar; ArgentinaFil: Cignoni, M.. Universitá di Bologna. Dipartimento di Astronomia; Italia;Fil: Bragaglia, A.. INAF-Osservatorio Astronomico di Bologna; Italia;Fil: Donati, P.. Universitá di Bologna. Dipartimento di Astronomia; Italia;Fil: Tosi, M.. INAF-Osservatorio Astronomico di Bologna; Italia;Fil: Marconi, G.. European Southern Observatory (ESO); Chile

A Soluble Phase Field Model

The kinetics of an initially undercooled solid-liquid melt is studied by means of a generalized Phase Field model, which describes the dynamics of an ordering non-conserved field phi (e.g. solid-liquid order parameter) coupled to a conserved field (e.g. thermal field). After obtaining the rules governing the evolution process, by means of analytical arguments, we present a discussion of the asymptotic time-dependent solutions. The full solutions of the exact self-consistent equations for the model are also obtained and compared with computer simulation results. In addition, in order to check the validity of the present model we confronted its predictions against those of the standard Phase field model and found reasonable agreement. Interestingly, we find that the system relaxes towards a mixed phase, depending on the average value of the conserved field, i.e. on the initial condition. Such a phase is characterized by large fluctuations of the phi field.Comment: 13 pages, 8 figures, RevTeX 3.1, submitted to Physical Review

Which is the temperature of granular systems? A mean field model of free cooling inelastic mixtures

We consider a mean field model describing the free cooling process of a two component granular mixture, a generalization of so called Maxwell model. The cooling is viewed as an ordering process and the scaling behavior is attributed to the presence of an attractive fixed point at $v=0$ for the dynamics. By means of asymptotic analysis of the Boltzmann equation and of numerical simulations we get the following results: 1)we establish the existence of two different partial granular temperatures, one for each component, which violates the Zeroth Law of Thermodynamics; 2) we obtain the scaling form of the two distribution functions; 3) we prove the existence of a continuous spectrum of exponents characterizing the inverse-power law decay of the tails of the velocity, which generalizes the previously reported value 4 for the pure model; 4) we find that the exponents depend on the composition, masses and restitution coefficients of the mixture; 5) we also remark that the reported distributions represent a dynamical realization of those predicted by the Non Extensive Statistical Mechanics, in spite of the fact that ours stem from a purely dynamical approach.Comment: 23 pages, 9 figures. submitted for publicatio

Steady state properties of a mean field model of driven inelastic mixtures

We investigate a Maxwell model of inelastic granular mixture under the influence of a stochastic driving and obtain its steady state properties in the context of classical kinetic theory. The model is studied analytically by computing the moments up to the eighth order and approximating the distributions by means of a Sonine polynomial expansion method. The main findings concern the existence of two different granular temperatures, one for each species, and the characterization of the distribution functions, whose tails are in general more populated than those of an elastic system. These analytical results are tested against Monte Carlo numerical simulations of the model and are in general in good agreement. The simulations, however, reveal the presence of pronounced non-gaussian tails in the case of an infinite temperature bath, which are not well reproduced by the Sonine method.Comment: 23 pages, 10 figures, submitted for publicatio

Obtención de variantes más resistentes a los contaminantes a partir de una cepa nativa de Chlorella vulgaris

El arroyo Cildánez ˜ (en la cuenca Matanza-Riachuelo) es uno de los cursos de agua más contaminados en Argentina. Este arroyo presenta contaminación mixta proveniente de residuos agrícolas e industriales. En este sentido, existe un amplio interés en la obtención de líneas de microalga más resistentes a la contaminación para usar en procesos de biorremediación. En este trabajo, se empleó la evolución adaptativa de laboratorio (ALE) y la mutagénesis aleatoria para obtener nuevas variantes de la cepa de Chlorella vulgaris LMPA-40 adaptadas para crecer en agua contaminada del arroyo Cildánez. ˜ El proceso ALE se realizó mediante 22 subcultivos sucesivos bajo presión selectiva (agua contaminada del arroyo Cildánez ˜ sola o con el agregado de fenol o H2O2), mientras que la mutagénesis aleatoria se realizó con radiación UV-C a 275 nm. No todas las líneas celulares obtenidas mediante ALE pudieron adaptarse lo suficiente para superar el estrés provocado por el agua contaminada del Cildánez, ˜ lo que indica que el proceso es bastante aleatorio y depende del estresor utilizado. Los mejores resultados se obtuvieron con las células adaptadas al agua contaminada del Cildánez ˜ (cepa Cild 3) que fueron más resistentes que la cepa original. La concentración de proteínas, clorofila A, clorofila B y carotenoides en la cepa evolucionada Cild 3 fue mayor que en la cepa control. Sin embargo, esta cepa Cild 3 exhibió la mitad del contenido de lípidos en comparación con la misma cepa control. Curiosamente, estas alteraciones y la tolerancia adquirida pueden revertirse con el tiempo durante el almacenamiento. Estos hallazgos sugieren que la adquisición de nuevas líneas celulares no podría ser permanente y este hecho debe tenerse en cuenta en ensayos futuros.Adaptivelaboratory evolution (ALE) and random mutagenesis, processes that allow to getnew variants from parental strains, were used to obtain Chlorella vulgaris LMPA-40 strains adapted to grow in watersubjected to mixed contamination from agricultural and industrial wastes. TheALE process was performed by 22 successive subcultures under selective pressure(phenol, H2O2 or Cildáñez wastewater) while randommutagenesis was performed with UV radiation at 275 nm. Not all the cell linesobtained after ALE could adapt enough to overcome the stress caused by Cildáñezwastewater indicating that the process is quite random and depends on thestressor used. The best results were obtained for the Cildáñez wastewateradapted cells (Cild 3 strain) that were more resistant than the originalstrain. The concentration of protein, Chlorophyll A, Chlorophyll B, andcarotenoid in the Cild 3 ALE evolved strain was higher than that of the controlstrain. However, this strain exhibited half the lipid content compared to thesame control strain. Interestingly, these alterations and the acquiredtolerance may be reversed over time during storage. These findings suggest thatthe acquisition of novel cell lines could not be permanent, and this fact mustbe considered for future trials.Fil: Trentini, Andrea Giannina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Maimónides. Área de Investigaciones Biomédicas y Biotecnológicas. Centro de Estudios Biomédicos, Biotecnológicos, Ambientales y de Diagnóstico; ArgentinaFil: Salvio, Uriel D.. Universidad Maimónides. Área de Investigaciones Biomédicas y Biotecnológicas. Centro de Estudios Biomédicos, Biotecnológicos, Ambientales y de Diagnóstico; ArgentinaFil: Sanchez Novoa, Juan Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Maimónides. Área de Investigaciones Biomédicas y Biotecnológicas. Centro de Estudios Biomédicos, Biotecnológicos, Ambientales y de Diagnóstico; ArgentinaFil: Groppa, María Daniela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Química y Físico-Química Biológicas "Prof. Alejandro C. Paladini". Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Instituto de Química y Físico-Química Biológicas; ArgentinaFil: Navarro Llorens, Juana M.. Universidad Complutense de Madrid; EspañaFil: Marconi, Patricia Laura. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Maimónides. Área de Investigaciones Biomédicas y Biotecnológicas. Centro de Estudios Biomédicos, Biotecnológicos, Ambientales y de Diagnóstico; Argentin

Massive Galaxies in COSMOS: Evolution of Black hole versus bulge mass but not versus total stellar mass over the last 9 Gyrs?

We constrain the ratio of black hole (BH) mass to total stellar mass of type-1 AGN in the COSMOS survey at 1<z<2. For 10 AGN at mean redshift z~1.4 with both HST/ACS and HST/NICMOS imaging data we are able to compute total stellar mass M_(*,total), based on restframe UV-to-optical host galaxy colors which constrain mass-to-light ratios. All objects have virial BH mass-estimates available from the COSMOS Magellan/IMACS and zCOSMOS surveys. We find zero difference between the M_BH--M_(*,total)-relation at z~1.4 and the M_BH--M_(*,bulge)-relation in the local Universe. Our interpretation is: (a) If our objects were purely bulge-dominated, the M_BH--M_(*,bulge)-relation has not evolved since z~1.4. However, (b) since we have evidence for substantial disk components, the bulges of massive galaxies (logM_(*,total)=11.1+-0.25 or logM_BH~8.3+-0.2) must have grown over the last 9 Gyrs predominantly by redistribution of disk- into bulge-mass. Since all necessary stellar mass exists in the galaxy at z=1.4, no star-formation or addition of external stellar material is required, only a redistribution e.g. induced by minor and major merging or through disk instabilities. Merging, in addition to redistributing mass in the galaxy, will add both BH and stellar/bulge mass, but does not change the overall final M_BH/M_(*,bulge) ratio. Since the overall cosmic stellar and BH mass buildup trace each other tightly over time, our scenario of bulge-formation in massive galaxies is independent of any strong BH-feedback and means that the mechanism coupling BH and bulge mass until the present is very indirect.Comment: Published in ApJL; 7 pages, 2 figures; updated to accepted version (methods changed, results unchanged

Accretion Rate and the Physical Nature of Unobscured Active Galaxies

We show how accretion rate governs the physical properties of a sample of unobscured broad-line, narrow-line, and lineless active galactic nuclei (AGNs). We avoid the systematic errors plaguing previous studies of AGN accretion rate by using accurate accretion luminosities (L_int) from well-sampled multiwavelength SEDs from the Cosmic Evolution Survey (COSMOS), and accurate black hole masses derived from virial scaling relations (for broad-line AGNs) or host-AGN relations (for narrow-line and lineless AGNs). In general, broad emission lines are present only at the highest accretion rates (L_int/L_Edd > 0.01), and these rapidly accreting AGNs are observed as broad-line AGNs or possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower specific accretion rates (L_int/L_Edd < 0.01) are unobscured and yet lack a broad line region. The disappearance of the broad emission lines is caused by an expanding radiatively inefficient accretion flow (RIAF) at the inner radius of the accretion disk. The presence of the RIAF also drives L_int/L_Edd < 10^-2 narrow-line and lineless AGNs to 10 times higher ratios of radio to optical/UV emission than L_int/L_Edd > 0.01 broad-line AGNs, since the unbound nature of the RIAF means it is easier to form a radio outflow. The IR torus signature also tends to become weaker or disappear from L_int/L_Edd < 0.01 AGNs, although there may be additional mid-IR synchrotron emission associated with the RIAF. Together these results suggest that specific accretion rate is an important physical "axis" of AGN unification, described by a simple model.Comment: Accepted for publication in the Astrophysical Journal. 15 pages, 9 figure