Rocking ratchets in 2D Josephson networks: collective effects and current reversal

A detailed numerical study on the directed motion of ac-driven vortices and antivortices in 2D Josephson junction arrays (JJA) with an asymmetric periodic pinning potential is reported. Dc-voltage rectification shows a strong dependence on vortex density as well as an inversion of the vortex flow direction with ac amplitude for a wide range of vortex density around $f$=1/2 ($f$=$Ha^2 / \Phi_0$), in good agreement with recent experiments by Shal\'om and Pastoriza [Phys. Rev. Lett. {\bf 94}, 177001 (2005)]. The study of vortex structures, spatial and temporal correlations, and vortex-antivortex pairs formation gives insight into a purely collective mechanism behind the current reversal effect.Comment: 4 pages, 5 figures. Accepted for publication in Phys. Rev. Let

The Role of Opacities in Stellar Pulsation

We examine the role of opacities in stellar pulsation with reference to Cepheids and RR Lyraes, and examine the effect of augmented opacities on the theoretical pulsation light curves in key temperature ranges. The temperature ranges are provided by recent experimental and theoretical work that have suggested that the iron opacities have been considerably underestimated. For Cepheids, we find that the augmented opacities have noticeable effects in certain period ranges (around $\log P \approx 1$) even though there is a degeneracy with mixing length. We also find significant effects in theoretical models of B-star pulsators.Comment: 6 pages, 3 Figures, Proceeding for the "Workshop on Astrophysical Opacities

Non-equilibrium fluctuations in a driven stochastic Lorentz gas

We study the stationary state of a one-dimensional kinetic model where a probe particle is driven by an external field E and collides, elastically or inelastically, with a bath of particles at temperature T. We focus on the stationary distribution of the velocity of the particle, and of two estimates of the total entropy production \Delta s_tot. One is the entropy production of the medium \Delta s_m, which is equal to the energy exchanged with the scatterers, divided by a parameter \theta, coinciding with the particle temperature at E=0. The other is the work W done by the external field, again rescaled by \theta. At small E, a good collapse of the two distributions is found: in this case the two quantities also verify the Fluctuation Relation (FR), indicating that both are good approximations of \Delta s_tot. Differently, for large values of E, the fluctuations of W violate the FR, while \Delta s_m still verifies it.Comment: 6 pages, 4 figure

Influence of Beams Distribution on the Dynamic and Seismic Linear Response of RC Frame Buildings

The present study compares the dynamic properties and seismic performances offered by reinforced concrete frame structures characterized by different beams distribution. The understanding of the influence of beams distribution on spatial frames is not only useful when dealing with the seismic vulnerability assessment of existing buildings that may show unusual layouts of beams, such as alternating beams at each storey, but also when facing the design of new buildings with fluid viscous dampers for which some structural flexibility is required. A systematic study is described in this paper. Four (2-, 3-, 6-, and 10-storey) regular frame buildings with rectangular plan are considered as reference structures. Different models are developed according to various layouts of the primary beams, exploring alternatives to the full three-dimensional organisation of beams and frames. For instance: beams placed along the longitudinal direction at the odd storeys and placed along the transversal direction at the even storeys, and vice versa; alternating beams every one and two storeys; beams just placed along one direction. Modal analysis has been conducted to evaluate the influence of beams distribution on the dynamic properties (periods of vibration and modal participating mass ratios). Response spectrum analysis and linear time-history dynamic analysis have been carried out to assess the effects of beams distribution on the fundamental seismic response parameters (shear forces, bending moments, top-storey displacements, interstorey drifts, and floor accelerations). On the contrary of what could be expected, the results indicate that structures with beams alternating every storey may show interesting advantages in terms of reduced total base shear, almost comparable bending moments and accelerations, within a still balanced overall behaviour along the two directions, with respect to the complete three-dimensional frame. Two effects are recognized: the period effect and the static scheme effect. The former acting basically on the storey shear forces; the latter acting mainly on the bending moments

Is there any evidence that ionised outflows quench star formation in type 1 quasars at z<1?

The aim of this paper is to test the basic model of negative AGN feedback. According to this model, once the central black hole accretes at the Eddington limit and reaches a certain critical mass, AGN driven outflows blow out gas, suppressing star formation in the host galaxy and self-regulating black hole growth. We consider a sample of 224 quasars selected from the SDSS at z<1 observed in the infrared band by Herschel. We evaluate the star formation rate in relation to several outflow signatures traced by the [OIII]4959,5007 and [OII]3726,3729 emission lines in about half of the sample with high quality spectra. Most of the quasars show asymmetric and broad wings in [OIII], which we interpret as outflow signatures. We separate the quasars in two groups, ``weakly'' and ``strongly'' outflowing, using three different criteria. When we compare the mean star formation rate in five redshift bins in the two groups, we find that the SFRs are comparable or slightly larger in the strongly outflowing quasars. We estimate the stellar mass from SED fitting and the quasars are distributed along the star formation main sequence, although with a large scatter. The scatter from this relation is uncorrelated with respect to the kinematic properties of the outflow. Moreover, for quasars dominated in the infrared by starburst or by AGN emission, we do not find any correlation between the star formation rate and the velocity of the outflow, a trend previously reported in the literature for pure starburst galaxies. We conclude that the basic AGN negative feedback scenario seems not to agree with our results. Although we use a large sample of quasars, we did not find any evidence that the star formation rate is suppressed in the presence of AGN driven outflows on large scale. A possibility is that feedback is effective over much longer timescales than those of single episodes of quasar activity.Comment: 18 pages, new version that implements the suggestions of the referee and matches the AA published versio

Interface pinning and slow ordering kinetics on infinitely ramified fractal structures

We investigate the time dependent Ginzburg-Landau (TDGL) equation for a non conserved order parameter on an infinitely ramified (deterministic) fractal lattice employing two alternative methods: the auxiliary field approach and a numerical method of integration of the equations of evolution. In the first case the domain size evolves with time as $L(t)\sim t^{1/d_w}$, where $d_w$ is the anomalous random walk exponent associated with the fractal and differs from the normal value 2, which characterizes all Euclidean lattices. Such a power law growth is identical to the one observed in the study of the spherical model on the same lattice, but fails to describe the asymptotic behavior of the numerical solutions of the TDGL equation for a scalar order parameter. In fact, the simulations performed on a two dimensional Sierpinski Carpet indicate that, after an initial stage dominated by a curvature reduction mechanism \`a la Allen-Cahn, the system enters in a regime where the domain walls between competing phases are pinned by lattice defects. The lack of translational invariance determines a rough free energy landscape, the existence of many metastable minima and the suppression of the marginally stable modes, which in translationally invariant systems lead to power law growth and self similar patterns. On fractal structures as the temperature vanishes the evolution is frozen, since only thermally activated processes can sustain the growth of pinned domains.Comment: 16 pages+14 figure

The multi-phase winds of Markarian 231: from the hot, nuclear, ultra-fast wind to the galaxy-scale, molecular outflow

We present the best sensitivity and angular resolution maps of the molecular disk and outflow of Mrk 231, as traced by CO observations obtained with IRAM/PdBI, and we analyze archival Chandra and NuSTAR observations. We constrain the physical properties of both the molecular disk and outflow, the presence of a highly-ionized ultra-fast nuclear wind, and their connection. The molecular outflow has a size of ~1 kpc, and extends in all directions around the nucleus, being more prominent along the south-west to north-east direction, suggesting a wide-angle biconical geometry. The maximum projected velocity of the outflow is nearly constant out to ~1 kpc, thus implying that the density of the outflowing material decreases from the nucleus outwards as $r^{-2}$. This suggests that either a large part of the gas leaves the flow during its expansion or that the bulk of the outflow has not yet reached out to ~1 kpc, thus implying a limit on its age of ~1 Myr. We find $\dot M_{OF}=[ 500-1000]~ M_{\odot}~yr^{-1}$ and $\dot E_{kin,OF}=[7-10]\times 10^{43}$ erg s$^{-1}$. Remarkably, our analysis of the X-ray data reveals a nuclear ultra-fast outflow (UFO) with velocity -20000 km s$^{-1}$, $\dot M_{UFO}=[0.3- 2.1] ~M_\odot yr^{-1}$, and momentum load $\dot P_{UFO}/\dot P_{rad}=[0.2-1.6]$.We find $\dot E_{kin,UFO}\sim \dot E_{kin,OF}$ as predicted for outflows undergoing an energy conserving expansion. This suggests that most of the UFO kinetic energy is transferred to mechanical energy of the kpc-scale outflow, strongly supporting that the energy released during accretion of matter onto super-massive black holes is the ultimate driver of giant massive outflows. We estimate a momentum boost $\dot P_{OF}/\dot P_{UFO}\approx [30-60]$. The ratios $\dot E_{kin, UFO}/L_{bol,AGN} =[ 1-5]\%$ and $\dot E_{kin,OF}/L_{bol,AGN} = [1-3]\%$ agree with the requirements of the most popular models of AGN feedback.Comment: 16 pages, 17 figures. Accepted for publication in A&

HST-FOS Observations of M87: Ly-a Emission from the Active Galactic Nucleus

The Faint Object Spectrograph on the Hubble Space Telescope was used to obtain spectra of the central region of M87. These spectra cover the wavelength range 1140 - 1606 Angstrom and have a resolution of about 1 Angstrom. The nuclear continuum is clearly visible in the spectra. The only strong line that is observed is Ly-alpha, which has a velocity width of about 3000 km/sec. There is also a marginal detection of C IV 1549. The ratio of Ly-alpha to C IV in the nuclear spectrum is at least a factor of 2 higher than in a spectrum taken at a position on the disk about 0.6" away from the nucleus by Dopita et al. This enhancement of Ly-alpha at the nucleus could point to significant differences in the properties of the emitting gas and/or the excitation mechanism between the outer and inner disk regions. The strength of the observed Ly-alpha places limits on the properties of the absorbing gas present within M87. For instance, if the hydrogen column at the systemic velocity of M87 is greater than about 10^18 cm^{-2} then it can cover only a small fraction of the line emitting region. Spectra separated by 5 days show a 60% difference in the Ly-alpha flux, but the same continuum level. This could be due to either a displacement between the aperture positions for the two sets of observations, or it could be due to intrinsic variability of the source. The current observations do not strongly favor either of these alternatives. The observations do show, however, that the Ly-alpha line is a useful tracer of kinematics in the M87 nucleus.Comment: 14 pages + 5 figures. LaTeX uses aaspp4.sty. Accepted for publication in the Astrophysical Journa