Correlations in Networks associated to Preferential Growth

Combinations of random and preferential growth for both on-growing and stationary networks are studied and a hierarchical topology is observed. Thus for real world scale-free networks which do not exhibit hierarchical features preferential growth is probably not the main ingredient in the growth process. An example of such real world networks includes the protein-protein interaction network in yeast, which exhibits pronounced anti-hierarchical features.Comment: 4 pages, 4 figure

Diffusion-annihilation processes in complex networks

We present a detailed analytical study of the $A+A\to\emptyset$ diffusion-annihilation process in complex networks. By means of microscopic arguments, we derive a set of rate equations for the density of $A$ particles in vertices of a given degree, valid for any generic degree distribution, and which we solve for uncorrelated networks. For homogeneous networks (with bounded fluctuations), we recover the standard mean-field solution, i.e. a particle density decreasing as the inverse of time. For heterogeneous (scale-free networks) in the infinite network size limit, we obtain instead a density decreasing as a power-law, with an exponent depending on the degree distribution. We also analyze the role of finite size effects, showing that any finite scale-free network leads to the mean-field behavior, with a prefactor depending on the network size. We check our analytical predictions with extensive numerical simulations on homogeneous networks with Poisson degree distribution and scale-free networks with different degree exponents.Comment: 9 pages, 5 EPS figure

Diffusion-annihilation processes in complex networks

We present a detailed analytical study of the $A+A\to\emptyset$ diffusion-annihilation process in complex networks. By means of microscopic arguments, we derive a set of rate equations for the density of $A$ particles in vertices of a given degree, valid for any generic degree distribution, and which we solve for uncorrelated networks. For homogeneous networks (with bounded fluctuations), we recover the standard mean-field solution, i.e. a particle density decreasing as the inverse of time. For heterogeneous (scale-free networks) in the infinite network size limit, we obtain instead a density decreasing as a power-law, with an exponent depending on the degree distribution. We also analyze the role of finite size effects, showing that any finite scale-free network leads to the mean-field behavior, with a prefactor depending on the network size. We check our analytical predictions with extensive numerical simulations on homogeneous networks with Poisson degree distribution and scale-free networks with different degree exponents.Comment: 9 pages, 5 EPS figure

Internal rotation of red giants by asteroseismology

We present an asteroseismic approach to study the dynamics of the stellar interior in red-giant stars by asteroseismic inversion of the splittings induced by the stellar rotation on the oscillation frequencies. We show preliminary results obtained for the red giant KIC4448777 observed by the space mission Kepler.Comment: 3 pages, 4 figures, the 40th Liege International Astrophysical Colloquium Liac40, 'Ageing low mass stars: from red giants to white dwarfs', to be published on EPJ Web of Conference

Oxygen Gas Phase Abundance Revisited

We present new measurements of the interstellar gas-phase oxygen abundance along the sight lines towards 19 early-type galactic stars at an average distance of 2.6 kpc. We derive O {\small I} column densities from {\it HST}/STIS observations of the weak 1355 \AA intersystem transition. We derive total hydrogen column densities [N(H {\small I})+2N(H$_2$)] using {\it HST}/STIS observations of \lya and {\it FUSE} observations of molecular hydrogen. The molecular hydrogen content of these sight lines ranges from f(H$_2$) = 2N(H$_2$)/[N(H {\small I})+2N(H$_2$)] = 0.03 to 0.47. The average $$ of 6.3$\times10^{21}$ cm$^{-2}$ mag$^{-1}$ with a standard deviation of 15% is consistent with previous surveys. The mean oxygen abundance along these sight lines, which probe a wide range of galactic environments in the distant ISM, is 10$^6$ \oh = $408 \pm 13$ (1 $\sigma$ in the mean). %$({\rm O/H})_{gas} = 408 \pm 14$(1 $\sigma$). We see no evidence for decreasing gas-phase oxygen abundance with increasing molecular hydrogen fraction and the relative constancy of \oh suggests that the component of dust containing the oxygen is not readily destroyed. We estimate that, if 60% of the dust grains are resilient against destruction by shocks, the distant interstellar total oxygen abundance can be reconciliated with the solar value derived from the most recent measurements %by Holweger and by Allende Prieto, Lambert & Asplund: of 10$^6$ \oh$_\odot$ = 517 $\pm$ 58 (1 $\sigma$). We note that the smaller oxygen abundances derived for the interstellar gas within 500 pc %by Meyer, Cardelli & Jura or from nearby B star surveys are consistent with a local elemental deficit.Comment: 9 figures, 37 page

Characterization of Kepler targets based on medium-resolution LAMOST spectra analyzed with ROTFIT

In this work we present the results of our analysis of 16,300 medium-resolution LAMOST spectra of late-type stars in the Kepler field with the aim of determining the stellar parameters, activity level, lithium atmospheric content, and binarity. We have used a version of the code ROTFIT specifically developed for these spectra. We provide a catalog with the atmospheric parameters (Teff, log(g), and [Fe/H]), radial velocity (RV), and projected rotation velocity (vsini). For cool stars (Teff < 6500 K), we also calculated the H-alpha and LiI-6708 equivalent width, which are important indicators of chromospheric activity and evolutionary stage, respectively. We have derived the RV and atmospheric parameters for 14,300 spectra of 7443 stars. Literature data were used for a quality control of the results. The Teff and log(g) values are in good agreement with the literature. The [Fe/H] values appear to be overestimated for metal-poor stars. We propose a relation to correct the [Fe/H] values derived with ROTFIT. We were able to identify double-lined binaries, stars with variable RVs, lithium-rich giants, and emission-line objects. Based on the H-alpha flux, we found 327 active stars. We detected the LiI-6708 line and measure its equivalent width for 1657 stars, both giants and stars on the main sequence. Regarding the latter, we performed a discrete age classification based on the atmospheric lithium abundance and the upper envelopes of a few open clusters. Among the giants, we found 195 Li-rich stars, 161 of which are reported here for the first time. No relationship is found between stellar rotation and lithium abundance, which allows us to rule out merger scenarios as the predominant explanation of the enrichment of Li in our sample. The fraction of Li-rich giants, about 4%, is higher than expected.Comment: 32 pages, 34 figures; accepted for publication in Astronomy & Astrophysic

Spectroscopic survey of Kepler stars. I. HERMES/Mercator observations of A- and F-type stars

The Kepler space mission provided near-continuous and high-precision photometry of about 207 000 stars, which can be used for asteroseismology. However, for successful seismic modeling it is equally important to have accurate stellar physical parameters. Therefore, supplementary ground-based data are needed. We report the results of the analysis of high-resolution spectroscopic data of A- and F-type stars from the Kepler field, which were obtained with the HERMES spectrograph on the Mercator telescope. We determined spectral types, atmospheric parameters and chemical abundances for a sample of 117 stars. Hydrogen Balmer, Fe i, and Fe ii lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. The atmospheric parameters obtained were compared with those from the Kepler Input Catalogue (KIC), confirming that the KIC effective temperatures are underestimated for A stars. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The analysed sample comprises stars with approximately solar chemical abundances, as well as chemically peculiar stars of the Am, Ap, and λ Boo types. The distribution of the projected rotational velocity, vsin i, is typical for A and F stars and ranges from 8 to about 280 km s−1, with a mean of 134 km s−1

Steady-State Dynamics of the Forest Fire Model on Complex Networks

Many sociological networks, as well as biological and technological ones, can be represented in terms of complex networks with a heterogeneous connectivity pattern. Dynamical processes taking place on top of them can be very much influenced by this topological fact. In this paper we consider a paradigmatic model of non-equilibrium dynamics, namely the forest fire model, whose relevance lies in its capacity to represent several epidemic processes in a general parametrization. We study the behavior of this model in complex networks by developing the corresponding heterogeneous mean-field theory and solving it in its steady state. We provide exact and approximate expressions for homogeneous networks and several instances of heterogeneous networks. A comparison of our analytical results with extensive numerical simulations allows to draw the region of the parameter space in which heterogeneous mean-field theory provides an accurate description of the dynamics, and enlights the limits of validity of the mean-field theory in situations where dynamical correlations become important.Comment: 13 pages, 9 figure

Hypothetical porous medium concept as a virtual swirl tape: A novel modelling technique towards efficient CFD simulation of swirl tape cooling pipe

The EU-DEMO divertor target cooling circuit is equipped with Swirl Tape (ST) inserts to improve its thermo-hydraulic performance in terms of heat transfer coefficient and critical heat flux. Due to the presence of the STs, accurate 3D CFD-based thermofluid-dynamic assessments of the divertor targets cooling circuit require a high computational cost and a laborious pre-processing modelling effort. To this end, a cost-efficient CFD simulation technique based on an equivalent porous medium concept, namely the Virtual Swirl Tape (VST) approach, has been developed. In this work, the mathematical formulation of different VSTs models is presented, and the porous media calibration procedure and validation are shown. This technique enables the reduction of computational costs by decreasing the number of volumes required for a single Plasma-Facing Unit (PFU) assembly cooling channel by a factor of 10, while lowering the calculation time by ≈86%. The results obtained show that it is possible to correctly reproduce the friction factor profile and pressure drop of a PFU assembly cooling channel, this latter with errors within 10% considering a wide range of coolant inlet velocities. Some limitations have been observed concerning the VST thermal performance, which is still unsatisfactory and requires further development. The VST approach has been studied using the commercial CFD code ANSYS CFX, coupled with a multi-objective optimization algorithm available in the ANSYS Direct Optimization tool

Multisite photometry of the pulsating Herbig Ae star V346 Ori

CONTEXT: The study of pulsation in Pre--Main--Sequence intermediate-mass stars represents an important tool for deriving information on fundamental stellar parameters and internal structure, as well as for testing current theoretical models. Interest in this class of variable stars has significantly increased during the last decade and about 30 members are presently known in the literature. AIMS: We have constructed the frequency spectrum of the oscillations in V346 Ori. We apply asteroseismic tools to these data to estimate the intrinsic parameters (mass, luminosity, effective temperature) of V346 Ori and to obtain information on its internal structure. METHODS: CCD time series photometry in the Johnson V filter has been obtained for a total of 145.7 h of observations distributed over 36 nights. The resulting light curves have been subjected to a detailed frequency analysis using updated numerical techniques. Photometric and spectroscopic data have also been acquired to determine reliable estimates of the stellar properties. RESULTS: We have identified 13 oscillation frequencies, 6 of which with higher significance. These have been compared with the predictions of non-radial adiabatic models. The resulting best fit model has a mass of 2.1$\pm$0.2 $M_{\odot}$, luminosity $\log{L/L_{\odot}}=1.37^{+0.11}_{-0.13}$, and effective temperature 7300$\pm$200 K. These values are marginally consistent with the association of V346 Ori to Orion OB1a. Alternatively, V346 Ori could be placed at a slightly larger distance than previously estimated.Comment: Accepted for publication on A&A. 14 Pages, 11 Figure