SiC(0001): a surface Mott-Hubbard insulator

We present ab-initio electronic structure calculations for the Si-terminated SiC(0001)$\sqrt{3}\times\sqrt{3}$ surface. While local density approximation (LDA) calculations predict a metallic ground state with a half-filled narrow band, Coulomb effects, included by the spin-polarized LDA+U method, result in a magnetic (Mott-Hubbard) insulator with a gap of 1.5 eV, comparable with the experimental value of 2.0 eV. The calculated value of the inter-site exchange parameter, J=30K, leads to the prediction of a paramagnetic Mott state, except at very low temperatures. The observed Si 2p surface core level doublet can naturally be explained as an on-site exchange splitting.Comment: RevTex, 4 pages, 4 eps-figure

An upper limit to the secular variation of the gravitational constant from white dwarf stars

A variation of the gravitational constant over cosmological ages modifies the main sequence lifetimes and white dwarf cooling ages. Using an state-of-the-art stellar evolutionary code we compute the effects of a secularly varying G on the main sequence ages and, employing white dwarf cooling ages computed taking into account the effects of a running G, we place constraints on the rate of variation of Newton's constant. This is done using the white dwarf luminosity function and the distance of the well studied open Galactic cluster NGC 6791. We derive an upper bound G'/G ~ -1.8 10^{-12} 1/yr. This upper limit for the secular variation of the gravitational constant compares favorably with those obtained using other stellar evolutionary properties, and can be easily improved if deep images of the cluster allow to obtain an improved white dwarf luminosity function.Comment: 15 pages, 4 figures, accepted for publication in JCA

Absolute proper motion of the Galactic open cluster M67

We derived the absolute proper motion (PM) of the old, solar-metallicity Galactic open cluster M67 using observations collected with CFHT (1997) and with LBT (2007). About 50 galaxies with relatively sharp nuclei allow us to determine the absolute PM of the cluster. We find (mu_alpha cos(delta),mu_delta)_J2000.0 = (-9.6+/-1.1,-3.7+/-0.8) mas/yr. By adopting a line-of-sight velocity of 33.8+/-0.2 km/s, and assuming a distance of 815+/-50 pc, we explore the influence of the Galactic potential, with and without the bar and/or spiral arms, on the galactic orbit of the cluster.Comment: 7 pages, 5 figures, and 3 tables. Published in Astronomy and Astrophysics, Volume 513, id.A51

A white dwarf cooling age of 8 Gyr for NGC 6791 from physical separation processes

NGC 6791 is a well studied open cluster1 that it is so close to us that can be imaged down to very faint luminosities. The main sequence turn-off age (~8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (~6 Gyr) are significantly different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22Ne, sinks in the deep interior of these stars. At lower temperatures, white dwarfs are expected to crystallise and phase separation of the main constituents of the core of a typical white dwarf, 12C and 16O, is expected to occur. This sequence of events is expected to introduce significant delays in the cooling times, but has not hitherto been proven. Here we report that, as theoretically anticipated, physical separation processes occur in the cores of white dwarfs, solving the age discrepancy for NGC 6791.Comment: 3 pages, 2 figures, published in Natur

THE DETERMINANTS OF FOOD SECURITY IN A GLOBALIZED WORLD: EVALUATION OF THE EFFECTS OF TRADE LIBERALIZATION WITH A SYNTHETIC CONTROL APPROACH

The food security issue is becoming more and more centre-stage in the political and social debate. Especially after the world food crisis of 2007-2008 and the consequent price spikes, it reached the top of the international agenda. The problems of accessibility, disposability and utilization of food, both in developing and poor countries, now draws the attention of policymakers and researchers. Of course, this dynamic and complex problem is influenced and determined by many \u2018macro\u2019 factors, analyzed with a case-study econometric approach. The focus of this thesis is to test whether trade liberalization could have a role on the increasing, or decreasing, of the under five mortality rate (WDI, World Bank), used as a proxy for food security. The statistical sample considered is composed of 80 developing countries, 39 of which have experienced an episode of trade liberalization, between 1960 and 2010. The 41 residuals countries are included as a counterfactual in the econometric approach: the Synthetic Control Method (SCM). What was particularly challenging at the beginning of this investigation concerned the isolation of the effect of commercial liberalization on child mortality, and, as a consequence, on the level of food security in each country. As for the methodological approach, what was defined firstly were the covariates: the variables implemented for the synthetic control. They were chosen as a function of their influence on the outcome variable. Above all, we included the level of development and economic growth, the presence of wars or conflicts, the population growth, the percentage of rural population and food supply (measured as the amount of kilo calories available per capita). As already mentioned above with reference to the statistical and the econometric approach, it was developed by Abadie and Garzeabal (2003; 2010), and called SCM. The underlying hypothesis is that it is possible to construct a weighted combination of potential control countries \u2013 the synthetic control \u2013 that approximate the most relevant characteristics of the country affected by the intervention. In our study, the \u2018treated\u2019 country is the one which experienced a trade reform \u2013 \u2018treatment\u2019. The situation which happened after the treatment (economic openness) can be estimated by the SCM. The SCM is able to calculate a counterfactual circumstance of the \u2018treated\u2019 country, in the absence of trade liberalization. The consequential result of the \u2018treatment\u2019 can be verified by looking at the outcome trend of the synthetic control. More precisely, the synthetic control algorithm estimates the missing counterfactual as a weighted average of the outcomes of potential controls. The weights are chosen so that the pretreatment outcome and the covariates of the synthetic control are, on average, very similar to those of the \u2018treated\u2019 country. Although this methodology is more flexible and transparent than others, there is a limitation regarding the inferential techniques. This is mainly due to the fact that the number of observations in the control pool and the number of periods covered by the sample are usually quite small in comparative case studies, like this. This is why, following the suggestions of the inventors, we implemented the placebo tests, based on permutation techniques. This means that the synthetic algorithm was sequentially applied to every country in the pool of potential controls and then used to compare the placebo with the baseline results. To sum up, we evaluated the dynamic of the under five mortality rate in a \u2018treated\u2019 country (which experienced an episode of trade liberalization in the decades considered), with the situation in a sample of control (the synthetic control), composed by a group of very similar countries to the one \u2018treated\u2019, but in which there was not any trade reform. It is important to notice that the synthetic control is chosen taking into account the covariates mentioned above; besides, the weight of each country of the synthetic control is selected in order to minimize the difference with the \u2018treated\u2019 country, in the pre-treatment period. With respect to the results, we decided to select a pre-treatment period of twenty years, and a post-treatment period of ten years. This is mainly because after a decade from the openness to trade, the effect of this phenomenon of child mortality tend to be very difficult to isolate and identify. The preliminary results obtained, show that the impacts of trade liberalization on under five mortality rate is, on average, positive. Moreover, in a few developing countries political and economic reforms happened simultaneously; so that, this overlapping may not permit the correct identification of the real effect of commercial openness on the outcome variable

Hubble Space Telescope astrometry of the closest brown dwarf binary system -- I. Overview and improved orbit

Located at ~2pc, the L7.5+T0.5 dwarfs system WISE J104915.57-531906.1 (Luhman16AB) is the third closest system known to Earth, making it a key benchmark for detailed investigation of brown dwarf atmospheric properties, thermal evolution, multiplicity, and planet-hosting frequency. In the first study of this series -- based on a multi-cycle Hubble Space Telescope (HST) program -- we provide an overview of the project and present improved estimates of positions, proper motions, annual parallax, mass ratio, and the current best assessment of the orbital parameters of the A-B pair. Our HST observations encompass the apparent periastron of the binary at 220.5+/-0.2 mas at epoch 2016.402. Although our data seem to be inconsistent with recent ground-based astrometric measurements, we also exclude the presence of third bodies down to Neptune masses and periods longer than a year.Comment: 19 pages, 9 figures, 9 tables. Accepted for publication in MNRAS on 2017 May

Low-complexity hardware and algorithm for joint communication and sensing

Joint Communication and Sensing (JCAS) is foreseen as one very distinctive feature of the emerging 6G systems providing, in addition to fast end reliable communication, the ability to obtain an accurate perception of the physical environment. In this paper, we propose a JCAS algorithm that exploits a novel beamforming architecture, which features a combination of wideband analog and narrowband digital beamforming. This allows accurate estimation of Time of Arrival (ToA), exploiting the large bandwidth and Angle of Arrival (AoA), exploiting the high-rank digital beamforming. In our proposal, we separately estimate the ToA and AoA. The association between ToA and AoA is solved by acquiring multiple non-coherent frames and adding up the signal from each frame such that a specific component is combined coherently before the AoA estimation. Consequently, this removes the need to use 2D and 3D joint estimation methods, thus significantly lowering complexity. The resolution performance of the method is compared with that of 2D MUltiple SIgnal Classification (2D-MUSIC) algorithm, using a fully-digital wideband beamforming architecture. The results show that the proposed method can achieve performance similar to a fully-digital high-bandwidth system, while requiring a fraction of the total aggregate sampling rate and having much lower complexity.Comment: 13 pages, 9 figures. Submitted to IEEE Transactions on Wireless Communication

V, J, H and K Imaging of the Metal Rich Globular Cluster NGC 6528

New near-infrared observations of NGC6528 are presented. The JHK observations complement a previous HST/NICMOS data set by Ortolani et al. (2001), in that they sample a larger area, contain a more numerous sample of red giant stars, and include the K band. Also, archival HST data sets (separated by 6.093 years) were used to proper-motion decontaminate the near-infrared sample and extract a clean VJHK catalogue. Using the present wide colour baseline, we compared the cleaned colour-magnitude diagrams of NGC6528 with those of NGC 6553 and NGC104 and derived new estimates of reddening and distance, E(B-V)=0.55 and (m-M)o=14.44 (7.7 kpc). Moreover, the morphology and location of the cleaned red giant branch were used to derive a photometric estimate of the cluster metallicity. The average of 10 metallicity indicators yields a mean value of [M/H] ~ 0.0, and [Fe/H] ~-0.20 and +0.08 on the Zinn & West (1984) and Carretta & Gratton (1997) revised metallicity scale, respectively. The best isochrone fit to the cleaned K,V-K diagram is obtained for a 12.6 Gyr and Z=0.02 isochrone, i.e. the derived metallicity of NGC6528 turns out to be very close to the mean of stars in the Baade's Window. Five AGB variable star candidates, whose membership has to be confirmed spectroscopically, are bolometrically as bright as the known long period variable stars in NGC6553. As discussed in Guarnieri et al. (1997) for NGC6553, this may indicate that an `intermediate age' population is not needed to account for the brightest stars in external galaxies such as M32.Comment: 11 pages, 9 figures, A&A accepte