Coulomb breakup of 22C in a four-body model

Breakup cross sections are determined for the Borromean nucleus 22C by using a four-body eikonal model, including Coulomb corrections. Bound and continuum states are constructed within a 20C + n + n three-body model in hyperspherical coordinates. We compute continuum states with the correct asymptotic behavior through the R-matrix method. For the n+ n potential, we use the Minnesota interaction. As there is no precise experimental information on 21C, we define different parameter sets for the 20C + n potentials. These parameter sets provide different scattering lengths, and resonance energies of an expected 3/2+ excited state. Then we analyze the 22C ground-state energy and rms radius, as well as E1 strength distributions and breakup cross sections. The E1 strength distribution presents an enhancement at low energies. Its amplitude is associated with the low binding energy, rather than with a three-body resonance. We show that the shape of the cross section at low energies is sensitive to the ground-state properties. In addition, we suggest the existence of a low-energy 2+ resonance, which should be observable in breakup experiments

Microscopic description of 7^7Li in the 7Li+12C^{7}\text{Li}+^{12}\text{C} and 7Li+28Si^7\text{Li}+^{28}\text{Si} elastic scattering at high energies

We employ a microscopic continuum-discretized coupled-channels reaction framework (MCDCC) to study the elastic angular distribution of the $^7$Li$=\alpha+t$ nucleus colliding with $^{12}$C and $^{28}$Si targets at $E_{\text{Lab}}$=350 MeV. In this framework, the $^7$Li projectile is described in a microscopic cluster model and impinges on non-composite targets. The diagonal and coupling potentials are constructed from nucleon-target interactions and $^7$Li microscopic wave functions. We obtain a fair description of the experimental data, in the whole angular range studied, when continuum channels are included. The inelastic and breakup angular distributions on the lightest target are also investigated. In addition, we compute $^{7}$Li$+^{12}$C MCDCC elastic cross sections at energies much higher than the Coulomb barrier and we use them as reference calculations to test the validity of multichannel eikonal cross sections.Comment: 9 Pages, 6 Figure

Lopsided dust rings in transition disks

Context. Particle trapping in local or global pressure maxima in protoplanetary disks is one of the new paradigms in the theory of the first stages of planet formation. However, finding observational evidence for this effect is not easy. Recent work suggests that the large ring-shaped outer disks observed in transition disk sources may in fact be lopsided and constitute large banana-shaped vortices. Aims. We wish to investigate how effective dust can accumulate along the azimuthal direction. We also want to find out if the size- sorting resulting from this can produce a detectable signatures at millimeter wavelengths. Methods. To keep the numerical cost under control we develop a 1+1D method in which the azimuthal variations are treated sepa- rately from the radial ones. The azimuthal structure is calculated analytically for a steady-state between mixing and azimuthal drift. We derive equilibration time scales and compare the analytical solutions to time-dependent numerical simulations. Results. We find that weak, but long-lived azimuthal density gradients in the gas can induce very strong azimuthal accumulations of dust. The strength of the accumulations depends on the P\'eclet number, which is the relative importance of advection and diffusion. We apply our model to transition disks and our simulated observations show that this effect would be easily observable with ALMA and in principle allows to put constraints on the strength of turbulence and the local gas density.Comment: 4 pages, 4 figures, accepted for publication in A&A Letter

Spirals in protoplanetary disks from photon travel time

Spiral structures are a common feature in scattered-light images of protoplanetary disks, and of great interest as possible tracers of the presence of planets. However, other mechanisms have been put foward to explain them, including self-gravity, disk-envelope interactions, and dead zone boundaries. These mechanisms explain many spirals very well, but are unable to easily account for very loosely wound spirals and single spiral arms. We study the effect of light travel time on the shape of a shadow cast by a clump orbiting close (within ${\sim}1\,$au) of the central star, where there can be significant orbital motion during the light travel time from the clump to the outer disk and then to the sky plane. This delay in light rays reaching the sky plane gives rise to a variety of spiral- and arc-shaped shadows, which we describe with a general fitting formula for a flared, inclined disk.Comment: Accepted for publication in A&A Letters. Videos available at dl.dropboxusercontent.com/u/3526708/spiralmovies.zi

Fingerprints of giant planets in the photospheres of Herbig stars

Around 2% of all A stars have photospheres depleted in refractory elements. This is hypothesized to arise from a preferential accretion of gas rather than dust, but the specific processes and the origin of the material -- circum- or interstellar -- are not known. The same depletion is seen in 30% of young, disk-hosting Herbig Ae/Be stars. We investigate whether the chemical peculiarity originates in a circumstellar disk. Using a sample of systems for which both the stellar abundances and the protoplanetary disk structure are known, we find that stars hosting warm, flaring group I disks typically have Fe, Mg and Si depletions of 0.5 dex compared to the solar-like abundances of stars hosting cold, flat group II disks. The volatile, C and O, abundances in both sets are identical. Group I disks are generally transitional, having radial cavities depleted in millimetre-sized dust grains, while those of group II are usually not. Thus we propose that the depletion of heavy elements emerges as Jupiter-like planets block the accretion of part of the dust, while gas continues to flow towards the central star. We calculate gas to dust ratios for the accreted material and find values consistent with models of disk clearing by planets. Our results suggest that giant planets of ~0.1 to 10 M_Jup are hiding in at least 30% of Herbig Ae/Be disks.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter

A tunnel and a traffic jam: How transition disks maintain a detectable warm dust component despite the presence of a large planet-carved gap

We combined hydrodynamical simulations of planet-disk interactions with dust evolution models that include coagulation and fragmentation of dust grains over a large range of radii and derived observational properties using radiative transfer calculations. We studied the role of the snow line in the survival of the inner disk of transition disks. Inside the snow line, the lack of ice mantles in dust particles decreases the sticking efficiency between grains. As a consequence, particles fragment at lower collision velocities than in regions beyond the snow line. This effect allows small particles to be maintained for up to a few Myrs within the first astronomical unit. These particles are closely coupled to the gas and do not drift significantly with respect to the gas. For lower mass planets (1$M_{\rm{Jup}}$), the pre-transition appearance can be maintained even longer because dust still trickles through the gap created by the planet, moves invisibly and quickly in the form of relatively large grains through the gap, and becomes visible again as it fragments and gets slowed down inside of the snow line. The global study of dust evolution of a disk with an embedded planet, including the changes of the dust aerodynamics near the snow line, can explain the concentration of millimetre-sized particles in the outer disk and the survival of the dust in the inner disk if a large dust trap is present in the outer disk. This behaviour solves the conundrum of the combination of both near-infrared excess and ring-like millimetre emission observed in several transition disks.Comment: Accepted for publication in A&A (including acknowledgments

Party System Institutionalization and Accountability for Corruption in Latin America: The Cases of Brazil and Chile

In the wake of the ``third wave'' of democratization, scholars and practitioners were optimistic regarding the ease with which countries could adopt democratic norms and institutions. However, after four decades of democracy, many competitive regimes have not improved government probity or developed strong forms of accountability. Levels of economic and political development and ill-advised institutional designs were cited to explain these phenomena, but the role of political parties and their dynamics of interaction and competition remained poorly understood. Has the institutionalization of parties and party systems contributed to the control of corruption in neo-democracies? This monograph analyzes the relationship between party systems and accountability for corruption. It claims that new democracies that have developed stable party systems and party organizations capable of incorporating societal demands are most able to build and maintain mechanisms of accountability. We test these arguments through a ``nested'' research design that includes an in-depth study of two Latin American democracies within a big-N statistical analysis. The monograph begins with a cross-country study that evaluates the association between party and party system institutionalization and the levels of political corruption across competitive democracies. It then traces the institutionalization of party systems in Brazil and Chile, and it explores specifics mechanisms through which parties and party elites hold politicians to account. The Chilean case represents a country that, after seventeen years of authoritarian rule, effectively institutionalized a democratic party system. Institutionalized stability in a context of parties and coalitions with distinguishable brands facilitated the responsiveness of parties and politicians in the face of accusations of political misdeeds. Partisan and institutional arrangements provided party elites with means and incentives to discipline politicians who threatened to damage parties' reputations. In Brazil, the consolidation of the post-authoritarian party system followed a more erratic path. An exceptionally permissive electoral system interacted with organizationally weak catch-all parties to give politicians and candidates high levels of autonomy from their organizations. ``Entrepreneurial'' politicians escaped accountability by cultivating electoral turfs in localized territories and by building personal instead of partisan reputations. Politicians involved in corruption found in this context broad opportunities to avoid responsibility for misdeeds. The study contributes to the literature by broadening the understanding of how party system institutionalization influences the mechanisms of accountability for corruption. It is underscored that the stabilization of party competition is a necessary but insufficient condition to control corruption in new democracies. To be responsive over time, parties must develop channels to incorporate societal demands, so that politicians are less able to evade accountability and counteract the reputational costs of misdeeds through personalistic and clientelist appeals

Sistema para el control de acceso a un centro comercial

Sequential logic systems over the years have allowed an improvement in control and access systems. This article looks for a way to use this type of logic to create an autonomous system in Verilog for the access of people to a shopping mall, which should allow the realization of counts, movement analysis, and temperature detections, which allow avoiding the direct relationship with the security employees, making this way a protection measure for situations like the one that is currently being experienced with the global pandemic. This system, due to its configuration and accessibility, must be easy to understand, therefore the operation of this project is visualized with a traffic light configuration where green represents a positive action of entry, represents an alert or emergency. The system will give a stop signal, either for high temperatures or for people over or equal to the limits.Los sistemas de lógica secuencial a lo largo de los años han permitido un mejoramiento en los sistemas de control y de acceso. Este artículo busca la manera de usar este tipo de lógica para crear un sistema autónomo en Verilog de acceso de personas a un centro comercial, el cual debe permitir la realización de conteos, análisis de movimiento y detecciones de temperatura, que permitan evitar la relación directa con los empleados de seguridad haciendo de esta forma una medida de protección para situaciones como la que se está viviendo actualmente con la pandemia mundial. Este sistema, debido a su configuración y accesibilidad, debe ser de fácil comprensión, por esto el funcionamiento de este proyecto se visualiza con una configuración de semáforo en donde el verde representa una acción positiva de ingreso, mientras que el rojo representa una alerta o emergencia.&nbsp

Rotationally resolved spectroscopy of (20000) Varuna in the near-infrared

Models of the escape and retention of volatiles by minor icy objects exclude any presence of volatile ices on the surface of TNOs smaller than ~1000km in diameter at the typical temperature in this region of the solar system, whereas the same models show that water ice is stable on the surface of objects over a wide range of diameters. Collisions and cometary activity have been used to explain the process of surface refreshing of TNOs and Centaurs. These processes can produce surface heterogeneity that can be studied by collecting information at different rotational phases. The aims of this work are to study the surface composition of (20000)Varuna, a TNO with a diameter ~650km and to search for indications of rotational variability. We observed Varuna during two consecutive nights in January 2011 with NICS@TNG obtaining a set of spectra covering the whole rotation period of Varuna. After studying the spectra corresponding to different rotational phases, we did not find any indication of surface variability. In all the spectra, we detect an absorption at 2{\mu}m, suggesting the presence of water ice on the surface. We do not detect any other volatiles on the surface, although the S/N is not high enough to discard their presence. Based on scattering models, we present two possible compositions compatible with our set of data and discuss their implications in the frame of the collisional history of the Kuiper Belt. We find that the most probable composition for the surface of Varuna is a mixture of amorphous silicates, complex organics, and water ice. This composition is compatible with all the materials being primordial. However, our data can also be fitted by models containing up to a 10% of methane ice. For an object with the characteristics of Varuna, this volatile could not be primordial, so an event, such as an energetic impact, would be needed to explain its presence on the surface.Comment: 6 pages, 5 figures, to be published in A&