The orbit of Beta Pic b as a transiting planet

In 1981, Beta Pictoris showed strong and rapid photometric variations possibly due to a transiting giant planet. Later, a planetary mass companion to the star, Beta Pic b, was identified using imagery. Observations at different epochs (2003 and 2009-2015) detected the planet at a projected distance of 6 to 9 AU from the star and showed that the planet is on an edge-on orbit. The observed motion is consistent with an inferior conjunction in 1981, and Beta Pic b can be the transiting planet proposed to explain the photometric event observed at that time. Assuming that the 1981 event is related to the transit or the inferior conjunction of Beta Pic b on an edge-on orbit, we search for the planetary orbit in agreement with all the measurements of the planet position published so far. We find two different orbits that are compatible with all these constraints: (i) an orbit with a period of 17.97$\pm$0.08 years along with an eccentricity of around 0.12 and (ii) an orbit with a period of 36.38$\pm$0.13 years and a larger eccentricity of about 0.32. In the near future, new imaging observations should allow us to discriminate between these two different orbits. We also estimate the possible dates for the next transits, which could take place as early as 2017 or 2018, even for a long-period orbit.Comment: Accepted for publication in A&

Optimal distillation of a GHZ state

We present the optimal local protocol to distill a Greenberger-Horne-Zeilinger (GHZ) state from a single copy of any pure state of three qubits.Comment: RevTex, 4 pages, 2 figures. Published version, some references adde

Entanglement renormalization and gauge symmetry

A lattice gauge theory is described by a redundantly large vector space that is subject to local constraints, and can be regarded as the low energy limit of an extended lattice model with a local symmetry. We propose a numerical coarse-graining scheme to produce low energy, effective descriptions of lattice models with a local symmetry, such that the local symmetry is exactly preserved during coarse-graining. Our approach results in a variational ansatz for the ground state(s) and low energy excitations of such models and, by extension, of lattice gauge theories. This ansatz incorporates the local symmetry in its structure, and exploits it to obtain a significant reduction of computational costs. We test the approach in the context of the toric code with a magnetic field, equivalent to Z2 lattice gauge theory, for lattices with up to 16 x 16 sites (16^2 x 2 = 512 spins) on a torus. We reproduce the well-known ground state phase diagram of the model, consisting of a deconfined and spin polarized phases separated by a continuous quantum phase transition, and obtain accurate estimates of energy gaps, ground state fidelities, Wilson loops, and several other quantities.Comment: reviewed version as published in PRB; this version includes a new section about the accuracy of the results several corrections and added citation

Approximate transformations and robust manipulation of bipartite pure state entanglement

We analyze approximate transformations of pure entangled quantum states by local operations and classical communication, finding explicit conversion strategies which optimize the fidelity of transformation. These results allow us to determine the most faithful teleportation strategy via an initially shared partially entangled pure state. They also show that procedures for entanglement manipulation such as entanglement catalysis [Jonathan and Plenio, Phys. Rev. Lett. 83, 3566 (1999)] are robust against perturbation of the states involved, and motivate the notion of non-local fidelity, which quantifies the difference in the entangled properties of two quantum states.Comment: 11 pages, 4 figure

Open source environment to define constraints in route planning for GIS-T

Route planning for transportation systems is strongly related to shortest path algorithms, an optimization problem extensively studied in the literature. To find the shortest path in a network one usually assigns weights to each branch to represent the difficulty of taking such branch. The weights construct a linear preference function ordering the variety of alternatives from the most to the least attractive.Postprint (published version

Jet precession in the active nucleus of M81. Ongoing VLBI monitoring

In a recent publication, we reported results of a multi-frequency VLBI campaign of observations of the Active Galactic Nucleus (AGN) in galaxy M\,81, phase-referenced to the supernova SN\,1993J. We were able to extract precise information on the relative astrometry of the AGN radio emission at different epochs and frequencies. We found strong evidence of precession in the AGN jet (i.e., a systematic evolution in the jet inclination at each frequency) coupled to changes in the overall flux density at the different frequencies. In these proceedings, we summarise the main contents of our previous publication and we report on (preliminary) new results from our follow-up VLBI observations, now phase-referenced to the young supernova SN2008iz. We also briefly discuss how these results match the picture of our previously-reported precession model.Comment: 6 pages, 3 figures. Proceedings of EVN meeting 201

Modeling magnesium escape from HD209458b atmosphere

Transit observations in the MgI line of HD209458b revealed signatures of neutral magnesium escaping the upper atmosphere of the planet, while no atmospheric absorption was found in the MgII doublet. Here we present a 3D particle model of the dynamics of neutral and ionized magnesium populations, coupled with an analytical modeling of the atmosphere below the exobase. Theoretical MgI absorption line profiles are directly compared with the absorption observed in the blue wing of the line during the planet transit. Observations are well-fitted with an escape rate of neutral magnesium in the range 2x10^7-3.4x10^7 g/s, an exobase close to the Roche lobe (Rexo in the range 2.1-4.3 Rp, where Rp is the planet radius) and a planetary wind velocity at the exobase vpl=25km/s. The observed velocities of the planet-escaping magnesium up to -60km/s are well explained by radiation pressure acceleration, provided that UV-photoionization is compensated for by electron recombination up to about 13Rp. If the exobase properties are constrained to values given by theoretical models of the deeper atmosphere (Rexo=2Rp and vpl=10km/s), the best fit to the observations is found at a similar electron density and escape rate within 2 sigma. In all cases, the mean temperature of the atmosphere below the exobase must be higher than about 6100 K. Simulations predict a redward expansion of the absorption profile from the beginning to the end of the transit. The spatial and spectral structure of the extended atmosphere is the result of complex interactions between radiation pressure, planetary gravity, and self-shielding, and can be probed through the analysis of transit absorption profiles in the MgI line.Comment: 16 pages, 24 figure

The MgI line: a new probe of the atmospheres of evaporating exoplanets

Transit observations of HD209458b in the UV revealed signatures of neutral magnesium escaping the planet's upper atmosphere. The absorption detected in the MgI line provides unprecedented information on the physical conditions at the altitude where the atmospheric blow-off takes place. Here we use a 3D model of atmospheric escape to estimate the transit absorption signatures in the MgI line of their host stars. The detectability of these signatures depends on the brightness of the star and the escape rate of neutral magnesium. We identify a sample of potentially evaporating exoplanets that covers a wide range of stellar and planetary properties, and whose extended exospheres might be detected through MgI line observations with current UV facilities, allowing further steps in comparative exoplanetology.Comment: 4 pages, 2 figure