A purely geometric distance to the binary star Atlas, a member of the Pleiades

We present radial velocity and new interferometric measurements of the double star Atlas, which permit, with the addition of published interferometric data, to precisely derive the orbital parameters of the binary system and the masses of the components. The derived semi-major axis, compared with its measured angular size, allows to determine a distance to Atlas of 132+-4 pc in a purely geometrical way. Under the assumption that the location of Atlas is representative of the average distance of the cluster, we confirm the distance value generally obtained through main sequence fitting, in contradiction with the early Hipparcos result (118.3+-3.5 pc).Comment: 5 pages, 3 figures, accepted for publication in A&A Letter

Balanced superprojective varieties

We first review the definition of superprojective spaces from the functor-of-points perspective. We derive the relation between superprojective spaces and supercosets in the framework of the theory of sheaves. As an application of the geometry of superprojective spaces, we extend Donaldson\u2019s definition of balanced manifolds to supermanifolds and we derive the new conditions of a balanced supermanifold. We apply the construction to superpoints viewed as submanifolds of superprojective spaces. We conclude with a list of open issues and interesting problems that can be addressed in the present context

Reduced dimensionality multiphysics model for efficient VCSEL optimization

The ICT scene is dominated by short-range intra-datacenter interconnects and networking, requiring high speed and stable operations at high temperatures. GaAs/AlGaAs vertical-cavity surface-emitting lasers (VCSELs) emitting at 850–980 nm have arisen as the main actors in this framework. Starting from our in-house 3D fully comprehensive VCSEL solver VENUS, in this work we present the possibility of downscaling the dimensionality of the simulation, ending up with a multiphysics 1D solver (D1ANA), which is shown to be capable of reproducing the experimental data very well. D1ANA is then extensively applied to optimize high-temperature operation, by modifying cavity detuning and distributed Bragg’s reflector lengths

Measurement of the temperature of an ultracold ion source using time-dependent electric fields

We report on a measurement of the characteristic temperature of an ultracold rubidium ion source, in which a cloud of laser-cooled atoms is converted to ions by photo-ionization. Extracted ion pulses are focused on a detector with a pulsed-field technique. The resulting experimental spot sizes are compared to particle-tracking simulations, from which a source temperature $T = (1 \pm 2)$ mK and the corresponding transversal reduced emittance $\epsilon_r = 7.9 X 10^{-9}$ m rad $\sqrt{\rm{eV}}$ are determined. We find that this result is likely limited by space charge forces even though the average number of ions per bunch is 0.022.Comment: 8 pages, 11 figure

A search for solar-like oscillations in the Am star HD 209625

The goal is to test the structure of hot metallic stars, and in particular the structure of a near-surface convection zone using asteroseismic measurements. Indeed, stellar models including a detailed treatement of the radiative diffusion predict the existence of a near-surface convection zone in order to correctly reproduce the anomalies in surface abundances that are observed in Am stars. The Am star HD 209625 was observed with the Harps spectrograph mounted on the 3.6-m telescope at the ESO La Silla Observatory (Chile) during 9 nights in August 2005. This observing run allowed us to collect 1243 radial velocity (RV) measurements, with a standard deviation of 1.35 m/s. The power spectrum associated with these RV measurements does not present any excess. Therefore, either the structure of the external layers of this star does not allow excitation of solar-like oscillations, or the amplitudes of the oscillations remain below 20-30 cm/s (depending on their frequency range).Comment: 5 pages, 4 figures, A&A accepte

Una stima degli effetti di uno scenario di trasporto pubblico locale gratuito in Italia

Il tema della gratuità del trasporto pubblico è nuovamente al centro del dibattito, con contributi che tuttavia solo in pochi casi appaiono basati su metodi di valutazione rigorosi. In questo contributo vengono presentati i risultati di alcune simulazioni, condotte con il modello di traffico i-TraM, che stimano gli effetti di un possibile annullamento delle tariffe del trasporto pubblico locale e di quello ferroviario regionale in tutta Italia. L’effetto stimato non è omogeneo. L’effetto appare più intenso al crescere delle distanze percorse, con incrementi minimi sulle autolinee urbane e massimi sui treni regionali veloci. L’aumento della domanda si concentra, in particolare, sugli spostamenti di medio raggio verso i principali centri urbani del Nord e, in parte, del Centro. In altri termini, la gratuità tende a favorire i contesti dove l’offerta è già competitiva, ponendo evidenti problemi di costi e di capacità del sistema; mentre determina effetti sostanzialmente nulli dove l’offerta è scarsa o inefficace

Ab initio electronic structure, optical, and magneto-optical properties of MnGaAs digital ferromagnetic heterostructures

We report on a theoretical study of the electronic, optical, and magneto-optical properties of digital ferromagnetic heterostructures based on Mn delta-doped GaAs. We consider different structures corresponding to Mn contents within the range 12%-50% and we study how the system changes as a function of the doping concentration. Our first-principles approach includes the spin-orbit interaction in a fully relativistic pseudopotential scheme and the local-field effect in the description of the optical absorption. We show that Mn d-doped GaAs shares many properties with the uniformly doped Ga1-xMnxAs system, i.e., half-metallicity, similar absorption spectra, and moderate Kerr rotation angles in the visible spectral region