1,809 research outputs found
The initial helium abundance of the Galactic globular cluster system
No full textIn this paper we estimate the initial He content in about 30% of the Galactic globular clusters (GGCs) from new star counts we have performed on the recently published HST snapshot database of Colour Magnitude Diagrams (Piotto et al. \cite{Piotto02}). More specifically, we use the so-called R-parameter and estimate the He content from a theoretical calibration based on a recently updated set of stellar evolution models. We performed an accurate statistical analysis in order to assess whether GGCs show a statistically significant spread in their initial He abundances, and whether there is a correlation with the cluster metallicity. As in previous works on the subject, we do not find any significant dependence of the He abundance on the cluster metallicity; this provides an important constraint for models of Galaxy formation and evolution. Apart from GGCs with the bluest Horizontal Branch morphology, the observed spread in the individual helium abundances is statistically compatible with the individual errors. This means that either there is no intrinsic abundance spread among the GGCs, or that this is masked by the errors. In the latter case we have estimated a firm 1σ upper limit of 0.019 to the possible intrinsic spread. In case of the GGCs with the bluest Horizontal Branch morphology we detect a significant spread towards higher abundances inconsistent with the individual errors; this can be fully explained by additional effects not accounted for in our theoretical calibrations, which do not affect the abundances estimated for the clusters with redder Horizontal Branch morphology. In the hypothesis that the intrinsic dispersion on the individual He abundances is zero, taking into account the errors on the individual R-parameter estimates, as well as the uncertainties on the cluster metallicity scale and theoretical calibration, we have determined an initial He abundance mass fraction YGGC=0.250±0.006. This value is in perfect agreement with current estimates based on Cosmic Microwave Background radiation analyses and cosmological nucleosynthesis computations.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555, and on observations retrieved with the ESO ST-ECF Archive
Image-Guided Mini-Invasive Treatments for Vascular and Oncologic Diseases: Embolization Therapy
No full textTranscatheter Embolization, also called Embolotherapy, is a mini-invasive, non-surgical therapeutic solution used in Interventional Radiology to close blood vessels deliberately. A wide range of embolic agents is available in clinical practice, including metal coils and liquid agents. More recent advances in biomaterials such as shape-memory foam and in-situ gelling solutions have led to the development of new pre-clinical embolic agents. This review offers a brief overview of current and emerging technologies in the field of endovascular embolization. The focus is on devices, materials and techniques
A micromachined bistable 1 x 2 switch for optical fibers
No full textUsing bulk micromachining techniques, a 1x2 optical fiber switch has been fabricated. The optical signal is carried by a standard monomodal fiber. Switching is based on asymmetrical Joule heating of a Y-shaped cantilever structure by means of two integrated chromium thin film resistors; both electrical and optical properties are promising. A lock-release mechanism, based on silicon micromachined hooks, has also been tested. This mechanism allows the fabrication of a mechanically bistable device, that needs electrical power only during switching
Functional comparison of acoustic admittance measurements with a CMOS-compatible p-v microprobe and a reference one
No full textRecent developments in MEMS technology made available a new class of thermo-resistive sensors to be used as functional devices for acoustic particle velocity, v, measurements (Bruschi and Piotto, IEEE Sensors Proceedings 1405–1408, 2011). A very interesting feature of this new generation of v-sensors—distinguishing them from the Microflown® ones (de Bree et al., Sens Actuators A Phys 54:552–557, 1996)—is their compatibility with standard CMOS industrial processes, so allowing to integrate in the same chip both the sensors and read-out electronic circuits. This added flexibility of v-sensors, combined with miniature or MEMS microphones, allows of setting up pressure–velocity (p–v) microprobes for specific applications, in particular when the reduction of production costs is decisive for marketing strategies. In many applications, in fact, carefully designed functional devices can be safely used, without prejudice to the reliability and the robustness of the required measurement process. In other words, the required measurement precision can be achieved despite the low signal-to-noise ratio or limited band frequency response of the used p–v microprobes. This article shows a first comparison between the two kinds of sensor
Superhydrophobic Coatings and Artificial Neural Networks: Design, Development and Optimization
No full textRecently, considerable attention has been devoted to the development of superhydrophobic surfaces due to their advantageous antifouling and antimicrobial capacity. While significant effort has been devoted to fabricating such surfaces, very few polymeric superhydrophobic surfaces can be considered durable against various externally imposed stresses. Pyrogenic hydrophobic silica nanoparticles were used to confer superhydrophobic properties to the coatings. 450 samples were prepared using a layer-by-layer approach, deposing an epoxy resin or PDMS layer as adhesive on a substrate (PC/ABS), followed by one or more layers of silica nanoparticles, or silica-resin mixed layers. The best coating obtained shows a contact angle of 157° and a tape peeling grade resistance. The applied method involves the spray deposition of a multilayer coating composed of: silica layer/epoxy resin layer/silica layer, followed by partial curing of the coating (15 min, 70 °C); another silica layer is then sprayed on the surface and is cured for 10 min. Given the high number of parameters involved, process optimization is quite tricky. Artificial Neural Networks are the best tool to deal with multivariate analysis problems and for this reason, data from all the prepared samples were collected into a matrix and was used to train a neural network capable of predicting the degree of hydrophobicity of a silica nanoparticles-based coating
Comparison between observed, theoretical Red Giant Branch luminosity functions of galactic globular clusters
No full textFabrication and characterization of a directional anemometer based on a single chip MEMS flow sensor
No full textA compact anemometer based on integrated flow sensors is presented. The device exploits a recently proposed approach, by which the pressure induced on the lateral surface of a cylinder exposed to the wind is converted into two air flow rates exhibiting sinusoidal dependence on the wind direction. A significant size reduction and assemblage simplification were achieved by sensing the two flows by means of a double channel flow meter, integrated on a single silicon chip. The results of measurements, performed over a wind velocity range 0.4-7.9 m/s, demonstrated the effectiveness of the proposed anemometer. (C) 2010 Elsevier B.V. All rights reserved
A fabrication process for a silicon tunnel barrier with self-aligned gate
No full textA process for fabricating a device based on tunneling through a very thin vertical silicon membrane is presented. The process has been developed on a < 110 > oriented silicon wafer using high resolution e-beam lithography and KOH anisotropic etching to define the structure. A single evaporation step allows the fabrication of both the source-drain contacts and a control gate self aligned to the top of the silicon membrane. A vertical silicon membrane with a thickness of 15 nm has been obtained. (c) 2006 Elsevier B.V. All rights reserved
Electrical and noise characterization of suspended silicon wires
No full textWe present the fabrication of suspended silicon nanowires using 1 mum resolution conventional photolithography, anisotropic wet etching and thermal oxidation. A minimum transverse dimension of about 50 nm was achieved in some of the wires. We characterized the wires from the electrical and noise points of view, comparing the behavior at room temperature and at 77 K, and discuss possible hypotheses to explain the observed differences. (C) 2002 Elsevier Science B.V. All rights reserved
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