Instituto Superior Técnico

About: Instituto Superior Técnico is a based out in . It is known for research contribution in the topics: Catalysis & Finite element method. The organization has 10085 authors who have published 30226 publications receiving 667524 citations. The organization is also known as: IST & Instituto Superior Tecnico.

A Measurement of Omega from the North American Test Flight of Boomerang.

Abstract: We use the angular power spectrum of the cosmic microwave background, measured during the North American test flight of the Boomerang experiment, to constrain the geometry of the universe. Within the class of cold dark matter models, we find that the overall fractional energy density of the universe Ω is constrained to be 0.85 ≤ Ω ≤ 1.25 at the 68% confidence level. Combined with the COBE measurement, the data on degree scales from the Microwave Anisotropy Telescope in Chile, and the high-redshift supernovae data, we obtain new constraints on the fractional matter density and the cosmological constant.

SN 2008S: An electron-capture SN from a super-AGB progenitor?

Abstract: We present comprehensive photometric and spectroscopic observations of the faint transient SN 2008S discovered in the nearby galaxy NGC 6946. SN 2008S exhibited slow photometric evolution and almost no spectral variability during the first nine months, implying a long photon diffusion time and a high-density circumstellar medium. Its bolometric luminosity (≃10^(41) erg s^(−)1 at peak) is low with respect to most core-collapse supernovae but is comparable to the faintest Type II-P events. Our quasi-bolometric light curve extends to 300 d and shows a tail phase decay rate consistent with that of ^(56)Co. We propose that this is evidence for an explosion and formation of ^(56)Ni (0.0014 ± 0.0003 M_⊙). Spectra of SN 2008S show intense emission lines of Hα, [Ca ii] doublet and Ca ii near-infrared (NIR) triplet, all without obvious P-Cygni absorption troughs. The large mid-infrared (MIR) flux detected shortly after explosion can be explained by a light echo from pre-existing dust. The late NIR flux excess is plausibly due to a combination of warm newly formed ejecta dust together with shock-heated dust in the circumstellar environment. We reassess the progenitor object detected previously in Spitzer archive images, supplementing this discussion with a model of the MIR spectral energy distribution. This supports the idea of a dusty, optically thick shell around SN 2008S with an inner radius of nearly 90 au and outer radius of 450 au, and an inferred heating source of 3000 K. The luminosity of the central star is L ≃ 10^(4.6) L_⊙ . All the nearby progenitor dust was likely evaporated in the explosion leaving only the much older dust lying further out in the circumstellar environment. The combination of our long-term multiwavelength monitoring data and the evidence from the progenitor analysis leads us to support the scenario of a weak electron-capture supernova explosion in a super-asymptotic giant branch progenitor star (of initial mass 6–8 M_⊙ ) embedded within a thick circumstellar gaseous envelope. We suggest that all of main properties of the electron-capture SN phenomenon are observed in SN 2008S and future observations may allow a definitive answer.

Nonlinear QCD meets data: A global analysis of lepton-proton scattering with running coupling Balitsky-Kovchegov evolution

Abstract: We perform a global fit to the structure function F{sub 2} measured in lepton-proton experiments at small values of Bjorken-x, x{<=}0.01, for all experimentally available values of Q{sup 2}, 0.045 GeV{sup 2}{<=}Q{sup 2}{<=}800 GeV{sup 2}. We show that the recent improvements resulting from the inclusion of running coupling corrections allow for a description of data in terms of nonlinear QCD evolution equations. In this approach F{sub 2} is calculated within the dipole model with all Bjorken-x dependence described by the running coupling Balitsky-Kovchegov equation. Two different initial conditions for the evolution are used, both yielding good fits to data with {chi}{sup 2}/d.o.f.<1.1. The proton longitudinal structure function F{sub L}, not included in the fits, is also well described. Our analysis allows to perform a first principle extrapolation of the proton-dipole scattering amplitude once the initial condition has been fitted to presently available data. We provide predictions for F{sub 2} and F{sub L} in the kinematical regions of interest for future colliders and ultra-high energy cosmic rays. A numerical implementation of our results down to x=10{sup -12} is released as a computer code for public use.

The dynamic effect of pipe-wall viscoelasticity in hydraulic transients. Part I—experimental analysis and creep characterization

Abstract: The mechanical behaviour of the pipe material determines the pressure response of a fluid system during the occurrence of transient events. in viscoelastic pipes, typically made of polyethylene (pe), maximum or minimum transient pressures are rapidly attenuated and the overall pressure wave is delayed in time. this is a result of the retarded deformation of the pipe-wall. this effect has been observed in transient data collected in a high-density pe pipe-rig, at imperial college (london, uk). several transient tests were carried out to collect pressure and circumferential strain data. the pipe material presented a typical viscoelastic mechanical behaviour with a sudden pressure drop immediately after the fast valve closure, a major dissipation and dispersion of the pressure wave, and transient mechanical hysteresis. the creep-function of the pipe material was experimentally determined by creep tests, and, its order-of-magnitude was estimated based on pressure-strain data collected from the pipe-rig. a goo...