Power Density Spectra of Gamma-Ray Bursts in the Internal Shock Model

We simulate Gamma-Ray Bursts arising from internal shocks in relativistic winds, calculate their power density spectrum (PDS), and identify the factors to which the PDS is most sensitive: the wind ejection features, which determine the wind dynamics and its optical thickness, and the energy release parameters, which give the pulse 50-300 keV radiative efficiency. For certain combinations of ejection features and wind parameters the resulting PDS exhibits the features observed in real bursts. We found that the upper limit on the efficiency of conversion of wind kinetic energy into 50-300 keV photons is $\sim$ 1%. Winds with a modulated Lorentz factor distribution of the ejecta yield PDSs in accord with current observations and have efficiencies closer to $10^{-3}$, while winds with a random, uniform Lorentz factor ejection must be optically thick to the short duration pulses to produce correct PDSs, and have an overall efficiency around $10^{-4}$.Comment: 6 pages, 4 figures, Latex, submitted to The Astrophysical Journal (05/04/99

Improved calibration of the radii of cool stars based on 3D simulations of convection: implications for the solar model

Main sequence, solar-like stars (M < 1.5 Msun) have outer convective envelopes that are sufficiently thick to affect significantly their overall structure. The radii of these stars, in particular, are sensitive to the details of inefficient, super-adiabatic convection occurring in their outermost layers. The standard treatment of convection in stellar evolution models, based on the Mixing-Length Theory (MLT), provides only a very approximate description of convection in the super-adiabatic regime. Moreover, it contains a free parameter, alpha_MLT, whose standard calibration is based on the Sun, and is routinely applied to other stars ignoring the differences in their global parameters (e.g., effective temperature, gravity, chemical composition) and previous evolutionary history. In this paper, we present a calibration of alpha_MLT based on three-dimensional radiation-hydrodynamics (3D RHD) simulations of convection. The value of alpha_MLT is adjusted to match the specific entropy in the deep, adiabatic layers of the convective envelope to the corresponding value obtained from the 3D RHD simulations, as a function of the position of the star in the (log g, log T_eff) plane and its chemical composition. We have constructed a model of the present-day Sun using such entropy-based calibration. We find that its past luminosity evolution is not affected by the entropy calibration. The predicted solar radius, however, exceeds that of the standard model during the past several billion years, resulting in a lower surface temperature. This illustrative calculation also demonstrates the viability of the entropy approach for calibrating the radii of other late-type stars.Comment: 16 pages, 14 figures, accepted for publication in the Astrophysical Journa

Analysis of Temporal Features of Gamma Ray Bursts in the Internal Shock Model

In a recent paper we have calculated the power density spectrum of Gamma-Ray Bursts arising from multiple shocks in a relativistic wind. The wind optical thickness is one of the factors to which the power spectrum is most sensitive, therefore we have further developed our model by taking into account the photon down-scattering on the cold electrons in the wind. For an almost optically thick wind we identify a combination of ejection features and wind parameters that yield bursts with an average power spectrum in agreement with the observations, and with an efficiency of converting the wind kinetic energy in 50-300 keV emission of order 1%. For the same set of model features the interval time between peaks and pulse fluences have distributions consistent with the log-normal distribution observed in real bursts.Comment: ApJ in press, 2000; with slight revisions; 12 pag, 6 fi

The Yale-Potsdam Stellar Isochrones (YaPSI)

We introduce the Yale-Potsdam Stellar Isochrones (YaPSI), a new grid of stellar evolution tracks and isochrones of solar-scaled composition. In an effort to improve the Yonsei-Yale database, special emphasis is placed on the construction of accurate low-mass models (Mstar < 0.6 Msun), and in particular of their mass-luminosity and mass-radius relations, both crucial in characterizing exoplanet-host stars and, in turn, their planetary systems. The YaPSI models cover the mass range 0.15 to 5.0 Msun, densely enough to permit detailed interpolation in mass, and the metallicity and helium abundance ranges [Fe/H] = -1.5 to +0.3, and Y = 0.25 to 0.37, specified independently of each other (i.e., no fixed Delta Y/Delta Z relation is assumed). The evolutionary tracks are calculated from the pre-main sequence up to the tip of the red giant branch. The isochrones, with ages between 1 Myr and 20 Gyr, provide UBVRI colors in the Johnson-Cousins system, and JHK colors in the homogeneized Bessell & Brett system, derived from two different semi-empirical Teff-color calibrations from the literature. We also provide utility codes, such as an isochrone interpolator in age, metallicity, and helium content, and an interface of the tracks with an open-source Monte Carlo Markov-Chain tool for the analysis of individual stars. Finally, we present comparisons of the YaPSI models with the best empirical mass- luminosity and mass-radius relations available to date, as well as isochrone fitting of well-studied steComment: 17 pages, 14 figures; accepted for publication in the Astrophysical Journa

Glacio and hydro-isostasy in the Mediterranean Sea: Clark’s zones and role of remote ice sheets

Solving the sea-level equation for a spherically symmetric Earth we study the relative sea-level curves in the Mediterranean Sea in terms of Clark’s zones and we explore their sensitivity to the time-history of Late-Pleistocene ice aggregates. Since the Mediterranean is an intermediate field region with respect to the former ice sheets, glacio- and hydro-isostasy both contribute to sea-level variations throughout the Holocene. In the bulk of the basin, subsidence of the sea floor results in a monotonous sea-level rise, whereas along continental margins water loading produces the effect of «continental levering», which locally originates marked highstands followed by a sea-level fall. To describe such peculiar pattern of relative sea-level in this and other mid-latitude closed basins we introduce a new Clark’s zone (namely, Clark’s zone VII). Using a suite of publicly available ice sheet chronologies, we identify for the first time a distinct sensitivity of predictions to the Antarctic ice sheet. In particular, we show that the history of mid to Late Holocene sea-level variations along the coasts of SE Tunisia may mainly reflect the melting of Antarctica, by a consequence of a mutual cancellation of the effects from the Northern Hemisphere ice-sheets at this specific site. Ice models incorporating a delayed melting of Antarctica may account for the observations across the Mediterranean, but fail to reproduce the SE Tunisia highstand

Microphones’ Directivity for the Localization of Sound Sources

In a recent paper [P. Rizzo, G. Bordoni, A. Marzani, and J. Vipperman, “Localization of Sound Sources by Means of Unidirectional Microphones, Meas. Sci. Tech., 20, 055202 (12pp), 2009] the proof-of-concept of an approach for the localization of acoustic sources was presented. The method relies on the use of unidirectional microphones and amplitude-based signals’ features to extract information about the direction of the incoming sound. By intersecting the directions identified by a pair of microphones, the position of the emitting source can be identified. In this paper we expand the work presented previously by assessing the effectiveness of the approach for the localization of an acoustic source in an indoor setting. As the method relies on the accurate knowledge of the microphones directivity, analytical expression of the acoustic sensors polar pattern were derived by testing them in an anechoic chamber. Then an experiment was conducted in an empty laboratory by using an array of three unidirectional microphones. The ability to locate the position of a commercial speaker placed at different positions in the room is discussed. The objective of this study is to propose a valid alternative to the common application of spaced arrays and therefore to introduce a new generation of reduced size sound detectors and localizers. The ability of the proposed methodology to locate the position of a commercial speaker placed at different positions in the room was evaluated and compared to the accuracy provided by a conventional time delay estimate algorithm

Elastoplastic Damaging Model for Adhesive Anchor Systems. I: Theoretical Formulation and Numerical Implementation

In this and in the companion paper, the mechanical response of adhesive anchor systems is theoretically and numerically predicted and experimentally observed. The theoretical prediction is on the basis of an elastoplastic damaging model formulated to predict the structural response associated with the development of a fracture in adhesive anchor systems. This part describes the analytical model developed in the framework of a thermodynamically consistent theory, which assumes adhesion where the structure is sound, and friction in correspondence with the fracture. Isotropic damage is considered. The model can predict the structural behavior at the interface between two surfaces of ductile, brittle, or quasi-brittle materials. The Helmholtz free energy is written to model the materials' hardening or softening. Isotropic damage is considered, and the possible effects of dilatancy are taken into account, including nonassociative flow rules. The formulation is implemented into the finite-element code FEAP. In the companion paper, the new model is adopted to predict the mechanical response to the pullout force of postinstalled rebar chemically bonded in concrete. The analytical model and the numerical implementation are experimentally validated by several pullout tests, which are monitored by using an acoustic-emission technique

Elastoplastic Damaging Model for Adhesive Anchor Systems. II: Numerical and Experimental Validation

This paper presents the numerical and experimental validation of the analytical elastoplastic damaging model proposed in the companion paper (Part I). The validation was carried out by describing the pullout failure of epoxy adhesive anchors. Pullout tests were simulated numerically and performed experimentally. Several specimens made of a rebar embedded in a hardened concrete cylinder by means of polyester resin were tested. Conventional strain gauges and acoustic emission (AE) sensors were used to evaluate the structural response of the system and to monitor the onset and progression of structural damage, respectively. The parametric analysis and the moment tensor analysis of the AE data were used to discriminate among different sources of damage. The results show the ability of the model to predict the response of the anchors and the suitability of the AE method to monitor damage onset and propagation and to discriminate among different source of damage