Effects of dark matter annihilation on the first stars

We study the evolution of the first stars in the universe (Population III) from the early pre-Main Sequence until the end of helium burning in the presence of WIMP dark matter annihilation inside the stellar structure. The two different mechanisms that can provide this energy source are the contemporary contraction of baryons and dark matter, and the capture of WIMPs by scattering off the gas with subsequent accumulation inside the star. We find that the first mechanism can generate an equilibrium phase, previously known as a "dark star", which is transient and present in the very early stages of pre-MS evolution. The mechanism of scattering and capture acts later, and can support the star virtually forever, depending on environmental characteristic of the dark matter halo and on the specific WIMP model.Comment: Proceedings of the IAU Symposium 255, "Low-Metallicity Star Formation: From the First Stars to Dwarf Galaxies"; L.K. Hunt, S. Madden and R. Schneider ed

A Meta-Heuristic Optimization Procedure for the Identification of the Nonlinear Model Parameters of Hydraulic Dampers Based on Experimental Dataset of Real Working Conditions

Hydralic dampers are widely implemented in railway vehicle suspension stages, especially in high-speed passenger trains. They are designed to be mounted in different positions to improve comfort, stability, and safety performances. Numerical simulations are often used to assist the design and optimization of these components. Unfortunately, hydraulic dampers are highly nonlinear due to the complex fluid dynamic phenomena taking place inside the chambers and through the by-pass orifices. This requires accurate damper models to be developed to estimate the influence of the nonlinearities of such components during the dynamic performances of the whole vehicle. This work aims at presenting a new parametric damper model based on a nonlinear lumped element approach. Moreover, a new model tuning procedure will be introduced. Differently from the typical sinusoidal characterization cycles, this routine is based on experimental tests of real working conditions. The set of optimal model parameters will be found through a metaheuristic iterative approach able to minimize the differences between numerical and experimental damper forces. The performances of the optimal model will be compared with the ones of the most common Maxwell model generally implemented in railway multibody software programs

First star formation with dark matter annihilation

We include an energy term based on Dark Matter (DM) self-annihilation during the cooling and subsequent collapse of the metal-free gas, in halos hosting the formation of the first stars in the Universe. We have found that the feedback induced on the chemistry of the cloud does modify the properties of the gas throughout the collapse. However, the modifications are not dramatic, and the typical Jeans mass within the halo is conserved throughout the collapse, for all the DM parameters we have considered. This result implies that the presence of Dark Matter annihilations does not substantially modify the Initial Mass Function of the First Stars, with respect to the standard case in which such additional energy term is not taken into account. We have also found that when the rate of energy produced by the DM annihilations and absorbed by the gas equals the chemical cooling (at densities yet far from the actual formation of a proto-stellar core) the structure does not halt its collapse, although that proceeds more slowly by a factor smaller than few per cent of the total collapse time.Comment: 12 pages, 8 figures, 3 tables; replaced with published version after minor change

ANALYSING MEDICAL ENGLISH LEXIS: A CORPUS-BASED RESEARCH. IMPLICATIONS FOR LEARNING AND TEACHING

Abstract In the last few decades medical specialized communication has become progressively dynamic and prolific with an ever growing number of researchers employing English as a lingua franca. Medical specialized communication unquestionably constitutes a challenging problem with non-native medical students and health care practitioners who are increasingly faced with the need to have an active command of Medical English, that vast set of standardized and non-standardized terms used to describe and represent the changes and the results accomplished in the medical field. My direct involvement in a language trial test aiming at investigating the productive vocabulary knowledge of a group of Italian medical undergraduates was the background that provided the incentive for this dissertation to investigate the language of medicine and devise strategies and materials that were specific enough to help non-native speakers from different medical fields acquire the English language skills they needed step by step. Trying to find the convergence between non-native medical doctors\u2019 vocabulary needs and pedagogy, my research has developed along two lines, purposefully called \u2018diagnosis\u2019 and \u2018remedy\u2019. Firstly, I endeavoured to \u2018diagnose\u2019 which key-words are homogeneously distributed across mainstream professional medical writings opportunely collected in the Medical English Corpus (MedEnCor), a specialized corpus extensively representative of current healthcare domains and biomedical topics. Secondly, I attempted to seek a \u2018remedy\u2019 to the non-native learners' lexical \u2018impairments\u2019 categorizing all the extracted key-words into semantic wordlists, suitably catalogued into the Medical English Corpus Lexical database (MedEnCor-Lex), a web-based monolingual glossary (www.medencor.com) meant to provide non-native users not only with denotative information on medical key-words, but also with appropriate instances of their collocational and phraseological context and use. Although this writing tool is currently being completed, my goal is equally educational and professional because by compiling a specialized lexical database I do not only mean to make the English used in medicine accessible to the healthcare community, but, first and foremost, to make non-native recipients familiarize themselves with the terms and expressions relevant to the scientific register. An essential skill for their career

Modelling and experimental characterization of secondary suspension elements for rail vehicle ride comfort simulation

Secondary suspensions play an essential role in the dynamic behaviour of rail vehicles. In particular, they are adopted to reduce the vibrations transmitted to the carbody, thus improving ride comfort. In this paper, an experimental characterization of the viscous damper and coil spring elements composing a vertical secondary suspension is presented. The elements are separately tested with the aim of analysing their dynamic behaviour. Then, modified prototypes are manufactured to reduce the transmitted force. The results of the experimental campaign are later adopted to tune the parameters of the mathematical model of the whole secondary suspension, including the dynamics of both the coil spring and the damper elements. This model allows discussing the effectiveness of the proposed modifications, proving the design of both the components to be fundamental for the improvement of ride comfort

Development of a fatigue test machine for high frequency applications

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Design and validation of a hardware-in-the-loop test bench for evaluating the performance of an active mass damper

The purpose of this study is to propose an innovative solution for evaluating the performance of a full-scale Active Mass Damper (AMD). The AMD adopted is a custom hydraulic actuator, developed for active control of existing buildings against earthquakes. For vibration control, a sky-hook algorithm was implemented. Its characteristics ensure good robustness, which is fundamental in structural engineering since buildings are subjected to significant variation in dynamic properties in presence of damage or ambient conditions. A Hardware-In-the-Loop (HIL) test bench was specifically designed to simulate the actual working condition of the anti-seismic system. The HIL setup consists of a shaking table moved by a hydraulic actuator in accordance with the roof's displacement, evaluated using a structural numerical model of the building to which the AMD is fixed. The presence of two distinct active systems (HIL and AMD) could generate control issues; therefore, a Triple Variable Control logic was introduced to reduce the interaction delay. The effectiveness of the proposed AMD is validated comparing the roof's displacement in an uncontrolled structure with that in a controlled one. Also, the robustness of the control algorithm was verified using a non-linear structural model and applying seismic excitation at different intensities

Numerical and Experimental Validation of Active Vibration Control Logic Performance of a Hybrid Noise Control-Based Brick

The limitations of active noise control (ANC) in coping with low frequencies and of passive noise control (PNC) in coping with middle-high frequencies are objects of research that present the potentialities of hybrid noise control (HBC). It aims at combining both of the behaviours by broadening the range of absorbed frequencies. Among the several application fields, the AEC (architecture, engineering, and construction) market can take advantage for those applications in which the noise conditions are caused by sound sources that tune in a broad frequencies range. In this frame, the paper describes the numerical and experimental validation of the active behaviour of an under-development project of a hybrid noise control-based acoustic bricks. The latter intends to embed the potentialities of active vibrational noise control (AVC) and passive destructive interference (PDI) in a unique design of an easy-to-mount, 3D-printed, customisable smart acoustic blocks. Active vibration control, the object of this paper, is provided by a 5-mm thick aluminium circular plate with an attached piezoelectric patch. The vibration of the latter, depending on a specific control law, defines the vibration of the plate itself achieving an abatement of the reflection coefficient. Through mathematical modelling and tests in an impedance tube, the results show that the control logic can reach an average abatement of the reflection coefficient of 82% in the frequency range 144-1007 Hz