826 research outputs found
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
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
Neoplastic tissue transfiguration in vivo by recombinant human transforming growth factor-β3
Human oral squamous cell carcinomas (hSCCs) are the most common head and neck cancers now presenting with more aggressive biological and clinical features due to smoking and alcohol together with widespread viremia. Transforming growth factor-β (TGF-β) proteins are powerful morphogens that induce rapid and substantial induction of endochondral bone formation but in primates only
Josephson effect in MgB_2 break junctions
We present the first observation of the DC and AC Josephson effect in MgB_2
break junctions. The junctions, obtained at 4.2 K in high-quality, high-density
polycrystalline metallic MgB_2 samples, show a non-hysteretic DC Josephson
effect. By irradiating the junctions with microwaves we observe clear Shapiro
steps spaced by the ideal value. The temperature dependence of the
DC Josephson current and the dependence of the height of the steps on the
microwave power are obtained. These results are a direct prove for the
existence of pairs with charge 2e in this new metallic superconductor and give
evidence of the superconductor-normal metal-superconductor weak link character
of these junctions.Comment: 4 RevTEX pages, 4 eps figure
Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements
In a type-II superconductor the generation of higher harmonics in the
magnetic response to an alternating magnetic field is a consequence of the
non-linearity in the I-V relationship. The shape of the current-voltage (I-V)
curve is determined by the current dependence of the thermal activation energy
U(J) and is thus related to the dynamical regimes governing the vortex motion.
In order to investigate the vortex dynamics in MgB2 bulk superconductors we
have studied the fundamental (chi1) and third (chi3) harmonics of the ac
magnetic susceptibility. Measurements have been performed as a function of the
temperature and the dc magnetic field, up to 9 T, for various frequencies and
amplitudes of the ac field. We show that the analysis of the behaviour in
frequency of chi3(T) and chi3(B) curves can provide clear information about the
non-linearity in different regions of the I-V characteristic. By comparing the
experimental curves with numerical simulations of the non-linear diffusion
equation for the magnetic field we are able to resolve the crossover between a
dissipative regime governed by flux creep and one dominated by flux flow
phenomena.Comment: to be published in "Horizons in Superconductivity Research" (Nova
Science Publishers, Inc., NY, 2003
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
EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE DYNAMICS OF THE FLOATING FLOOR AND PASSENGER SEATS OF A RAILWAY VEHICLE
On a semiclassical formula for non-diagonal matrix elements
Let be a Schr\"odinger operator on the real
line, be a bounded observable depending only on the coordinate and
be a fixed integer. Suppose that an energy level intersects the potential
in exactly two turning points and lies below
. We consider the semiclassical limit
, and where is the th
eigen-energy of . An asymptotic formula for , the
non-diagonal matrix elements of in the eigenbasis of , has
been known in the theoretical physics for a long time. Here it is proved in a
mathematically rigorous manner.Comment: LaTeX2
Measurement and Processing of Road Irregularity for Surface Generation and Tyre Dynamics Simulation in NVH Context
Nowadays, finite element tyre models are often used to perform vehicle NVH (noise, vibration, harshness) simulations. To
account for the specific operating conditions, a road surface must be properly included in the model. This paper deals with a
methodology to experimentally evaluate and process road irregularity measurements, so as to generate a road surface input.
These surfaces are used to simulate the tyre/road interaction at the footprint, which is modelled as a contact surface in finite
element tyre models. For this reason, a linear profile of the road surface is not suitable for these simulations and the whole
surface must be considered. Starting from the measurements taken through a test equipment specifically designed to carry
laser sensors and scan road profiles, the Power Spectral Density (PSD) of a specific track is estimated and then interpolated
considering piecewise functions. Finally, a model to generate a road surface starting from the measured PSD is developed,
discussed and validated
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