10,397 research outputs found
Lyapunov stabilization of discrete-time feedforward dynamics
The paper discusses stabilization of nonlinear discrete-time dynamics in feedforward form. First it is shown how to define a Lyapunov function for the uncontrolled dynamics via the construction of a suitable cross-term. Then, stabilization is achieved in terms of u-average passivity. Several constructive cases are analyzed
Discrete port-controlled Hamiltonian dynamics and average passivation
The paper discusses the modeling and control of port-controlled Hamiltonian dynamics in a pure discrete-time domain. The main result stands in a novel differential-difference representation of discrete port-controlled Hamiltonian systems using the discrete gradient. In these terms, a passive output map is exhibited as well as a passivity based damping controller underlying the natural involvement of discrete-time average passivity
Coming out in the workplace: a comparative study between Italy and England
Introduction: The study aims to analyze the main characteristics and differences relating to the lived experiences of coming out in the workplace in two very different territorial contexts (Italy and England), looking at universities as a case study.
Method: 30 sexual and gender minority university teachers (15 from Italy and 15 from England), ranging from 29 to 56 years of age (M = 41.66; SD = 10.13), were recruited in 2019 to participate in a qualitative research study. Data were collected through semi-structured interviews and analyzed in line with the assumptions of the six-step thematic analysis approach.
Result: The propensity to come out in the workplace is mainly affected by legislative safeguards. However, policies and the regulatory framework alone are not enough. Results suggested that the cultural context and the working climate have an equally significant impact.
Conclusion: The stigma toward LGBT + people cannot be eradicated overnight and changing cultural beliefs is a slow process that requires imaginative and creative methods to get students and the university community to interact with LGBT + people.
Practical implications: Stimulating a bottom-up change within individual institutions can represent a possible strategy to activate a process of change and social inclusion toward the LGBT + community with an echo on the wider environmental and cultural context
A spectroscopic study of the globular Cluster NGC 4147
Indexación: Web of ScienceWe present the abundance analysis for a sample of 18 red giant branch stars in the metal-poor globular cluster NGC 4147 based on medium- and high-resolution spectra. This is the first extensive spectroscopic study of this cluster. We derive abundances of C, N, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Y, Ba, and Eu. We find a metallicity of [Fe/H] = -1.84 +/- 0.02 and an alpha-enhancement of +0.38 +/- 0.05 (errors on the mean), typical of halo globular clusters in this metallicity regime. A significant spread is observed in the abundances of light elements C, N, O, Na, and Al. In particular, we found an Na-O anticorrelation and Na-Al correlation. The cluster contains only similar to 15 per cent of stars that belong to the first generation (Na-poor and O-rich). This implies that it suffered a severe mass-loss during its lifetime. Its [Ca/Fe] and [Ti/Fe] mean values agree better with the Galactic halo trend than with the trend of extragalactic environments at the cluster metallicity. This possibly suggests that NGC 4147 is a genuine Galactic object at odd with what claimed by some author that proposed the cluster to be member of the Sagittarius dwarf galaxy. An antirelation between the light s-process element Y and Na may also be present.https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stw114
Molecular dynamics simulation study of the high frequency sound waves in the fragile glass former ortho-terphenyl
Using a realistic flexible molecule model of the fragile glass former
orthoterphenyl, we calculate via molecular dynamics simulation the collective
dynamic structure factor, recently measured in this system by Inelastic X-ray
Scattering. The comparison of the simulated and measured dynamic structure
factor, and the study of its properties in an extended momentum, frequency and
temperature range allows: i) to conclude that the utilized molecular model
gives rise to a dynamic structure factor in agreement with the experimental
data, for those thermodynamic states and momentum values where the latter are
available; ii) to confirm the existence of a slope discontinuity on the
T-dependence of the sound velocity that, at finite Q, takes place at a
temperature T_x higher than the calorimetric glass transition temperature T_g;
iii) to find that the values of T_x is Q-dependent and that its vanishing Q
limit is consistent with T_g. The latter finding is interpreted within the
framework of the current description of the dynamics of supercooled liquids in
terms of exploration of the potential energy landscape.Comment: RevTex, 9 pages, 10 eps figure
The cosmological Lithium problem outside the Galaxy: the Sagittarius globular cluster M54
The cosmological Li problem is the observed discrepancy between Li abundance,
A(Li), measured in Galactic dwarf, old and metal-poor stars (traditionally
assumed to be equal to the initial value A(Li)_0), and that predicted by
standard Big Bang Nucleosynthesis calculations (A(Li)_{BBN}). Here we attack
the Li problem by considering an alternative diagnostic, namely the surface Li
abundance of red giant branch stars that in a colour magnitude diagram populate
the region between the completion of the first dredge-up and the red giant
branch bump. We obtained high-resolution spectra with the FLAMES facility at
the Very Large Telescope for a sample of red giants in the globular cluster
M54, belonging to the Sagittarius dwarf galaxy. We obtain A(Li)=+0.93+-0.11
dex, translating -- after taking into account the dilution due to the dredge
up-- to initial abundances (A(Li)_0) in the range 2.35--2.29 dex, depending on
whether or not atomic diffusion is considered. This is the first measurement of
Li in the Sagittarius galaxy and the more distant estimate of A(Li)_0 in old
stars obtained so far. The A(Li)_0 estimated in M54 is lower by ~0.35 dex than
A(Li)_{BBN}, hence incompatible at a level of ~3sigma. Our result shows that
this discrepancy is a universal problem concerning both the Milky Way and
extra-galactic systems. Either modifications of BBN calculations, or a
combination of atomic diffusion plus a suitably tuned additional mixing during
the main sequence, need to be invoked to solve the discrepancy.Comment: Accepted by MNRAS, 10 pages, 5 figures, 1 tabl
How does gas cool in DM halos?
In order to study the process of cooling in dark-matter (DM) halos and assess
how well simple models can represent it, we run a set of radiative SPH
hydrodynamical simulations of isolated halos, with gas sitting initially in
hydrostatic equilibrium within Navarro-Frenk-White (NFW) potential wells. [...]
After having assessed the numerical stability of the simulations, we compare
the resulting evolution of the cooled mass with the predictions of the
classical cooling model of White & Frenk and of the cooling model proposed in
the MORGANA code of galaxy formation. We find that the classical model predicts
fractions of cooled mass which, after about two central cooling times, are
about one order of magnitude smaller than those found in simulations. Although
this difference decreases with time, after 8 central cooling times, when
simulations are stopped, the difference still amounts to a factor of 2-3. We
ascribe this difference to the lack of validity of the assumption that a mass
shell takes one cooling time, as computed on the initial conditions, to cool to
very low temperature. [...] The MORGANA model [...] better agrees with the
cooled mass fraction found in the simulations, especially at early times, when
the density profile of the cooling gas is shallow. With the addition of the
simple assumption that the increase of the radius of the cooling region is
counteracted by a shrinking at the sound speed, the MORGANA model is also able
to reproduce for all simulations the evolution of the cooled mass fraction to
within 20-50 per cent, thereby providing a substantial improvement with respect
to the classical model. Finally, we provide a very simple fitting function
which accurately reproduces the cooling flow for the first ~10 central cooling
times. [Abridged]Comment: 15 pages, accepted by MNRA
An Analytical Approach to Inhomogeneous Structure Formation
We develop an analytical formalism that is suitable for studying
inhomogeneous structure formation, by studying the joint statistics of dark
matter halos forming at two points. Extending the Bond et al. (1991) derivation
of the mass function of virialized halos, based on excursion sets, we derive an
approximate analytical expression for the ``bivariate'' mass function of halos
forming at two redshifts and separated by a fixed comoving Lagrangian distance.
Our approach also leads to a self-consistent expression for the nonlinear
biasing and correlation function of halos, generalizing a number of previous
results including those by Kaiser (1984) and Mo & White (1996). We compare our
approximate solutions to exact numerical results within the excursion-set
framework and find them to be consistent to within 2% over a wide range of
parameters. Our formalism can be used to study various feedback effects during
galaxy formation analytically, as well as to simply construct observable
quantities dependent on the spatial distribution of objects. A code that
implements our method is publicly available at
http://www.arcetri.astro.it/~evan/GeminiComment: 41 Pages, 11 figures, published in ApJ, 571, 585. Reference added,
Figure 2 axis relabele
UVES radial velocity accuracy from asteroid observations. Implications for the fine structure constant variability
High resolution observations of the asteroids Iris and Juno have been
performed by means of the UVES spectrograph at the ESO VLT to obtain the
effective accurac y of the spectrograph's radial velocity. The knowledge of
this quantity has impo rtant bearings on studies searching for a variability of
the fine structure cons tant carried on with this instrument. Asteroids provide
a precise radial velocit y reference at the level of 1 m/s which allows
instrumental calibration and the recognition of small instrumental drifts and
calibration systematics. In particu lar, radial velocity drifts due to non
uniform slit illumination and slit optica l misalignment in the two UVES
spectrograph arms can be investigated. The positi on of the solar spectrum
reflected by the asteroids are compared with the solar wavelength positions or
with that of asteroid observations at other epochs or wi th the twilight to
asses UVES instrumental accuracy . Radial velocities offsets in the range
10--50 m/s are generally observed likely due to a non uniform slit
illumination. However, no radial velocity patterns with wavelength are detected
and the two UVES arms provide consistent radial velocities. These results
suggest that the detected alpha variability by Levshakov et al. (2007) deduced
from a drift of -180 (+/- 85) m/s at z =1.84, between two sets of FeII lines
falling in the two UVES arms may be real or induced by other kinds of
systematics than those investigated here. The proposed technique allows real
time quality check of the spectrograph and should be followed for very accurate
measurements.Comment: Accepted A&
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