3,676 research outputs found
High resolution spectroscopy of the three dimensional cosmic web with close QSO groups
We study the three-dimensional distribution of matter at z~2 using high
resolution spectra of QSO pairs and simulated spectra drawn from cosmological
hydro-dynamical simulations. We present a sample of 15 QSOs, corresponding to
21 baselines of angular separations evenly distributed between ~1 and 14
arcmin, observed with the Ultraviolet and Visual Echelle Spectrograph (UVES) at
the European Southern Observatory-Very Large Telescope (ESO-VLT). The observed
correlation functions of the transmitted flux in the HI Lya forest transverse
to and along the line of sight are in agreement, implying that the distortions
in redshift space due to peculiar velocities are relatively small and - within
the relatively large error bars - not significant. The clustering signal is
significant up to velocity separations of ~300 km/s, corresponding to about 5
h^{-1} comoving Mpc. Compatibility at the 2 sigma level has been found both for
the Auto- and Cross-correlation functions and for the set of the Cross
correlation coefficients. The analysis focuses in particular on two QSO groups
of the sample. Searching for alignments in the redshift space between Lya
absorption lines belonging to different lines of sight, it has been possible to
discover the presence of a wide HI structures extending over about ten Mpc in
comoving space, and give constraints on the sizes of two cosmic under-dense
regions in the intergalactic medium.Comment: Accepted by MNRAS, version matching the published on
Dissecting the effects of free fatty acids on the thermodynamic stability of complex model membranes mimicking insulin secretory granules
A stepwise micro-DSC study of Small, Large and Giant Unilamellar Vesicles prepared as pure and mixed systems of DMPC, DPPC, DSPC and DOPC was performed, achieving the preparation of final model membranes whose phospholipid compositions represent the 75% in terms of the phospholipids tails and the 50% headgroups of the Insulin Secretory Granules (vesicles located in the pancreatic Langerhans \u3b2-cells and which are responsible for insulin and amylin storage and secretion in response to nutrient intake). Moreover, the effect of Free Fatty Acids, whose levels are recurrently altered in diabetic and/or obese subjects, on the thermodynamic stability of the final membranes was eventually investigated. The results allowed to discriminate each single thermodynamic contribution among the main factors that dictate the overall thermodynamic stability of these complex unilamellar systems evidencing mainly entropic effects hierarchically summarized as phospholipid unsaturations > phospholipid tail length > membrane curvature. The effect of the Free Fatty Acids highlighted a strong stabilizing effect on the membranes as well as more pronounced phase segregations in the case of saturated acids (palmitic and stearic), whereas the opposite effect was observed in the case of an unsaturated one (oleic)
Probing 3-D matter distribution at z~2 with QSO multiple lines of sight
We investigate the 3-D matter distribution at z~2 with high resolution (R ~
40000) spectra of QSO pairs and groups obtained with the UVES spectrograph at
ESO VLT. Our sample is unique for the number density of objects and the variety
of separations, between 0.5 and 7 proper Mpc. We compute the real space
cross-correlation function of the Lyman-alpha forest transmitted fluxes. There
is a significant clustering signal up to ~2 proper Mpc, which is still present
when absorption lines with high column density (log N > 13.8) are excluded.Comment: Poster paper presented at the IAU Colloquium #199 on "Probing
Galaxies through Quasar Absorption Lines" held in Shanghai, China from March
14th to 18th, 200
Tomography of the intergalactic medium with Ly-alpha forests in close QSO pairs
We study the three-dimensional distribution of non virialised matter at z~2
using high resolution spectra of QSO pairs and simulated spectra drawn from
cosmological hydrodynamical simulations. We have collected the largest sample
of QSO pairs ever observed with UVES at the ESO-VLT, with angular separations
between ~1 and 14 arcmin. The observed correlation functions of the transmitted
flux in the HI Lyman alpha forest along and transverse to the lines of sight
are in good agreement implying that the distortions in redshift space due to
peculiar velocities are small. The clustering signal is significant up to
velocity separations of ~200 km/s, or about 3 h^{-1} comoving Mpc. The regions
at lower overdensity (rho/ < 6.5) are still clustered but on smaller
scales (Delta v < 100 km/s). The observed and simulated correlation functions
are compatible at the 3 sigma level. A better concordance is obtained when only
the low overdensity regions are selected for the analysis or when the effective
optical depth of the simulated spectra is increased artificially, suggesting a
deficiency of strong lines in the simulated spectra. We found that also a lower
value of the power-law index of the temperature-density relation for the Lyman
alpha forest gas improves the agreement between observed and simulated results.
If confirmed, this would be consistent with other observations favouring a late
HeII reionization epoch (at z~3). We remark the detection of a significant
clustering signal in the cross correlation coefficient at a transverse velocity
separation Delta v_{\perp} ~500 km/s whose origin needs further investigation.Comment: Accepted for publication in MNRAS, revised version matching the
accepted on
Effects of disorder on the optical gap of (Zn,Mg)(S,Se)
The electronic properties and optical gap of (Zn,Mg)(S,Se) wide-gap solid solutions are studied using ab initio techniques and starting from the previously determined atomistic structure of the alloy. Compositional disorder is shown to close the gap substantially with respect to the predictions of the virtual-crystal approximation. The bowing of the fundamental gap versus composition predicted by our calculations is in very good agreement with experiments available for the Zn(S,Se) pseudobinary alloy. At temperatures typical of molecular-beam epitaxy growth, the quaternary alloy displays a rather large amount of short-range order whose effect is to slightly but unmistakably open the gap. Our results agree well with recent experimental data for the quaternary alloy. (C) 1999 American Institute of Physics. [S0003-6951(99)02044-6]
Structure and stability of graphene nanoribbons in oxygen, carbon dioxide, water, and ammonia
We determine, by means of density functional theory, the stability and the
structure of graphene nanoribbon (GNR) edges in presence of molecules such as
oxygen, water, ammonia, and carbon dioxide. As in the case of
hydrogen-terminated nanoribbons, we find that the most stable armchair and
zigzag configurations are characterized by a non-metallic/non-magnetic nature,
and are compatible with Clar's sextet rules, well known in organic chemistry.
In particular, we predict that, at thermodynamic equilibrium, neutral GNRs in
oxygen-rich atmosphere should preferentially be along the armchair direction,
while water-saturated GNRs should present zigzag edges. Our results promise to
be particularly useful to GNRs synthesis, since the most recent and advanced
experimental routes are most effective in water and/or ammonia-containing
solutions.Comment: accepted for publication in PR
Structure, Stability, Edge States and Aromaticity of Graphene Ribbons
We determine the stability, the geometry, the electronic and magnetic
structure of hydrogen-terminated graphene-nanoribbons edges as a function of
the hydrogen content of the environment by means of density functional theory.
Antiferromagnetic zigzag ribbons are stable only at extremely-low ultra-vacuum
pressures. Under more standard conditions, the most stable structures are the
mono- and di-hydrogenated armchair edges and a zigzag edge reconstruction with
one di- and two mono-hydrogenated sites. At high hydrogen-concentration
``bulk'' graphene is not stable and spontaneously breaks to form ribbons, in
analogy to the spontaneous breaking of graphene into small-width nanoribbons
observed experimentally in solution. The stability and the existence of exotic
edge electronic-states and/or magnetism is rationalized in terms of simple
concepts from organic chemistry (Clar's rule)Comment: 4 pages, 3 figures, accepted for publication by Physical Review
Letter
A new and efficient approach to time-dependent density-functional perturbation theory for optical spectroscopy
Using a super-operator formulation of linearized time-dependent
density-functional theory, the dynamical polarizability of a system of
interacting electrons is given a matrix continued-fraction representation whose
coefficients can be obtained from the non-symmetric block-Lanczos method. The
resulting algorithm allows for the calculation of the {\em full spectrum} of a
system with a computational workload which is only a few times larger than that
needed for {\em static} polarizabilities within time-independent
density-functional perturbation theory. The method is demonstrated with the
calculation of the spectrum of benzene, and prospects for its application to
the large-scale calculation of optical spectra are discussed.Comment: 4 pages, 2 figure
Status of the Golden Eagle Aquila chrysaetos in Sicily
In the present work, we review all the relevant information
since the first Regional Atlas of breeding birds (Massa
1985), and we add field data on the species occurrence and site occupancy relative to the period January 2014-December
2016, in order to update the species’ status in Sicily
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