4,806 research outputs found
Late movement of basin-edge lobate scarps on Mercury
Basin-edge lobate scarps are a sub-type of tectonic shortening structure on the surface of Mercury that have formed at the edge of volcanic units that fill or partly fill impact basins. We have performed a global survey of these features and find that they are widespread in basins across the planet. We obtained model ages from crater size–frequency distribution analysis for a subset of our surveyed basins, for both the smooth plains infill and for the last resolvable tectonic activity on the associated basin-edge scarps. Our results indicate that some of these lobate scarps were still accumulating strain in the late Mansurian (approximately 1 Ga). From a photogeological assessment, we find that the orientations of these basin-edge lobate scarps are similar to those reported for the global population of lobate scarps in earlier studies, appearing to align ∼north–south at low latitudes and ∼east–west at higher latitudes. However, reassessing these landforms’ orientation with artificially illuminated topographic data does not allow us to rule out the effect of illumination bias. We propose that these landforms, the result of crustal shortening in response to global contraction, formed along the interface between the basin floor and the smooth plains unit, which acted as a mechanical discontinuity along which shortening strains were concentrated
The global mass function of M15
Data obtained with the NICMOS instrument on board the Hubble Space Telescope
(HST) have been used to determine the H-band luminosity function (LF) and mass
function (MF) of three stellar fields in the globular cluster M15, located ~7'
from the cluster centre. The data confirm that the cluster MF has a
characteristic mass of ~0.3 Msolar, as obtained by Paresce & De Marchi (2000)
for a stellar field at 4.6' from the centre. By combining the present data with
those published by other authors for various radial distances (near the centre,
at 20" and at 4.6'), we have studied the radial variation of the LF due to the
effects of mass segregation and derived the global mass function (GMF) using
the Michie-King approach. The model that simultaneously best fits the LF at
various locations, the surface brightness profile and the velocity dispersion
profile suggests that the GMF should resemble a segmented power-law with the
following indices: x ~ 0.8 for stars more massive than 0.8 Msolar, x ~ 0.9 for
0.3 - 0.8 Msolar and x ~ -2.2 at smaller masses (Salpeter's IMF would have
x=1.35). The best fitting model also suggests that the cluster mass is ~5.4
10^5 Msolar and that the mass-to-light ratio is on average M/L_V ~ 2.1, with
M/L_V ~ 3.7 in the core. A large amount of mass (~ 44 %) is found in the
cluster core in the form of stellar heavy remnants, which may be sufficient to
explain the mass segregation in M15 without invoking the presence of an
intermediate-mass black hole.Comment: 12 pages, 10 figures, accepted for publication in A&
The Cratering History of Asteroid (2867) Steins
The cratering history of main belt asteroid (2867) Steins has been
investigated using OSIRIS imagery acquired during the Rosetta flyby that took
place on the 5th of September 2008. For this purpose, we applied current models
describing the formation and evolution of main belt asteroids, that provide the
rate and velocity distributions of impactors. These models coupled with
appropriate crater scaling laws, allow the cratering history to be estimated.
Hence, we derive Steins' cratering retention age, namely the time lapsed since
its formation or global surface reset. We also investigate the influence of
various factors -like bulk structure and crater erasing- on the estimated age,
which spans from a few hundred Myrs to more than 1Gyr, depending on the adopted
scaling law and asteroid physical parameters. Moreover, a marked lack of
craters smaller than about 0.6km has been found and interpreted as a result of
a peculiar evolution of Steins cratering record, possibly related either to the
formation of the 2.1km wide impact crater near the south pole or to YORP
reshaping.Comment: Accepted by Planetary and Space Scienc
Resonating Valence Bond wave function with molecular orbitals: application to first-row molecules
We introduce a method for accurate quantum chemical calculations based on a simple variational wave function, defined by a single geminal that couples all the electrons into singlet pairs, combined with a real space correlation factor. The method uses a constrained variational optimization, based on an expansion of the geminal in terms of molecular orbitals. It is shown that the most relevant non-dynamical correlations are correctly reproduced once an appropriate number n of molecular orbitals is considered. The value of n is determined by requiring that, in the atomization limit, the atoms are described by Hartree-Fock Slater determinants with Jastrow correlations. The energetics, as well as other physical and chemical properties, are then given by an efficient variational approach based on standard quantum Monte Carlo techniques. We test this method on a set of homonuclear (Be2, B2, C2, N2, O2, and F2) and heteronuclear (LiF, and CN) dimers for which strong non-dynamical correlations and/or weak van der Waals interactions are present
Romanesque and territory. The construction materials of Sardinian medieval churches: new approaches to the valorization, conservation and restoration
This paper is intended to illustrate a multidisciplinary research project devoted to the study of the constructive materials of the Romanesque churches in Sardinia during the “Giudicati” period (11th -13th centuries). The project focuses on the relationship between a selection of monuments and their territory, both from a historical-architectural perspective and from a more modern perspective addressing future restoration works. The methodologies of the traditional art-historical research (study of bibliographic, epigraphic and archival sources, formal reading of artifacts) are flanked by new technologies: digital surveys executed with a 3D laser-scanner, analyses of the materials (stones, mortars, bricks) with different instrumental methods: X-ray fluorescence (XRF) and inductively coupled mass spectrometry (ICP-MS) for chemical composition, X-ray diffractometer (XRD) to determine the alteration phases (e.g., soluble salts), optical microscopy and electronic (SEM) to study textures, mineral assemblages and microstructures, termogravimetric/differential scanning, calorimetric analysis (TG/DTA) for the composition of the binder mortars.
This multidisciplinary approach allows the achieving of important results in an archaeometric context: 1) from a historical point of view, with the possible identification of ancient traffics, trade routes, sources of raw materials, construction phases, wall textures; 2) from a conservative point of view, by studying chemical and physical weathering processes of stone materials compatible for replacement in case of future restoration works.
Sardinian Romanesque architectural heritage is particularly remarkable: about 200 churches of different types and sizes, with the almost exclusive use of cut stones. Bi- or poly-chromy, deriving from the use of different building materials, characterizes many of these monuments, becoming also a vehicle for political and cultural meanings. The paper will present some case studies aimed to illustrate the progress of the project and the results achieved
Heating of near-Earth objects and meteoroids due to close approaches to the Sun
It is known that near-Earth objects (NEOs) during their orbital evolution may
often undergo close approaches to the Sun. Indeed it is estimated that up to
~70% of them end their orbital evolution colliding with the Sun. Starting from
the present orbital properties, it is possible to compute the most likely past
evolution for every NEO, and to trace its distance from the Sun. We find that a
large fraction of the population may have experienced in the past frequent
close approaches, and thus, as a consequence, a considerable Sun-driven
heating, not trivially correlated to the present orbits. The detailed dynamical
behaviour, the rotational and the thermal properties of NEOs determine the
exact amount of the resulting heating due to the Sun. In the present paper we
discuss the general features of the process, providing estimates of the surface
temperature reached by NEOs during their evolution. Moreover, we investigate
the effects of this process on meteor-size bodies, analyzing possible
differences with the NEO population. We also discuss some possible effects of
the heating which can be observed through remote sensing by ground-based
surveys or space missions.Comment: 8 pages, 5 figures, accepted by MNRA
On the Globular Cluster IMF below 1 Solar Mass
(Abridged) Accurate luminosity functions (LF) for a dozen globular clusters
have now been measured at or just beyond their half-light radius using HST.
They span almost the entire cluster main sequence below ~ 0.75 Msolar. All
these clusters exhibit LF that rise continuously from an absolute I magnitude
M_I ~ 6 to a peak at M_I ~ 8.5-9 and then drop with increasing M_I.
Transformation of the LF into mass functions (MF) by means of the most recent
mass luminosity relations that are consistent with all presently available data
on the physical properties of low mass, low metallicity stars shows that all
the LF observed so far can be obtained from MF having the shape of a log-normal
distribution with characteristic mass m_c=0.33 +/- 0.03 Msolar and standard
deviation sigma = 1.81 +/- 0.19. After correction for the effects of mass
segregation, the variation of the ratio of the number of higher to lower mass
stars with cluster mass or any simple orbital parameter or the expected time to
disruption recently computed for these clusters shows no statistically
significant trend over a range of this last parameter of more than a factor of
100. We conclude that the global MF of these clusters have not been measurably
modified by evaporation and tidal interactions with the Galaxy and, thus,
should reflect the initial distribution of stellar masses. Since the log-normal
function that we find is also very similar to the one obtained independently
for much younger clusters and to the form expected theoretically, the
implication seems to be unavoidable that it represents the true stellar IMF for
this type of stars in this mass range.Comment: Accepted for publication in The Astrophysical Journal. Contains 28
pages with 6 figure
Elimination of congenital rubella: a seroprevalence study of pregnant women and women of childbearing age in Italy
Prevention of congenital rubella is achieved by vaccination of susceptible women of childbearing age. In Italy, the National Plan for Measles and Congenital Rubella Elimination 2010–2015 implemented catch-up vaccination activities targeting susceptible adolescents and young adults, including women of childbearing age. The aim of this study was to assess the immunity against rubella in women of childbearing age in Tuscany (Central Italy) and Apulia (Southern Italy) and pregnant women in Apulia after the implementation of the National Plan for Measles and Congenital Rubella Elimination. Overall, anti-rubella IgG prevalence in women of childbearing age samples was 88.6% in Tuscany and 84.3% in Apulia. The lowest prevalence was observed in samples of 26–35 years old women of childbearing age in Apulia with 77.8%. Only 62.7% of samples from 26–35 years old pregnant women had IgG against rubella, and one sample out of 95 was positive to IgM. The findings of this study highlight the need for increasing awareness on the risk of contracting rubella infection during pregnancy and implement vaccination strategies to create opportunities for administration of rubella containing vaccines in young girls and women of childbearing age
Correlation Energy and the Spin Susceptibility of the Two-Valley Two-dimensional Electron Gas
We find that the spin susceptibility of a two-dimensional electron system
with valley degeneracy does not grow critically at low densities, at variance
with experimental results [A. Shashkin et al., Phys. Rev. Lett. 96, 036403
(2006)]. We ascribe this apparent discrepancy to the weak disorder present in
experimental samples. Our prediction is obtained from accurate correlation
energies computed with state of-the-art diffusion Monte Carlo simulations and
fitted with an analytical expression which also provides a local spin density
functional for the system under investigation.Comment: 7 pages, 3 figures, accepted for publication in Phys. Rev.
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