11,896 research outputs found
Double lenses
The analysis of the shear induced by a single cluster on the images of a
large number of background galaxies is all centered around the curl-free
character of a well-known vector field that can be derived from the data. Such
basic property breaks down when the source galaxies happen to be observed
through two clusters at different redshifts, partially aligned along the line
of sight. In this paper we address the study of double lenses and obtain five
main results. (i) First we generalize the procedure to extract the available
information, contained in the observed shear field, from the case of a single
lens to that of a double lens. (ii) Then we evaluate the possibility of
detecting the signature of double lensing given the known properties of the
distribution of clusters of galaxies. (iii) As a different astrophysical
application, we demonstrate how the method can be used to detect the presence
of a dark cluster that might happen to be partially aligned with a bright
cluster studied in terms of statistical lensing. (iv) In addition, we show that
the redshift distribution of the source galaxies, which in principle might also
contribute to break the curl-free character of the shear field, actually
produces systematic effects typically two orders of magnitude smaller than the
double lensing effects we are focusing on. (v) Remarkably, a discussion of
relevant contributions to the noise of the shear measurement has brought up an
intrinsic limitation of weak lensing analyses, since one specific contribution,
associated with the presence of a non-vanishing two-galaxy correlation
function, turns out not to decrease with the density of source galaxies (and
thus with the depth of the observations).Comment: 40 pages, 15 figures. Accepted for publication in ApJ main journa
The astroclimatological comparison of the Paranal Observatory and El Roque de Los Muchachos Observatory
The new extremely large telescope projects need accurate evaluation of the
candidate sites. In this paper we present the astroclimatological comparison
between the Paranal Observatory, located on the coast of the Atacama Desert
(Chile), and the Observatorio del Roque de Los Muchachos (ORM), located in La
Palma (Canary Islands). We apply a statistical analysis using long term
databases from Paranal and Carlsberg Meridian Telescope (CAMC) weather
stations. Significant differences between the two analyzed sites have been
found.Comment: Accepted for publication in MNRAS. 11 pages, 12 figures, 12 table
Fraction of clear skies above astronomical sites: a new analysis from the GOES12 satellite
Comparing the number of clear nights (cloud free) available for astronomical
observations is a critical task because it should be based on homogeneous
methodologies. Current data are mainly based on different judgements based on
observer logbooks or on different instruments. In this paper we present a new
homogeneous methodology on very different astronomical sites for modern optical
astronomy, in order to quantify the available night time fraction. The data are
extracted from night time GOES12 satellite infrared images and compared with
ground based conditions when available. In this analysis we introduce a wider
average matrix and 3-Bands correlation in order to reduce the noise and to
distinguish between clear and stable nights. Temporal data are used for the
classification. In the time interval 2007-2008 we found that the percentage of
the satellite clear nights is 88% at Paranal, 76% at La Silla, 72.5% at La
Palma, 59% at Mt. Graham and 86.5% at Tolonchar. The correlation analysis of
the three GOES12 infrared bands B3, B4 and B6 indicates that the fraction of
the stable nights is lower by 2% to 20% depending on the site
Changing the University System Management: a study of the Italian scenario
Over recent years, the Italian University System has been handling a phase of deep
changes, which have had significant impact on its mission and on the way it operates.
The most important of these changes have been to the organisation of universities, their
recruitment procedures and in terms of improvements to the quality and efficiency of
the university system itself. In this perspective, the objective of this research was to carry
out a critical analysis of the process of change, with special reference to improving
efficiency by making the transition from cash-based accounting to accrual accounting. In
order to achieve this objective, the starting point was the legislation of reference that
sets out the terms for the move to financial accrual accounting. A comparative analysis
was then carried out at an international level, with the purpose of highlighting the
strengths and weaknesses identified during the implementation of these new
procedures within the public field. This was followed by an analysis of the details of the
theory defining the accounting principles to be used in the process of preparing
university’s financial statements. Finally, the study identified the main critical points
relating to implementation of the new accounting system, offering, at the same time,
several thoughts concerning possible subsequent analyses on this topic
Improved nanopatterning for YBCO nanowires approaching the depairing current
An improved nanopatterning procedure has been developed to obtain YBCO
nanowires with cross sections as small as 50x50 nm^2, protected by an Au
capping layer. To probe the effective role of the Au protecting layer, we have
measured the current-voltage characteristics and the resistive transition in
temperature of the nanowires. Critical current densities up to 10^8 A/cm^2 have
been achieved at T=4.2 K, approaching the theoretical depairing current limit.
The resistance, measured as a function of temperature close to Tc, has been
fitted with a thermal activated phase slip model, including the effect of the
gold layer. The extracted values of the superconducting coherence length and of
the London penetration depth give current densities consistent with the
measured ones. These results cannot be achieved with same nanowires, without
the Au capping layer.Comment: ASC 2012 conference contributio
Mixing in massive stellar mergers
The early evolution of dense star clusters is possibly dominated by close
interactions between stars, and physical collisions between stars may occur
quite frequently. Simulating a stellar collision event can be an intensive
numerical task, as detailed calculations of this process require hydrodynamic
simulations in three dimensions. We present a computationally inexpensive
method in which we approximate the merger process, including shock heating,
hydrodynamic mixing and mass loss, with a simple algorithm based on
conservation laws and a basic qualitative understanding of the hydrodynamics of
stellar mergers. The algorithm relies on Archimedes' principle to dictate the
distribution of the fluid in the stable equilibrium situation. We calibrate and
apply the method to mergers of massive stars, as these are expected to occur in
young and dense star clusters. We find that without the effects of microscopic
mixing, the temperature and chemical composition profiles in a collision
product can become double-valued functions of enclosed mass. Such an unphysical
situation is mended by simulating microscopic mixing as a post-collision
effect. In this way we find that head-on collisions between stars of the same
spectral type result in substantial mixing, while mergers between stars of
different spectral type, such as type B and O stars (10 and 40\msun
respectively), are subject to relatively little hydrodynamic mixing.Comment: Accepted by MNRA
El Roque de Los Muchachos Site Characteristics. III. Analysis of Atmospheric Dust and Aerosol Extinction
Canary Islands are normally interested by dominant North-East winds that, in
some meteorological conditions, can transport sand at high altitude from the
Sahara desert. The dust may affect the efficiency of the telescopes and
decreases the transparency of the sky. In order to maximize the scientific
return of the telescopes located at the ORM, we present an analysis of the
atmospheric dust content and its effects on astronomical observations. B, V and
I dust aerosol astronomical extinction are derived. Using a 5 years series
database of data taken from the four channel TNG dust monitor, we compute a
mean hourly and daily values of the dust content. We have detected particles
having size 0.3, 0.5, 1.0 and 5.0 um. Using a power law we have derived the
content of 10.0 um particles. We found a typical local dust concentration
ranging from 3x10^6 particles per cubic meter at 0.3 um, to 10^3 at 5.0 um and
10 at 10.0 um, increasing up to 3 order of magnitudes during the dust storms,
with a relative higher increase of 1.0, 5.0 and 10.0 um particles. The number
of local dust storm events is the same in winter- and summertime, but, the
average background and storm-related increases in the dust concentration in
summer are significantly higher than in winter. In a uniform approximation,
during the dust storms, an average height of the dust layer of 2.5 km above the
telescope is inferred. During the sand storms La Palma Island is affected by an
almost uniform layer extending up to 5 km above the sea level, down, at least
the height of the telescope. The visible extinction is dominated by particles
at 1.0, 5.0 and 10.0 um. In agreement with the results from Carlsberg Automatic
Meridian Circle (CAMC) we find a typical extinction during dust storms of about
0.2 mag/airmass.Comment: Accepted for publication in A&A. 9 pages, 11 figures. This work is
the continuation of a series of papers concerning a detailed study of the
Astroclimatology at ORM. The two previous papers (both Lombardi et al.) have
reference PASP.2006.118.1198-1204 and PASP.2007.119.292-30
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