13,995 research outputs found
The effect of stellar migration on Galactic chemical evolution: a heuristic approach
In the last years, stellar migration in galactic discs has been the subject
of several investigations. However, its impact on the chemical evolution of the
Milky Way still needs to be fully quantified. In this paper, we aim at imposing
some constraints on the significance of this phenomenon by considering its
influence on the chemical evolution of the Milky Way thin disc. We do not
investigate the physical mechanisms underlying the migration of stars. Rather,
we introduce a simple, heuristic treatment of stellar migration in a detailed
chemical evolution model for the thin disc of the Milky Way, which already
includes radial gas flows and reproduces several observational constraints for
the solar vicinity and the whole Galactic disc. When stellar migration is
implemented according to the results of chemo-dynamical simulations by Minchev
et. al. (2013) and finite stellar velocities of 1 km s are taken into
account, the high-metallicity tail of the metallicity distribution function of
long-lived thin-disc stars is well reproduced. By exploring the velocity space,
we find that the migrating stars must travel with velocities in the range 0.5
-2 km s to properly reproduce the high-metallicity tail of the
metallicity distribution. We confirm previous findings by other authors that
the observed spread in the age-metallicity relation of solar neighbourhood
stars can be explained by the presence of stars which originated at different
Galactocentric distances, and we conclude that the chemical properties of stars
currently observed in the solar vicinity do suggest that stellar migration is
present to some extent.Comment: Accepted for publication by Ap
The accretion environment of Supergiant Fast X-ray Transients probed with XMM-Newton
Supergiant fast X-ray transients (SFXTs) are characterized by a remarkable
variability in the X-ray domain, widely ascribed to the accretion from a clumpy
stellar wind. In this paper we performed a systematic and homogeneous analysis
of sufficiently bright X-ray flares from the SFXTs observed with XMM-Newton to
probe spectral variations on timescales as short as a few hundred of seconds.
Our ultimate goal is to investigate if SFXT flares and outbursts are triggered
by the presence of clumps and eventually reveal whether strongly or mildly
dense clumps are required. For all sources, we employ a technique developed by
our group, making use of an adaptive rebinned hardness ratio to optimally
select the time intervals for the spectral extraction. A total of twelve
observations performed in the direction of five SFXTs are reported. We show
that both strongly and mildly dense clumps can trigger these events. In the
former case, the local absorption column density may increase by a factor of
>>3, while in the latter case, the increase is only by a factor of 2-3 (or
lower). Overall, there seems to be no obvious correlation between the dynamic
ranges in the X-ray fluxes and absorption column densities in SFXTs, with an
indication that lower densities are recorded at the highest fluxes. This can be
explained by the presence of accretion inhibition mechanism(s). We propose a
classification of the flares/outbursts from these sources to drive future
observational investigations. We suggest that the difference between the
classes of flares/outbursts is related to the fact that the mechanism(s)
inhibiting accretion can be overcome more easily in some sources compared to
others. We also investigate the possibility that different stellar wind
structures, rather than clumps, could provide the means to temporarily overcome
the inhibition of accretion in SFXTs.Comment: Accepted for publication on A&
Satellite observations of reconnection between emerging and pre-existing small-scale magnetic fields
We report multi-wavelength ultraviolet observations taken with the IRIS
satellite, concerning the emergence phase in the upper chromosphere and
transition region of an emerging flux region (EFR) embedded in the unipolar
plage of active region NOAA 12529. The photospheric configuration of the EFR is
analyzed in detail benefitting from measurements taken with the
spectropolarimeter aboard the Hinode satellite, when the EFR was fully
developed. In addition, these data are complemented by full-disk, simultaneous
observations of the SDO satellite, relevant to the photosphere and the corona.
In the photosphere, magnetic flux emergence signatures are recognized in the
fuzzy granulation, with dark alignments between the emerging polarities,
cospatial with highly inclined fields. In the upper atmospheric layers, we
identify recurrent brightenings that resemble UV bursts, with counterparts in
all coronal passbands. These occur at the edges of the EFR and in the region of
the arch filament system (AFS) cospatial to the EFR. Jet activity is also found
at chromospheric and coronal levels, near the AFS and the observed brightness
enhancement sites. The analysis of the IRIS line profiles reveals the heating
of dense plasma in the low solar atmosphere and the driving of bi-directional
high-velocity flows with speeds up to 100 km/s at the same locations.
Furthermore, we detect a correlation between the Doppler velocity and line
width of the Si IV 1394 and 1402 \AA{} line profiles in the UV burst pixels and
their skewness. Comparing these findings with previous observations and
numerical models, we suggest evidence of several long-lasting, small-scale
magnetic reconnection episodes between the emerging bipole and the ambient
field. This process leads to the cancellation of a pre-existing photospheric
flux concentration of the plage with the opposite polarity flux patch of the
EFR. [...]Comment: 4 pages, 2 figures, to be published in "Nuovo Cimento C" as
proceeding of the Third Meeting of the Italian Solar and Heliospheric
Communit
Orchestrating Forest Policy in Italy: Mission Impossible?
In the Italian political and economic agenda the forest sector occupies a marginal role. The forest sector in Italy is characterized by a high institutional fragmentation and centralized decision-making processes dominated by Public Forest Administrations. Public participation in forest policy processes has been implemented since the 1990s at national, regional and local levels in several cases. However, today no significant changes have been observed in the overall governance of the forest sector and stakeholders' involvement in Italian forest policy decision-making is still rather limited. The aims of this paper are to describe the state of forest-related participatory processes in Italy at various levels (national, regional and local) and identify which factors and actors hinder or support the establishment and implementation of participatory forest-related processes in the country. The forest-related participatory processes are analyzed adopting a qualitative-based approach and interpreting interactive, complex and non-linear participatory processes through the lens of panarchy theory
A cluster theory for a Janus fluid
Recent Monte Carlo simulations on the Kern and Frenkel model of a Janus fluid
have revealed that in the vapour phase there is the formation of preferred
clusters made up of a well-defined number of particles: the micelles and the
vesicles. A cluster theory is developed to approximate the exact clustering
properties stemming from the simulations. It is shown that the theory is able
to reproduce the micellisation phenomenon.Comment: 27 pages, 8 figures, 6 table
Height dependence of the penumbral fine-scale structure in the inner solar atmosphere
We studied the physical parameters of the penumbra in a large and
fully-developed sunspot, one of the largest over the last two solar cycles, by
using full-Stokes measurements taken at the photospheric Fe I 617.3 nm and
chromospheric Ca II 854.2 nm lines with the Interferometric Bidimensional
Spectrometer. Inverting measurements with the NICOLE code, we obtained the
three-dimensional structure of the magnetic field in the penumbra from the
bottom of the photosphere up to the middle chromosphere. We analyzed the
azimuthal and vertical gradient of the magnetic field strength and inclination.
Our results provide new insights on the properties of the penumbral magnetic
fields in the chromosphere at atmospheric heights unexplored in previous
studies. We found signatures of the small-scale spine and intra-spine structure
of both the magnetic field strength and inclination at all investigated
atmospheric heights. In particular, we report typical peak-to-peak variations
of the field strength and inclination of G and , respectively, in the photosphere, and of G and
in the chromosphere. Besides, we estimated the vertical
gradient of the magnetic field strength in the studied penumbra: we find a
value of G km between the photosphere and the middle
chromosphere. Interestingly, the photospheric magnetic field gradient changes
sign from negative in the inner to positive in the outer penumbra.Comment: 14 page, 9 figures, accepted for Ap
Thermodynamic properties of a classical d-dimensional spin-S Heisenberg ferromagnet with long-range interactions via the spectral density method
The thermodynamic properties of a classical d-dimensional spin-S Heisenberg
ferromagnet, with long-range interactions decaying as and in the
presence of an external magnetic field, is investigated by means of the
spectral density method in the framework of classical statistical mechanics. We
find that long-range order exists at finite temperature for with
and for with , consistently with known theorems. Besides,
the related critical temperature is determined and a study of the critical
properties is performed.Comment: 27 pages, 2 figures, Submitted to Physica
Nonclassical Light in Interferometric Measurements
It is shown that the even and odd coherent light and other nonclassical
states of light like superposition of coherent states with different phases may
replace the squeezed light in interferometric gravitational wave detector to
increase its sensitivity. (Contribution to the Second Workshop on Harmonic
Oscillator, Cocoyoc, Mexico, March 1994)Comment: 8 pages,LATEX,preprint of Naples University,
INFN-NA-IV-94/30,DSF-T-94/3
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