12,145 research outputs found
Chemical composition of stellar populations in Omega Centauri
We derive abundances of Fe, Na, O, and s-elements from GIRAFFE@VLT spectra
for more than 200 red giant stars in the Milky Way satellite Omega Centauri.
Our preliminary results are that: (i) we confirm that Omega Centauri exibiths
large star-to-star metallicity variations ( 1.4 dex); (ii) the
metallicity distribution reveals the presence of at least five stellar
populations with different [Fe/H]; (iii) a clear Na-O anticorrelation is
clearly observed for the metal-poor and metal-intermediate populations while
apparently the anticorrelation disappears for the most metal-rich populations.
Interestingly the Na level grows with iron.Comment: 2 pages, 2 figures. To appear in the proceedings of IAU Symp. 268
"Light elements in the Universe" (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds., Cambridge Univ. Press
Inhomogeneous mechanical losses in micro-oscillators with high reflectivity coating
We characterize the mechanical quality factor of micro-oscillators covered by
a highly reflective coating. We test an approach to the reduction of mechanical
losses, that consists in limiting the size of the coated area to reduce the
strain and the consequent energy loss in this highly dissipative component.
Moreover, a mechanical isolation stage is incorporated in the device. The
results are discussed on the basis of an analysis of homogeneous and
non-homogeneous losses in the device and validated by a set of Finite-Element
models. The contributions of thermoelastic dissipation and coating losses are
separated and the measured quality factors are found in agreement with the
calculated values, while the absence of unmodeled losses confirms that the
isolation element integrated in the device efficiently uncouples the dynamics
of the mirror from the support system. Also the resonant frequencies evaluated
by Finite-Element models are in good agreement with the experimental data, and
allow the estimation of the Young modulus of the coating. The models that we
have developed and validated are important for the design of oscillating
micro-mirrors with high quality factor and, consequently, low thermal noise.
Such devices are useful in general for high sensitivity sensors, and in
particular for experiments of quantum opto-mechanics
Detection of weak stochastic force in a parametrically stabilized micro opto-mechanical system
Measuring a weak force is an important task for micro-mechanical systems,
both when using devices as sensitive detectors and, particularly, in
experiments of quantum mechanics. The optimal strategy for resolving a weak
stochastic signal force on a huge background (typically given by thermal noise)
is a crucial and debated topic, and the stability of the mechanical resonance
is a further, related critical issue. We introduce and analyze the parametric
control of the optical spring, that allows to stabilize the resonance and
provides a phase reference for the oscillator motion, yet conserving a free
evolution in one quadrature of the phase space. We also study quantitatively
the characteristics of our micro opto-mechanical system as detector of
stochastic force for short measurement times (for quick, high resolution
monitoring) as well as for the longer term observations that optimize the
sensitivity. We compare a simple, naive strategy based on the evaluation of the
variance of the displacement (that is a widely used technique) with an optimal
Wiener-Kolmogorov data analysis. We show that, thanks to the parametric
stabilization of the effective susceptibility, we can more efficiently
implement Wiener filtering, and we investigate how this strategy improves the
performance of our system. We finally demonstrate the possibility to resolve
stochastic force variations well below 1% of the thermal noise
A unique model for the variety of multiple populations formation(s) in globular clusters: a temporal sequence
We explain the multiple populations recently found in the 'prototype'
Globular Cluster (GC) NGC 2808 in the framework of the asymptotic giant branch
(AGB) scenario. The chemistry of the five -or more- populations is
approximately consistent with a sequence of star formation events, starting
after the supernovae type II epoch, lasting approximately until the time when
the third dredge up affects the AGB evolution (age ~90-120Myr), and ending when
the type Ia supernovae begin exploding in the cluster, eventually clearing it
from the gas. The formation of the different populations requires episodes of
star formation in AGB gas diluted with different amounts of pristine gas. In
the nitrogen-rich, helium-normal population identified in NGC 2808 by the UV
Legacy Survey of GCs, the nitrogen increase is due to the third dredge up in
the smallest mass AGB ejecta involved in the star formation of this population.
The possibly-iron-rich small population in NGC 2808 may be a result of
contamination by a single type Ia supernova. The NGC 2808 case is used to build
a general framework to understand the variety of 'second generation' stars
observed in GCs. Cluster-to-cluster variations are ascribed to differences in
the effects of the many processes and gas sources which may be involved in the
formation of the second generation. We discuss an evolutionary scheme, based on
pollution by delayed type II supernovae, which accounts for the properties of
s-Fe-anomalous clusters.Comment: 20 pages, 7 figures, in press on MNRA
An ultra-low dissipation micro-oscillator for quantum opto-mechanics
Generating non-classical states of light by opto-mechanical coupling depends
critically on the mechanical and optical properties of micro-oscillators and on
the minimization of thermal noise. We present an oscillating micro-mirror with
a mechanical quality factor Q = 2.6x10^6 at cryogenic temperature and a Finesse
of 65000, obtained thanks to an innovative approach to the design and the
control of mechanical dissipation. Already at 4 K with an input laser power of
2 mW, the radiation-pressure quantum fluctuations become the main noise source,
overcoming thermal noise. This feature makes our devices particularly suitable
for the production of pondero-motive squeezing.Comment: 21 pages including Supplementary Informatio
Four stellar populations and extreme helium variation in the massive outer-halo globular cluster NGC 2419
Recent work revealed that both the helium variation within globular clusters
(GCs) and the relative numbers of first and second-generation stars (1G, 2G)
depend on the mass of the host cluster. Precise determination of the internal
helium variations and of the fraction of 1G stars are crucial constraints to
the formation scenarios of multiple populations (MPs). We exploit multi-band
Hubble Space Telescope photometry to investigate MPs in NGC 2419, which is one
of the most-massive and distant GCs of the Galaxy, almost isolated from its
tidal influence. We find that the 1G hosts the ~37% of the analyzed stars, and
identified three populations of 2G stars, namely 2GA, 2GB, and 2GC, which
comprise the ~20%, ~31% and ~12% of stars, respectively. We compare the
observed colors of these four populations with the colors derived from
appropriate synthetic spectra to infer the relative helium abundances. We find
that 2GA, 2GB, and 2GC stars are enhanced in helium mass fraction by deltaY
~0.01, 0.06, and 0.19 with respectto 1G stars that have primordial helium
(Y=0.246). The high He enrichment of 2GC stars is hardly reconcilable with most
of the current scenarios for MPs. Furthermore, the relatively larger fraction
of 1G stars (~37%) compared to other massive GCs is noticeable. By exploiting
literature results, we find that the fractions of 1G stars of GCs with large
perigalactic distance are typically higher than in the other GCs with similar
masses. This suggests that NGC 2419, similarly to other distant GCs, lost a
lower fraction of 1G stars.Comment: 10 pages, 8 figures, submitted to MNRAS January 22n
Extended main sequence turnoff as a common feature of Milky Way open clusters
We present photometric analysis of twelve Galactic open clusters and show
that the same multiple-population phenomenon observed in Magellanic Clouds
(MCs) is present in nearby open clusters. Nearly all the clusters younger than
2.5 Gyr of both MCs exhibit extended main-sequence turnoffs (eMSTOs) and
all the cluster younger than 700 Myr show broadened/split main sequences
(MSs). High-resolution spectroscopy has revealed that these clusters host stars
with a large spread in the observed projected rotations. In addition to
rotation, internal age variation is indicated as a possible responsible for the
eMSTOs, making these systems the possible young counterparts of globular
clusters with multiple populations. Recent work has shown that the
eMSTO+broadened MSs are not a peculiarity of MCs clusters. Similar photometric
features have been discovered in a few Galactic open clusters, challenging the
idea that the color-magnitude diagrams (CMDs) of these systems are similar to
single isochrones and opening new windows to explore the eMSTO phenomenon. We
exploit photometry+proper motions from Gaia DR2 to investigate the CMDs of open
clusters younger than 1.5 Gyr. Our analysis suggests that: (i) twelve
open clusters show eMSTOs and/or broadened MSs, that cannot be due neither to
field contamination, nor binaries; (ii) split/broadened MSs are observed in
clusters younger than 700 Myr, while older objects display only an eMSTO,
similarly to MCs clusters; (iii) the eMSTO, if interpreted as a pure age
spread, increases with age, following the relation observed in MCs clusters and
demonstrating that rotation is the responsible for this phenomenon.Comment: 17 pages, 42 figures, 1 table, accepted for publication in ApJ
(31/10/2018
Dynamical two-mode squeezing of thermal fluctuations in a cavity opto-mechanical system
We report the experimental observation of two-mode squeezing in the
oscillation quadratures of a thermal micro-oscillator. This effect is obtained
by parametric modulation of the optical spring in a cavity opto-mechanical
system. In addition to stationary variance measurements, we describe the
dynamic behavior in the regime of pulsed parametric excitation, showing
enhanced squeezing effect surpassing the stationary 3dB limit. While the
present experiment is in the classical regime, our technique can be exploited
to produce entangled, macroscopic quantum opto-mechanical modes
Fertilizer Nitrogen and Morphogenetic Response in Avena Sativa and Lolium Multiflorum
A field experiment was carried out at the EEA Balcarce, INTA, Argentina (37° 45’LS, 58° 18’LW) to determine whether Leaf Appearance Rate (LAR) was affected by N fertilization in Italian ryegrass (Lolium multiflorum Lam.) and forage oats (Avena sativa). N treatments (0, 50, 100, 150, 200 and 250 kg N ha-1) were applied in winter 1995, after a defoliation. Subsequently, number of leaves per tiller was determined on 45 labelled tillers in each treatment twice a week. LAR was calculated as the slope of the linear regression of number of leaves on thermal time (air temperature, base 0 °C). Leaf appearance was more rapid with N fertilization in ryegrass, but was not in oats. In situations in which N applied did not affect LAR similar phyllochrons of 112 (± 4,6) and 113 (± 3,6) GDD leaf1 were found for Italian ryegrass and oats, respectively
Frequency noise cancellation in optomechanical systems for ponderomotive squeezing
Ponderomotive squeezing of the output light of an optical cavity has been
recently observed in the MHz range in two different cavity optomechanical
devices. Quadrature squeezing becomes particularly useful at lower spectral
frequencies, for example in gravitational wave interferometers, despite being
more sensitive to excess phase and frequency noise. Here we show a
phase/frequency noise cancellation mechanism due to destructive interference
which can facilitate the production of ponderomotive squeezing in the kHz range
and we demonstrate it experimentally in an optomechanical system formed by a
Fabry-P\'{e}rot cavity with a micro-mechanical mirror.Comment: 11 pages, 9 figures. Physical explanation expanded. Modified figure
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