100 research outputs found
A Sterescopic System to Measure Water Waves in Laboratories
A new system for estimating the synthetic parameters of sea states during physical investigations has been implemented. The technique proposed herein is based on stereographic analysis of digital images acquired with optical sensors. A series of ad hoc floating markers has been made and properly moored to the bottom of a large wave tank to estimate the synthetic parameters of generated waves. The implemented acquisition system and the proposed algorithm provide automatic recognition of all markers by a pair of optical sensors that synchronously captures their instantaneous location and tracks their movements over time. After transformation from the image to the real-world coordinates, water surface elevation time series have been obtained. Several experimental tests have been carried out to assess the feasibility and reliability of the proposed approach. The estimated wave synthetic parameters have been then compared with those obtained by employing standard resistive probes. The deviation were found to be equal to ~6% for the significant wave height and 1% for peak, mean, and significant wave periods
22Ne and 23Na ejecta from intermediate-mass stars: The impact of the new LUNA rate for 22Ne(p,gamma)23Na
We investigate the impact of the new LUNA rate for the nuclear reaction NeNa on the chemical ejecta of intermediate-mass stars, with particular focus on the thermally-pulsing asymptotic giant branch (TP-AGB) stars that experience hot-bottom burning. To this aim we use the PARSEC and COLIBRI codes to compute the complete evolution, from the pre-main sequence up to the termination of the TP-AGB phase, of a set of stellar models with initial masses in the range , and metallicities , , and . We find that the new LUNA measures have much reduced the nuclear uncertainties of the Ne and Na AGB ejecta, which drop from factors of to only a factor of few for the lowest metallicity models. Relying on the most recent estimations for the destruction rate of Na, the uncertainties that still affect the Ne and Na AGB ejecta are mainly dominated by evolutionary aspects (efficiency of mass-loss, third dredge-up, convection). Finally, we discuss how the LUNA results impact on the hypothesis that invokes massive AGB stars as the main agents of the observed O-Na anti-correlation in Galactic globular clusters. We derive quantitative indications on the efficiencies of key physical processes (mass loss, third dredge-up, sodium destruction) in order to simultaneously reproduce both the Na-rich, O-poor extreme of the anti-correlation, and the observational constraints on the CNO abundance. Results for the corresponding chemical ejecta are made publicly available
The impact of the revised 17 O(p, \u3b1)14 N reaction rate on 17 O stellar abundances and yields
Context. Material processed by the CNO cycle in stellar interiors is enriched in 17O. When mixing processes from the stellar surface
reach these layers, as occurs when stars become red giants and undergo the first dredge up, the abundance of 17O increases. Such an
occurrence explains the drop of the 16O/17O observed in RGB stars with mass larger than solar mass 1:5M solar mass. As a consequence, the interstellar
medium is continuously polluted by the wind of evolved stars enriched in 17O .
Aims. Recently, the Laboratory for Underground Nuclear Astrophysics (LUNA) collaboration released an improved rate of the
17O(p; a)14N reaction. In this paper we discuss the impact that the revised rate has on the 16O/17O ratio at the stellar surface and
on 17O stellar yields.
Methods.We computed stellar models of initial mass between 1 and 20M solar mass and compared the results obtained by adopting the revised
rate of the 17O(p; a)14N to those obtained using previous rates.
Results. The post-first dredge up 16O/17O ratios are about 20% larger than previously obtained. Negligible variations are found in the
case of the second and the third dredge up. In spite of the larger 17O(p; a)14N rate, we confirm previous claims that an extra-mixing
process on the red giant branch, commonly invoked to explain the low carbon isotopic ratio observed in bright low-mass giant stars,
marginally affects the 16O/17O ratio. Possible effects on AGB extra-mixing episodes are also discussed. As a whole, a substantial
reduction of 17O stellar yields is found. In particular, the net yield of stars with mass ranging between 2 and 20 solar mass is 15 to 40%
smaller than previously estimated.
Conclusions. The revision of the 17O(p; a)14N rate has a major impact on the interpretation of the 16O/17O observed in evolved
giants, in stardust grains and on the 17O stellar yields
22Ne and 23Na ejecta from intermediate-mass stars: the impact of the new LUNA rate for 22Ne(p, \u3b3)23Na
We investigate the impact of the new LUNA rate for the nuclear reaction 22Ne(p, \u3b3)23Na on the chemical ejecta of intermediate-mass stars, with particular focus on the thermally pulsing asymptotic giant branch (TP-AGB) stars that experience hot-bottom burning. To this aim, we use the PARSEC and COLIBRI codes to compute the complete evolution, from the premain sequence up to the termination of the TP-AGB phase, of a set of stellar models with initial masses in the range 3.0-6.0M 99 and metallicities Zi = 0.0005, 0.006 and 0.014. We find that the new LUNA measures have much reduced the nuclear uncertainties of the 22Ne and 23Na AGB ejecta that drop from factors of 4310 to only a factor of few for the lowest metallicity models. Relying on the most recent estimations for the destruction rate of 23Na, the uncertainties that still affect the 22Ne and 23Na AGB ejecta are mainly dominated by the evolutionary aspects (efficiency of mass-loss, third dredge-up, convection). Finally, we discuss how the LUNA results impact on the hypothesis that invokes massive AGB stars as the main agents of the observed O-Na anticorrelation in Galactic globular clusters. We derive quantitative indications on the efficiencies of key physical processes (mass-loss, third dredgeup, sodium destruction) in order to simultaneously reproduce both the Na-rich, O-poor extreme of the anticorrelation and the observational constraints on the CNO abundance. Results for the corresponding chemical ejecta are made publicly available. \ua9 2016 The Authors
First direct limit on the 334 keV resonance strength in the Ne({\alpha},{\gamma})Mg reaction
In stars, the fusion of Ne and He may produce either Mg,
with the emission of a neutron, or Mg and a ray. At high
temperature, the () channel dominates, while at low temperature, it
is energetically hampered. The rate of its competitor, the
Ne(,)Mg reaction, and, hence, the minimum
temperature for the () dominance, are controlled by many nuclear
resonances. The strengths of these resonances have hitherto been studied only
indirectly. The present work aims to directly measure the total strength of the
resonance at _{r}334keV (corresponding to
_{x}10949keV in Mg). The data reported here have been
obtained using high intensity He beam from the INFN LUNA 400 kV
underground accelerator, a windowless, recirculating, 99.9% isotopically
enriched Ne gas target, and a 4 bismuth germanate summing
-ray detector. The ultra-low background rate of less than 0.5
counts/day was determined using 67 days of no-beam data and 7 days of
He beam on an inert argon target. The new high-sensitivity setup
allowed to determine the first direct upper limit of 4.010
eV (at 90% confidence level) for the resonance strength. Finally, the
sensitivity of this setup paves the way to study further
Ne(,)Mg resonances at higher energy.Comment: Submitted to Eur. Phys. J.
Underground experimental study finds no evidence of low-energy resonance in the 6Li(p,Îł)7Be reaction
The astrophysical Li6(p,\u3b3)Be7 reaction occurs during Big Bang nucleosynthesis and the pre-main sequence and main sequence phases of stellar evolution. The low-energy trend of its cross section remains uncertain, since different measurements have provided conflicting results. A recent experiment reported a resonancelike structure at center-of-mass energy 195 keV, associated to a positive-parity state of Be7. The existence of such resonance is still a matter of debate. We report a new measurement of the Li6(p,\u3b3)Be7 cross section performed at the Laboratory for Underground Nuclear Astrophysics, covering the center-of-mass energy range E=60-350 keV. Our results rule out the existence of low-energy resonances. The astrophysical S-factor varies smoothly with energy, in agreement with theoretical models
Characterization of the LUNA neutron detector array for the measurement of the 13C(α,n)16O reaction
We introduce the LUNA neutron detector array developed for the investigation of the 13C(\u3b1, n)16O reaction towards its astrophysical s-process Gamow peak in the low-background environment of the Laboratori Nazionali del Gran Sasso (LNGS). Eighteen 3He counters are arranged in two different configurations (in a vertical and a horizontal orientation) to optimize neutron detection efficiency, target handling and target cooling over the investigated energy range E\u3b1,lab=300 12400 keV (En=2.2 122.6MeV in emitted neutron energy). As a result of the deep underground location, the passive shielding of the setup and active background suppression using pulse shape discrimination, we reached a total background rate of 1.23\ub10.12 counts/hour. This resulted in an improvement of two orders of magnitude over the state of the art allowing a direct measurement of the 13C(\u3b1, n)16O cross-section down to E\u3b1,lab=300 keV. The absolute neutron detection efficiency of the setup was determined using the 51V(p,n)51Cr reaction and an AmBe radioactive source, and completed with a Geant4 simulation. We determined a (34 \ub1 3)% and (38 \ub1 3)% detection efficiency for the vertical and horizontal configurations, respectively, for En=2.4MeV neutrons
Low-energy resonances in the 18O (p,΄) 19F reaction
Background: Shell hydrogen burning during the asymptotic giant branch (AGB) phase through the oxygen
isotopes has been indicated as a key process that is needed to understand the observed 18O/16O relative abundance
in presolar grains and in stellar atmospheres. This ratio is strongly influenced by the relative strengths of the
reactions 18O(p,\u3b1) 15N and 18O(p,\u3b3 ) 19F in low-mass AGB stars. While the former channel has been the focus
of a large number of measurements, the (p,\u3b3 ) reaction path has only recently received some attention and its
stellar reaction rate over a wide temperature range rests on only one measurement.
Purpose: Our aim is the direct measurement of states in 19F as populated through the reaction 18O(p,\u3b3 ) 19F
to better determine their influence on the astrophysical reaction rate, and more generally to improve the
understanding of the nuclear structure of 19F.
Method: Branchings and resonance strengths were measured in the proton energy range Elab
p = 150\u2013400 keV,
using a high-purity germanium detector inside a massive lead shield. The measurement took place in the ultralow-
background environment of the Laboratory for Underground Nuclear Astrophysics (LUNA) experiment at
the Gran Sasso National Laboratory, leading to a highly increased sensitivity.
Results: The uncertainty of the \u3b3 branchings and strengths was improved for all four resonances in the studied
energy range; many new transitions were observed in the case of the 334 keV resonance, and individual \u3b3 decays
of the 215 keV resonance were measured for the first time. In addition a number of transitions to intermediate
states that decay through \u3b1 emission were identified. The strengths of the observed resonances are generally in
agreement with literature values.
Conclusions: Our measurements substantially confirm previous determinations of the relevant resonance
strengths. Therefore the 18O(p,\u3b3 ) 19F reaction rate does not change with respect to the reaction rate reported
in the compilations commonly adopted in the extant computations of red-giant branch and AGB stellar models.
Nevertheless, our measurements definitely exclude a nonstandard scenario for the fluorine nucleosynthesis and
a nuclear physics solution for the 18O depletion observed in Group 2 oxygen-rich stardust grains
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