511 research outputs found
Precision luminosity measurement at ILC
In these proceedings a novel approach to deal with the beam-induced effects
in luminosity measurement is presented. Based on the relativistic kinematics of
the collision frame of the Bhabha process, the beam-beam related uncertainties
can be reduced to the permille level independently of a precision with which
the beam parameters are known. Specific event selection combined with the
corrective methods we introduce, leads to the systematic uncertainty from the
beam-induced effects to be at a few permille level in the peak region above the
80% of the nominal centre-of-mass energies at ILC.Comment: Talk presented on behalf of the FCAL Collaboration at the
International Workshop on Future Linear Colliders (LCWS13) Tokyo, Japan,
11-15 November 201
Beryllium abundances and the formation of the halo and the thick disk
The single stable isotope of beryllium is a pure product of cosmic-ray
spallation in the ISM. Assuming that the cosmic-rays are globally transported
across the Galaxy, the beryllium production should be a widespread process and
its abundance should be roughly homogeneous in the early-Galaxy at a given
time. Thus, it could be useful as a tracer of time. In an investigation of the
use of Be as a cosmochronometer and of its evolution in the Galaxy, we found
evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate
into two components. One is consistent with predictions of evolutionary models
while the other is chemically indistinguishable from the thick-disk stars. This
is interpreted as a difference in the star formation history of the two
components and suggests that the local halo is not a single uniform population
where a clear age-metallicity relation can be defined. We also found evidence
that the star formation rate was lower in the outer regions of the thick disk,
pointing towards an inside-out formation.Comment: 6 pages, 5 figures, To appear in the Proceedings of IAU Symp. 268 -
Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds
A view of the Galactic halo using beryllium as a time scale
Beryllium stellar abundances were suggested to be a good tracer of time in
the early Galaxy. In an investigation of its use as a cosmochronometer, using a
large sample of local halo and thick-disk dwarfs, evidence was found that in a
log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components.
One is consistent with predictions of evolutionary models while the other is
chemically indistinguishable from the thick-disk stars. This is interpreted as
a difference in the star formation history of the two components and suggests
that the local halo is not a single uniform population where a clear
age-metallicity relation can be defined.Comment: To appear in Proceedings of the International Astronomical Union, IAU
Symposium, Volume 265, Chemical abundances in the Universe: connecting first
stars to planets, K. Cunha, M. Spite and B. Barbuy, eds. 2 Pages, 2 figure
Luminosity measurement at ILC
In this paper we describe a method of luminosity measurement at the future
linear collider ILC that estimates and corrects for the impact of the dominant
sources of systematic uncertainty originating from the beam-induced effects and
the background from physics processes. Based on the relativistic kinematics of
the collision frame of the Bhabha process, the beam-beam related uncertainty is
reduced to a permille independently of the precision with which the beam
parameters are known. With the specific event selection, different from the
isolation cuts based on topology of the signal used at LEP, combined with the
corrective methods we introduce, the overall systematic uncertainty in the peak
region above 80% of the nominal center-of-mass energy meets the physics
requirements to be at the few permille level at all ILC energies.Comment: Accepted for publication in JINST (submission JINST_016P_0413
Mixing at young ages: Beryllium abundances in cool main-sequence stars of the open clusters IC 2391 and IC 2602
The determination of lithium abundances in stars of young clusters have shown
that they deplete Li by different degrees during their pre-main sequence phase.
Beryllium abundances are complementary to the lithium ones, and can help
tracing the mixing processes in the stellar interiors. Our aim is to derive
beryllium abundances in a sample of G- and K-type stars of two young pre-main
sequence open clusters, IC 2391 and IC 2602. The Be abundances are used to
investigate the mixing of internal material in these stars. The reliability of
the Be lines as abundance indicators in low-temperatures is also investigated
in detail. We derived Be abundances from high-resolution, high signal-to-noise
UVES/VLT spectra using spectrum synthesis and model atmospheres. Atmospheric
parameters and other elemental abundances are adopted from a previous work. The
sample stars have masses in the range between 0.80 < M/Msun < 1.20. They have
been shown to differ in lithium abundance by about 0.60 dex, with lower A(Li)
in cooler and lower mass stars. Here, we find that all the stars have the same
Be abundance within the uncertainties. These observations show that the Be
abundance is not affected by the mixing events in the pre-main sequence, in
this mass range, in agreement with the expectation of evolutionary models. A
comparison with Be abundances in older clusters shows that, contrary to the
models, cool stars deplete Be during their main-sequence lifetime, confirming
what has been previously suggested in the literature.Comment: To appear in A&A, 12 pages, 12 figure
Beryllium abundances and star formation in the halo and in the thick disk
[abridged] Beryllium is a pure product of cosmic ray spallation. This implies
a relatively simple evolution in time of the beryllium abundance and suggests
its use as a time-like observable. We study the evolution of Be in the early
Galaxy and its dependence on kinematic and orbital parameters. We investigate
the formation of the halo and the thick disk of the Galaxy and the use of Be as
a cosmochronometer. Beryllium abundances are determined from high resolution,
high signal to noise UVES spectra with spectrum synthesis in the largest sample
of halo and thick disk stars analyzed to date. We present our observational
results in various diagrams. 1) In a log(Be/H) vs [Fe/H] diagram we find a
marginal statistical detection of a real scatter, above what expected from
measurement errors, with a larger scatter among halo stars. The detection of
the scatter is further supported by the existence of pairs of stars with
identical atmospheric parameters and different Be abundances. 2) In an
log(Be/H) vs [alpha/Fe] diagram, the halo stars separate into two components;
one is consistent with predictions of evolutionary models, while the other has
too high alpha and Be abundances and is chemically indistinguishable from thick
disk stars. This suggests that the halo is not a single uniform population
where a clear age-metallicity relation can be defined. 3) In diagrams of Rmin
vs [alpha/Fe] and log(Be/H) the thick disk stars show a possible decrease of
[alpha/Fe] with Rmin, whereas no dependence of Be with Rmin is seen. This
anticorrelation suggests that the star formation rate was lower in the outer
regions of the thick disc, pointing towards an inside-out formation. The lack
of correlation for Be indicates that it is insensitive to the local conditions
of star formation.Comment: 34 pages, A&A accepted, revised versio
On the Accented/Unaccented Distinction in Western Basque and the Typology of Accentual Systems
Proceedings of the Twenty-Sixth Annual Meeting of the Berkeley Linguistics Society: General Session and Parasession on Aspect (2000
The Gaia-ESO Survey: the selection function of the Milky Way field stars
The Gaia-ESO Survey was designed to target all major Galactic components
(i.e., bulge, thin and thick discs, halo and clusters), with the goal of
constraining the chemical and dynamical evolution of the Milky Way. This paper
presents the methodology and considerations that drive the selection of the
targeted, allocated and successfully observed Milky Way field stars. The
detailed understanding of the survey construction, specifically the influence
of target selection criteria on observed Milky Way field stars is required in
order to analyse and interpret the survey data correctly. We present the target
selection process for the Milky Way field stars observed with VLT/FLAMES and
provide the weights that characterise the survey target selection. The weights
can be used to account for the selection effects in the Gaia-ESO Survey data
for scientific studies. We provide a couple of simple examples to highlight the
necessity of including such information in studies of the stellar populations
in the Milky Way.Comment: 18 pages, 19 figures, Accepted for publication in MNRAS (April 25,
2016
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