526 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
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
Thermohaline mixing and the photospheric composition of low-mass giant stars
We compute full evolutionary sequences of red giant branch stars close to the
luminosity bump by including state of the art composition transport
prescriptions for the thermohaline mixing regimes. In particular we adopt a
self-consistent double-diffusive convection theory, that allows to handle the
instabilities that arise when thermal and composition gradients compete against
each other, and a very recent empirically motivated and parameter free
asymptotic scaling law for thermohaline composition transport. In agreement
with previous works, we find that during the red giant stage, a thermohaline
instability sets in shortly after the hydrogen burning shell (HBS) encounters
the chemical discontinuity left behind by the first dredge-up. We also find
that the thermohaline unstable region, initially appearing at the exterior wing
of the HBS, is unable to reach the outer convective envelope, with the
consequence that no mixing of elements that produces a non-canonical
modification of the stellar surface abundances occurs. Also in agreement with
previous works, we find that by artificially increasing the mixing efficiency
of thermohaline regions it is possible to connect both unstable regions, thus
affecting the photospheric composition. However, we find that in order to
reproduce the observed abundances of red giant branch stars close to the
luminosity bump, thermohaline mixing efficiency has to be artificially
increased by about 4 orders of magnitude from that predicted by recent 3D
numerical simulations of thermohaline convection close to astrophysical
environments. From this we conclude the chemical abundance anomalies of red
giant stars cannot be explained on the basis of thermohaline mixing alone.Comment: 7 pages, 6 figures, accepted for publication in A&
Ruthenium and hafnium abundances in giant and dwarf barium stars
We present abundances for Ru and Hf, compare them to abundances of other
heavy elements, and discuss the problems found in determining Ru and Hf
abundances with laboratory gf-values in the spectra of barium stars. We
determined Ru and Hf abundances in a sample of giant and dwarf barium stars, by
the spectral synthesis of two RuI (4080.574A and 4757.856A) and two HfII
(4080.437A and 4093.155A) transitions. The stellar spectra were observed with
FEROS/ESO, and the stellar atmospheric parameters lie in the range 4300 <
Teff/K < 6500, -1.2 < [Fe/H] <= 0 and 1.4 <= log g < 4.6. The HfII 4080A and
the RuI 4758A observed transitions result in a unreasonably high solar
abundance, given certain known uncertainties, when fitted with laboratory
gf-values. For these two transitions we determined empirical gf-values by
fitting the observed line profiles of the spectra of the Sun and Arcturus. For
the sample stars, this procedure resulted in a good agreement of Ru and Hf
abundances given by the two available lines. The resulting Ru and Hf abundances
were compared to those of Y, Nd, Sm and Eu. In the solar system Ru, Sm and Eu
are dominated by the r-process and Hf, Nd and Y by the s-process, and all of
these elements are enhanced in barium stars since they lie inside the s-process
path. Ru abundances show large scatter when compared to other heavy elements,
whereas Hf abundances show less scatter and closely follow the abundances of Sm
and Nd, in good agreement with theoretical expectations. We also suggest a
possible, unexpected, correlation of Ru and Sm abundances. The observed
behaviour in abundances is probably due to variations in the 13C pocket
efficiency in AGB stars, and, though masked by high uncertainties, hint at a
more complex scenario than proposed by theory.Comment: 11 pages, 7 figures and 7 tables. accepted to A&
Enhancing oxygen evolution functionality through anodization and nitridation of compositionally complex alloy
Compositionally complex materials (CCMs) have recently attracted great interest in electrocatalytic applications. To date, very few materials were systematically developed and tested due to the highly difficult preparation of high-surface-area CCMs. In this work, a surface of a compositionally complex FeCoNiCuZn alloy (CCA) was nitridated with subsequent anodization leading to morphological and compositional modifications. Notably, the electrochemical surface area and surface roughness as well as the electrocatalytic activity of the anodized material exhibit significant enhancement. Oxygen evolution reaction (OER) activity by the anodized CCN (CCNâAO) proceeds with remarkably small overpotential (233 mV) at 10 mA cmâ2 in 1 M KOH. Experimental characterization indicates that the oxidation state of Co plays a critical role in the FeâCoâNi electrocatalyst. The developed approach and design strategy open up immense prospects in the preparation of a new, affordable, scalable and effective type of complex and high-performance electrocatalytic electrodes with tunable properties
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
HD 11397 and HD 14282 - Two new barium stars?
We have performed a detailed abundance analysis of the content of s-process
elements of two dwarf stars with suspected overabundace of those elements. Such
stars belong to a special kinematic sample of the solar neighborhood, with
peculiar kinematics and different chemical abundances when compared to "normal"
disk stars. We aim to define if those stars can be identified as barium stars,
based on their s-process elements abundances, and their classification, i.e.,
if they share their chemical profile with strong or mild barium stars. We also
intend to shed light on the possible origins of the different kinds of barium
stars. Spectra have been taken by using the FEROS spectrograph at the 1.52m
telescope of ESO, La Silla. Abundances have been derived for 18 elements, by
matching the synthetic profile with the observed spectrum. We have found that
HD 11397 shows a mild enhancement for most of the s-process elements as well as
for some r-process elements. This star seems to share its abundance profile
with the mild Ba-stars. Although showing some slight chemical anomalies for Y,
Sr, Mo, and Pb, HD 14282 depicts a chemical pattern similar to the normal stars
with slight s-process enhancements.Comment: 11 pages, 5 figure
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