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&

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

The Gaia-ESO Survey: Detailed Abundances in the Metal-poor Globular Cluster NGC 4372

We present the abundance analysis for a sample of 7 red giant branch stars in the metal-poor globular cluster NGC 4372 based on UVES spectra acquired as part of the Gaia-ESO Survey. This is the first extensive study of this cluster from high resolution spectroscopy. We derive abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, Fe, Cr, Ni, Y, Ba, and La. We find a metallicity of [Fe/H] = -2.19 $\pm$ 0.03 and find no evidence for a metallicity spread. This metallicity makes NGC 4372 one of the most metal-poor galactic globular clusters. We also find an {\alpha}-enhancement typical of halo globular clusters at this metallicity. Significant spreads are observed in the abundances of light elements. In particular we find a Na-O anti-correlation. Abundances of O are relatively high compared with other globular clusters. This could indicate that NGC 4372 was formed in an environment with high O for its metallicity. A Mg-Al spread is also present which spans a range of more than 0.5 dex in Al abundances. Na is correlated with Al and Mg abundances at a lower significance level. This pattern suggests that the Mg-Al burning cycle is active. This behavior can also be seen in giant stars of other massive, metal-poor clusters. A relation between light and heavy s-process elements has been identified.Comment: 14 pages, 14 figures, accepted for publication in A&