Context. A key science goal of the Gaia-ESO survey (GES) at the VLT is to use the kinematics of low-mass stars in young clusters and star forming regions to probe their dynamical histories and how they populate the field as they become unbound. The clustering of low-mass stars around the massive Wolf-Rayet binary system γ 2 Velorum was one of the first GES targets. Aims. We empirically determine the radial velocity precision of GES data, construct a kinematically unbiased sample of cluster members and characterise their dynamical state. Methods. Targets were selected from colour-magnitude diagrams and intermediate resolution spectroscopy was used to derive radial velocities and assess membership from the strength of the Li i 6708 Å line. The radial velocity distribution was analysed using a maximum likelihood technique that accounts for unresolved binaries. Results. The GES radial velocity precision is about 0.25 km s −1 and sufficient to resolve velocity structure in the low-mass population around γ 2 Vel. The structure is well fitted by two kinematic components with roughly equal numbers of stars; the first has an intrinsic dispersion of 0.34 ± 0.16 km s −1 , consistent with virial equilibrium. The second has a broader dispersion of 1.60 ± 0.37 km s −1 and is offset from the first by 2 km s −1 . The first population is older by 1-2 Myr based on a greater level of Li depletion seen among its M-type stars and is probably more centrally concentrated around γ 2 Vel. Conclusions. We consider several formation scenarios, concluding that the two kinematic components are a bound remnant of the original, denser cluster that formed γ 2 Vel, and a dispersed population from the wider Vela OB2 association, of which γ 2 Vel is the most massive member. The apparent youth of γ 2 Vel compared to the older (≥10 Myr) low-mass population surrounding it suggests a scenario in which the massive binary formed in a clustered environment after the formation of the bulk of the low-mass stars.

Aims. Observational studies of the Milky Way bulge are providing increasing evidence of its complex chemo-dynamical patterns and morphology. Our intent is to use the iDR1 Gaia-ESO Survey (GES) data set to provide new constraints on the metallicity and kinematic trends of the Galactic bulge, exploring the viability of the currently proposed formation scenarios. Methods. We analyzed the stellar parameters and radial velocities of ∼1200 stars in five bulge fields wich are located in the region −10 • < l < 7 • and −10 • < b < −4 • . We use VISTA Variables in the Via Lactea (VVV) photometry to verify the internal consistency of the atmospheric parameters recommended by the consortium. As a by-product, we obtained reddening values using a semi-empirical T eff -color calibration. We constructed the metallicity distribution functions and combined them with photometric and radial velocity data to analyze the properties of the stellar populations in the observed fields. Results. From a Gaussian decomposition of the metallicity distribution functions, we unveil a clear bimodality in all fields, with the relative size of components depending of the specific position on the sky. In agreement with some previous studies, we find a mild gradient along the minor axis (−0.05 dex/deg between b = −6 • and b = −10 • ) that arises from the varying proportion of metal-rich and metal-poor components. The number of metal-rich stars fades in favor of the metal-poor stars with increasing b. The K-magnitude distribution of the metal-rich population splits into two peaks for two of the analyzed fields that intersects the near and far branches of the X-shaped bulge structure. In addition, two lateral fields at (l, b) = (7, −9) and (l, b) = (−10, −8) present contrasting characteristics. In the former, the metallicity distribution is dominated by metal-rich stars, while in the latter it presents a mix of a metal-poor population and and a metal-intermediate one, of nearly equal sizes. Finally, we find systematic differences in the velocity dispersion between the metal-rich and the metal-poor components of each field. Conclusions. The iDR1 bulge data show chemo-dynamical distributions that are consistent with varying proportions of stars belonging to (i) a metal-rich boxy/peanut X-shaped component, with bar-like kinematics; and (ii) a metal-poor more extended rotating structure with a higher velocity dispersion that dominates far from the Galactic plane. These first GES data already allow studying the detailed spatial dependence of the Galactic bulge populations, thanks to the analysis of individual fields with relatively high statistics.

Slow neutron captures are responsible for the production of about 50% of elements heavier than iron, mainly, occurring during the asymptotic giant branch phase of low-mass stars (1 M/M ⊙ 3), where the main neutron source is the 13 C(α,n) 16 O reaction. This last is activated from locally-produced 13 C, formed by partial mixing of hydrogen into the He-rich layers. We present here the first attempt at describing a physical mechanism for the formation of the 13 C reservoir, studying the mass circulation induced by magnetic buoyancy and without adding new free parameters to those already involved in stellar modelling.Our approach represents the application, to the stellar layers relevant for s-processing, of recent exact, analytical 2D and 3D models for magneto-hydrodynamic processes at the base of convective envelopes in evolved stars in order to promote downflows of envelope material for mass conservation, during the occurrence of a dredge-up phenomenon. We find that the proton penetration is characterized by small concentrations, but extended over a large fractional mass of the He-layers, thus producing 13 C reservoirs of several 10 −3 M ⊙ .The ensuing 13 C-enriched zone has an almost flat profile, while only a limited production of 14 N occurs. In order to verify the effects of our new findings we show how the abundances of the main s-component nuclei can be accounted for in solar proportions and how our large 13 C-reservoir allows us to solve a few so far unexplained features in the abundance distribution of post-AGB objects.

Aims. The nature of the thick disc and its relation to the thin disc is presently an important subject of debate. In fact, the structural and chemodynamical transition between disc populations can be used as a test of the proposed models of Galactic disc formation and evolution. Methods. We used the atmospheric parameters, [α/Fe] abundances, and radial velocities, which were determined from the Gaia-ESO Survey GIRAFFE spectra of FGK-type stars (first nine months of observations) to provide a chemo-kinematical characterisation of the disc stellar populations. We focussed on a subsample of 1016 stars with high-quality parameters, covering the volume |Z| < 4.5 kpc and R in the range 2-13 kpc. Results. We have identified a thin to thick disc separation in the [α/Fe] vs. [M/H] plane, thanks to the presence of a low-density region in the number density distribution. The thick disc stars seem to lie in progressively thinner layers above the Galactic plane, as metallicity increases and [α/Fe] decreases. In contrast, the thin disc population presents a constant value of the mean distance to the Galactic plane at all metallicities. In addition, our data confirm the already known correlations between V φ and [M/H] for the two discs. For the thick disc sequence, a study of the possible contamination by thin disc stars suggests a gradient up to 64 ± 9 km s −1 dex −1 . The distributions of azimuthal velocity, vertical velocity, and orbital parameters are also analysed for the chemically separated samples. Concerning the gradients with galactocentric radius, we find, for the thin disc, a flat behaviour of the azimuthal velocity, a metallicity gradient equal to −0.058 ± 0.008 dex kpc Conclusions. Our chemo-kinematical analysis suggests a picture where the thick disc seems to have experienced a settling process, during which its rotation increased progressively and, possibly, the azimuthal velocity dispersion decreased. At [M/H] ≈ −0.25 dex and [α/Fe] ≈ 0.1 dex, the mean characteristics of the thick disc in vertical distance to the Galactic plane, rotation, rotational dispersion, and stellar orbits' eccentricity agree with that of the thin disc stars of the same metallicity, suggesting a possible connection between these two populations at a certain epoch of the disc evolution. Finally, the results presented here, based only on the first months of the Gaia ESO Survey observations, confirm how crucial large high-resolution spectroscopic surveys outside the solar neighbourhood are today for our understanding of the Milky Way history.