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The trends of chemical abundances and abundance ratios observed in stars of different ages, kinematics, and metallicities bear the imprints of several physical processes which concur, thus shaping the host galaxy properties. By inspecting these trends, we obtain precious information on stellar nucleosynthesis, the stellar mass spectrum, the timescale of structure formation, the efficiency of star formation, as well as any inward or outward flows of gas. In this paper, we analyse recent determinations of carbon-to-iron and carbon-to-oxygen abundance ratios in different environments (the Milky Way and elliptical galaxies), using our latest chemical evolution models that implement up-to-date stellar yields and rely on the tight constraints provided by asteroseismic stellar ages, whenever available. A scenario where most carbon is produced by rotating massive stars, with yields largely dependent on the metallicity of the parent proto-star clouds, allowed us to simultaneously fit the high-quality data available for the local Galactic components (thick and thin discs) and for microlensed dwarf stars in the Galactic bulge, as well as the abundance ratios inferred for massive elliptical galaxies. Nevertheless, more efforts are needed from both observers and theoreticians in order to base these conclusions on firmer ground.

Using the UVBLUE library of synthetic stellar spectra we have computed a set of mid-UV line and continuum spectroscopic indices. We explore their behavior in terms of the leading stellar parameters (T eff , log g). The overall result is that synthetic indices follow the general trends depicted by those computed from empirical databases. Separately we also examine the index sensitivity to changes in chemical composition, an analysis only feasible under a theoretical approach. In this respect, lines indices Fe i 3000, BL 3096 and Mg i 2852 and the continuum index 2828/2921 are the least sensitive features, an important characteristic to be taken into account for the analyses of integrated spectra of stellar systems. We also quantify the effects of instrumental resolution on the indices and find that indices display variations up to 0.1 mag in the resolution interval between 6-10Å of FWHM. We discuss the extent to which synthetic indices are compatible with indices measured in spectra collected by the International Ultraviolet Explorer (IUE). Five line and continuum indices (Fe i 3000, 2110/2570, 2828/2921, S2850, and S2850L) display a remarkable good correlation with observations. The rest of the indices are either underestimated or overestimated, however, two of them, Mg Wide and BL 3096, display only marginal discrepancies. For 11 indices we give the coefficients to convert synthetic indices to the IUE system. This work represents the first attempt to synthesize mid-UV indices from high resolution theoretical spectra and foresees important applications for the study of the ultraviolet morphology of old stellar aggregates.

This paper focuses on carbon that is one of the most abundant elements in the Universe and is of high importance in the field of nucleosynthesis and galactic and stellar evolution. Even nowadays, the origin of carbon and the relative importance of massive and low-to intermediate-mass stars in producing it is still a matter of debate. In this paper we aim at better understanding the origin of carbon by studying the trends of [C/H], [C/Fe],and [C/Mg] versus [Fe/H], and [Mg/H] for 2133 FGK dwarf stars from the fifth Gaia-ESO Survey internal data release (GES iDR5). The availability of accurate parallaxes and proper motions from Gaia DR2 and radial velocities from GES iDR5 allows us to compute Galactic velocities, orbits and absolute magnitudes and, for 1751 stars, ages via a Bayesian approach. Three different selection methodologies have been adopted to discriminate between thin and thick disk stars. In all the cases, the two stellar groups show different abundance ratios, [C/H], [C/Fe], and [C/Mg], and span different age intervals, with the thick disk stars being, on average, older than those in the thin disk. The behaviours of [C/H], [C/Fe], and [C/Mg] versus [Fe/H], [Mg/H], and age all suggest that C is primarily produced in massive stars like Mg. The increase of [C/Mg] for young thin disk Corresponding author: Mariagrazia Franchini Mariagrazia.franchini@inaf.it * Based on observations collected with the FLAMES instrument at VLT/UT2 telescope (Paranal Observatory, ESO, Chile), for the Gaia-ESO Large Public Spectroscopic Survey (188.B-3002, 193.B-0936). Franchini et al.stars indicates a contribution from low-mass stars or the increased C production from massive stars at high metallicities due to the enhanced mass loss. The analysis of the orbital parameters R med and |Z max | support an "inside-out" and "upside-down" formation scenario for the disks of Milky Way.

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