Context. The Cepheid period-luminosity (PL) relation is unquestionably one of the most powerful tools at our disposal for determining the extragalactic distance scale. While significant progress has been made in the past few years towards its understanding and characterization both on the observational and theoretical sides, the debate on the influence that chemical composition may have on the PL relation is still unsettled. Aims. With the aim to assess the influence of the stellar iron content on the PL relation in the V and K bands, we have related the V-band and the K-band residuals from the standard PL relations of Freedman et al. (2001, ApJ, 553, 47) and Persson et al. (2004, AJ, 128, 2239, respectively, to [Fe/H]. Methods. We used direct measurements of the iron abundances of 68 Galactic and Magellanic Cepheids from FEROS and UVES high-resolution and high signal-to-noise spectra. Results. We find a mean iron abundance ([Fe/H]) about solar (σ = 0.10) for our Galactic sample (32 stars), ∼−0.33 dex (σ = 0.13) for the Large Magellanic Cloud (LMC) sample (22 stars) and ∼−0.75 dex (σ = 0.08) for the Small Magellanic Cloud (SMC) sample (14 stars). Our abundance measurements of the Magellanic Cepheids double the number of stars studied up to now at high resolution. The metallicity affects the V-band Cepheid PL relation and metal-rich Cepheids appear to be systematically fainter than metal-poor ones. These findings depend neither on the adopted distance scale for Galactic Cepheids nor on the adopted LMC distance modulus. Current data do not allow us to reach a firm conclusion concerning the metallicity dependence of the K-band PL relation. The new Galactic distances indicate a small effect, whereas the old ones support a marginal effect. Conclusions. Recent robust estimates of the LMC distance and current results indicate that the Cepheid PL relation is not Universal.

We present homogeneous and accurate iron abundances for 42 Galactic Cepheids based on high resolution (R ∼ 38 000) high signal-to-noise ratio (S /N ≥ 100) optical spectra collected with UVES at VLT (128 spectra). The above abundances were complemented with high-quality iron abundances provided either by our group (86) or available in the literature. We were careful to derive a common metallicity scale and ended up with a sample of 450 Cepheids. We also estimated accurate individual distances for the entire sample by using homogeneous near-infrared photometry and the reddening free period-Wesenheit relations. The new metallicity gradient is linear over a broad range of Galactocentric distances (R G ∼ 5-19 kpc) and agrees quite well with similar estimates available in the literature (-0.060 ± 0.002 dex/kpc). We also uncover evidence that suggests that the residuals of the metallicity gradient are tightly correlated with candidate Cepheid groups (CGs). The candidate CGs have been identified as spatial overdensities of Cepheids located across the thin disk. They account for a significant fraction of the residual fluctuations, and also for the large intrinsic dispersion of the metallicity gradient. We performed a detailed comparison with metallicity gradients based on different tracers: OB stars and open clusters. We found very similar metallicity gradients for ages younger than 3 Gyr, while for older ages we found a shallower slope and an increase in the intrinsic spread. The above findings rely on homogeneous age, metallicity, and distance scales. Finally, by using a large sample of Galactic and Magellanic Cepheids for which accurate iron abundances are available, we found that the dependence of the luminosity amplitude on metallicity is vanishing.

The detailed abundances of 23 chemical elements in nine bright RGB stars in the Carina dwarf spheroidal galaxy are presented based on high resolution spectra gathered at the VLT and Magellan telescopes. A spherical model atmospheres analysis is applied using standard methods (LTE and plane parallel radiative transfer) to spectra ranging from 380 to 680 nm. Stellar parameters are found to be consistent between photometric and spectroscopic analyses, both at moderate and high resolution. The stars in this analysis range in metallicity from −2.9 < [Fe/H] < −1.3, and adopting the ages determined by Lemasle et al. (2012), we are able to examine the chemical evolution of Carina's old and intermediate-aged populations. One of the main results from this work is the evidence for inhomogeneous mixing in Carina; a large dispersion in [Mg/Fe] indicates poor mixing in the old population, an offset in the [α/Fe] ratios between the old and intermediate-aged populations (when examined with previously published results) suggests that the second star formation event occurred in α-enriched gas, and one star, Car-612, seems to have formed in a pocket enhanced in SN Ia/II products. This latter star provides the first direct link between the formation of stars with enhanced SN Ia/II ratios in dwarf galaxies to those found in the outer Galactic halo . Another important result is the potential evidence for SN II driven winds. We show that the very metal-poor stars in Carina have not been enhanced in AGB or SN Ia products, and therefore their very low ratios of [Sr/Ba] suggests the loss of contributions from the early SNe II. Low ratios of [Na/Fe], [Mn/Fe], and [Cr/Fe] in two of these stars support this scenario, with additional evidence from the low [Zn/Fe] upper limit for one star. It is interesting that the chemistry of the metal-poor stars in Carina is not similar to those in the Galaxy, most of the other dSphs, or the UFDs, and suggests that Carina may be at the critical mass where some chemical enrichments are lost through SN II driven winds.

Context. The ages of individual Red Giant Branch stars can range from 1 Gyr old to the age of the Universe, and it is believed that the abundances of most chemical elements in their photospheres remain unchanged with time (those that are not affected by the first dredge-up). This means that they trace the interstellar medium in the galaxy at the time the star formed, and hence the chemical enrichment history of the galaxy. Aims. Colour-Magnitude Diagram analysis has shown the Carina dwarf spheroidal to have had an unusually episodic star formation history and this is expected to be reflected in the abundances of different chemical elements. Methods. We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode (R≈20000) to measure the abundances of several chemical elements, including Fe, Mg, Ca and Ba, in a sample of 35 individual Red Giant Branch stars in the Carina dwarf spheroidal galaxy. We also combine these abundances with photometry to derive age estimates for these stars. This allows us to determine which of two distinct star formation episodes the stars in our sample belong to, and thus to define the relationship between star formation and chemical enrichment during these two episodes.

We have combined deep photometry in the B, V and I bands from CTIO/MOSAIC of the Sculptor dwarf spheroidal galaxy, going down to the oldest main sequence turn-offs, with spectroscopic metallicity distributions of red giant branch stars. This allows us to obtain the most detailed and complete star formation history to date, as well as an accurate timescale for chemical enrichment. The star formation history shows that Sculptor is dominated by old (>10 Gyr), metal-poor stars, but that younger, more metal-rich populations are also present. Using star formation histories determined at different radii from the centre we show that Sculptor formed stars with an increasing central concentration with time. The old, metal-poor populations are present at all radii, while more metalrich, younger stars are more centrally concentrated. We find that within an elliptical radius of 1 degree, or 1.5 kpc from the centre, a total mass in stars of 7.8 × 106 M was formed, between 14 and 7 Gyr ago, with a peak at 13−14 Gyr ago. We use the detailed star formation history to determine age estimates for individual red giant branch stars with high resolution spectroscopic abundances. Thus, for the first time, we can directly determine detailed timescales for the evolution of individual chemical elements. We find that the trends in alpha-elements match what is expected from an extended, relatively uninterrupted period of star formation continuing for 6−7 Gyr. The knee in the alpha-element distribution occurs at an age of 10.9 ± 1Gyr, suggesting that SNe Ia enrichment began ≈2 ± 1 Gyr after the start of star formation in Sculptor.