We here propose a new model-independent technique to overcome the circularity problem affecting the use of gamma-ray bursts (GRBs) as distance indicators through the use of Ep−Eiso correlation. We calibrate the Ep−Eiso correlation and find the GRB distance moduli that can be used to constrain dark energy models. We use observational Hubble data to approximate the cosmic evolution through Bézier parametric curve obtained through the linear combination of Bernstein basis polynomials. In doing so, we build up a new data set consisting of 193 GRB distance moduli. We combine this sample with the supernova JLA data set to test the standard ΛCDM model and its wCDM extension. We place observational constraints on the cosmological parameters through Markov Chain Monte Carlo numerical technique. Moreover, we compare the theoretical scenarios by performing the Akaike and Deviance Information statistical criteria.the 2σ level, while for the wCDM model we obtain $\Omega _m=0.34^{+0.13}_{-0.15}$ and $w=-0.86^{+0.36}_{-0.38}$ at the 2σ level. Our analysis suggests that ΛCDM model is statistically favoured over the wCDM scenario. No evidence for extension of the ΛCDM model is found.

Context. Classical Cepheids are the most popular distance indicators and tracers of young stellar populations. The key advantage is that they are bright and they can be easily identified in Local Group and Local Volume galaxies. Their evolutionary and pulsation properties depend on their chemical abundances. Aims. The main aim of this investigation is to perform a new and accurate abundance analysis of 20 calibrating Galactic Cepheids. We used high spectral resolution (R ~ 40 000–115 000) and high S/N spectra (~400), covering the entire pulsation cycle. Methods. We focused our attention on plausible systematics that would affect the estimate of atmospheric parameters and elemental abundances along the pulsation cycle. We cleaned the line list by using atomic transition parameters based on laboratory measurements and by removing lines that are either blended or that display abundance variations along the pulsation cycle. Results. The spectroscopic approach we developed brings forward small dispersions in the variation of the atmospheric parameters (σ(Teff) ~ 50 K, σ(log g) ~ 0.2 dex, and σ(ξ) ~ 0.2 kms−1) as well as in the abundance of both iron (≲0.05 dex) and α elements (≲0.10 dex) over the entire pulsation cycle. We also provide new and accurate effective temperature templates by splitting the calibrating Cepheids into four different period bins, ranging from short to long periods. For each period bin, we performed an analytical fit with Fourier series providing θ = 5040/Teff as a function of the pulsation phase. Conclusions. The current findings are a good viaticum for tracing the chemical enrichment of the Galactic thin disk by using classical Cepheids as a fundamental stepping stone for further investigations into the more metal-poor regime that is typical of Magellanic Cepheids.

We have used B, V time-series photometry collected with the Large Binocular Telescope to undertake the first study of variable stars in the Milky Way ultra-faint dwarf (UFD) satellites Pisces II and Pegasus III. In Pisces II we have identified an RRab star, one confirmed and a candidate SX Phoenicis star, and a variable with uncertain classification. In Pegasus III we confirmed the variability of two sources: an RRab star and a variable with uncertain classification, similar to the case found in Pisces II. Using the intensity-averaged apparent magnitude of the bona fide RRab star in each galaxy, we estimate distance moduli of (m − M)0 = 21.22 ± 0.14 mag (d = 175 ± 11 kpc) and 21.21 ± 0.23 mag (d = 174 ± 18 kpc) for Pisces II and Pegasus III, respectively. Tests performed to disentangle the actual nature of variables with an uncertain classification led us to conclude that they most likely are bright, long-period, and very metal-poor RRab members of their respective hosts. This may indicate that Pisces II and Pegasus III contain a dominant old stellar population (t > 12 Gyr) with metallicity 〈[Fe/H]〉 − 1.8 dex along with, possibly, a minor, more metal-poor component, as supported by the V, B – V color–magnitude diagrams of the two UFDs and their spectroscopically confirmed members. The metallicity spread that we derived from our data sample is ≳0.4 dex in both systems. Lastly, we built isodensity contour maps that do not reveal any irregular shape, thus making the existence of a physical connection between these UFDs unlikely.