Context. The most primitive metal-poor stars are important for studying the conditions of the early galaxy and are also relevant to big bang nucleosynthesis. Aims. Our objective is to find the brightest (V < 14) most metal-poor stars. Methods. Candidates were selected using a new method, which is based on the mismatch between spectral types derived from colors and observed spectral types. They were observed first at low resolution with EFOSC2 at the NTT to obtain an initial set of stellar parameters. The most promising candidate, 2MASS J18082002−5104378 (V = 11.9), was observed at high resolution (R = 50 000) with UVES at the VLT, and a standard abundance analysis was performed. Results. We found that 2MASS J18082002−5104378 is an ultra metal-poor star with stellar parameters T eff = 5440 K, log g = 3.0 dex, v t = 1.5 km s −1 , [Fe/H] = −4.1 dex. The star has [C/Fe] < +0.9 in a 1D analysis, or [C/Fe] +0.5 if 3D effects are considered; its abundance pattern is typical of normal (non-CEMP) ultra metal-poor stars. Interestingly, the star has a binary companion. Conclusions. 2MASS J1808−5104 is the brightest (V = 11.9) metal-poor star of its category, and it could be studied further with even higher S/N spectroscopy to determine additional chemical abundances, thus providing important constraints to the early chemical evolution of our Galaxy.
The aim of Galactic archaeology is to recover the history of our Galaxy through the information encoded in stars. An unprobed assumption of this field is that the chemical composition of a star is an immutable marker of the gas from which it formed. It is vital to test this assumption on open clusters, group of stars formed from the same gas. Previous investigations have shown that unevolved stars in clusters are chemically homogeneous within the typical uncertainties of these analysis, i.e. 15% of the elemental abundances. Our strictly differential analysis on five members of the Pleiades allows us to reach precisions of 5% for most elements and to unveil chemical anomalies within the cluster that could be explained by planet engulfment events. These results reveal that the evolution of planetary systems may alter the chemical composition of stars, challenging our capability of tagging them to their native environments, and also paving the way for the study of planetary architectures and their evolution, through the chemical pattern of their host stars.
The photometric monitoring of planetary transits is an important method for the characterization of exoplanets. Transiting exoplanets allow measurements of the planetary radius and mass, providing information on the physical structure of the planet. Therefore, the parameters obtained with the transit method become fundamental for a comparative study of exoplanets in different planetary systems. In this work, we present the results obtained from observations of a transit event of the exoplanet HATS-24b using the new SPARC4 CCD mounted on the 1.6 m telescope of the Pico dos Dias Observatory (OPD). We have used Bayesian statistical inference to determine the physical parameters of this exoplanet, where we obtained a radius of 1.395±0.057 R J , mass of 2.26±0.17 M J , and density of 1.03±0.15 g cm −3 . We have combined our measurements of the central time of transit and orbital period with values from the literature to obtain an improved ephemeris for the transits of HATS-24b, T c = (2457948.709321±0.000039)+E(1.3484978±0.0000009). We have applied the differential analysis using a solar spectrum to recalculate the stellar surface parameters of HATS-24, where we obtained T eff = 6125 ± 94 K, log g = 4.370 ± 0.045 cm s−2, and [Fe/H]=−0.229 ± 0.058 dex. This allowed us to estimate an equilibrium temperature for HATS-24b of T eq = 2166 ± 53 K. The mass and radius of HATS-24b are consistent with the theoretical model of a pure helium-hydrogen planet at 1 Gy.
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