Context. We are carrying out a search for planets around a sample of solar twin stars using the HARPS spectrograph. The goal of this project is to exploit the advantage offered by solar twins to obtain chemical abundances of unmatched precision. This survey will enable new studies of the stellar composition -planet connection. Aims. We determine the fundamental parameters of the 88 solar twin stars that have been chosen as targets for our experiment. Methods. We used the MIKE spectrograph on the Magellan Clay Telescope to acquire high resolution, high signal-to-noise ratio spectra of our sample stars. We measured the equivalent widths of iron lines and used strict differential excitation/ionization balance analysis to determine atmospheric parameters of unprecedented internal precision: σ(T eff ) = 7 K, σ(log g) = 0.019, σ([Fe/H]) = 0.006 dex, σ(v t ) = 0.016 km s −1 . Reliable relative ages and highly precise masses were then estimated using theoretical isochrones. Results. The spectroscopic parameters we derived are in good agreement with those measured using other independent techniques. There is even better agreement if the sample is restricted to those stars with the most internally precise determinations of stellar parameters in every technique involved. The root-mean-square scatter of the differences seen is fully compatible with the observational errors, demonstrating, as assumed thus far, that systematic uncertainties in the stellar parameters are negligible in the study of solar twins. We find a tight activity-age relation for our sample stars, which validates the internal precision of our dating method. Furthermore, we find that the solar cycle is perfectly consistent both with this trend and its star-to-star scatter. Conclusions. We present the largest sample of solar twins analyzed homogeneously using high quality spectra. The fundamental parameters derived from this work will be employed in subsequent work that aims to explore the connections between planet formation and stellar chemical composition.

We study with unprecedented detail the chemical composition and stellar parameters of the solar twin 18 Sco in a strictly differential sense relative to the Sun. Our study is mainly based on high resolution (R ∼ 110 000) high S/N (800-1000) VLT UVES spectra, which allow us to achieve a precision of about 0.005 dex in differential abundances. The effective temperature and surface gravity of 18 Sco are T eff = 5823±6 K and log g = 4.45±0.02 dex, i.e., 18 Sco is 46±6 K hotter than the Sun and log g is 0.01±0.02 dex higher. Its metallicity is [Fe/H] = 0.054±0.005 dex and its microturbulence velocity is +0.02±0.01 km s −1 higher than solar. Our precise stellar parameters and differential isochrone analysis show that 18 Sco has a mass of 1.04±0.02M ⊙ and that it is ∼1.6 Gyr younger than the Sun. We use precise HARPS radial velocities to search for planets, but none were detected. The chemical abundance pattern of 18 Sco displays a clear trend with condensation temperature, showing thus higher abundances of refractories in 18 Sco than in the Sun. Intriguingly, there are enhancements in the neutron-capture elements relative to the Sun. Despite the small element-to-element abundance differences among nearby n-capture elements (∼0.02 dex), we successfully reproduce the r-process pattern in the solar system. This is independent evidence for the universality of the r-process. Our results have important implications for chemical tagging in our Galaxy and nucleosynthesis in general.

Context. Solar twins are stars with similar stellar (surface) parameters to the Sun that can have a wide range of ages. This provides an opportunity to analyze the variation of their chemical abundances with age. Nissen (2015, A&A, 579, A52) recently suggested that the abundances of the s-process element Y and the α-element Mg could be used to estimate stellar ages. Aims. This paper aims to determine with high precision the Y, Mg, and Fe abundances for a sample of 88 solar twins that span a broad age range (0.3-10.0 Gyr) and investigate their use for estimating ages. Methods. We obtained high-quality Magellan Inamori Kyocera Echelle (MIKE) spectra and determined Y and Mg abundances using equivalent widths and a line-by-line differential method within a 1D LTE framework. Stellar parameters and iron abundances were measured in Paper I of this series for all stars, but a few (three) required a small revision.Results. The [Y/Mg] ratio shows a strong correlation with age. It has a slope of −0.041 ± 0.001 dex/Gyr and a significance of 41σ. This is in excellent agreement with the relation first proposed by Nissen (2015). We found some outliers that turned out to be binaries where mass transfer may have enhanced the yttrium abundance. Given a precise measurement of [Y/Mg] with typical error of 0.02 dex in solar twins, our formula can be used to determine a stellar age with ∼0.8 Gyr precision in the 0 to 10 Gyr range.

tugal * Based on observations obtained at the European Southern Observatory (observing programs 083.D-0871 and 188.C-0265).-3 - ABSTRACTWe present the first detailed chemical abundance analysis of the old 8.2 Gyr solar twin, HIP 102152. We derive differential abundances of 21 elements relative to the Sun with precisions as high as 0.004 dex ( 1%), using ultra high-resolution (R = 110,000), high S/N UVES spectra obtained on the 8.2-m Very Large Telescope. Our determined metallicity of HIP 102152 is [Fe/H] = -0.013 ± 0.004.The atmospheric parameters of the star were determined to be 54 K cooler than the Sun, 0.09 dex lower in surface gravity, and a microturbulence identical to our derived solar value. Elemental abundance ratios examined vs. dust condensation temperature reveal a solar abundance pattern for this star, in contrast to most solar twins. The abundance pattern of HIP 102152 appears to be the most similar to solar of any known solar twin. Abundances of the younger, 2.9Gyr solar twin, 18 Sco, were also determined from UVES spectra to serve as a comparison for HIP 102152. The solar chemical pattern of HIP 102152 makes it a potential candidate to host terrestrial planets, which is reinforced by the lack of giant planets in its terrestrial planet region. The following non-local thermodynamic equilibrium Li abundances were obtained for HIP 102152, 18 Sco, and the Sun: log ǫ (Li) = 0.48 ± 0.07, 1.62 ± 0.02, and 1.07 ± 0.02, respectively.The Li abundance of HIP 102152 is the lowest reported to date for a solar twin, and allows us to consider an emerging, tightly constrained Li-age trend for solar twin stars.

Photometric data in the UBV(RI) C system have been acquired for 80 solar analog stars for which we have previously derived highly precise atmospheric parameters T eff , log g, and [Fe/H] using high-resolution, high signal-to-noise ratio spectra. UBV and (RI) C data for 46 and 76 of these stars, respectively, are published for the first time. Combining our data with those from the literature, colors in the UBV(RI) C system, with 0.01 mag precision, are now available for 112 solar analogs. Multiple linear regression is used to derive the solar colors from these photometric data and the spectroscopically derived T eff , log g, and [Fe/H] values. To minimize the impact of systematic errors in the model-dependent atmospheric parameters, we use only the data for the 10 stars that most closely resemble our Sun, i.e., the solar twins, and derive the following solar colors: (B − V ) = 0.653 ± 0.005, (U − B) = 0.166 ± 0.022, (V − R) = 0.352 ± 0.007, and (V − I ) = 0.702 ± 0.010. These colors are consistent, within the 1σ errors, with those derived using the entire sample of 112 solar analogs. We also derive the solar colors using the relation between spectral-line-depth ratios and observed stellar colors, i.e., with a completely model-independent approach, and without restricting the analysis to solar twins. We find (B − V ) = 0.653 ± 0.003, (U − B) = 0.158 ± 0.009, (V − R) = 0.356 ± 0.003, and (V − I ) = 0.701 ± 0.003, in excellent agreement with the model-dependent analysis.

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The physical processes driving chemical evolution in the Milky Way can be probed using the distribution of abundances in low-mass FGK type stars in space phase at different times. During their final stages of evolution, stars experience nucleosynthesis several times, each at different timescales and producing different chemical elements. Finding abundance ratios that have simple variations across cosmic times therefore remains a challenge. Using the sample of 80 solar twins for which ages and abundances of 30 elements have been measured with high precision, we searched for all possible abundance ratio combinations that show linear trends with age. We found 55 such ratios, all combining an n-capture element and another element produced by different nucleosynthesis channels. We recovered the ratios of [Y/Mg], [Ba/Mg], and [Al/Y] that have been reported previously in the literature, and found that [C/Ba] depends most strongly on age, with a slope of 0.049 ± 0.003 dex Gyr −1 . This imposes constraints on the magnitude of the time dependency of abundance ratios in solar twins. Our results suggest that s-process elements, in lieu of Fe, should be used as a reference for constraining chemical evolution models of the solar neighbourhood. Our study illustrates that a wide variety of chemical elements measured in high-resolution spectra is key to meeting the current challenges in understanding the formation and evolution of our Galaxy.

Context. Well studied Open Clusters (OCs) of the Solar neighbourhood are frequently used as reference objects to test galactic and stellar theories. For that purpose their chemical composition needs to be known with a high level of confidence. It is also important to clarify if each OC is chemically homogeneous and if it has a unique chemical signature. Aims. The aims of this work are (1) to determine accurate and precise abundances of 22 chemical species (from Na to Eu) in the Hyades, Praesepe and Rupecht 147 using a large number of stars at different evolutionary states, (2) to evaluate the level of chemical homogeneity of these OCs, (3) to compare their chemical signatures. Methods. We gathered ∼800 high resolution and high signal-to-noise spectra of ∼100 members in the three clusters, obtained with the latest memberships based on Gaia DR2 data. We build a pipeline which computes atmospheric parameters and strictly line-by-line differential abundances among twin stars in our sample. With this method we are able to reach a very high precision in the abundances (0.01-0.02 dex in most of the elements). Results. We find large differences in the absolute abundances in some elements, which can be attributed to diffusion, NLTE effects or systematics in the analysis. For the three OCs, we find strong correlations in the differential abundances between different pairs of elements. According to our experiment with synthetic data, this can be explained by some level of chemical inhomogeneity. We compare differential abundances of several stars from the Hyades and Praesepe tails: the stars that differ more in chemical abundances also have distinct kinematics, even though they have been identified as members of the tail. Conclusions. It is possible to obtain high precision abundances using a differential analysis even when mixing spectra from different instruments. With this technique we find that the Hyades and Preasepe have the same chemical signature when G dwarfs and K giants are considered. Despite a certain level of inhomogeneity in each cluster, it is still possible to clearly distinguish the chemical signature of the older cluster Ruprecht 147 when compared to the Hyades and Praesepe.