High-precision abundances of elements in solar twin stars: Trends with stellar age and elemental condensation temperature

HARPS spectra with S/N > 600 for 21 solar twin stars are used to determine very precise (sigma ~ 0.01 dex) differential abundances of C, O, Na, Mg, Al, Si, S, Ca, Ti, Cr, Fe, Ni, Zn, and Y in order to see how well [X/Fe] is correlated with elemental condensation temperature, Tc. In addition, precise (sigma < 0.8 Gyr) stellar ages are obtained by interpolating between Yonsei-Yale isochrones in the logg - Teff diagram. It is confirmed that the ratio between refractory and volatile elements is lower in the Sun than in most of the solar twins, but for many stars, the relation between [X/Fe] and Tc is not well defined. For several elements there is, instead, an astonishingly tight correlation between [X/Fe] and stellar age with amplitudes up to 0.2 dex over an age interval of 8 Gyr in contrast to the lack of correlation between [Fe/H] and age. While [Mg/Fe] increases with age, the s-process element yttrium shows the opposite behavior so that [Y/Mg] can be used as a sensitive chronometer for Galactic evolution. [Na/Fe] and [Ni/Fe] are not well correlated with stellar age, but define a tight Ni-Na relation similar to that previously found for more metal-poor stars. These results provide new constraints on supernovae yields and Galactic evolution. Furthermore, it is found that the C/O ratio evolves very little with time, which is of interest for discussions of the composition of exoplanets.Comment: 13 pages, 14 figures, and 2 on-line tables. To appear in A&

The most metal-poor damped Lyα systems: insights into chemical evolution in the very metal-poor regime

We present a high spectral resolution survey of the most metal-poor damped Lyα absorption systems (DLAs) aimed at probing the nature and nucleosynthesis of the earliest generations of stars. Our survey comprises 22 systems with iron abundance less than 1/100 solar; observations of seven of these are reported here for the first time. Together with recent measures of the abundances of C and O in Galactic metal-poor stars, we reinvestigate the trend of C/O in the very metal-poor (VMP) regime and we compare, for the first time, the O/Fe ratios in the most metal-poor DLAs and in halo stars. We confirm the near-solar values of C/O in DLAs at the lowest metallicities probed, and find that their distribution is in agreement with that seen in Galactic halo stars. We find that the O/Fe ratio in VMP DLAs is essentially constant, and shows very little dispersion, with a mean [〈O/Fe〉]=+0.39 ± 0.12, in good agreement with the values measured in Galactic halo stars when the oxygen abundance is measured from the [O i] λ6300 line. We speculate that such good agreement in the observed abundance trends points to a universal origin for these metals. In view of this agreement, we construct the abundance pattern for a typical VMP DLA and compare it to model calculations of Population II and Population III nucleosynthesis to determine the origin of the metals in VMP DLAs. Our results suggest that the most metal-poor DLAs may have been enriched by a generation of metal-free stars; however, given that abundance measurements are currently available for only a few elements, we cannot yet rule out an additional contribution from Population II stars

Carbon and oxygen in metal-poor halo stars

Carbon and oxygen are key tracers of the Galactic chemical evolution; in particular, a reported upturn in [C/O] towards decreasing [O/H] in metal-poor halo stars could be a signature of nucleosynthesis by massive Population III stars. We reanalyse carbon, oxygen, and iron abundances in 39 metal-poor turn-off stars. For the first time, we take into account 3D hydrodynamic effects together with departures from local thermodynamic equilibrium (LTE) when determining both the stellar parameters and the elemental abundances, by deriving effective temperatures from 3D non-LTE Hβ profiles, surface gravities from Gaia parallaxes, iron abundances from 3D LTE Fe II equivalent widths, and carbon and oxygen abundances from 3D non-LTE C I and O I equivalent widths. We find that [C/Fe] stays flat with [Fe/H], whereas [O/Fe] increases linearly up to 0.75 dex with decreasing [Fe/H] down to −3.0 dex. Therefore [C/O] monotonically decreases towards decreasing [C/H], in contrast to previous findings, mainly because the non-LTE effects for O I at low [Fe/H] are weaker with our improved calculations.AMA and KL acknowledge funds from the Alexander von Humboldt Foundation in the framework of the Sofja Kovalevskaja Award endowed by the Federal Ministry of Education and Research, and KL also acknowledges funds from the Swedish Research Council (grant 2015-004153) and Marie Skłodowska Curie Actions (cofund project INCA 600398). Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (grant DNRF106). MA gratefully acknowledges funding from the Australian Research Council (grants FL110100012 and DP150100250). Parts of this research were conducted by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. PSB acknowledges financial support from the Swedish Research Council and the project grant “The New Milky Way” from the Knut and Alice Wallenberg Foundation. This work was based on observations collected at the European Southern Observatory under ESO programmes 67.D-0106 and 73.D-0024

Lithium isotopic abundances in metal-poor halo stars

Very high-quality spectra of 24 metal-poor halo dwarfs and subgiants have been acquired with ESO's VLT/UVES for the purpose of determining Li isotopic abundances. The derived 1D, non-LTE 7Li abundances from the LiI 670.8nm line reveal a pronounced dependence on metallicity but with negligible scatter around this trend. Very good agreement is found between the abundances from the LiI 670.8nm line and the LiI 610.4nm line. The estimated primordial 7Li abundance is $7Li/H = 1.1-1.5 x 10^-10, which is a factor of three to four lower than predicted from standard Big Bang nucleosynthesis with the baryon density inferred from the cosmic microwave background. Interestingly, 6Li is detected in nine of our 24 stars at the >2sigma significance level. Our observations suggest the existence of a 6Li plateau at the level of log 6Li = 0.8; however, taking into account predictions for 6Li destruction during the pre-main sequence evolution tilts the plateau such that the 6Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of 6Li in the very metal-poor star LP815-43. Such a high 6Li abundance during these early Galactic epochs is very difficult to achieve by Galactic cosmic ray spallation and alpha-fusion reactions. It is concluded that both Li isotopes have a pre-Galactic origin. Possible 6Li production channels include proto-galactic shocks and late-decaying or annihilating supersymmetric particles during the era of Big Bang nucleosynthesis. The presence of 6Li limits the possible degree of stellar 7Li depletion and thus sharpens the discrepancy with standard Big Bang nucleosynthesis.Comment: Replaced with version accepted by ApJ. Minor changes compared with previous version (some discussion and references added

Hydrogen bond rotations as a uniform structural tool for analyzing protein architecture

Proteins fold into three-dimensional structures, which determine their diverse functions. The conformation of the backbone of each structure is locally at each Cα effectively described by conformational angles resulting in Ramachandran plots. These, however, do not describe the conformations around hydrogen bonds, which can be non-local along the backbone and are of major importance for protein structure. Here, we introduce the spatial rotation between hydrogen bonded peptide planes as a new descriptor for protein structure locally around a hydrogen bond. Strikingly, this rotational descriptor sampled over high-quality structures from the protein data base (PDB) concentrates into 30 localized clusters, some of which correlate to the common secondary structures and others to more special motifs, yet generally providing a unifying systematic classification of local structure around protein hydrogen bonds. It further provides a uniform vocabulary for comparison of protein structure near hydrogen bonds even between bonds in different proteins without alignment