Bengt Edvardsson

Chemical abundances in 43 metal-poor stars

We have derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Ni, and Ba for 43 metal-poor field stars in the solar neighbourhood, most of them subgiants or turn-off-point stars with iron abundances (Fe/H) ranging from −0. 4t o−3.0. About half of this sample has not been spectroscopically analysed in detail before. Effective temperatures were estimated from uvby photometry, and surface gravities primarily from Hipparcos parallaxes. The analysis is differential relative to the Sun, and was carried out with plane-parallel MARCS models. Various sources of error are discussed and found to contribute a total error of about 0.1-0.2 dex for most elements, while relative abundances, such as (Ca/Fe), are most probably more accurate. For the oxygen abundances, determined in an NLTE analysis of the 7774 A triplet lines, the errors may be somewhat larger. We made a detailed comparison with similar studies and traced the reasons for the, in most cases, relatively small differences. Among the results we find that (O/Fe) possibly increases beyond (Fe/H) = -1.0, though considerably less so than in results obtained by others from abundances based on OH lines. We did not trace any tendency toward strong overionization of iron, and find the excesses, relative to Fe and the Sun, of the α elements Mg, Si, and Ca to be smaller than those of O. We discuss some indications that also the abundances of different α elements relative to Fe vary and the possibility that some of the scatter around the trends in abundances relative to iron may be real. This may support the idea that the formation of Halo stars occurred in smaller systems with different star formation rates. We verify the finding by Gratton et al. (2003b, A&A, 406, 131) that stars that do not participate in the rotation of the galactic disk show a lower mean and larger spread in (α/Fe) than stars participating in the general rotation. The latter stars also seem to show some correlation between (α/Fe) and rotation speed. We trace some stars with peculiar abundances, among these two Ba stars, HD 17072 and HD 196944, the second already known to be rich in s elements. Finally we advocate that a spectroscopic study of a larger sample of halo stars with well-defined selection criteria is very important, in order to add to the very considerable efforts that various groups have already made.

Boron in Very Metal-poor Stars*

We have observed the B I λ2497 line to derive the boron abundances of two very metal-poor stars selected to help in tracing the origin and evolution of this element in the early Galaxy: BD +23°3130 and HD 84937. The observations were conducted using the Goddard High Resolution Spectrograph on board the Hubble Space Telescope. A very detailed abundance analysis via spectral synthesis has been carried out for these two stars, as well as for two other metal-poor objects with published spectra, using both Kurucz and OSMARCS model photospheres and taking into account consistently the non-LTE (NLTE) effects on the line formation. We have also reassessed all published boron abundances of old disk and halo unevolved stars. Our analysis shows that the combination of high effective temperature (Teff 6000 K, for which boron is mainly ionized) and low metallicity ([Fe/H] -1) makes it difficult to obtain accurate estimates of boron abundances from the B I λ2497 line. This is the case of HD 84937 and three other published objects (including two stars with [Fe/H] ~ -3), for which only upper limits can be established. BD +23°3130, with [Fe/H] ~ -2.9 and log N(B)NLTE = 0.05 ± 0.30, appears then as the most metal-poor star for which a firm measurement of the boron abundance presently exists. The evolution of the boron abundance with metallicity that emerges from the seven remaining stars with Teff < 6000 K and [Fe/H] < -1, for which beryllium abundances were derived using the same stellar parameters, shows a linear increase with a slope of ~1. Furthermore, the B/Be ratio found is constant at a value of ~20 for stars in the range -3 < [Fe/H] < -1. These results point to spallation reactions of ambient protons and α-particles with energetic particles enriched in CNO as the origin of boron and beryllium in halo stars.

FINDING HIGH-REDSHIFT DARK STARS WITH THE JAMES WEBB SPACE TELESCOPE

The first stars in the history of the universe are likely to form in the dense central regions of similar to 10(5)-10(6) M-circle dot cold dark matter halos at z approximate to 10-50. The annihilat ...

First detection of beryllium in a very metal poor star : a test of the standard big bang model

The primordial abundances of 9 Be and 11 B are the most sensitive indicators of the importance of inhomogeneity in the early universe which have not yet been measured. Additionally, their abundances in the oldest stars are a sensitive measure of the star formation and cosmic-ray spallation history of the early Milky Way. We report here the first determination of the abundance of 9 Be in a very metal poor star, showing that such measurements are possible, and opening the way to a new investigation of the structure of the very early universe and the evolution of the early Galaxy.

Isochrones for old (>5 Gyr) stars and stellar populations. I. Models for –2.4 ≤ [Fe/H] ≤+0.6, 0.25 ≤ Y ≤ 0.33, and –0.4 ≤ [α/Fe] ≤+0.4

Canonical grids of stellar evolutionary sequences have been computed for the helium mass-fraction abundances

Europium abundances in F and G disk dwarfs

Y = 0.25

A Constraint on Z☉ from Fits of Isochrones to the Color-Magnitude Diagram of M67

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Calibration of Strömgren uvby-H

0.29

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photometry for late-type stars – a model atmosphere approach

0.33