Lyudmila Mashonkina

A non-LTE study of neutral and singly-ionized iron line spectra in 1D models of the Sun and selected late-type stars ?

Aims. We evaluate non-local thermodynamical equilibrium (non-LTE) line formation for the two ions of iron and check the ionization equilibrium between Fe i and Fe ii in model atmospheres of the cool reference stars based on the best available complete model atom for neutral and singly-ionized iron. Methods. We present a comprehensive model atom for Fe with more than 3000 measured and predicted energy levels. As a test and first application of the improved model atom, iron abundances are determined for the Sun and five stars with well determined stellar parameters and high-quality observed spectra. The efficiency of inelastic collisions with hydrogen atoms in the statistical equilibrium of iron is empirically estimated from inspection of their different influence on the Fe i and Fe ii lines in the selected stars. Results. Non-LTE leads to systematically depleted total absorption in the Fe i lines and to positive abundance corrections in agreement with the previous studies, however, the magnitude of such corrections is smaller compared to the earlier results. These non-LTE corrections do not exceed 0.1 dex for the solar metallicity and mildly metal-deficient stars, and they vary within 0.21 dex and 0.35 dex in the very metal-poor stars HD 84937 and HD 122563, respectively, depending on the assumed efficiency of collisions with hydrogen atoms. Based on the analysis of the Fe i/Fe ii ionization equilibrium in these two stars, we recommend to apply the Drawin formalism in non-LTE studies of Fe with a scaling factor of 0.1. For the Fe ii lines non-LTE corrections do not exceed 0.01 dex in absolute value over the whole range of stellar parameters that are considered. This study reveals two problems. The first one is that gf -values available for the Fe i and Fe ii lines are not accurate enough to pursue high-accuracy absolute stellar abundance determinations. For the Sun, the mean non-LTE abundance obtained from 54 Fe i lines is 7.56 ± 0.09 and the mean abundance from 18 Fe ii lines varies between 7.41 ± 0.11 and 7.56 ± 0.05 depending on the source of the gf -values. The second problem is that lines of Fe i give, on average, a 0.1 dex lower abundance compared with those of Fe ii lines for HD 61421 and HD 102870, even when applying a differential line-by-line analysis with regard to the Sun. A disparity between neutral atoms and first ions points to problems of stellar atmosphere modelling or/and effective temperature determination.

ATOMIC DIFFUSION AND MIXING IN OLD STARS. I. VERY LARGE TELESCOPE FLAMES-UVES OBSERVATIONS OF STARS IN NGC 6397

We present a homogeneous photometric and spectroscopic analysis of 18 stars along the evolutionary sequence of the metal-poor globular cluster NGC 6397 ([Fe/H] ≈ -2), from the main-sequence turnoff point to red giants below the bump. The spectroscopic stellar parameters, in particular stellar parameter differences between groups of stars, are in good agreement with broadband and Stromgren photometry calibrated on the infrared flux method. The spectroscopic abundance analysis reveals, for the first time, systematic trends of iron abundance with evolutionary stage. Iron is found to be 30% less abundant in the turnoff point stars than in the red giants. An abundance difference in lithium is seen between the turnoff point and warm subgiant stars. The impact of potential systematic errors on these abundance trends (stellar parameters, the hydrostatic and LTE approximations) is quantitatively evaluated and found not to alter our conclusions significantly. Trends for various elements (Li, Mg, Ca, Ti, and Fe) are compared with stellar structure models including the effects of atomic diffusion and radiative acceleration. Such models are found to describe the observed element-specific trends well, if extra (turbulent) mixing just below the convection zone is introduced. It is concluded that atomic diffusion and turbulent mixing are largely responsible for the subprimordial stellar lithium abundances of warm halo stars. Other consequences of atomic diffusion in old metal-poor stars are also discussed.

Heavy element abundances in cool dwarf stars: An implication for the evolution of the Galaxy

We present revised strontium, barium and europium abundances for 63 cool stars with metallicities (Fe/H) ranging from 2:20 to 0:25. The stellar sample has been extracted from Fuhrmanns lists (1998, 2001). It is confined to main-sequence and turnoff stars. The results are based on NLTE line formation obtained in differential model atmosphere analyses of spectra that have a typical S/N of 200 and a resolution of 40000 to 60000. The element abundance ratios reveal a distinct chemical history of the halo and thick disk compared with that of the thin disk. Europium is overabundant relative to iron and barium in halo and thick disk stars suggesting that during the formation of these galactic populations high-mass stars exploding as SNe II dominated nucleosynthesis on a short time scale of the order of 1 Gyr. We note the importance of (Eu/Mg) determinations for halo stars. Our analysis leads to the preliminary conclusion that Eu/Mg ratios found in halo stars do not support current theoretical models of the r-process based on low-mass SNe; instead they seem to point at a halo formation time much shorter than 1 Gyr. A steep decline of (Eu/Fe) and a slight decline of (Eu/Ba) with increasing metallicity have been first obtained for thick disk stars. This indicates the start of nucleosynthesis in the lower mass stars, in SN I and AGB stars, which enriched the interstellar gas with iron and the most abundant s-process elements. From a decrease of the Eu/Ba ratio by 0:10 ::: 0 :15 dex the time interval corresponding to the thick disk formation phase can be estimated. The step-like change of element abundance ratios at the thick to thin disk transition found in our previous analysis (Mashonkina & Gehren 2000) is confirmed in this study: (Eu/Ba) and (Eu/Fe) are reduced by 0:25 dex and 0:15 dex, respectively; (Ba/Fe) increases by 0:1 dex. This is indicative of an intermediate phase before the early stage of the thin disk developed, during which only evolved middle and low mass (< 8M) stars contributed to nucleosynthesis. Our data provide an independent method to calculate the duration of this phase. The main s-process becomes dominant in the production of heavy elements beyond the iron group during the thin disk evolution. We find that in the thin disk stars Ba/Fe ratios increase with time from (Ba/Fe) = 0:06 in stars older than 8 Gyr to (Ba/Fe) = 0:06 in stars that are between 2 and 4 Gyr old.

A primordial star in the heart of the Lion

Context: The discovery and chemical analysis of extremely metal-poor stars permit a better understanding of the star formation of the first generation of stars and of the Universe emerging from the Big Bang. aims: We report the study of a primordial star situated in the centre of the constellation Leo (SDSS J102915+172027). method: The star, selected from the low resolution-spectrum of the Sloan Digital Sky Survey, was observed at intermediate (with X-Shooter at VLT) and at high spectral resolution (with UVES at VLT). The stellar parameters were derived from the photometry. The standard spectroscopic analysis based on 1D ATLAS models was completed by applying 3D and non-LTE corrections. results: An iron abundance of [Fe/H]=--4.89 makes SDSS J102915+172927 one of the lowest [Fe/H] stars known. However, the absence of measurable C and N enhancements indicates that it has the lowest metallicity, Z<= 7.40x10^{-7} (metal-mass fraction), ever detected. No oxygen measurement was possible. conclusions: The discovery of SDSS J102915+172927 highlights that low-mass star formation occurred at metallicities lower than previously assumed. Even lower metallicity stars may yet be discovered, with a chemical composition closer to the composition of the primordial gas and of the first supernovae.

Non-LTE line formation for heavy elements in four very metal-poor stars

Aims. Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba, and Eu are determined for four very metal-poor (VMP) stars (−2.15 ≥ [Fe/H] ≥− 2.66). For two of them, HD 84937 and HD 122563, the fraction of the odd isotopes of Ba derived for the first time. Methods. Determination of an effective temperature, surface gravity, and element abundances was based on non-local thermodynamic equilibrium (non-LTE) line formation and analysis of high-resolution (R ∼ 60 000 and 90 000) high signal-to-noise (S/N ≥ 200) observed spectra. A model atom for H i is presented. An effective temperature was obtained from the Balmer Hα and Hβ line wing fits. The surface gravity was calculated from the Hipparcos parallax if available and the non-LTE ionization balance between Ca i and Ca ii. Based on the hyperfine structure affecting the Ba ii resonance line λ 4554, the fractional abundance of the odd isotopes of Ba was derived from a requirement that Ba abundances from the resonance line and subordinate lines of Ba ii must be equal. Results. For each star, non-LTE leads to a consistency of Teff from two Balmer lines and to a higher temperature compared to the LTE case, by up to 60 K. Non-LTE effects are important in spectroscopic determination of surface gravity from the ionization balance between Ca i and Ca ii. For each star with a known trigonometric surface gravity, non-LTE abundances from the lines of two ionization stages, Ca i and Ca ii, agree within the error bars, while a difference in the LTE abundances consists of 0.23 dex to 0.40 dex for different stars. Departures from LTE are found to be significant for all investigated atoms, and they strongly depend on stellar parameters. For HD 84937, the Eu/Ba ratio is consistent with the relative solar system r-process abundances, and the fraction of the odd isotopes of Ba, fodd, equals 0.43 ± 0.14. The latter can serve as an observational constraint on r-process models. The lower Eu/Ba ratio and fodd = 0.22 ± 0.15 found for HD 122563 suggest that the s-process or the unknown process has contributed significantly to the Ba abundance in this star.

Mg, Ba and Eu abundances in thick disk and halo stars

Our sample of cool dwarf stars from previous papers (Mashonkina & Gehren 2000, 2001) is extended in this study including 15 moderately metal-deficient stars. The samples of halo and thick disk stars have overlapping metallicities with (Fe/H) in the region from −0. 9t o−1.5, and we compare chemical properties of these two kinematically different stellar popu- lations independent of their metallicity. We present barium, europium and magnesium abundances for the new sample of stars. The results are based on NLTE line formation obtained in differential model atmosphere analyses of high resolution spectra observed mainly using the UVES spectrograph at the VLT of the European Southern Observatory. We confirm the overabun- dance of Eu relative to Mg in halo stars as reported in our previous papers. Eight halo stars show (Eu/Mg) values between 0.23 and 0.41, whereas stars in the thick and thin disk display a solar europium to magnesium ratio. The (Eu/Ba) values found in the thick disk stars to lie between 0.35 and 0.57 suggest that during thick disk formation evolved low-mass stars started to enrich the interstellar gas by s-nuclei of Ba, and the s-process contribution to barium thus varies from 30% to 50%. Based on these results, and using the chemical evolution calculations by Travaglio et al. (1999), we estimate that the thick disk stellar population formed on a timescale between 1.1 to 1.6 Gyr from the beginning of the protogalactic collapse. In the halo stars the (Eu/Ba) values are found mostly between 0.40 and 0.67, which suggests a duration of the halo formation of about 1.5 Gyr. For the whole sample of stars we present the even-to-odd Ba isotope ratios as determined from hyperfine structure seen in the Ba  resonance line λ 4554. As expected, the solar ratio 82 : 18 (Cameron 1982) adjusts to observations of the Ba  lines in the thin disk stars. In our halo stars the even-to-odd Ba isotope ratios are close to the pure r-process ratio 54 : 46 (Arlandini et al. 1999), and in the thick disk stars the isotope ratio is around 65 : 35 ( ±10%). Based on these data we deduce for thick disk stars the ratio of the s/r-process contribution to barium as 30 : 70 (±30%), in agreement with the results obtained from the (Eu/Ba) values.

Nucleosynthesis Modes in The High-Entropy Wind of Type II Supernovae: Comparison of Calculations With Halo-Star Observations

While the high-entropy wind (HEW) of Type II supernovae remains one of the more promising sites for the rapid neutron-capture (r-) process, hydrodynamic simulations have yet to reproduce the astrophysical conditions under which the latter occurs. We have performed large-scale network calculations within an extended parameter range of the HEW, seeking to identify or to constrain the necessary conditions for a full reproduction of all r-process residuals N r,☉ = N ☉–N s,☉ by comparing the results with recent astronomical observations. A superposition of weighted entropy trajectories results in an excellent reproduction of the overall N r,☉ pattern beyond Sn. For the lighter elements, from the Fe group via Sr-Y-Zr to Ag, our HEW calculations indicate a transition from the need for clearly different sources (conditions/sites) to a possible co-production with r-process elements, provided a range of entropies are contributing. This explains recent halo-star observations of a clear noncorrelation of Zn and Ge and a weak correlation of Sr-Zr with heavier r-process elements. Moreover, new observational data on Ru and Pd also seem to confirm a partial correlation with Sr as well as the main r-process elements (e.g., Eu).

Statistical equilibrium of silicon in the solar atmosphere

Aims. The statistical equilibrium of neutral and ionised silicon in the solar photosphere is investigated. Line formation is discussed and the solar silicon abundance determined. Methods. High-resolution solar spectra were used to determine solar log g f eSi values by comparison with Si line synthesis based on LTE and NLTE level populations. The results will be used in a forthcoming paper for differential abundance analyses of metalpoor stars. A detailed analysis of silicon line spectra leads to setting up realistic model atoms, which are exposed to interactions in plane-parallel solar atmospheric models. The resulting departure coefficients are entered into a line-by-line analysis of the visible and near-infrared solar silicon spectrum. Results. The statistical equilibrium of Si i turns out to depend marginally on bound-free interaction processes, both radiative and collisional. Bound-bound interaction processes do not play a significant role either, except for hydrogen collisions, which have to be chosen adequately for fitting the cores of the near-infrared lines. Except for some near-infrared lines, the NLTE influence on the abundances is weak. Conclusions. Taking the deviations from LTE in silicon into account, it is possible to calculate the ionisation equilibrium from neutral and ionised lines. The solar abundance based on the experimental f -values of Garz corrected for the Becker et al.’s measurement is 7.52 ± 0.05. Combined with an extended line sample with selected NIST f -values, the solar abundance is 7.52 ± 0.06, with a nearly perfect ionisation equilibrium of Δ log e� (Siii/Sii) = −0.01.

Statistical equilibrium of silicon in the atmospheres of metal-poor stars

Aims. We discuss the statistical equilibrium of neutral and ionized silicon in the atmospheres of nearby metal-poor stars. We investigated the effects of non-local thermodynamic equilibrium (NLTE) and determined the silicon abundances. Methods. We used high resolution, high signal-to-noise ratio spectra from the FOCES spectragraph at the DSAZ telescope. Lineformation calculations of Si i and Si ii in the atmospheres of nearby metal-poor stars are presented. All abundance results are derived from local thermodynamic equilibrium (LTE) and NLTE statistical equilibrium calculations and spectrum synthesis methods. Results. We find that NLTE effects for Si ii optical lines are important for warm stars, and that they depend on effective temperature. The Si abundances of thin and thick disc stars follow distinct trends, as in the case of Mg. We find that [Si/Fe] gradually increases as [Fe/H] decreases in thin disc stars, while it remains around at ∼+0.30 dex for halo and thick disc stars, the halo stars showing larger scatter. Conclusions. The derived dependence between [Si/Fe] and [Fe/H] is inconsistent with the theoretical predictions of published model calculations for the chemical evolution of the Galaxy. The nearly constant [Si/Mg] ratio with some scatter for halo and thick disc stars suggests that the nucleosynthesis of silicon is closely coupled to that of Mg. In addition, our results do not support the suggestion that type Ia supernove produce significant amounts of silicon.

The Hamburg/ESO R-process enhanced star survey (HERES) - V. Detailed abundance analysis of the r-process enhanced star HE 2327-5642

Aims. We present a detailed abundance analysis of a strongly r-process enhanced giant star discovered in the HERES project, HE 2327−5642, for which [Fe/H] = −2.78, [r/Fe] =+ 0.99. Methods. We determined the stellar parameters and element abundances by analyzing the high-quality VLT/UVES spectra. The surface gravity was calculated from the non-local thermodynamic equilibrium (NLTE) ionization balance between Fe i and Fe ii ,a nd Cai and Ca ii. Results. Accurate abundances for a total of 40 elements and for 23 neutron-capture elements beyond Sr and up to Th were determined in HE 2327−5642. For every chemical species, the dispersion in the single line measurements around the mean does not exceed 0.11 dex. The heavy element abundance pattern of HE 2327−5642 is in excellent agreement with those previously derived for other strongly r-process enhanced stars, such as CS 22892-052, CS 31082-001, and HE 1219-0312. Elements in the range from Ba to Hf match the scaled Solar r-process pattern very well. No firm conclusion can be drawn about the relationship between the fisrt neutroncapture peak elements, Sr to Pd, in HE 2327−5642 and the Solar r-process, due to the uncertainty in the Solar r-process. A clear distinction in Sr/Eu abundance ratios was found between the halo stars of different europium enhancement. The strongly r-process enhanced stars contain a low Sr/Eu abundance ratio at [Sr/Eu] = −0.92 ± 0.13, while the stars with 0 < [Eu/Fe] < 1a nd [Eu/Fe] < 0 have 0.36 dex and 0.93 dex higher Sr/Eu values, respectively. Radioactive dating for HE 2327−5642 with the observed thorium and rare-earth element abundance pairs results in an average age of 13.3 Gyr, when based on the high-entropy wind calculations, and 5.9 Gyr, when using the Solar r-residuals. We propose that HE 2327−5642 is a radial-velocity variable based on our high-resolution spectra covering ∼4.3 years.