Abundances Of Iron-peak Elements Research Articles

Abundances of iron-peak elements in accreted and in situ born Galactic halo stars

Context. Studies of the element abundances and kinematics of stars belonging to the Galactic halo have revealed the existence of two distinct populations: accreted stars with a low [α/Fe] ratio and in situ born stars with a higher ratio. Aims. Previous work on the abundances of C, O, Na, Mg, Si, Ca, Ti, Cr, Mn, Fe, Ni, Cu, and Zn in high-α and low-α halo stars is extended to include the abundances of Sc, V, and Co, enabling us to study the nucleosynthesis of all iron-peak elements along with the lighter elements. Methods. The Sc, V, and Co abundances were determined from a 1D MARCS model-atmosphere analysis of equivalent widths of atomic lines in high signal-to-noise, high resolution spectra assuming local thermodynamic equilibrium (LTE). In addition, new 3D and/or non-LTE calculations were used to correct the 1D LTE abundances for several elements including consistent 3D non-LTE calculations for Mg. Results. The two populations of accreted and in situ born stars are well separated in diagrams showing [Sc/Fe], [V/Fe], and [Co/Fe] as a function of [Fe/H]. The [X/Mg] versus [Mg/H] trends for high-α and low-α stars were used to determine the yields of core-collapse and Type Ia supernovae. The largest Type Ia contribution occurs for Cr, Mn, and Fe, whereas Cu is a pure core-collapse element. Sc, Ti, V, Co, Ni, and Zn represent intermediate cases. A comparison with yields calculated for supernova models shows poor agreement for the core-collapse yields. The Ia yields suggest that sub-Chandrasekhar-mass Type Ia supernovae provide a dominant contribution to the chemical evolution of the host galaxies of the low-α stars. A substructure in the abundances and kinematics of the low-α stars suggests that they arise from at least two different satellite accretion events, Gaia-Sausage-Enceladus and Thamnos.

Abundances of Iron-peak Elements in the B8 Mn Star HD 110073

The synthesis of spectral intervals in the optical range allows the determination of LTE abundances of several iron-peak elements for the chemically peculiar B8 Mn star HD 110073. Titanium, chromium and manganese are found to be overabundant by factors of 19, 4 and 250 times their respective solar abundances. Iron and nickel are deficient by factors of 0.25 and 0.10. Weak lines of Cr ii formed deep in the atmosphere yield lower chromium abundances than strong lines do by a factor of 2. This supports that chromium is probably stratified vertically in the atmosphere of HD 110073. Several weak flat-bottomed lines which could be caused by vertical stratification of the abundances are present throughout the spectra. The new abundance determinations differ from previous determinations partly because the atomic data have changed. A non-uniform distribution of the elements over the surface of HD 110073 could also account for variation of the lines with time.

Most “Young” α-rich Stars Have High Masses but are Actually Old

Abstract Recent observations have revealed a population of α-element abundances, enhanced giant stars with unexpected high masses (≳1 M ⊙) from asteroseismic analysis and spectroscopy. Assuming single-star evolution, their masses imply young ages (τ < 6 Gyr) incompatible with the canonical Galactic chemical evolution scenario. Here we study the chemistry and kinematics of a large sample of such α-rich, high-mass red giant branch (RGB) stars drawn from the LAMOST spectroscopic surveys. Using LAMOST and Gaia, we found these stars share the same kinematics as the canonical high-α old stellar population in the Galactic thick disk. The stellar abundances show that these high-α massive stars have α- and iron-peak element abundances similar to those of the high-α old thick-disk stars. However, a portion of them exhibit higher [(N+C)/Fe] and [Ba/Fe] ratios, which implies they have gained C- and Ba-rich materials from extra sources, presumably asymptotic giant branch (AGB) companions. The results support the previous suggestion that these RGB stars are products of binary evolution. Their high masses thus mimic “young” single stars, yet in fact they belong to an intrinsic old stellar population. To fully explain the stellar abundance patterns of our sample stars, a variety of binary evolution channels, such as main-sequence (MS) + RGB, MS + AGB, RGB + RGB, and RGB + AGB, are required, pointing to diverse formation mechanisms of these seemly rejuvenated cannibals. With this larger sample, our results confirm earlier findings that most, if not all, α-rich stars in the Galactic disk seem to be old.

A New Set of Chisels for Galactic Archeology: Sc, V, and Zn as Taggers of Accreted Globular Clusters*

Abstract Chemical tagging is a powerful tool to reveal the origin of stars and globular clusters (GCs), especially when dynamics alone cannot provide robust answers. So far, mostly α-elements and neutron capture elements have been used to distinguish stars born in the Milky Way (MW) from those born in external environments such as that of dwarf galaxies. Here, instead, we use iron-peak element abundances to investigate the origin of a sample of metal-rich GCs. By homogeneously analyzing high-resolution UVES spectra of giant stars belonging to four metal-rich GCs (namely NGC 5927, NGC 6388, NGC 6441, and NGC 6496), we find that while the α-elements Si and Ca have similar abundance ratios for all four GCs, and Ti and neutron capture elements (La, Ba, and Eu) only show a marginal discrepancy, a stark difference is found when considering the abundances of some iron-peak elements (Sc, V, and Zn). In particular, NGC 6388 and NGC 6441 have abundance ratios for these iron-peak elements significantly lower (by ∼0.5 dex) than those measured in NGC 5927 and NGC 6496, which are clearly identified as born in situ MW clusters through an analysis of their orbital properties. These measurements indicate that the environment in which these clusters formed is different, and they provide robust evidence supporting an accreted origin from the same progenitor for NGC 6388 and NGC 6441.

Chemical Tagging N-rich Field Stars with High-resolution Spectroscopy

Abstract We measure chemical abundances for over 20 elements of 15 N-rich field stars with high-resolution (R ∼ 30,000) optical spectra. We find that Na, Mg, Al, Si, and Ca abundances of our N-rich field stars are mostly consistent with those of stars from globular clusters (GCs). Seven stars are estimated to have [Al/Fe ] > 0.5, which is not found in most GC “first generation” stars. On the other hand, α element abundances (especially Ti) could show distinguishable differences between in situ stars and accreted stars. We discover that one interesting star, with consistently low [Mg/Fe], [Si/Fe], [Ca/Fe], [Ti/Fe], [Sc/Fe], [V/Fe], and [Co/Fe], show similar kinematics and [Ba/Eu] as other stars from the dissolved dwarf galaxy “Gaia–Sausage–Enceladus.” The α-element abundances and the iron-peak element abundances of the N-rich field stars with metallicities − 1.25 ≤ [Fe/H] ≤ − 0.95 show consistent values with Milky Way field stars (we refer to Milky Way field stars as Milky Way halo field stars unless otherwise specified in this paper) rather than stars from dwarf galaxies, indicating that they were formed in situ. In addition, the neutron-capture elements of N-rich field stars show that most of them could be enriched by asymptotic giant branch stars with masses around 3–5 M ⊙.

Cobalt and copper abundances in 56 Galactic bulge red giants

Context. The Milky Way bulge is an important tracer of the early formation and chemical enrichment of the Galaxy. The abundances of different iron-peak elements in field bulge stars can give information on the nucleosynthesis processes that took place in the earliest supernovae. Cobalt (Z = 27) and copper (Z = 29) are particularly interesting. Aims. We aim to identify the nucleosynthesis processes responsible for the formation of the iron-peak elements Co and Cu. Methods. We derived abundances of the iron-peak elements cobalt and copper in 56 bulge giants, 13 of which were red clump stars. High-resolution spectra were obtained using FLAMES-UVES at the ESO Very Large Telescope by our group in 2000–2002, which appears to be the highest quality sample of optical high-resolution data on bulge red giants obtained in the literature to date. Over the years we have derived the abundances of C, N, O, Na, Al, Mg; the iron-group elements Mn and Zn; and neutron-capture elements. In the present work we derive abundances of the iron-peak elements cobalt and copper. We also compute chemodynamical evolution models to interpret the observed behaviour of these elements as a function of iron. Results. The sample stars show mean values of [Co/Fe] ~ 0.0 at all metallicities, and [Cu/Fe] ~ 0.0 for [Fe/H] ≥−0.8 and decreasing towards lower metallicities with a behaviour of a secondary element. Conclusions. We conclude that [Co/Fe] varies in lockstep with [Fe/H], which indicates that it should be produced in the alpha-rich freezeout mechanism in massive stars. Instead [Cu/Fe] follows the behaviour of a secondary element towards lower metallicities, indicating its production in the weak s-process nucleosynthesis in He-burning and later stages. The chemodynamical models presented here confirm the behaviour of these two elements (i.e. [Co/Fe] vs. [Fe/H] ~constant and [Cu/Fe] decreasing with decreasing metallicities).

Chemically Peculiar A and F Stars with Enhanced s-process and Iron-peak Elements: Stellar Radiative Acceleration at Work

Abstract We present ≳15,000 metal-rich ([Fe/H] > −0.2 dex) A and F stars whose surface abundances deviate strongly from solar abundance ratios and cannot plausibly reflect their birth material composition. These stars are identified by their high [Ba/Fe] abundance ratios ([Ba/Fe] > 1.0 dex) in the LAMOST DR5 spectra analyzed by Xiang et al. They are almost exclusively main-sequence and subgiant stars with T eff ≳ 6300 K. Their distribution in the Kiel diagram (T eff– ) traces a sharp border at low temperatures along a roughly fixed-mass trajectory (around 1.4 M ⊙) that corresponds to an upper limit in convective envelope mass fraction of around 10−4. Most of these stars exhibit distinctly enhanced abundances of iron-peak elements (Cr, Mn, Fe, Ni) but depleted abundances of Mg and Ca. Rotational velocity measurements from GALAH DR2 show that the majority of these stars rotate slower than typical stars in an equivalent temperature range. These characteristics suggest that they are related to the so-called Am/Fm stars. Their abundance patterns are qualitatively consistent with the predictions of stellar evolution models that incorporate radiative acceleration, suggesting they are a consequence of stellar internal evolution, particularly involving the competition between gravitational settling and radiative acceleration. These peculiar stars constitute 40% of the whole population of stars with mass above 1.5 M ⊙, affirming that “peculiar” photospheric abundances due to stellar evolution effects are a ubiquitous phenomenon for these intermediate-mass stars. This large sample of Ba-enhanced, chemically peculiar A/F stars with individual element abundances provides the statistics to test more stringently the mechanisms that alter the surface abundances in stars with radiative envelopes.

Detailed Iron-peak Element Abundances in Three Very Metal-poor Stars*

Abstract We have obtained new detailed abundances of the Fe-group elements Sc through Zn (Z = 21–30) in three very metal-poor ([Fe/H] ≈ −3) stars: BD+03o740, BD−13o3442, and CD−33o1173. High-resolution ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra in the wavelength range 2300–3050 Å were gathered, and complemented by an assortment of optical echelle spectra. The analysis featured recent laboratory atomic data for a number of neutral and ionized species for all Fe-group elements except Cu and Zn. A detailed examination of scandium, titanium, and vanadium abundances in large-sample spectroscopic surveys indicates that they are positively correlated in stars with [Fe/H] ≤ −2. The abundances of these elements in BD+03o740, BD−13o3442, CD−33o1173, and HD 84937 (studied in a previous paper of this series) are in accord with these trends and lie at the high end of the correlations. Six elements have detectable neutral and ionized features, and generally their abundances are in reasonable agreement. For Cr we find only minimal abundance disagreement between the neutral (mean of [Cr i/Fe] = +0.01) and ionized species (mean of [Cr ii/Fe] = +0.08), unlike most studies in the past. The prominent exception is Co, for which the neutral species indicates a significant overabundance (mean of [Co i/H] = −2.53), while no such enhancement is seen for the ionized species (mean of [Co ii/H] = −2.93). These new stellar abundances, especially the correlations among Sc, Ti, and V, suggest that models of element production in early high-mass metal-poor stars should be revisited.

HD 54361: evidence for the status of a low mass TP-AGB star

The mass of the cool carbon star HD 54361 was suspected to be high, assuming the membership in the stellar association CMa OB1, in conflict with the theory of stellar evolution. The Gaia parallax and a high-resolution spectrum in the wavelength region from about 4800 to 6900 A was employed in order to update the distance, atmospheric parameters and abundances using the method of spectral synthesis and new model atmospheres. Absorption lines are broad in the spectrum presumably because of a large macroturbulence, $\xi _{\mathit{RT}} = 10 \pm 1~\mbox{km}\,\mbox{s}^{-1}$. Radial-velocity monitoring revealed a scatter of about 4 km s−1, which reflects probably a large scale convection in the atmosphere. The effective temperature was found to be about 3200 K by modelling the wings of Na i D doublet, a somewhat higher than that estimated from the angular diameter. The difference in the temperature could be due to the light/colour variation. The abundances of iron peak elements calculated from less blended lines blueward of 5000 A are near solar, $[\text{M/H}] = 0.0 \pm 0.2~\mbox{dex}$. The neutron-capture elements are enhanced up to 1 dex. The parallax leads to the distance of $554^{+ 49}_{-41}~\mbox{pc}$, the absolute magnitude, $M_{K} = -7.7 \pm 0.2~\mbox{mag}$, and the bolometric magnitude, $M_{\mathit{bol}}(K) = -4.6\pm 0.2~\mbox{mag}$. The observed stellar parameters and abundances are typical values for the low mass ${\sim} 1.8~M_{\odot }$ star on the TP-AGB branch according to the isochrone calculated for $\log \mathit{Age}(\mathit{yr}) = 9.20$ with solar initial metallicity. HD 54361 possibly belongs to the spike in the TP-AGB star numbers associated with the AGB-boosting effect.

Iron-peak elements Sc, V, Mn, Cu, and Zn in Galactic bulge globular clusters

Aims. Globular clusters are tracers of the history of star formation and chemical enrichment in the early Galaxy. Their abundance pattern can help understanding their chemical enrichment processes. In particular, the iron-peak elements have been relatively little studied so far in the Galactic bulge. Methods. The main aim of this work is to verify the strength of abundances of iron-peak elements for chemical tagging in view of identifying different stellar populations. Besides, the nucleosynthesis processes that build these elements are complex, therefore observational data can help constraining theoretical models, as well as give suggestions as to the kinds of supernovae that enriched the gas before these stars formed. Results. The abundances of iron-peak elements are derived for the sample clusters, and compared with bulge field, and thick disk stars. We derived abundances of the iron-peak elements Sc, V, Mn, Cu, and Zn in individual stars of five bulge globular clusters (NGC 6528, NGC 6553, NGC 6522, NGC 6558, HP 1), and of the reference thick disk/or inner halo cluster 47 Tucanae (NGC 104). High resolution spectra were obtained with the UVES spectrograph at the Very Large Telescope over the years. Conclusions. The sample globular clusters studied span metallicities in the range –1.2 ≲ [Fe/H] ≲ 0.0. V and Sc appear to vary in lockstep with Fe, indicating that they are produced in the same supernovae as Fe. We find that Mn is deficient in metal-poor stars, confirming that it is underproduced in massive stars; Mn-over-Fe steadily increases at the higher metallicities due to a metallicity-dependent enrichment by supernovae of type Ia. Cu behaves as a secondary element, indicating its production in a weak-s process in massive stars. Zn has an alpha-like behaviour at low metallicities, which can be explained in terms of nucleosynthesis in hypernovae. At the metal-rich end, Zn decreases with increasing metallicity, similarly to the alpha-elements.

Investigation of the dependence of rare-earth element abundances on the effective temperature and magnetic field in the atmospheres of chemically peculiar (Ap) stars

We have derived the abundances of the rare-earth elements (REEs) Ce, Pr, Nd, and Eu in the atmospheres of 26 magnetic peculiar (Ap) stars in the range of effective temperatures 7000–10 000 K from spectra with resolutions R = 48 000 and 80 000 and investigated the dependence of the CePrNdEu anomalies (the difference in the element abundances determined separately from lines of the first and second ionization stages) on the effective temperature. The REE anomaly is shown to decrease with increasing effective temperature virtually to the point of disappearance for all of the investigated elements, except Eu. For the best-studied element Nd the Nd anomaly has also been found to decrease with increasing magnetic field strength for cool stars. For hot stars there is no Nd anomaly in a wide range of magnetic field strengths. Since the presence of anomalies in cool Ap stars is associated with the REE concentration in the upper atmospheric layers, the lower boundary of the REE layer apparently sinks into deeper layers with increasing effective temperature and magnetic field, causing the anomalies to disappear. We have detected an anticorrelation between the abundances of iron-peak elements and rare-earth elements, which serves as additional evidence for different stratification of these elements in the atmospheres of Ap stars.

Two groups of red giants with distinct chemical abundances in the bulge globular cluster NGC 6553 through the eyes of APOGEE

Multiple populations revealed in globular clusters (GCs) are important windows to the formation and evolution of these stellar systems. The metal-rich GCs in the Galactic bulge are an indispensable part of this picture, but the high optical extinction in this region has prevented extensive research. In this work, we use the high resolution near-infrared (NIR) spectroscopic data from APOGEE to study the chemical abundances of NGC 6553, which is one of the most metal-rich bulge GCs. We identify ten red giants as cluster members using their positions, radial velocities, iron abundances, and NIR photometry. Our sample stars show a mean radial velocity of $-0.14\pm5.47$ km s$^{-1}$, and a mean [Fe/H] of $-0.15\pm 0.05$. We clearly separate two populations of stars in C and N in this GC for the first time. NGC 6553 is the most metal-rich GC where the multiple stellar population phenomenon is found until now. Substantial chemical variations are also found in Na, O, and Al. However, the two populations show similar Si, Ca, and iron-peak element abundances. Therefore, we infer that the CNO, NeNa, and MgAl cycles have been activated, but the MgAl cycle is too weak to show its effect on Mg. Type Ia and Type II supernovae do not seem to have significantly polluted the second generation stars. Comparing with other GC studies, NGC 6553 shows similar chemical variations as other relatively metal-rich GCs. We also confront current GC formation theories with our results, and suggest possible avenues for improvement in the models.

THE ORIGIN OF LOW [α/Fe] RATIOS IN EXTREMELY METAL-POOR STARS

We show that the low ratios of $\alpha$ elements (Mg, Si, and Ca) to Fe recently found for a small fraction of extremely metal-poor stars can be naturally explained with the nucleosynthesis yields of core-collapse supernovae, i.e., $13-25M_\odot$ supernovae, or hypernovae. For the case without carbon enhancement, the ejected iron mass is normal, consistent with observed light curves and spectra of nearby supernovae. On the other hand, the carbon enhancement requires much smaller iron production, and the low [$\alpha$/Fe] of carbon enhanced metal-poor stars can also be reproduced with $13-25M_\odot$ faint supernovae or faint hypernovae. Iron-peak element abundances, in particular Zn abundances, are important to put further constraints on the enrichment sources from galactic archaeology surveys.

Fluorine abundances in dwarf stars of the solar neighbourhood

In spite of many observational efforts aiming to characterize the chemical evolution of our Galaxy, not much is known about the origin of fluorine (F). Models suggest that the F found in the Galaxy might have been produced mainly in three different ways, namely, Type II supernovae, asymptotic giant branch nucleosynthesis, or in the core of Wolf-Rayet stars. Only a few observational measurements of F abundances are available in the literature and mostly for objects whose characteristics might hamper an accurate determination of fluorine abundance (e.g.,complex mixing and nucleosynthesis processes, external/internal contamination). We derive the F abundances for a set of nine cool main-sequence dwarfs in the solar neighbourhood, based on an unblended line of the HF molecule at 2.3 microns. In addition, we study the s-process elements of five of these stars. We acquire data using the high-resolution IR-spectrograph CRIRES and gather FEROS data from the European Southern Observatory archive. Several of the analysed stars seem to be slightly fluorine enhanced with respect to the Sun, although no correlation is found between the F abundance and the iron content. In addition, the most fluorine enriched stars are also yttrium and zirconium enriched, which suggests that AGB fluorine nucleosynthesis is the dominant source of fluorine production for the observed stars. Nevertheless, the correlation between [F/Fe] and the s-elements is rather weak and possibly masked by the uncertainties in the F abundance measurements. Finally, we compare our derived F abundances to previous measurements of alpha-element and iron-peak element abundances. Type II core collapse Supernovae do not appear to be the main site of F production for our targets, as no correlation seems to exist between the [F/Fe] and the [alpha/Fe] ratios.

Elemental abundance analyses with DAO spectrograms: XXXII. HR 6455 (A3 III), δ Aqr (A3 V), η Lep (F2 V), and 1 Boo (A1 V)

AbstractWe examine the sharp‐lined stars HR 6455 (A3 III, v sin i = 8.7 km s–1) and η Lep (F2 V, v sin i = 13.5 km s–1) as well as δ Aqr (A3 V, v sin i = 81 km s–1) and 1 Boo (A1 V, v sin i = 59 km s–1) to increase the number consistently analyzed A and F stars using high dispersion and high S/N (≥200) spectrograms obtained with CCD detectors at the long Coudé camera of the 1.22‐m telescope of the Dominion Astrophysical Observatory. Such studies contribute to understanding systematic abundance differences between normal and non‐magnetic main‐sequence band chemically peculiar A and early F stars. LTE fine analyses of HR 6455, δ Aqr, and 1 Boo using Kurucz's ATLAS suite programs show the same general elemental abundance trends with differences in the metal richness. Light and iron‐peak element abundances are generally solar or overabundant while heavy element and rare earth element abundances are overabundant. HR 6455 is an evolved Am star while δ Aqr and 1 Boo show the phenomenon to different extents. Most derived abundances of η Lep are solar (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The abundance of iron-peak elements and the dust composition in η Carinae: manganese

We study the chemical abundances of the strontium filament found in the ejecta of η Carinae. In particular, we derive the abundances of iron-peak elements from the spectra of their singly ionized ions present in the optical/infrared (IR) spectra. In this paper we analyse the spectrum of Mn II using a new non-local thermodynamic equilibrium (non-LTE) model for this system. In constructing this models we carried out theoretical calculations of radiative transition rates and electron impact excitation rate coefficients. We find that relative to Ni the gas-phase abundance ratio of Mn is roughly solar, similar to the Cr abundance but in contrast to the large enhancements in the abundances of Sc and Ti. We interpret this result as an indication of non-equilibrium condensation in the ejecta of η Carinae.

Chemical abundances in LMC stellar populations

The advent of the new class 8 meters telescopes allows for the first time the detailed study of numerous stars of galaxies other than our own. The Large Magellanic Cloud is one of the most interesting targets due to its proximity and its relatively face-on disk, permitting to separate populations pertaining to the different structures of the galaxy. We have used FLAMES (the Fibre Large Array Multi Element Spectrograph) at the VLT-UT2 telescope to obtain spectra of a large sample of stars from the Inner Disk of the LMC, $\sim$2 kpc from the center of the galaxy. We investigate the chemical abundances of iron-peak elements, heavy and light $s$-process elements, Cu, Na, Sc and $\alpha$-elements for a sample of red giant stars. Metallicities for the sample stars range from [Fe/H] = $-$1.76 to $-$0.02. LMC inner disk stars show a definitely different chemical pattern when compared to galactic stars of the same metallicity.

The Chemical Compositions of the Extreme Halo Stars HE 0107-5240 and HE 1327-2326 Inferred from Three-dimensional Hydrodynamical Model Atmospheres

We investigate the impact of realistic three-dimensional (3D) hydrodynamical model stellar atmospheres on the determination of elemental abundances in the carbon-rich, hyper-iron-poor stars HE 0107-5240 and HE 1327-2326. We derive the chemical compositions of the two stars by means of a detailed 3D analysis of spectral lines under the assumption of local thermodynamic equilibrium (LTE). The lower temperatures of the line-forming regions of the hydrodynamical models cause changes in the predicted spectral line strengths. In particular, we find the 3D abundances of C, N, and O to be lower by about -0.8 dex (or more) than estimated from a 1D analysis. The 3D abundances of iron peak elements are also decreased but by smaller factors (about -0.2 dex). We caution, however, that the neglected non-LTE effects might actually be substantial for these metals. We finally discuss possible implications for studies of early Galactic chemical evolution.

Abundance analysis of 5 early-type stars in the young open cluster IC 2391

Aims. It is unclear whether chemically peculiar stars of the upper main sequence represent a class completely distinct from normal A-type stars, or whether there exists a continuous transition from the normal to the most peculiar late F- to early B-type stars. A systematic abundance analysis of open cluster early-type stars would help to relate the observed differences of the chemical abundances of the photospheres to other stellar characteristics, without being concerned by possible different original chemical composition. Furthermore, if a continuous transition region from the very peculiar to the so called normal A–F stars exists, it should be possible to detect objects with mild peculiarities. Methods. As a first step of a larger project, an abundance analysis of 5 F–A type stars in the young cluster IC 2391 was performed using high resolution spectra obtained with the UVES instrument of the ESO VLT. Results. Our targets seem to follow a general abundance pattern: close to solar abundance of the light elements and iron peak elements, heavy elements are slightly overabundant with respect to the sun, similar to what was found in previous studies of normal field A-type stars of the galactic plane. We detected a weakly chemically peculiar star, HD 74044. Its element pattern contains characteristics of CP1 as well as CP2 stars, enhanced abundances of iron peak elements and also higher abundances of Sc, Y, Ba and Ce. We did not detect a magnetic field in this star (detection limit was 2 kG). We also studied the star SHJM 2, proposed as a pre-main sequence object in previous works. Using spectroscopy we found a high surface gravity, which suggests that the star is very close to the ZAMS.

Abundances of neutron capture elements in Population II stars

The paper presents new abundance results for 20 very metal poor Population II stars. High-resolution low-noise spectra in selected wavelength regions were obtained for each program star, and the spectra were examined using complete model atmosphere, line analysis, and spectrum synthesis techniques. The abundances of heavy elements formed predominantly by neutron capture processes are taken into account. It is shown that real star-to-star differences exist in the ratios of neutron capture element to iron-peak element abundances. Below stellar metallicities Fe/H of less than -2, all of the heavy-element abundance patterns indicate the presence of r-process synthesis events. 41 references.