Abundances In Metal-poor Stars Research Articles

Neutron-Capture Elements in the Early Galaxy

The content of neutron-capture (trans-iron-peak) elements in the low-metallicity Galactic halo varies widely from star to star. The differences are both in bulk amount of the neutron-capture elements with respect to lighter ones and in element-to-element ratios among themselves. Several well-defined abundance distributions have emerged that reveal characteristic rapid and slow neutron-capture nucleosynthesis patterns. In this review we summarize these observed metal-poor star's abundances, contrasting them with the Solar-system values, comparing them to theoretical predictions, using them to assess the types of stars responsible for their specific anomalies, and speculating on the timing and nature of early Galactic nucleosynthesis.

β-decay studies of r-process nuclei at NSCL

Observed neutron-capture elemental abundances in metal-poor stars, along with ongoing analysis of the extremely metal-poor Eu-enriched sub-class provide new guidance for astrophysical models aimed at finding the r-process sites. The present paper emphasizes the importance of nuclear physics parameters entering in these models, particularly β-decay properties of neutron-rich nuclei. In this context, several r-process motivated β-decay experiments performed at the National Superconducting Cyclotron Laboratory (NSCL) are presented, including a summary of results and impact on model calculations.

Chemical Evolution of Heavy Elements in the Early Galaxy: Implications for Stellar Sources

AbstractAn overview of the sources for heavy elements in the early Galaxy is given. It is shown that observations of abundances in metal-poor stars can be used along with a basic understanding of stellar models to guide the search for the source of the heavy r-process nuclei (r-nuclei). Observations show that this source produces very little of the elements from C through Zn, including Fe. This strongly suggests that O–Ne–Mg core-collapse supernovae (SNe) from progenitors of ∼8–11 M⊙ are the source for the heavy r-nuclei. It is shown that a two-component model based on the abundances of Fe (from Fe core-collapse SNe) and Eu (from O–Ne–Mg core-collapse SNe) gives very good quantitative predictions for the abundances of all the other elements in metal-poor stars.

Nucleosynthesis in the Early Galaxy

Recent observations of r-process-enriched metal-poor star abundances reveal a non-uniform abundance pattern for elements $Z\leq47$. Based on non-correlation trends between elemental abundances as a function of Eu-richness in a large sample of metal-poor stars, it is shown that the mixing of a consistent and robust light element primary process (LEPP) and the r-process pattern found in r-II metal-poor stars explains such apparent non-uniformity. Furthermore, we derive the abundance pattern of the LEPP from observation and show that it is consistent with a missing component in the solar abundances when using a recent s-process model. As the astrophysical site of the LEPP is not known, we explore the possibility of a neutron capture process within a site-independent approach. It is suggested that scenarios with neutron densities $n_{n}\leq10^{13}$ $cm^{-3}$ or in the range $n_{n}\geq10^{24}$ $cm^{-3}$ best explain the observations.

A Search for Nitrogen‐enhanced Metal‐poor Stars

Theoretical models of very metal-poor intermediate-mass Asymptotic Giant Branch (AGB) stars predict a large overabundance of primary nitrogen. The very metal-poor, carbon-enhanced, s-process-rich stars, which are thought to be the polluted companions of now-extinct AGB stars, provide direct tests of the predictions of these models. Recent studies of the carbon and nitrogen abundances in metal-poor stars have focused on the most carbon-rich stars, leading to a potential selection bias against stars that have been polluted by AGB stars that produced large amounts of nitrogen, and hence have small [C/N] ratios. We call these stars Nitrogen-Enhanced Metal-Poor (NEMP) stars, and define them as having [N/Fe] > +0.5 and [C/N] < -0.5. In this paper, we report on the [C/N] abundances of a sample of 21 carbon-enhanced stars, all but three of which have [C/Fe] < +2.0. If NEMP stars were made as easily as Carbon-Enhanced Metal-Poor (CEMP) stars, then we expected to find between two and seven NEMP stars. Instead, we found no NEMP stars in our sample. Therefore, this observational bias is not an important contributor to the apparent dearth of N-rich stars. Our [C/N] values are in the same range as values reported previously in the literature (-0.5 to +2.0), and all stars are in disagreement with the predicted [C/N] ratios for both low-mass and high-mass AGB stars. We suggest that the decrease in [C/N] from the low-mass AGB models is due to enhanced extra-mixing, while the lack of NEMP stars may be caused by unfavorable mass ratios in binaries or the difficulty of mass transfer in binary systems with large mass ratios.

Lithium abundances in metal-poor stars

Aims. Lithium abundances for 19 metal-poor stars are determined using high-resolution spectroscopy. The abundances of stars on the lithium plateau are discussed. Methods. All abundance results are derived from NLTE statistical equilibrium calculations and spectrum synthesis methods. Results. In agreement with previous analyses it is found that excitation and de-excitation due to hydrogen collisions are negligible for the lithium line formation process, while charge transfer reactions are an important source of thermalization. However, the resulting NLTE effects on the determination of lithium abundances for metal-poor stars are negligible (<0.06 dex). Conclusions. The mean lithium abundance for stars on the lithium plateau determined from NLTE analyses is A(Li) ∼ 2.26, while it is 2.21 dex when charge transfer reactions are included. The latter result enhances the discrepancy between the observed lithium abundances and the primordial lithium abundance as inferred by the WMAP analysis of the cosmic microwave background. This discrepancy may be explained by metal diffusion.

Fundamental parameters and abundances of metal-poor stars: the SDSS standard BD +17 4708

The atmospheric parameters and iron abundance of the Sloan Digital Sky Survey (SDSS) spectrophotometric standard star BD +17 4708 are critically examined using up-to-date Kurucz model atmospheres, LTE line formation calculations, and reliable atomic data. We find Teff = 6141+-50 K, log g = 3.87+-0.08, and [Fe/H]=-1.74+-0.09. The line-of-sight interstellar reddening, bolometric flux, limb-darkened angular diameter, stellar mass, and the abundances of Mg, Si, and Ca are also obtained. This star is a unique example of a moderately metal-poor star for which the effective temperature can be accurately constrained from the observed spectral energy distribution (corrected for reddening). Such analysis leads to a value that is higher than most spectroscopic results previously reported in the literature (~5950 K). We find that the ionization balance of Fe lines is satisfied only if a low Teff (~5950 K) is adopted. With our preferred Teff (6141 K), the mean iron abundance we obtain from the FeII lines is lower by about 0.15 dex than that from the FeI lines, and therefore, the discrepancy between the mean iron abundance from FeI and FeII lines cannot be explained by overionization by UV photons as the main non-LTE effect. We also comment on non-LTE effects and the importance of inelastic collisions with neutral H atoms in the determination of oxygen abundances in metal-poor stars from the 777 nm OI triplet. (Abridged)

Oxygen abundances in metal-poor subgiants as determined from [O I], O I and OH lines

The debate on the oxygen abundances of metal-poor stars has its origin in contradictory results obtained using different abundance indicators. To achieve a better understanding of the problem we have acquired high quality spectra with the Ultraviolet and Visual Echelle Spectrograph at VLT, with a signal-to-noise of the order of 100 in the near ultraviolet and 500 in the optical and near infrared wavelength range. Three different oxygen abundance indicators, OH ultraviolet lines around 310.0 nm, the [OI] line at 630.03 nm and the OI lines at 777.1-5 nm were observed in the spectra of 13 metal-poor subgiants with -3.0<=[Fe/H]<=-1.5. Oxygen abundances were obtained from the analysis of these indicators which was carried out assuming local thermodynamic equilibrium and plane-parallel model atmospheres. Abundances derived from OI were corrected for departures from local thermodynamic equilibrium. Stellar parameters were computed using Teff-vs-color calibrations based on the infrared flux method and Balmer line profiles, Hipparcos parallaxes and FeII lines. [O/Fe] values derived from the forbidden line at 630.03 nm are consistent with an oxygen/iron ratio that varies linearly with [Fe/H] as [O/Fe]}=-0.09(+/-0.08)[Fe/H]+0.36(+/-0.15). Values based on the OI triplet are on average 0.19+/-0.22 dex(s.d.) higher than the values based on the forbidden line while the agreement between OH ultraviolet lines and the forbidden line is much better with a mean difference of the order of -0.09+/-0.25 dex(s.d.). In general, our results follow the same trend as previously published results with the exception of the ones based on OH ultraviolet lines. In that case our results lie below the values which gave rise to the oxygen abundance debate for metal-poor stars.

Estimation of Carbon Abundances in Metal-Poor Stars. I. Application to the Strong G-Band Stars of Beers, Preston, and Shectman

We develop and test a method for the estimation of metallicities ([Fe/H]) and carbon abundance ratios ([C/Fe]) for carbon-enhanced metal-poor (CEMP) stars based on the application of artificial neural networks, regressions, and synthesis models to medium-resolution (1–2 A) spectra and J - K colors. We calibrate this method by comparison with metallicities and carbon abundance determinations for 118 stars with available high-resolution analyses reported in the recent literature. The neural network and regression approaches make use of a previously defined set of line-strength indices quantifying the strength of the Ca II K line and the CH G band, in conjunction with J - K colors from the Two Micron All Sky Survey Point Source Catalog. The use of near-IR colors, as opposed to broadband B - V colors, is required because of the potentially large affect of molecular carbon bands on bluer color indices. We also explore the practicality of obtaining estimates of carbon abundances for metal-poor stars from the spectral information alone, i.e., without the additional information provided by photometry, as many future samples of CEMP stars may lack such data. We find that although photometric information is required for the estimation of [Fe/H], it provides little improvement in our derived estimates of [C/Fe], and hence, estimates of carbon-to-iron ratios based solely on line indices appear sufficiently accurate for most purposes. Although we find that the spectral synthesis approach yields the most accurate estimates of [C/Fe], in particular for the stars with the strongest molecular bands, it is only marginally better than is obtained from the line index approaches. Using these methods we are able to reproduce the previously measured [Fe/H] and [C/Fe] determinations with an accuracy of ≈0.25 dex for stars in the metallicity interval -5.5 ≤ [Fe/H] ≤ -1.0 and with 0.2 ≤ (J - K)0 ≤ 0.8. At higher metallicity, the Ca II K line begins to saturate, especially for the cool stars in our program, and hence, this approach is not useful in some cases. As a first application, we estimate the abundances of [Fe/H] and [C/Fe] for the 56 stars identified as possibly carbon-rich, relative to stars of similar metal abundance, in the sample of strong G-band stars discussed by Beers, Preston, and Shectman.

Errata: Sulfur Abundances in Metal-Poor Stars Based on OAO-1.88m/HIDES Spectra

Errata: Sulfur Abundances in Metal-Poor Stars Based on OAO-1.88m/HIDES Spectra

First enrichment of r-process elements in our Galaxy

Observed large scatters in Eu, Ba, and Sr abundances of metal-poor stars provide clues to determine the site(s) of r -process. In particular, our observation using Subaru HDS gives sub-solar values of [Eu/Fe] for three extremely metal-poor stars. Using an inhomogeneous chemical evolution model, we show that the dominant source of Eu to be the low-mass end of the supernova mass range to account for the observation. On the other hand, scatters in [Sr/Ba] suggest existence of “weak” r -process, which produces lighter nuclei. We also discuss the yield of weak r -process, using new data of Pd abundances.

Early star formation in the Galaxy from beryllium and oxygen abundances

We investigate the evolution of the star formation rate in the early Galaxy using beryllium and oxygen abundances in metal poor stars. Specifically, we show that stars belonging to two previously identified kinematical classes (the so-called ``accretion'' and ``dissipative'' populations) are neatly separated in the [O/Fe] vs. Log(Be/H) diagram. The dissipative population follows the predictions of our model of Galactic evolution for the thick disk component, suggesting that the formation of this stellar population occurred on a timescale significantly longer (by a factor ~5-10) than the accretion component. The latter shows a large scatter in the [O/Fe] vs. Log(Be/H) diagram, probably resulting from the inhomogeneous enrichment in oxygen and iron of the protogalactic gas. Despite the limitation of the sample, the data suggest that the combined use of products of spallation reactions (like beryllium) and elemental ratios of stellar nucleosynthesis products (like [O/Fe]) can constrain theoretical models for the formation and early evolution of our Galaxy.

Sulfur Abundances in Metal-Poor Stars Based on OAO-1.88 m/HIDES Spectra

Abstract The LTE abundances of sulfur (S) of 21 metal-poor stars and one normal star were explored in the metallicity range of $-3 &amp;lt; \mathrm{[Fe/H]} \leq 0$, based on the equivalent widths of the S I (1) 9212, 9237 Å and S I (6) 8693, 9894 Å lines measured on high-resolution spectra, which were observed by the OAO 1.88-m telescope equipped with HIDES. Our main results are: (1) The abundances derived from the S I (6) lines are consistent with those from the S I (1) lines among our sample stars in the range of $\mathrm{[Fe/H]} \gt-2 \mathrm{[Fe/H]} \gt-2$ with an average difference of $+0.03 \pm 0.05 \,\mathrm{dex}$, whereas a significant discrepancy is observed in the range of $\mathrm{[Fe/H]} \leq-2$. (2) The behavior of $\mathrm{[S(6)/Fe]}$ versus $\mathrm{[Fe/H]}$ of our halo sample stars exhibits a nearly flat trend with an average of $+0.62 \pm 0.09 \,\mathrm{dex}$ in the range of $-3 &amp;lt; \mathrm{[Fe/H]} &amp;lt;-1.25$, and shows a distribution around $+0.29 \,\mathrm{dex}$ in $-1.25 \leq\mathrm{[Fe/H]}\leq-0.7$. Our sample stars with $-1.25 \leq \mathrm{[Fe/H]} \leq-0.5$ follow an increasing trend with decreasing $\mathrm{[Fe/H]}$. The behavior of $\mathrm{[S(1)/Fe]}$ of our sample stars also shows essentially the same trend as $\mathrm{[S(6)/Fe]}$, though it is quantitatively different. (3) The S behavior in the range of $-3 &amp;lt; \mathrm{[Fe/H]} \leq 0$ inferred from the abundances of multiplets 6 and 1 are qualitatively consistent with each other, and may be represented by a combination of a nearly flat trend and a linearly increasing trend with decreasing $\mathrm{[Fe/H]}$. A transition of the trend is likely to occur at $\mathrm{[Fe/H]} \sim-1.5 \,\mathrm{dex}$.

Nuclear physics and astrophysics of the r-process

Some nuclear and astrophysical aspects of the r-process are discussed. Particular attention is paid to observations of abundances in metal-poor stars and their implications for the astrophysical site and yield patterns of the r-process. The effects of supernova neutrinos and related nuclear processes on the yield patterns are explored. The uncertainties in the theoretical nuclear input for the r-process are discussed and the need for experimental data is emphasized.

Abundances for metal-poor stars with accurate parallaxes

Abundances for elements and Fe in about 150 field subdwarfs and early subgiants with accurate parallaxes and kinematic data are used to discuss the run of abundance ratios in metal-poor stars in the solar neighborhood. Based on kinematics, we separated stars into two populations: the first one has a positive velocity of rotation around the galactic center, and it is likely to be related to the dissipational collapse of the galaxy; the second one has either negligible or negative rotational velocity, and it is likely related to an accretion component. The two populations show a large overlap in metallicity. However, they show distinct chemical properties. For the first population we found that there are close correlations (with small scatters around) of the rotational velocity with metallicity and with the Fe/α abundance ratio: this might be a signature of a not very fast collapse of the progenitor clouds, with enough time for a significant contribution by SNe Ia, although this result needs to be confirmed by a 3-D/non-LTE study. On the other side, the second population exhibits a larger scatter in both the above mentioned relations, and on average, a larger Fe/α ratio at a given metallicity, suggesting a larger scatter in ages. We argue that the lack of stars with moderate rotational velocities and high Fe/α abundance ratios is due to the short merging time for protogalactic clouds with prograde motion, while the presence of a group of counter-rotating stars with this characteristics indicates a much longer typical lifetimes for protogalactic fragments having such a motion. Finally, we found that perigalactic distances correlate with the Fe/α abundance ratios better than the apogalactic distances.

Abundances for metal-poor stars with accurate parallaxes

We present element-to-element abundance ratios measured from high dispersion spectra for 150 field subdwarfs and early subgiants with accurate Hipparcos parallaxes (errors <20%). For 50 stars new spectra were obtained with the UVES on Kueyen (VLT UT2), the McDonald 2.7 m telescope, and SARG at TNG. Additionally, literature equivalent widths were taken from the works by Nissen & Schuster, Fulbright, and Prochaska et al. to complement our data. The whole sample includes both thick disk and halo stars (and a few thin disk stars); most stars have metallicities in the range −2 < (Fe/H) < −0.6. We found our data, that of Nissen & Schuster, and that of Prochaska to be of comparable quality; results from Fulbright scatter a bit more, but they are still of very good quality and are extremely useful due to the large size of his sample. The results of the present analysis will be used in forthcoming papers to discuss the chemical properties of the dissipational collapse and accretion components of our Galaxy.

Stellar Sources for Heavyr‐Process Nuclei

The stellar sites and the complete mechanism of r-process nucleosynthesis are still unresolved issues. From consideration of the observed abundances in metal-poor stars, it is proposed that the production of heavy r-process nuclei (r-nuclei with mass numbers A > 130) is not related to the production of Fe group elements or of elements with lower atomic numbers: Na, Mg, Al, Si, Ca, Sc, and Ti. This requires that the production of heavy r-nuclei not occur in supernovae with extended shell structure but be associated with either bare neutron stars or Type II supernovae (SNe II) in the mass range 8 M_☉ < M < 10 M_☉. From the observations of stars with [Fe/H] ~ -3 but with high abundances of r-elements, it is clear that these r-process enrichments cannot represent the composition of the interstellar medium from which the stars were formed but must represent very local contamination from binary companions. Further evidence for very high enrichments of s-process elements in metal-poor stars also requires binary systems for explanation. We propose that the accretion-induced collapse (AIC) of a white dwarf into a neutron star in a binary system may be associated with the production of heavy r-nuclei and may provide occasional coupling of high r-process and high s-process enrichments in the envelopes of low-mass stars with low [Fe/H]. If we assume that the bulk of the heavy r-nuclei are produced in AIC events, then these events would have produced ~1.6 × 109 neutron stars in the Galaxy. A much larger number of white dwarf binaries would have resulted from the evolution of other binary systems. The AIC scenario removes the assignment in our earlier model that SNe II provide the bulk of heavy r-nuclei and relegates r-process production in SNe II to light r-nuclei with A ≾ 130. This new assignment gives Fe yields that are in accord with the observed values for most SNe II.

Oxygen line formation in late-F through early-K disk/halo stars

In order to investigate the formation of O i 7771–5 and [O i] 6300/6363 lines, extensive non-LTE calculations for neutral atomic oxygen were carried out for wide ranges of model atmosphere parameters, which are applicable to early-K through late-F halo/disk stars of various evolutionary stages. The formation of the triplet O i lines was found to be well described by the classical two-level-atom scattering model, and the non-LTE correction is practically determined by the parameters of the line-transition itself without any significant relevance to the details of the oxygen atomic model. This simplifies the problem in the sense that the non-LTE abundance correction is essentially determined only by the line-strength (), if the atmospheric parameters of Teff, , and ξ are given, without any explicit dependence of the metallicity; thus allowing a useful analytical formula with tabulated numerical coefficients. On the other hand, our calculations lead to the robust conclusion that LTE is totally valid for the forbidden [O i] lines. An extensive reanalysis of published equivalent-width data of O i 7771–5 and [O i] 6300/6363 taken from various literature resulted in the conclusion that, while a reasonable consistency of O i and [O i] abundances was observed for disk stars (), the existence of a systematic abundance discrepancy was confirmed between O i and [O i] lines in conspicuously metal-poor halo stars () without being removed by our non-LTE corrections, i.e., the former being larger by ~0.3 dex at . An inspection of the parameter-dependence of this discordance indicates that the extent of the discrepancy tends to be comparatively lessened for higher stars, suggesting the preference of dwarf (or subgiant) stars for studying the oxygen abundances of metal-poor stars.

The Status of Oxygen Abundances in Metal-Poor Stars

AbstractThe sensitivity of various methods of oxygen-abundance determination to the structure of the model atmosphere in a classical analysis is described, with emphasis on the treatment of convection. Oxygen-abundance results presented during this meeting are outlined. Because the oxygen diagnostics are sensitive in different ways to the choice of model stellar parameters and to the treatment of convection, it is not surprising that different methods applied by different groups give different results. Most are consistent with an increase in [O/Fe] as [Fe/H] declines, but the slope is uncertain still, and the scatter is large at very low abundances. The trend may continue to low [Fe/H], but more stars and more checks are needed.

On OH line formation and oxygen abundances in metal-poor stars

The formation of the UV OH spectral lines has been investigated for a range of stellar parameters in the light of 3D hydrodynamical model atmospheres. The low atmospheric temperatures encountered at low metallicities compared with the radiative equilibrium values enforced in classical 1D hydrostatic model atmospheres have a profound impact on the OH line strengths. As a consequence, the derived O abundances using 3D models are found to be systematically lower by more than 0.6 dex at [Fe/H] compared with previous 1D analyses, casting doubts on the recent claims for a monotonic increase in [O/Fe] towards lower metallicities. In fact, taken at face value the resulting 3D LTE trend is in rough agreement with the conventional [O/Fe] plateau. Caution must, however, be exercised in view of the remaining assumptions in the 3D calculations. We have verified that the stellar parameters remain essentially unchanged with 3D model atmospheres provided that the infrared flux method ( K), Hipparcos parallaxes () and Fe ii lines ( dex) are utilised, leaving the 3D O abundances from OH lines largely intact ( dex). Greater concern stems from possible departures from LTE in both the line formation and the molecular equilibrium, which, if present, would increase the derived O abundances again. Non-LTE line formation calculations with 1D model atmospheres suggest no significant steepening of the [O/Fe] trend even if the abundance corrections amount to about 0.2 dex for all investigated stellar parameters. We note, however, that the 3D case may not necessarily be as metallicity-independent. The apparent lack of laboratory or theoretical rate coefficients at the relevant temperatures for the involved molecular reactions unfortunately prevents a quantitative discussion on the possible effects of non-equilibrium chemistry.