Abundance Patterns Of Light Elements Research Articles

Signature of a Massive Rotating Metal-poor Star Imprinted in the Phoenix Stellar Stream*

Abstract The Phoenix stellar stream has a low intrinsic dispersion in velocity and metallicity that implies the progenitor was probably a low-mass globular cluster. In this work we use Magellan/Magellan Inamori Kyocera Echelle (MIKE) high-dispersion spectroscopy of eight Phoenix stream red giants to confirm this scenario. In particular, we find negligible intrinsic scatter in metallicity ( σ ( [ Fe II / H ] ) = 0.04 − 0.03 + 0.11 ) and a large peak-to-peak range in [Na/Fe] and [Al/Fe] abundance ratios, consistent with the light element abundance patterns seen in the most metal-poor globular clusters. However, unlike any other globular cluster, we also find an intrinsic spread in [Sr ii/Fe] spanning ∼1 dex, while [Ba ii/Fe] shows nearly no intrinsic spread ( σ ( [ Ba II / H ] ) = 0.03 − 0.02 + 0.10 ). This abundance signature is best interpreted as slow-neutron-capture element production from a massive fast-rotating metal-poor star (15–20 M ⊙, v ini/v crit = 0.4, [Fe/H] = −3.8). The low inferred cluster mass suggests the system would have been unable to retain supernovae ejecta, implying that any massive fast-rotating metal-poor star that enriched the interstellar medium must have formed and evolved before the globular cluster formed. Neutron-capture element production from asymptotic giant branch stars or magneto-rotational instabilities in core-collapse supernovae provide poor fits to the observations. We also report one Phoenix stream star to be a lithium-rich giant (A(Li) = 3.1 ± 0.1). At [Fe/H ] = −2.93; it is among the most metal-poor lithium-rich giants known.

Heavy-element Abundances in P-rich Stars: A New Site for the s-process?

Abstract The recently discovered phosphorus-rich stars pose a challenge to stellar evolution and nucleosynthesis theory, as none of the existing models can explain their extremely peculiar chemical abundances pattern. Apart from the large phosphorus enhancement, such stars also show enhancement in other light (O, Mg, Si, and Al) and heavy (e.g., Ce) elements. We have obtained high-resolution optical spectra of two optically bright phosphorus-rich stars (including a new P-rich star), for which we have determined a larger number of elemental abundances (from C to Pb). We confirm the unusual light-element abundance pattern with very large enhancements of Mg, Si, Al, and P, and possibly some Cu enhancement, but the spectrum of the new P-rich star are the only ones to reveal some C(+N) enhancement. When compared to other appropriate metal-poor and neutron-capture enhanced stars, the two P-rich stars show heavy-element overabundances similar to low neutron density s-process nucleosynthesis, with high first-peak (Sr, Y, and Zr) and second-peak (Ba, La, Ce, and Nd) element enhancements (even some Pb enhancement in one star) and a negative [Rb/Sr] ratio. However, this s-process is distinct from the one occurring in asymptotic giant branch (AGB) stars. The notable distinctions encompass larger [Ba/La] and lower Eu and Pb than their AGB counterparts. Our observations should guide stellar nucleosynthesis theoreticians and observers to identify the P-rich star progenitor, which represents a new site for s-process nucleosynthesis, with important implications for the chemical evolution of our Galaxy.

OBSERVATIONAL CONSTRAINTS ON FIRST-STAR NUCLEOSYNTHESIS. II. SPECTROSCOPY OF AN ULTRA METAL-POOR CEMP-no STAR*

ABSTRACT We report on the first high-resolution spectroscopic analysis of HE 0020–1741, a bright (V = 12.9), ultra metal-poor ( = −4.1), carbon-enhanced ( = +1.7) star selected from the Hamburg/ESO Survey. This star exhibits low abundances of neutron-capture elements ( = −1.1) and an absolute carbon abundance A(C) = 6.1; based on either criterion, HE 0020–1741 is subclassified as a carbon-enhanced metal-poor star without enhancements in neutron-capture elements (CEMP-no). We show that the light-element abundance pattern of HE 0020–1741 is consistent with predicted yields from a massive (M = ), primordial-composition, supernova (SN) progenitor. We also compare the abundance patterns of other ultra metal-poor stars from the literature with available measures of C, N, Na, Mg, and Fe abundances with an extensive grid of SN models (covering the mass range ), in order to probe the nature of their likely stellar progenitors. Our results suggest that at least two classes of progenitors are required at , as the abundance patterns for more than half of the sample studied in this work (7 out of 12 stars) cannot be easily reproduced by the predicted yields.

Gas expulsion versus gas retention: what process dominates in young massive clusters?

The ability of young stellar clusters to expel or retain the gas left over after a first episode of star formation is a central issue in all models aiming to explain multiple stellar populations and the peculiar light element abundance patterns in globular clusters. Recent attempts to detect the gas left over from star formation in present day clusters with masses similar to those of globular clusters did not reveal a significant amount of gas in the majority of them, which strongly restricts the scenarios of multiple stellar population formation. Here the conditions required to retain the gas left over from star formation within the natal star forming cloud are revised. It is shown that the usually accepted concept regarding the thermalization of the star cluster kinetic energy due to nearby stellar winds and SNe ejecta collisions must be taken with care in the case of very compact and dense star forming clouds where three star formation regimes are possible if one considers different star formation efficiencies and mass concentrations. The three possible regimes are well separated in the half-mass radius and in the natal gas central density vs pre-stellar cloud mass parameter space. The two gas free clusters in the Antennae galaxies and the gas rich cluster with a similar mass and age in the galaxy NGC 5253 appear in different zones in these diagrams. The critical lines obtained for clusters with a solar and a primordial gas metallicity are compared.

CHEMICAL ABUNDANCES IN NGC 5024 (M53): A MOSTLY FIRST GENERATION GLOBULAR CLUSTER

ABSTRACT We present the Fe, Ca, Ti, Ni, Ba, Na, and O abundances for a sample of 53 red giant branch stars in the globular cluster (GC) NGC 5024 (M53). The abundances were measured from high signal-to-noise medium resolution spectra collected with the Hydra multi-object spectrograph on the Wisconsin–Indiana–Yale–NOAO 3.5 m telescope. M53 is of interest because previous studies based on the morphology of the cluster’s horizontal branch suggested that it might be composed primarily of first generation (FG) stars and differ from the majority of other GCs with multiple populations, which have been found to be dominated by the second generation (SG) stars. Our sample has an average [Fe/H] = −2.07 with a standard deviation of 0.07 dex. This value is consistent with previously published results. The alpha-element abundances in our sample are also consistent with the trends seen in Milky Way halo stars at similar metallicities, with enhanced [Ca/Fe] and [Ti/Fe] relative to solar. We find that the Na–O anti-correlation in M53 is not as extended as other GCs with similar masses and metallicities. The ratio of SG to the total number of stars in our sample is approximately 0.27 and the SG generation is more centrally concentrated. These findings further support that M53 might be a mostly FG cluster and could give further insight into how GCs formed the light element abundance patterns we observe in them today.

The light element abundance distribution in NGC 5128 from planetary nebulae

The light element abundance pattern from many planetary nebulae (PNe) covering the upper 4 mag. of the [O III] luminosity function was observed with ESO VLT FORS1 multi-slit. Spectra of 51 PNe over the wavelength range 3500-7500 Angstrom were obtained in three fields at 4, 8 and 17 kpc, for a distance of 3.8 Mpc. Emission line ratios are entirely typical of PN such as in the Milky Way. The temperature sensitive [O III]4363A line was weakly detected in 10 PNe, both [O II] and [O III] lines were detected in 30 PNe, and only the bright [O III]5007A line in 7 PN. Cloudy photoionization models were run to match the spectra by a spherical, constant density nebula ionized by a black body central star. He, N, O and Ne abundances with respect to H were determined and, for brighter PNe, S and Ar; central star luminosities and temperatures are also derived. For 40 PNe with Cloudy models, from the upper 2 mag. of the luminosity function, the most reliably estimated element, oxygen, has a mean 12+log(O/H) of 8.52. No obvious radial gradient is apparent in O/H over a range 2-20 kpc. Comparison of the PN abundances with the stellar population, from the spectra of the integrated starlight on the multi-slits and photometric studies, suggests [Fe/H]=-0.4 and [O/Fe]=0.25. The masses of the PN central stars in NGC 5128 from model tracks imply an epoch of formation more recent than for the minority young population from colour-magnitude studies. The PNe progenitors may belong to the young tail of a recent, minor, star formation episode or derive from other evolutionary channels.[Abridged]

A DIFFERENTIAL CHEMICAL ELEMENT ANALYSIS OF THE METAL-POOR GLOBULAR CLUSTER NGC 6397

We present chemical abundances in three red giants and two turn-off stars in the metal poor Galactic globular cluster (GC) NGC 6397 based on spectroscopy obtained with the MIKE high resolution spectrograph on the Magellan 6.5-m Clay telescope. Our results are based on a line-by-line differential abundance analysis relative to the well-studied red giant Arcturus and the Galactic halo field star Hip 66815. At a mean of -2.10 +/- 0.02 (stat.) +/- 0.07 (sys.) the differential iron abundance is in good agreement with other studies in the literature based on gf-values. As in previous, differential works we find a distinct departure from ionization equilibrium in that the abundances of Fe I and Fe II differ by ~0.1 dex, with opposite sign for the RGB and TO stars. The alpha-element ratios are enhanced to 0.4 (RGB) and 0.3 dex (TO), respectively, and we also confirm strong variations in the O, Na, and Al/Fe abundance ratios. Accordingly, the light-element abundance patterns in one of the red giants can be attributed to pollution by an early generation of massive SNe II. TO and RGB abundances are not significantly different, with the possible exception of Mg and Ti, which is, however, amplified by the patterns in one TO star, additionally belonging to this early generation of GC stars. We discuss interrelations of these light elements as a function of the GC metallicity.

A holistic abundance analysis of r-rich stars

The chemical abundances of metal-poor stars are an excellent test bed by which to set new constraints on models of neutron-capture processes at low metallicity. Some r-process-rich (hereafter r-rich) metal-poor stars, such as HD221170, show an overabundance of the heavier neutron-capture elements and excesses of lighter neutron-capture elements. The study of these r-rich stars could give us a better understanding of weak and main r-process nucleosynthesis at low metallicity. Based on conclusions from the observation of metal-poor stars and neutron-capture element nucleosynthesis theory, we set up a model to determine the relative contributions from weak and main r-processes to the heavy-element abundances in metal-poor stars. Using this model, we find that the abundance patterns of light elements for most sample stars are close to the pattern of weak r-process stars, and those of heavier neutron-capture elements very similar to the pattern of main r-process stars, while the lighter neutron-capture elements can be fitted by the mixing of weak and main r-process material. The production of weak r-process elements appears to be associated with the light elements, while the production of main r-process elements is almost decoupled from that of the light elements. We compare our results with the observed data at low metallicities, showing that the predicted trends are in good agreement with the observed trends, at least for the metallicity range [Fe/H] < -2.1. For most sample stars, the abundance patterns of both neutron-capture elements and light elements could be best explained by a star formed in a molecular cloud that has been polluted by both weak and main r-process material.

CHEMICAL ABUNDANCES FOR 855 GIANTS IN THE GLOBULAR CLUSTER OMEGA CENTAURI (NGC 5139)

We present elemental abundances for 855 red giant branch (RGB) stars in the globular cluster Omega Centauri (w Cen) from spectra obtained with the Blanco 4m telescope and Hydra multifiber spectrograph. The sample includes nearly all RGB stars brighter than V=13.5, and span's w Cen's full metallicity range. The heavy alpha elements (Si, Ca, and Ti) are generally enhanced by ~+0.3 dex, and exhibit a metallicity dependent morphology that may be attributed to mass and metallicity dependent Type II supernova (SN) yields. The heavy alpha and Fe-peak abundances suggest minimal contributions from Type Ia SNe. The light elements (O, Na, and Al) exhibit >0.5 dex abundance dispersions at all metallicities, and a majority of stars with [Fe/H]>-1.6 have [O/Fe], [Na/Fe], and [Al/Fe] abundances similar to those in monometallic globular clusters, as well as O-Na, O-Al anticorrelations and the Na-Al correlation in all but the most metal-rich stars. A combination of pollution from intermediate mass asymptotic giant branch (AGB) stars and in situ mixing may explain the light element abundance patterns. A large fraction (27%) of w Cen stars are O-poor ([O/Fe]<0) and are preferentially located within 5-10' of the cluster center. The O-poor giants are spatially similar, located in the same metallicity range, and are present in nearly equal proportions to blue main sequence stars. This suggests the O-poor giants and blue main sequence stars may share a common origin. [La/Fe] increases sharply at [Fe/H]>-1.6, and the [La/Eu] ratios indicate the increase is due to almost pure s-process production.

LIGHT ELEMENT ABUNDANCES IN TWO CHEMICALLY PECULIAR STARS: HD 104340 AND HD 206983

We derive carbon, nitrogen, oxygen, and lithium abundances as well as 12C/13C isotopic ratios in two chemically peculiar red giant stars: HD 104340 and HD 206983. The abundances were determined via spectrum synthesis of optical high-resolution spectra containing C2, CH, CN, [O I], and Li I lines. Our results indicate that HD 206983 is a barium star, while HD 104340, although showing enhancements of s-process elements, should not be considered as a classical barium star: its barium star nature can be explained by internal nucleosynthesis. The low metallicity giant HD 104340 can experience deeper convective mixing and, consequently, a larger dredge-up of CNO-cycle products compared to normal red giants. The light element abundance pattern of HD 104340 resembles anomalies resulting from the appearance on the stellar surface of material enriched by triple-α and CNO cycling.

CNO and Li abundances in Barium-enriched stars

AbstractCNO and Li abundances and 12C/13C isotopic ratios have been derived for two metal-poor Ba giants, HD 104340 and HD 206983, and two CH subgiants, HD 50264 and HD 87080. High resolution spectra obtained with the 1.52 m telescope and the FEROS echelle spectrograph at ESO, La Silla, Chile, were used in this study. CNO and Li abundances so as 12C/13C isotopic ratios were determined by applying the synthetic spectrum method to the lines of C2, CH, CN, [O I’ and Li I. Our analysis showed that the giant stars studied here have quite different natures: HD 206983 is a metal-poor barium star while HD 104340, although showing enhancement of s-process elements, should not be considered as a classical barium star: its barium syndrome can be explained by internal nucleosynthesis. The low metallicity giant HD 104340 can experience deeper convective mixing and, consequently, a larger dredge-up of CNO-cycle products compared to normal red giants. Light element abundance pattern of HD 104340 resembles anomalies resulting from the appearance on the stellar surface of material enriched in triple-α and CNO cycling.

Beryllium in Lithium‐deficient F and G Stars

We present the results of an extensive search, conducted at the Canada-France-Hawaii 3.6-m telescope, for beryllium (Be) in the atmospheres of lithium-deficient F and G dwarfs. We also report revised lithium (Li) estimates for the entire sample using previously published equivalent widths and updated, consistently calculated stellar parameters. Abundances derived from an LTE analysis of the Li and Be line-forming regions confirm the suspicion that F stars which deplete Li by factors of 10-200 may also be beryllium deficient. Photospheric Be concentrations range from near meteoritic levels in G dwarfs to factors of 10-100 below this assumed initial abundance in hotter stars. Moreover, significant Be deficiencies appear in stars that populate a 600 K wide effective temperature window centered on 6500 K. This Be abundance gap is reminiscent of the Li gap observed in open clusters. Also, the discovery of 12 probable "110 Herculis" stars, objects that exhibit a depleted, but detected, surface concentration of both Li and Be, provides a powerful means of differentiating between the possible physical processes responsible for observed light element abundance patterns. Indeed, the Be data presented here, in conjunction with the newly calculated Li abundances, lead to the following conclusions regarding the hypothesized, light element depletion scenarios: Mass loss cannot account for stars with severely depleted (but detected) Li and moderate Be deficiencies. The predicted timescales for surface depletion due to microscopic diffusion are too long for significant Li and Be deficiencies to develop in cool (Teff ≤ 6200) stars; nevertheless, underabundances are observed in these stars. Diffusion theory also predicts Li and Be depletion rates to be comparable, but it is evident that Li and Be depletion proceed at different speeds. Models of mixing induced by internal gravity waves cannot explain mild Be deficiencies in cool dwarfs. A key meridional circulation prediction regarding the efficiency and severity of Li and Be dilution is shown to be fallible. However, rotationally induced mixing, a turbulent blending of material beneath the surface convection zone due to the onset of instabilities from superficial angular momentum loss, predicts both the observed light element depletion morphology as well as the existence of 110 Her analogs. These "Yale" mixing models provide, therefore, the most plausible explanation, of those presented, for the observed Li and Be abundances.