Abundances Of Giants Research Articles

Lithium Abundances in Giants as a Function of Stellar Mass: Evidence for He Flash as the Source of Li Enhancement in Low-mass Giants

In this work, we studied the distribution of lithium abundances in giants as a function of stellar mass. We used a sample of 1240 giants common among Kepler photometric and LAMOST medium-resolution (R ≈ 7500) spectroscopic survey fields. The asteroseismic ΔP–Δν diagram is used to define core He-burning red clump giants and red giant branch stars with an inert He core. Li abundances have been derived using spectral synthesis for all sample stars. Directly measured values of asteroseismic parameters ΔP (or ΔΠ1) and Δν are either taken from the literature or measured in this study. Of the 777 identified red clump giants, we found 668 low-mass (≤2 M ⊙) primary red clump giants and 109 high-mass (>2 M ⊙) secondary red clump giants. Observed Li abundances in secondary red clump giants agree with the theoretical model predictions. The lack of Li-rich giants among secondary red clump giants and the presence of Li-rich, including super Li-rich, giants among primary red clump stars reinforces the idea that helium flash holds the key for Li enrichment among low-mass giants. The results will further constrain theoretical models searching for a physical mechanism for Li enhancement among low-mass red clump giants. Results also serve as observational evidence that only giants with mass less than ≈2 M ⊙ develop a degenerate He core and undergo He flash.

On Lithium-6 as a Diagnostic of the Lithium-enrichment Mechanism in Red Giants

Abstract High lithium-7 (7Li) abundances in giants are indicative of nonstandard physical processes affecting the star. Mechanisms that could produce this signature include contamination from an external source, such as planets, or internal production and subsequent mixing to the stellar surface. However, distinguishing between different families of solutions has proven challenging, and there is no current consensus model that explains all the data. The lithium-6 (6Li) abundance may be a potentially important discriminant, as the relative 6Li and 7Li abundances are expected to be different if the enrichment were to come from internal production or from engulfment. In this work, we model the 6Li and 7Li abundances of different giants after the engulfment of a substellar mass companion. Given that 6Li is more strongly affected by Galactic chemical evolution than 7Li, 6Li is not a good discriminant at low metallicities, where it is expected to be low in both star and planet. For modeled metallicities ([Fe/H] > −0.5), we use a “best-case” initial 6Li/7Li ratio equal to the solar value. 6Li increases significantly after the engulfment of a companion. However, at metallicities close to solar and higher, the 6Li signal does not last long in the stellar surface. As such, detection of surface 6Li in metal-rich red giants would most likely indicate the action of a mechanism for 6Li enrichment other than planet engulfment. At the same time, 6Li should not be used to reject the hypothesis of engulfment in a 7Li-enriched giant or to support a particular 7Li-enhancement mechanism.

APOGEE: the Sloan Digital Sky Survey Apache Point Observatory Galactic Evolution Experiment. Insights into the Galactic Disk: A Review

AbstractThe SDSS Apache Point Observatory Galactic Evolution Experiment (APOGEE) has collected high resolution near-IR spectra for several hundred thousand stars throughout the Milky Way. We review some of the results related to chemistry of stars in the disk, where APOGEE has a particular advantage by virtue of being able to work in more obscured areas. The ability to measure carbon and nitrogen abundances in giants in the near-IR provides insight into stellar ages. We summarize results on the variation of mean metallicity, metallicity distribution functions, and the [α/Fe]–[Fe/H] relation across the Galactic disk, as well as results on the structural parameters in mono-abundance populations. Many of these results suggest that radial migration has played a significant role in the Galactic disk. It may be possible to disentangle radial mixing using multi-element abundance patterns.

Abundances and physical parameters for stars in the open clusters NGC 5822 and IC 4756

Classical chemical analyses may be affected by systematic errors that would cause observed abundance differences between dwarfs and giants. For some elements, however, the abundance difference could be real. We address the issue by observing 2 solar--type dwarfs in NGC 5822 and 3 in IC 4756, and comparing their composition with that of 3 giants in either of the aforementioned clusters. We determine iron abundance and stellar parameters of the dwarf stars, and the abundances of calcium, sodium, nickel, titanium, aluminium, chromium, silicon and oxygen for both the giants and dwarfs. We acquired UVES high-resolution, of high signal--to--noise ratio (S/N) spectra. The width of the cross correlation profiles was used to measure rotation velocities. For abundance determinations, the standard equivalent width analysis was performed differentially with respect to the Sun. For lithium and oxygen, we derived abundances by comparing synthetic spectra with observed line features. We find an iron abundance for dwarf stars equal to solar to within the margins of error for IC 4756, and slightly above for NGC 5822 ([Fe/H]= 0.01 and 0.05 dex respectively). The 3 stars in NG 4756 have lithium abundances between Log N(Li) 2.6 and 2.8 dex, the two stars in NGC 5822 have Log N(Li) ~ 2.8 and 2.5, respectively. For sodium, silicon, and titanium, we show that abundances of giants are significantly higher than those of the dwarfs of the same cluster (about 0.15, 0.15, and 0.35 dex).

STELLAR NUCLEOSYNTHESIS IN THE HYADES OPEN CLUSTER

We report a comprehensive light-element (Li, C, N, O, Na, Mg, and Al) abundance analysis of three solar-type main sequence (MS) dwarfs and three red giant branch (RGB) clump stars in the Hyades open cluster using highresolution and high signal-to-noise spectroscopy. The abundances have been derived in a self-consistent fashion, and for each group (MS or RGB), the CNO abundances are found to be in excellent star-to-star agreement. Using the dwarfs to infer the initial composition of the giants, the combined abundance patterns confirm that the giants have undergone the first dredge-up and that material processed by the CN cycle has been mixed to the surface layers. The observed abundances are compared to predictions of a standard stellar model based on the ClemsonAmerican University of Beirut (CAUB) stellar evolution code. The model reproduces the observed evolution of the N and O abundances, as well as the previously derived 12 C/ 13 C ratio, but it fails to predict by a factor of 1.5 the observed level of 12 C depletion. A similar discord appears to exist in previously reported observed and modeled C abundances of giants in the Galactic disk. Random uncertainties in the mean abundances and uncertainties related to possible systematic errors in the Hyades dwarf and giant parameter scales cannot account for the discrepancy in the observed and modeled abundances. Li abundances are derived to determine if noncanonical extra mixing, like that seen in low-mass metal-poor giants, has occurred in the Hyades giants. The Li abundance of the giant γ Tau is in good accord with the predicted level of surface Li dilution, but a ∼0.35 dex spread in the giant Li abundances is found and cannot be explained by the stellar model. Possible sources of the spread are discussed; however, it is apparent that the differential mechanism responsible for the Li dispersion must be unrelated to the uniformly low 12 C abundances of the giants. Na, Mg, and Al abundances are derived as an additional test of our stellar model. All three elements are found to be overabundant by 0.2–0.5 dex in the giants relative to the dwarfs. Such large enhancements of these elements are not predicted by the stellar model, and non-LTE effects significantly larger (and, in some cases, of opposite sign) than those implied by extant literature calculations are the most likely cause.

First stars XII. Abundances in extremely metal-poor turnoff stars, and comparison with the giants

<i>Context. <i/>The detailed chemical abundances of extremely metal-poor (EMP) stars are key guides to understanding the early chemical evolution of the Galaxy. Most existing data, however, treat giant stars that may have experienced internal mixing later.<i>Aims. <i/>We aim to compare the results for giants with new, accurate abundances for all observable elements in 18 EMP turnoff stars.<i>Methods. <i/>VLT/UVES spectra at <i>R<i/> ~ 45 000 and <i>S<i/>/<i>N<i/> ~ 130 per pixel ( 330-1000 nm) are analysed with OSMARCS model atmospheres and the TURBOSPECTRUM code to derive abundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, and Ba.<i>Results. <i/>For Ca, Ni, Sr, and Ba, we find excellent consistency with our earlier sample of EMP giants, at all metallicities. However, our abundances of C, Sc, Ti, Cr, Mn and Co are ~0.2 dex larger than in giants of similar metallicity. Mg and Si abundances are ~0.2 dex lower (the giant [Mg/Fe] values are slightly revised), while Zn is again ~0.4 dex higher than in giants of similar [Fe/H] (6 stars only).<i>Conclusions. <i/>For C, the dwarf/giant discrepancy could possibly have an astrophysical cause, but for the other elements it must arise from shortcomings in the analysis. Approximate computations of granulation (3D) effects yield smaller corrections for giants than for dwarfs, but suggest that this is an unlikely explanation, except perhaps for C, Cr, and Mn. NLTE computations for Na and Al provide consistent abundances between dwarfs and giants, unlike the LTE results, and would be highly desirable for the other discrepant elements as well. Meanwhile, we recommend using the giant abundances as reference data for Galactic chemical evolution models.

First stars VI – Abundances of C, N, O, Li, and mixing in extremely metal-poor giants. Galactic evolution of the light elements

We have investigated the poorly-understood origin of nitrogen in the early Galaxy by determining N abundances in 35 extremely metal-poor halo giants (22 stars have [Fe/H]<-3.0) using the C and O abundances determined in Paper V. Because any dredge-up of CNO processed material to the surface may complicate the interpretation of CNO abundances in giants, we have also measured the surface abundance of lithium. Our sample shows a clear dichotomy between two groups of stars. The first group shows evidence of C to N conversion through CN cycling and strong Li dilution, a signature of mixing. The second group shows no evidence for C to N conversion, and Li is only moderately diluted, and we conclude that their C and N abundances are very close to those of the gas from which they formed in the early Galaxy. These "unmixed" stars reflect the abundances in the early Galaxy: the [C/Fe] ratio is constant (about +0.2 dex) and the [C/Mg] ratio is close to solar at low metallicity, favouring a high C production by massive zero-metal supernovae. The [N/Fe] and [N/Mg] ratios scatter widely. The larger values of these ratios define a flat upper plateau ([N/Mg]= 0.0, [N/Fe]= +0.1), which could reflect higher values within a wide range of yields of zero-metal Sne II. Alternatively, by analogy with the DLA's, the lower abundances ([N/Mg]= -1.1, [N/Fe]= -0.7) could reflect generally low yields from the first Sne II, the other stars being N enhanced by winds of massive Asymptotic Giant Branch (AGB) stars. At present it cannot be decided whether primary N is produced primarily in SNe II or in massive AGB stars, or in both. The stellar N abundances and [N/O] ratios are compatible with those found in Damped Lyman-alpha (DLA) systems.

Lithium in Giant Stars

Lithium continues to be a surprising element and more than ever the study of its creation, destruction and distribution is giving us tools to understand, not only the chemical evolution of this element, but also the nature of mass loss of evolved giants and nucleosynthesis of other elements in cases of very low-metal giants. It also helps to set constraints for cosmological models. The presence of very strong lithium lines in some giant stars of different spectral types and stages of evolution has been considered up to the end of the nineties as a puzzle. To solve this problem, non standard evolutionary mechanisms must be invoked. We review here all the mechanisms presented in the literature and which are divided into internal and external processes of lithium enrichment. We will also discuss the observational tests which are being performed in order to discard (or not) some of them. In any case, the more realistic values of the lithium abundances in giants are, as we will see, the main test of these proposed scenarios. Because of this importance we discuss here the state of art of the Non-LTE determinations of lithium abundances in strong lithium giants. Evolved giants, with lithium abundances larger than that of the interstellar medium and with their important mass losses can be considered the most realistic sources of lithium in the Galaxy. We believe that a complete physical picture of this problem will give a powerful tool to understand the chemical evolution of a large part of all light elements.

Elemental Abundances in Giants in NGC 3201, a Globular Cluster with a Retrograde Orbit

We report the results of an analysis of high-quality spectroscopic and photometric observations of 18 giants in NGC 3201. We have included α Boo and the cool metal-poor field star HDE 232078 as comparison objects. We find evidence of a significant extinction gradient across the cluster with a range in E(B-V) of about 0.1 mag, which has not been taken into account in previous studies. The mean value of [Fe/H] for the cluster is -1.42 ± 0.03 (σ = 0.14). The Na and Al abundances correlate with each other and anticorrelate with the O abundances in a pattern similar to that reported recently for other globular clusters. The [O/H], [Na/H], and [Al/H] values range from -1.5 to -0.8, -0.7 to -1.5, and -0.5 to -1.5, respectively. The α-elements (even-even nuclei from Mg to Ti) show the usual excess relative to iron for metal-poor stars. The heavy-element abundance patterns are dominated by contributions from an r-process reservoir. The relative s-process contribution to the NGC 3201 giants is less than or equal to that in α Boo, and it is significantly less than in the Sun. In contrast, the relative s-process contribution to the halo star HDE 232078 is nearly as large as in the Sun. There is a hint that the relative r- and s-process contributions correlate with the O abundance (in the sense that the low-O stars have a larger relative s-process component). Lee 1312, the star for which our analysis is most complete, has a very similar distribution of s-process species, as does α Boo. Although NGC 3201 has an unusual Galactic orbit, we find no new evidence from our analysis that it is a captured cluster.

Investigation of Suspected Red Horizontal Branch Stars in the Galactic Field

The chemical composition of 11 suspected RHB stars in the galactic field was recently analyzed using high-resolution spectra obtained with the 6-meter telescope of the Special Astrophysical Observatory, North Caucasus. We summarize here some of the main conclusions obtained from comparison of the abundances of the RHB stars with available abundances of giants and dwarfs (hereafter G and D).

The lithium abundance in stars

The observations of Li abundances in pre-Main-Sequence, Main-Sequence (Population I and II), sub-giant, and giant stars are reviewed in order to show how Li can be used as a constraint on stellar hydrodynamics and in particular on particle transport processes in stars. Important observational results include the tight Li abundance dependence on T eff in the Hyades, the time dependence of the Li abundance below T eff = 6000 K, the presence of a Li gap at T eff = 6700 K in young clusters and the large Li abundance in some peculiar giants. The observed abundances are compared to models which include progressively more physical processes. The ‘standard’ stellar evolution model is compatible with the upper envelope of the observations in young clusters such as the Pleiades and α Per. The observed Li underabundances is then caused by Li burning on the pre-Main Sequence. The large abundance spread observed is not understood. It does not appear to be simply related to rotation since the Pleiades stars rotate more slowly but have larger Li abundances than many stars of α Per. The Li abundance gap observed in clusters is not explained by the ‘standard’ model. Models involving diffusion seem to explain it in a natural way, though meridional circulation could also be involved. Evolutionary effects and the interaction between diffusion and meridional circulation should, however, be taken more fully into account in those models. The Li abundances in giants show that additional destruction processes are involved beyond those included in the ‘standard’ evolutionary models. Meridional circulation is compatible with most of those observations, without any arbitrary parameter being adjusted. While turbulence is nearly certainly present in stars, it is poorly understood and we suggest that it should be invoked to explain only those phenomena that the better understood processes cannot explain. Its description always involves arbitrary parameters. Turbulence appears to be required to explain the Li abundances in the Sun and in G stars of the Hyades and older clusters. In halo stars, the observed Li abundance has probably been reduced from the original by a factor of 2 so that the original abundance was probably equal to log N(Li) = 2.5. More calculations are needed to better establish this value. The large Li abundances observed in some peculiar giants are not understood.

Carbon isotope ratios and lithium abundances in open cluster giants

New high-resolution, high S/N spectra of CN lines at 8000 A and Li lines at 6707 A have been obtained for giants in about 20 Galactic open clusters and C-12/C-13 ratios and Li abundances determined for these stars. The ages of the clusters vary from about 50 million years to 5 billion years, and their turn-off masses vary from 1 solar mass to about 6 solar masses. The ages and turn-off masses were determined by fitting theoretical isochrones to the cluster color-magnitude diagrams. Correlation of the isotope ratio and the Li abundances with the cluster turn-off masses indicates the following: (1) the C-12/C-13 ratio increases steeply with the turn-off mass until a mass of approximately 2.2 solar masses when the ratio levels off abruptly to a value near 26; (2) older clusters with turn-off masses lower than about 2.2 solar masses in general exhibit C-12/C-13 ratios that are considerably lower than the theoretically predicted values while those with larger turn-off masses show ratios close to standard predictions; and (3) no strong correlation exists between the Li abundances and the cluster turn-off mass, and the Li abundances in giants are, in general, lower than theoretically predicted values. Various theories to explainmore » the observed abundance trends are discussed. 66 refs.« less

Carbon and nitrogen abundances in giant stars of the metal-poor globular cluster M15

view Abstract Citations (122) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Carbon and nitrogen abundances in giant stars of the metal-poor globular cluster M15. Trefzger, D. V. ; Langer, G. E. ; Carbon, D. F. ; Suntzeff, N. B. ; Kraft, R. P. Abstract The authors derive carbon and nitrogen abundances for 33 giant-star members of the globular cluster M15, thus extending their earlier work in M92 to another cluster of comparably low metallicity ([M/H] ⪉ -2). They describe a technique for obtaining C and N abundances which is based on spectrophotometric indices that measure the strengths of NH, CN, and CH bands in low-resolution scanner spectra. The spectrophotometric indices are calibrated using scanner spectra of M92 giant stars in which estimates of C and N abundances had previously been obtained by the method of spectrum synthesis. The complex behavior of C and N abundances in M15 giants cannot be adequately described by models based on classical evolutionary theory. The scenarios already advanced to explain these abundance patterns in the case of M92 are discussed briefly in connection with M15. Publication: The Astrophysical Journal Pub Date: March 1983 DOI: 10.1086/160765 Bibcode: 1983ApJ...266..144T Keywords: Abundance; Giant Stars; Globular Clusters; Stellar Evolution; Stellar Spectrophotometry; Carbon Stars; Interstellar Extinction; Late Stars; Metallicity; Nitrogen; Stellar Spectra; Astrophysics full text sources ADS | data products SIMBAD (28)

Evidence for Primordial Inhomogeneities From Abundance of Giants in M5, M13, and M22

There is spectroscopic evidence for initial abundance inhomogeneities from star to star within M13 and M22. Among three stars in M13, there is an order-of-magnitude range in the sodium-to-iron ratio, which lies well beyond all known random and systematic errors; details will be given in a paper in preparation. In M22, the spread discussed below in logarithmic iron-to-hydrogen ratios, [Fe/H], is 0.6 dex, which is far larger than that found here for any other globular cluster.