First Stars. III. A detailed elemental abundance study of four extremely metal-poor giant stars (original) (raw)
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Detailed Iron-peak Element Abundances in Three Very Metal-poor Stars
The Astrophysical Journal, 2020
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+03 o 740, BD−13 o 3442, and CD−33 o 1173. High-resolution ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra in the wavelength range 2300-3050Å 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+03 o 740, BD−13 o 3442, CD−33 o 1173, 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.
Chemical Abundance Patterns of Extremely Metal-Poor Stars with [Fe/H]l -3.5
Proceedings of the International Astronomical Union, 2005
We have obtained high resolution (R =60,000), high quality (S/N >100) spectroscopy using Subaru/HDS for ∼20 candidate extremely metal-poor stars ([Fe/H]< −3) to determine their chemical abundance patterns. In the observing program we found HE1327-2326, which has [Fe/H]= −5.4, the lowest Fe abundance known in normal stars. This star shows extremely large excesses of C and N, and also excesses of other light elements (e.g. O, Na). The low iron abundance and the peculiar abundance pattern provide a signature of the nucleosynthesis by the first generation stars in the Universe. We also present the preliminary results for other stars: (1) The iron abundances of our sample, except for HE 1327−2326, are higher than [Fe/H]= −4. There is a gap of iron abundance between [Fe/H]= −4 and −5. This would indicate that the nucleosynthesis, or formation processes that are responsible for the stars with [Fe/H]< −5 and others are quite different. (2) Six stars of our sample, including HE1327-2326, show clear excesses of carbon. Only two of them show excesses of the heavy neutron-capture element Ba, suggesting a large contribution of AGB nucleosynthesis. The origin of carbon-excesses in other stars are still unclear, but the existence of these stars is a remarkable feature only found at the extremely low metallicity.
Proceedings of the …, 2005
As part of a study of the detailed abundance patterns in extremely metal-poor stars, we have compared our samples of giants and dwarfs with two samples of dwarfs measured by different teams. For most elements the abundances are in good agreement, but for C, Na, and Al we show that the atmospheric abundances are different in dwarfs and in giants. For C the difference could be explained by "atmospheric effects" or by the influence of the first dredge-up, but for Na and Al deep mixing inside the stars must be invoked. Until now, such deep mixing has not been observed in metal-poor field stars. An excess scatter in [Mg/Fe] in giants remains unexplained.
Chemical abundance patterns of extremely metal-poor stars
Proceedings of International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX — PoS(NIC-IX), 2010
We have obtained high resolution (R =60,000), high quality (S/N >100) spectroscopy using Subaru/HDS for ∼20 candidate extremely metal-poor stars ([Fe/H]< −3) to determine their chemical abundance patterns. In the observing program we found HE1327-2326, which has [Fe/H]= −5.4, the lowest Fe abundance known in normal stars. This star shows extremely large excesses of C and N, and also excesses of other light elements (e.g. O, Na). The low iron abundance and the peculiar abundance pattern provide a signature of the nucleosynthesis by the first generation stars in the Universe. We also present the preliminary results for other stars: (1) The iron abundances of our sample, except for HE 1327−2326, are higher than [Fe/H]= −4. There is a gap of iron abundance between [Fe/H]= −4 and −5. This would indicate that the nucleosynthesis, or formation processes that are responsible for the stars with [Fe/H]< −5 and others are quite different. (2) Six stars of our sample, including HE1327-2326, show clear excesses of carbon. Only two of them show excesses of the heavy neutron-capture element Ba, suggesting a large contribution of AGB nucleosynthesis. The origin of carbon-excesses in other stars are still unclear, but the existence of these stars is a remarkable feature only found at the extremely low metallicity.
First stars XII. Abundances in extremely metal-poor turnoff stars, and comparison with the giants
Astronomy and Astrophysics, 2009
Context. 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 are, however, for giant stars which may have experienced internal mixing later. Aims. We aim to compare the results for giants with new, accurate abundances for all observable elements in 18 EMP turnoff stars. Methods. VLT/UVES spectra at R ∼ 45, 000 and S/N∼ 130 per pixel (λλ 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. Results. 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).
Chemical Abundance Patterns of Extremely Metal-Poor Stars with [Fe/H]$\lesssim -3.5$
Proceedings of the International Astronomical Union, 2005
We have obtained high resolution (R =60,000), high quality (S/N >100) spectroscopy using Subaru/HDS for ∼20 candidate extremely metal-poor stars ([Fe/H]< −3) to determine their chemical abundance patterns. In the observing program we found HE1327-2326, which has [Fe/H]= −5.4, the lowest Fe abundance known in normal stars. This star shows extremely large excesses of C and N, and also excesses of other light elements (e.g. O, Na). The low iron abundance and the peculiar abundance pattern provide a signature of the nucleosynthesis by the first generation stars in the Universe. We also present the preliminary results for other stars: (1) The iron abundances of our sample, except for HE 1327−2326, are higher than [Fe/H]= −4. There is a gap of iron abundance between [Fe/H]= −4 and −5. This would indicate that the nucleosynthesis, or formation processes that are responsible for the stars with [Fe/H]< −5 and others are quite different. (2) Six stars of our sample, including HE1327-2326, show clear excesses of carbon. Only two of them show excesses of the heavy neutron-capture element Ba, suggesting a large contribution of AGB nucleosynthesis. The origin of carbon-excesses in other stars are still unclear, but the existence of these stars is a remarkable feature only found at the extremely low metallicity.
The Astrophysical Journal, 2001
High-resolution, high signal-to-noise ratio (SS/NT \ 85) spectra have been obtained for Ðve stars, CD [24¡17504, CD [38¡245, CS 22172[002, CS 22885[096, and CS 22949[037, having [Fe/H] [ [3.5 according to previous lower S/N material. LTE model atmosphere techniques are used to determine [Fe/H] and relative abundances, or their limits, for some 18 elements, and to constrain more tightly the early enrichment history of the Galaxy than is possible based on previous analyses. We compare our results with high-quality higher abundance literature data for other metal-poor stars and with the canonical Galactic chemical enrichment results of Timmes and colleagues and obtain the following basic results. (1) Large supersolar values of [C/Fe] and [N/Fe], not predicted by the canonical models, exist at lowest abundance. For C at least, the result is difficult to attribute to internal mixing e †ects. (2) We conÐrm that there is no upward trend in [a/Fe] as a function of [Fe/H], in contradistinction to some reports of the behavior of [O/Fe]. (3) The abundances of aluminum, after correction for non-LTE e †ects, are in fair accord with theoretical prediction. (4) We conÐrm earlier results concerning the Fe peak elements that [Cr/Fe] and [Mn/Fe] decrease at lowest abundance while [Co/Fe] increases, behaviors that had not been predicted. We Ðnd, however, that [Ni/Fe] does not vary with [Fe/H], and at [Fe/H] D [3.7, [Ni/Fe] \ 0.08^0.06. This result appears to be inconsistent with the supernova models of Nakamura and colleagues that seek to understand the observed behavior of the Fe peak elements by varying the position of the model mass cut relative to the Si-burning regions. (5) The heavy neutron capture elements Sr and Ba exhibit a large scatter, with the e †ect being larger for Sr than Ba. The disparate behavior of these two elements has been attributed to the existence of (at least) two di †erent mechanisms for their production. (6) For the remarkable object CS 22949[037, we conÐrm the result of McWilliam and colleagues that [C/Fe], [Mg/Fe], and [Si/Fe] are supersolar by D1.0 dex. Further, we Ðnd [N/Fe] \ 2.7^0.4. None of these results are understandable within the framework of standard models. We discuss them in terms of partial ejection of supernova mantles and massive (200È500 M _) zero heavy-element hypernovae. The latter model actually predicted overproduction of N and underproduction of Fe peak elements. (7) We use robust techniques to determine abundance trends as a function of [Fe/H]. In most cases one sees an apparent upturn in the dispersion of relative abundance [X/Fe] for [Fe/H] \ [2.5. It remains unclear whether this is a real e †ect or one driven by observational error. The question needs to be resolved with a much larger and homogeneous data set, both to improve the quality of the data and to understand the role of unusual stars such as CS 22949[037.
The Astrophysical Journal, 2012
ABSTRACT Recent work has shown that most globular clusters have at least two chemically distinct components, as well as cluster-to-cluster differences in the mean [O/Fe], [Mg/Fe], and [Si/Fe] ratios at similar [Fe/H] values. In order to investigate the implications of variations in the abundances of these and other metals for H-R diagrams and predicted ages, grids of evolutionary sequences have been computed for scaled solar and enhanced alpha-element mixtures, and for mixtures in which the assumed [m/Fe] value for each of the metals C, N, O, Ne, Na, Mg, Si, S, Ca, and Ti has been increased, in turn, by 0.4 dex at constant [Fe/H]. These tracks, together with isochrones for ages from 6 to 14 Gyr, have been computed for -3.0 < [Fe/H] < -0.6, with helium abundances Y = 0.25, 0.29, and 0.33 at each [Fe/H] value, using upgraded versions of the Victoria stellar structure program and the Regina interpolation code, respectively. Turnoff luminosity versus age relations from isochrones are found to depend almost entirely on the importance of the CNO-cycle, and thereby mainly on the abundance of oxygen. Since C, N, and O, as well as Ne and S, do not contribute significantly to the opacities at low temperatures and densities, variations in their abundances do not impact the Teff scale of red giants. The latter is a strong function of the abundances of only Mg and Si (and Fe, possibly to a lesser extent), because they are so abundant and because they are strong sources of opacity at low temperatures. For these reasons, Mg and Si also have important effects on the temperatures of main-sequence stars. Due to their low abundances, Na, Ca, and Ti are of little consequence for stellar models. The effects of varying the adopted solar metals mix and the helium abundance at a fixed [Fe/H] are also briefly discussed.
High-resolution abundance analysis of very metal-poor r-I stars
Astronomy & Astrophysics, 2014
Context. Moderately r-process-enriched stars (r-I; +0.3 ≤ [Eu/Fe] ≤ +1.0) are at least four times as common as those that are greatly enriched in r-process elements (r-II; [Eu/Fe] > +1.0), and the abundances in their atmospheres are important tools for obtaining a better understanding of the nucleosynthesis processes responsible for the origin of the elements beyond the iron peak. Aims. The main aim of this work is to derive abundances for a sample of seven metal-poor stars with −3.4 ≤ [Fe/H] ≤ −2.4 classified as r-I stars, to understand the role of these stars for constraining the astrophysical nucleosynthesis event(s) that is (are) responsible for the production of the r-process, and to investigate whether they differ, in any significant way, from the r-II stars. Methods. We carried out a detailed abundance analysis based on high-resolution spectra obtained with the VLT/UVES spectrograph, using spectra in the wavelength ranges 3400-4500 Å, 6800-8200 Å, and 8700-10 000 Å, with resolving power R ∼ 40 000 (blue arm) and R ∼ 55 000 (red arm). The OSMARCS LTE 1D model atmosphere grid was employed, along with the spectrum synthesis code Turbospectrum. Results. We have derived abundances of the light elements Li, C, and N, the α-elements Mg, Si, S, Ca, and Ti, the odd-Z elements Al, K, and Sc, the iron-peak elements V, Cr, Mn, Fe, Co, and Ni, and the trans-iron elements from the first peak (Sr, Y, Zr, Mo, Ru, and Pd), the second peak