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تسجيل مستخدم جديدThe Chemical Abundances of Stars in the Halo (CASH) Project. II. A Sample of 16 Extremely Metal-poor Stars
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Julie K. Hollek
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We present a comprehensive abundance analysis of 20 elements for 16 new low-metallicity stars from the Chemical Abundances of Stars in the Halo (CASH) project. The abundances have been derived from both Hobby-Eberly Telescope High Resolution Spectrograph snapshot spectra (R~15,000) and corresponding high-resolution (R~35,000) Magellan MIKE spectra. The stars span a metallicity range from [Fe/H] from -2.9 to -3.9, including four new stars with [Fe/H]<-3.7. We find four stars to be carbon-enhanced metal-poor (CEMP) stars, confirming the trend of increasing [C/Fe] abundance ratios with decreasing metallicity. Two of these objects can be classified as CEMP-no stars, adding to the growing number of these objects at [Fe/H]<-3. We also find four neutron-capture enhanced stars in the sample, one of which has [Eu/Fe] of 0.8 with clear r-process signatures. These pilot sample stars are the most metal-poor ([Fe/H]<-3.0) of the brightest stars included in CASH and are used to calibrate a newly-developed, automated stellar parameter and abundance determination pipeline. This code will be used for the entire ~500 star CASH snapshot sample. We find that the pipeline results are statistically identical for snapshot spectra when compared to a traditional, manual analysis from a high-resolution spectrum.
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We present a detailed abundance analysis of 23 elements for a newly discovered carbon-enhanced metal-poor (CEMP) star, HE 0414-0343, from the Chemical Abundances of Stars in the Halo (CASH) Project. Its spectroscopic stellar parameters are Teff = 486
3 K, log g = 1.25, vmic = 2.20 km/s, and [Fe/H] = -2.24. Radial velocity measurements covering seven years indicate HE 0414-0343 to be a binary. HE 0414-0343 has [C/Fe] = 1.44 and is strongly enhanced in neutron-capture elements but its abundances cannot be reproduced by a solar-type s-process pattern alone. Traditionally, it could be classified as CEMP-r/s star. Based on abundance comparisons with AGB star nucleosynthesis models, we suggest a new physically-motivated origin and classification scheme for CEMP-s stars and the still poorly-understood CEMP-r/s. The new scheme describes a continuous transition between these two so-far distinctly treated subgroups: CEMP-sA, CEMP-sB, and CEMP-sC. Possible causes for a continuous transition include the number of thermal pulses the AGB companion underwent, the effect of different AGB star masses on their nucleosynthetic yields, and physics that is not well approximated in 1-D stellar models such as proton ingestion episodes and rotation. Based on a set of detailed AGB models, we suggest the abundance signature of HE 0414-0343 to have arisen from a >1.3 Msun mass AGB star and a late-time mass transfer, that transformed HE 0414-0343 into a CEMP-sC star. We also find the [Y/Ba] ratio well parametrizes the classification and can thus be used to easily classify any future such stars.
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 (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). CONCLUSIONS: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.
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Reconstructing the chemical evolution of the Milky Way is crucial for understanding the formation of stars, planets, and galaxies throughout cosmic time. Different studies associated with element production in the early universe and how elements are
incorporated into gas and stars are necessary to piece together how the elements evolved. These include establishing chemical abundance trends, as set by metal-poor stars, comparing nucleosynthesis yield predictions with stellar abundance data, and theoretical modeling of chemical evolution. To aid these studies, we have collected chemical abundance measurements and other information such as stellar parameters, coordinates, magnitudes, and radial velocities, for extremely metal-poor stars from the literature. The database, JINAbase, contains 1658 unique stars, 60% of which have [Fe/H]<2.5. This information is stored in an SQL database, together with a user-friendly queryable web application (http://jinabase.pythonanywhere.com). Objects with unique chemical element signatures (e.g., r-process stars, s-process and CEMP stars) are labeled or can be classified as such. The web application enables fast selection of customized comparison samples from the literature for the aforementioned studies and many more. Using the multiple entries for three of the most well studied metal-poor stars, we evaluate systematic uncertainties of chemical abundances measurements. We provide a brief guide on the selection of chemical elements for model comparisons for non- spectroscopists who wish to learn about metal-poor stars and the details of chemical abundances measurements.
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