Browsing by Subject "r-process"

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18 Sco: A Solar Twin Rich In Refractory And Neutron-Capture Elements. Implications For Chemical Tagging
(2014-08) Melendez, Jorge; Ramirez, Ivan; Karakas, Amanda I.; Yong, David; Monroe, TalaWanda R.; Bedell, Megan; Bergemann, Maria; Asplund, Martin; Maia, Marcelo Tucci; Bean, Jacob; do Nascimento, Jose-Dias; Bazot, Michael; Alves-Brito, Alan; Freitas, Fabricio C.; Castro, Matthieu; Ramirez, Ivan
We study with unprecedented detail the chemical composition and stellar parameters of the solar twin 18 Sco in a strictly differential sense relative to the Sun. Our study is mainly based on high-resolution (R similar to 110,000), high signal-to-noise ratio (800-1,000) Very Large Telescope UVES spectra, which allow us to achieve a precision of about 0.005 dex in differential abundances. The effective temperature and surface gravity of 18 Sco are T-eff = 5823 +/- 6 K and log g = 4.45 +/- 0.02 dex, i.e., 18 Sco is 46 +/- 6 K hotter than the Sun and log g is 0.01 +/- 0.02 dex higher. Its metallicity is [Fe/H] = 0.054 +/- 0.005 dex, and its microturbulence velocity is +0.02 +/- 0.01 km s-1 higher than solar. Our precise stellar parameters and differential isochrone analysis show that 18 Sco has a mass of 1.04 +/- 0.02 M-circle dot and that it is similar to 1.6 Gyr younger than the Sun. We use precise High Accuracy Radial velocity Planet Searcher (HARPS) radial velocities to search for planets, but none are detected. The chemical abundance pattern of 18 Sco displays a clear trend with condensation temperature, thus showing higher abundances of refractories in 18 Sco than in the Sun. Intriguingly, there are enhancements in the neutron-capture elements relative to the Sun. Despite the small element-to-element abundance differences among nearby n-capture elements (similar to 0.02 dex), we successfully reproduce the r-process pattern in the Solar System. This is independent evidence for the universality of the r process. Our results have important implications for chemical tagging in our Galaxy and nucleosynthesis in general.
The First Carbon-Enhanced Metal-Poor Star Found In The Sculptor Dwarf Spheroidal OCR Issue
(2015-02) Skuladottir, A.; Tolstoy, E.; Salvadori, S.; Hill, V.; Pettini, M.; Shetrone, M. D.; Starkenburg, E.; Shetrone, Matthew D.
The origin of carbon-enhanced metal-poor (CEMP) stars and their possible connection with the chemical elements produced by the first stellar generation is still highly debated. In contrast to the Galactic halo, not many CFMP stars have been found in the dwarf spheroidal galaxies around the Milky Way. Here we present detailed abundances from ESO VET/IVES high resolution spectroscopy for ET0097, the first CEMP star found in the Sculptor dwarf spheroidal, which is one of the best studied dwarf galaxies in the Local Group, This star has [Fe/H] = 2.03 +/- 0.10, [C/Fe] = 0.51 +/- 0.10 and [N/Fe]) = 1.18 +/- . 0.20, which is the first nitrogen measurement in this galaxy. The traditional definition of CEIVIP Stars is [C/Fe] >= 0.70, but taking into account that this luminous red giant branch star has undergone mixing, it was intrinsically less nitrogen enhanced and more carbon rich when it was formed, and so it falls under the definition of CEMP stars, as proposed by Aoki et al. (2007. ApJ, 655. 492) to account for this effect. By making corrections for this mixing. we conclude that the star had EC/Fel 0.8 during its earlier evolutionary stages. Apart from the enhanced C and N abundances, ET0097 shows no peculiarities in other elements lighter than Zn. and no enhancement of the heavier neutron capture elements (Ba, La, Ce, Ncl, Sm Eu, Dy). making this a CEMP-no star. However, the star does show signs of the weak hprocess. with an overabundance of the lighter neutron-capture elements (Sr, Y, Zr). To explain the abundance pattern observed in ET0097, we explore the possibility that this star was enriched by primordial stars. In addition to the detailed abundances for ET0097. we present estimates and upper limits for C abundances in 85 other stars in Sculptor derived from UN molecular lines. including 11 stars with [Fe/H] <= -2. Combining these limits with observations horn the literature, the fraction of CEMP-no stars in Sculptor seems to be significantly lower than in the Galactic halo.
Improved yttrium and zirconium abundances in metal-poor stars
(2012-01) Violante, Renata
We present new abundances of the lighter n-capture elements, Yttrium (Z=30) and Zirconium (Z=40) in the very metal-poor, r-process rich stars BD+17 3248 and HD 221170. Very accurate abundances were obtained by use of the new transition probabilities for Y II published by Biemont et al. 2011, and Zr II by Malcheva et al. 2006, and by expanding the number of transitions employed for each element. For example, in BD+17 3248, we find log(Epsilon)=-0.03 +/-0.03 (Sigma=0.15 from 23 lines) for Y II. As for Zr II, log(Epsilon)=0.65 +/- 0.03 (Sigma=0.1 from 13 lines). The resulting abundance ratio is [Y/Zr]=-0.68 +/- 0.05. The results for HD 221170 are in accord with those of BD+17 3248. The quantity of lines used to form the abundance means has increased significantly since the original studies of these stars, resulting in more trustworthy abundances. These observed abundance ratios are in agreement with the r-process only value predicted from stellar models, but is under-abundant compared to an empirical model derived from direct analyses of meteoritic material. This ambiguity should stimulate further nucleosynthetic analysis to explain this abundance ratio.