Browsing by Subject "zirconium"

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    Hot Bottom Burning And S-Process Nucleosynthesis In Massive Agb Stars At The Beginning Of The Thermally-Pulsing Phase
    (2013-07) Garcia-Hernandez, D. A.; Zamora, O.; Yague, A.; Uttenthaler, S.; Karakas, A. I.; Lugaro, M.; Ventura, P.; Lambert, D. L.; D. L. Lambert
    We report the first spectroscopic identification of massive Galactic asymptotic giant branch (AGB) stars at the beginning of the thermal pulse (TP) phase. These stars are the most Li-rich massive AGBs found to date, super Li-rich AGBs with log epsilon (Li) similar to 3-4. The high Li overabundances are accompanied by weak or no s-process element (i.e. Rb and Zr) enhancements. A comparison of our observations with the most recent hot bottom burning (HBB) and s-process nucleosynthesis models confirms that HBB is strongly activated during the first TPs but the Ne-22 neutron source needs many more TP and third dredge-up episodes to produce enough Rb at the stellar surface. We also show that the short-lived element Tc, usually used as an indicator of AGB genuineness, is not detected in massive AGBs, which is in agreement with the theoretical predictions when the Ne-22 neutron source dominates the s-process nucleosynthesis.
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    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.