10 research outputs found
Galactic Chemical Evolution of the s Process from AGB Stars
We follow the chemical evolution of the Galaxy for the s elements using a
Galactic chemical evolution (GCE) model, as already discussed by Travaglio et
al. (1999, 2001, 2004), with a full updated network and refined asymptotic
giant branch (AGB) models. Calculations of the s contribution to each isotope
at the epoch of the formation of the solar system is determined by following
the GCE contribution by AGB stars only. Then, using the r-process residual
method we determine for each isotope their solar system r-process fraction, and
recalculate the GCE contribution of heavy elements accounting for both the s
and r process. We compare our results with spectroscopic abundances at various
metallicities of [Sr,Y,Zr/Fe], of [Ba,La/Fe], of [Pb/Fe], typical of the three
s-process peaks, as well as of [Eu/Fe], which in turn is a typical r-process
element. Analysis of the various uncertainties involved in these calculations
are discussed.Comment: 8 pages, 6 figures, 1 tabl
The NLTE Barium Abundance in Dwarf Stars in the Metallicity Range of -1 < [Fe/H] < +0.3
We present the results of determination of the barium abundance considering
the non-LTE (NLTE) effects in 172 dwarf stars in the metallicity range of -1<
[Fe/H] <+0.3, assigned to different Galactic substructures by kinematic
criteria. We used a model of the Ba atom with 31 levels of Ba I and 101 levels
of Ba II. The atmosphere models for the investigated stars were computed using
the ATLAS9 code modified by new opacity distribution functions. The NLTE
profiles of the unblended Ba II (4554 A, 5853 A, 6496 A) were computed and then
compared to those observed. The line 6141 A was also used, but with an
allowance for its correlation with the iron line. The average barium abundances
in the thin and thick discs are 0.01 +/- 0.08 and -0.03 +/- 0.07, respectively.
The comparison to the calculations of the Galactic chemical evolution by
Serminato et al. (2009) was conducted. The trend obtained for the Ba abundance
versus [Fe/H] suggests a complex barium production process in the thin and
thick discs
s-Process in Low Metallicity Stars. III. Individual analysis of CEMP-s and CEMP-s/r with AGB models
We provide an individual analysis of 94 carbon enhanced metal-poor stars
showing an s-process enrichment (CEMP-s) collected from the literature. The
s-process enhancement observed in these stars is ascribed to mass transfer by
stellar winds in a binary system from a more massive companion evolving faster
toward the asymptotic giant branch (AGB) phase. The theoretical AGB
nucleosynthesis models have been presented in Paper I. Several CEMP-s stars
show an enhancement in both s and r-process elements (CEMP-s/r). In order to
explain the peculiar abundances observed in CEMP-s/r stars, we assume that the
molecular cloud from which CEMP-s formed was previously enriched in r-elements
by Supernovae pollution.
A general discussion and the method adopted in order to interpret the
observations have been provided in Paper II. We present in this paper a
detailed study of spectroscopic observations of individual stars. We consider
all elements from carbon to bismuth, with particular attention to the three
s-process peaks, ls (Y, Zr), hs (La, Nd, Sm) and Pb, and their ratios [hs/ls]
and [Pb/hs]. The presence of an initial r-process contribution may be typically
evaluated by the [La/Eu] ratio. We found possible agreements between
theoretical predictions and spectroscopic data. In general, the observed
[Na/Fe] (and [Mg/Fe]) provide information on the AGB initial mass, while
[hs/ls] and [Pb/hs] are mainly indicators of the s-process efficiency. A range
of 13C-pocket strengths is required to interpret the observations. However,
major discrepancies between models and observations exist. We highlight star by
star the agreements and the main problems encountered and, when possible, we
suggest potential indications for further studies. These discrepancies provide
starting points of debate for unsolved problems ...Comment: 40 pages, 54 figures, 3 tables (Appendix A as supplementary
material), accepted Monthly Notices of the Royal Astronomical Society, 201
The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models
High-resolution spectroscopic observations of a hundred metal-poor Carbon and
s-rich stars (CEMP-s) collected from the literature are compared with the
theoretical nucleosynthesis models of asymptotic giant branch (AGB) presented
in Paper I (M = 1.3, 1.4, 1.5, 2 Msun, -3.6 < [Fe/H] < -1.5). The s-process
enhancement detected in these objects is associated to binary systems: the more
massive companion evolved faster through the thermally pulsing AGB phase
(TP-AGB), synthesising in the inner He-intershell the s-elements, which are
partly dredged-up to the surface during the third dredge-up (TDU) episode. The
secondary observed low mass companion became CEMP-s by mass transfer of C and
s-rich material from the primary AGB.
We analyse the light elements as C, N, O, Na and Mg, as well as the two
s-process indicators, [hs/ls] (where ls = is the the light-s peak at N
= 50 and hs = the heavy-s peak at N = 82), and [Pb/hs]. We
distinguish between CEMP-s with high s-process enhancement, [hs/Fe] > 1.5
(CEMP-sII), and mild s-process enhanced stars, [hs/Fe] < 1.5 (CEMP-sI). To
interpret the observations, .... .
Detailed analyses for individual stars will be provided in Paper III.Comment: 35 pages, 17 figures, 11 Tables, (Appendix A as Supplementary
material