209 research outputs found
The first determination of the actinide Th abundance for a red giant of the Ursa Minor dwarf galaxy
The Thorium abundance for the red giant COS82 in the Ursa Minor dwarf
spheroidal galaxy is determined based on a high resolution spectrum. This is
the first detection of actinides in an extra Galactic object. A detailed
abundance pattern is determined for 12 other neutron-capture elements from the
atomic number 39 to 68. These elements are significantly over-abundant with
respect to other metals like Fe (> 1 dex) and their abundance pattern agrees
well with those of the r-process-enhanced, very metal-poor stars known in the
Galactic halo, while the metallicity of this object ([Fe/H] ~ -1.5) is much
higher than these field stars ([Fe/H] ~ -3.0). The results indicate that the
mechanism and the astrophysical site that are responsible for neutron-capture
elements in COS82 is similar to that for field r-process-enhanced stars, while
the condition of low mass star formation is quite different. An estimate of the
age of this object based on the Th abundance ratio is discussed.Comment: 5 pages, 2 figures, 1 table, to appear in PAS
Extreme Enhancements of r-process Elements in the Cool Metal-Poor Main-Sequence Star SDSS J2357-0052
We report the discovery of a cool metal-poor, main-sequence star exhibiting
large excesses of r-process elements. This star is one of two newly discovered
cool subdwarfs (effective temperatures of 5000 K) with extremely low
metallicity ([Fe/H]<-3) identified from follow-up high-resolution spectroscopy
of metal-poor candidates from the Sloan Digital Sky Survey. SDSS J2357-0052 has
[Fe/H]=-3.4 and [Eu/Fe]=+1.9, and exhibits a scaled solar r-process abundance
pattern of heavy neutron-capture elements. This is the first example of an
extremely metal-poor, main-sequence star showing large excesses of r-process
elements; all previous examples of the large r-process-enhancement phenomena
have been associated with metal-poor giants. The metallicity of this object is
the lowest, and the excess of Eu ([Eu/Fe]) is the highest, among the
r-process-enhanced stars found so far. We consider possible scenarios to
account for the detection of such a star, and discuss techniques to enable
searches for similar stars in the future.Comment: 16 pages, 3 figures, 2 tables, ApJL in pres
Interpretation of CEMP(s) and CEMP(s + r) Stars with AGB Models
Asymptotic Giant Branch (AGB) stars play a fundamental role in the s-process
nucleosynthesis during their thermal pulsing phase. The theoretical predictions
obtained by AGB models at different masses, s-process efficiencies, dilution
factors and initial r-enrichment, are compared with spectroscopic observations
of Carbon-Enhanced Metal-Poor stars enriched in s-process elements, CEMP(s),
collected from the literature. We discuss here five stars as example, CS
22880-074, CS 22942-019, CS 29526-110, HE 0202-2204, and LP 625-44. All these
objects lie on the main-sequence or on the giant phase, clearly before the
TP-AGB stage: the hypothesis of mass transfer from an AGB companion, would
explain the observed s-process enhancement. CS 29526-110 and LP 625-44 are
CEMP(s+r) objects, and are interpreted assuming that the molecular cloud, from
which the binary system formed, was already enriched in r-process elements by
SNII pollution. In several cases, the observed s-process distribution may be
accounted for AGB models of different initial masses with proper 13C-pocket
efficiency and dilution factor. Na (and Mg), produced via the neutron capture
chain starting from 22Ne, may provide an indicator of the initial AGB mass.Comment: 8 pages, 6 figures, 2 table
Chemical compositions of six metal-poor stars in the ultra-faint dwarf spheroidal galaxy Bo\"otes I
Ultra-faint dwarf galaxies recently discovered around the Milky Way (MW)
contain extremely metal-poor stars, and might represent the building blocks of
low-metallicity components of the MW. Among them, the Bo\"otes I dwarf
spheroidal galaxy is of particular interest because of its exclusively old
stellar population. We determine chemical compositions of six red giant stars
in Bo\"otes I, based on the high-resolution spectra obtained with the High
Dispersion Spectrograph mounted on the Subaru Telescope. Abundances of 12
elements, including C, Na, alpha, Fe-peak, and neutron capture elements, were
determined for the sample stars. The abundance results were compared to those
in field MW halo stars previously obtained using an abundance analysis
technique similar to the present study. We confirm the low metallicity of
Boo-094 ([Fe/H]=-3.4). Except for this star, the abundance ratios ([X/Fe]) of
elements lighter than zinc are generally homogeneous with small scatter around
the mean values in the metallicities spanned by the other five stars
(-2.7-2.7 show
no significant enhancement of carbon. The [Mg/Fe] and [Ca/Fe] ratios are almost
constant with a modest decreasing trend with increasing [Fe/H] and are slightly
lower than the field halo stars. The [Sr/Fe] and [Sr/Ba] ratios also tend to be
lower in the Bo\"otes I stars than in the halo stars. Our results of small
scatter in the [X/Fe] ratios for elements lighter than zinc suggest that these
abundances were homogeneous among the ejecta of prior generation(s) of stars in
this galaxy.Comment: 16 pages, 12 figures. Accepted to A&A, language correcte
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