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ABSTRACT: We show that SDSS J170733.93+585059.7 (hereafter SDSS J1707+58), previously
identified by Aoki and collaborators as a carbon-enhanced metal-poor star (with
s-process-element enhancements; CEMP-s), on the assumption that it is a
main-sequence turn-off star, is the RR Lyrae star VIII-14 identified by the
Lick Astrograph Survey. Revised abundances for SDSS J1707+58 are [Fe/H] =
-2.92, [C/Fe] = +2.79, and [Ba/Fe] = +2.83. It is thus one of the most
metal-poor RR Lyrae stars known, and has more extreme [C/Fe] and [Ba/Fe] than
the only other RR Lyrae star known to have a CEMP-s spectrum (TY Gru). Both
stars are Oosterhoff II stars with prograde kinematics, in contrast to stars
with [C/Fe] < +0.7, such as KP Cyg and UY CrB, which are disk stars. Twelve
other RR Lyrae stars with [C/Fe] >= +0.7 are presented as CEMP candidates for
further study.
07/2012;
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ABSTRACT: We present the abundance analysis of 97 nearby metal-poor (-3.3<[Fe/H]<-0.5)
stars having kinematics characteristics of the Milky Way (MW) thick disk,
inner, and outer stellar halos. The high-resolution, high-signal-to-noise
optical spectra for the sample stars have been obtained with the High
Dispersion Spectrograph mounted on the Subaru Telescope. Abundances of Fe, Mg,
Si, Ca and Ti have been derived using a one-dimensional LTE abundance analysis
code with Kurucz NEWODF model atmospheres. By assigning membership of the
sample stars to the thick disk, inner or outer halo components based on their
orbital parameters, we examine abundance ratios as a function of [Fe/H] and
kinematics for the three subsamples in wide metallicity and orbital parameter
ranges.
We show that, in the metallicity range of -1.5<[Fe/H]<= -0.5, the thick disk
stars show constantly high mean [Mg/Fe] and [Si/Fe] ratios with small scatter.
In contrast, the inner, and the outer halo stars show lower mean values of
these abundance ratios with larger scatter. The [Mg/Fe], [Si/Fe] and [Ca/Fe]
for the inner and the outer halo stars also show weak decreasing trends with
[Fe/H] in the range [Fe/H]$>-2$. These results favor the scenarios that the MW
thick disk formed through rapid chemical enrichment primarily through Type II
supernovae of massive stars, while the stellar halo has formed at least in part
via accretion of progenitor stellar systems having been chemically enriched
with different timescales.
05/2012;
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ABSTRACT: We provide measurements of the Ba isotopic fractions for five metal-poor
stars derived with an LTE analysis using 1D model stellar atmospheres. We use
high resolution (R\equiv{\lambda}/\Delta{\lambda}=90000-95000), very high
signal-to-noise (S/N>500) spectra to determine the fraction of odd Ba isotopes
(fodd) by measuring subtle asymmetries in the profile of the Ba ii line at 4554
{\AA}. We also use two different macroturbulent broadening techniques, Gaussian
and radial-tangential, to model the Fe lines of each star, and propagate each
technique to model macroturbulent broadening in the Ba 4554 {\AA} line. We
conduct a 1D non-LTE (NLTE) treatment of the Fe lines in the red giant HD122563
and the subgiant HD140283 in an attempt to improve the fitting. We determine
[Ba/Eu] ratios for the two giants in our study, HD122563 and HD88609, which can
also be used to determine the relative contribution of the s- and r-processes
to heavy-element nucleosynthesis, for comparison with fodd. We find fodd for
HD122563, HD88609 and HD84937, BD+26\circ3578 and BD-04\circ3208 to be
-0.12\pm0.07, -0.02\pm0.09, and -0.05\pm0.11, 0.08\pm0.08 and 0.18\pm0.08
respectively. This means that all stars examined here show isotopic fractions
more compatible with an s-process dominated composition. The [Ba/Eu] ratios in
HD122563 and HD88609 are found to be -0.20\pm0.15 and -0.47\pm0.15
respectively, which indicate instead an r-process signature. We report a better
statistical fit to the majority of Fe profiles in each star when employing a
radial-tangential broadening technique during our 1D LTE investigation. We have
shown that, from a statistical point of view, one must consider using a
radial-tangential broadening technique rather than a Gaussian one to model Fe
line macroturbulences when working in 1D. No improvement to Fe line fitting is
seen when employing a NLTE treatment.
01/2012;
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ABSTRACT: There is recent evidence that some SiC X grains from the Murchison meteorite
may contain supernova-produced {\nu}-process 11B and or 7Li encapsulated in the
grains. The synthesis of 11B and 7Li via neutrino-induced nucleon emission (the
{\nu} -process) in supernovae is sensitive to the neutrino mass hierarchy for
finite sin^2(2{\theta}13) > 0.001}. This sensitivity arises because, when there
is 13 mixing, the average electron neutrino energy for charged-current neutrino
reactions is larger for a normal mass hierarchy than for an inverted hierarchy.
Recent constraints on {\theta}13 from the Daya Bay, Double Chooz, MINOS, RENO
and T2K collaborations all suggest that indeed sin^2(2{\theta}13) > 0.001}. We
examine the possible implications of these new results based upon a Bayesian
analysis of the uncertainties in the measured meteoritic material and the
associated supernova nucleosynthesis models. We show that although the
uncertainties are large, they hint at a marginal preference for an inverted
neutrino mass hierarchy. We discuss the possibility that an analysis of more X
grains enriched in Li and B along with a better understanding of the relevant
stellar nuclear and neutrino reactions could eventually reveal the neutrino
mass hierarchy.
08/2011;
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ABSTRACT: Current theory regarding heavy element nucleosynthesis in metal-poor environments states that the r-process would be dominant. The star HD140283 has been the subject of debate after it appeared in some studies to be dominated by the s-process. We provide an independent measure of the Ba isotope mixture in HD140283 using an extremely high quality spectrum and an extensive chi^2 analysis. We exploit hyperfine splitting of the BaII 4554 \AA\ and 4934 \AA\ resonance lines in an effort to constrain the isotope ratio in 1D LTE. Using the code ATLAS in conjunction with KURUCZ06 model atmospheres we analyse 93 Fe lines to determine the star's macroturbulence. With this information we construct a grid of Ba synthetic spectra and, using a \chi^2 code, fit these to our observed data to determine the isotopic ratio, fodd, which represents the ratio of odd to even isotopes. We also analyse the Eu lines. We set a new upper limit of the rotation of HD140283 at vsin{i}\leq3.9\kms, a new upper limit on [Eu/H] < -2.80 and abundances [Fe/H] = -2.59\pm0.09, [Ba/H] = -3.46\pm0.11. This leads to a new lower limit on [Ba/Eu] > -0.66. We find that, in the framework of a 1D LTE analysis, the isotopic ratios of Ba in HD140283 indicate fodd=0.02\pm0.06, a purely s-process signature. This implies that observations and analysis do not validate currently accepted theory. We speculate that a 1D code, due to simplifying assumptions, is not adequate when dealing with observations with high levels of resolution and S/N because of the turbulent motions associated with a 3D stellar atmosphere. New approaches to analysing isotopic ratios, in particular 3D hydrodynamics, need to be considered when dealing with the levels of detail required to properly determine them. However published 3D results exacerbate the disagreement between theory and observation. Comment: 16 pages, 10 figures, 7 tables, 1 online appendix Accepted by A&A
08/2010;
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ABSTRACT: We study r‐process nucleosynthesis in neutrino‐driven winds of Type II supernovae (SNe), binary neutron‐star mergers, and magneto‐hydrodynamic jets in view of recent astronomical observations of r‐process elements in metal‐deficient stars and new nuclear reaction data. Universality in observed abundance pattern and total ejected yields of the r‐elements from single episode of each candidate site are used to identify the astrophysical site of the r‐process. Neutrinos play the critical roles in light element synthesis as well as r‐process. Elemental abundances are affected strongly by neutrino oscillations (MSW effect) through the SN ν‐process nucleosynthesis. We find that unknown neutrino oscillation parameters, i.e. mass hierarchy and mixing angle θ13, are simultaneously constrained by the Li∕B ratio from SN nucleosynthesis. Gamma ray burst (GRB) nucleosynthesis in contrast is expected to be relatively free from thermal neutrino effects because of black hole (BH) formation insteard of neutron star. We find that the abundance pattern is totally different from ordinary SN nucleosynthesis.
AIP Conference Proceedings. 05/2010; 1235(1):341-348.
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T Hashimoto,
K Ohta,
K Aoki,
I Tanaka,
K. Yabe,
N Kawai, W. Aoki,
H. Furusawa,
T Hattori,
M. Iye, [......],
Y. Shirasaki,
M Suzuki,
A. Tajitsu,
T Takata,
T Tamagawa,
H Terada,
T. Totani,
J Watanabe,
T Yamada,
A Yoshida
[show abstract]
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ABSTRACT: We present optical and near infrared observations of GRB 080325 classified as a "Dark GRB". Near-infrared observations with Subaru/MOIRCS provided a clear detection of afterglow in Ks band, although no optical counterpart was reported. The flux ratio of rest-wavelength optical to X-ray bands of the afterglow indicates that the dust extinction along the line of sight to the afterglow is Av = 2.7 - 10 mag. This large extinction is probably the major reason for optical faintness of GRB 080325. The J - Ks color of the host galaxy, (J - Ks = 1.3 in AB magnitude), is significantly redder than those for typical GRB hosts previously identified. In addition to J and Ks bands, optical images in B, Rc, i', and z' bands with Subaru/Suprime-Cam were obtained at about one year after the burst, and a photometric redshift of the host is estimated to be z_{photo} = 1.9. The host luminosity is comparable to L^{*} at z \sim 2 in contrast to the sub-L^{*} property of typical GRB hosts at lower redshifts. The best-fit stellar population synthesis model for the host shows that a large dust extinction (Av = 0.8 mag) attributes to the red nature of the host and that the host galaxy is massive (M_{*} = 7.0 \times 10^{10} Msun) which is one of the most massive GRB hosts previously identified. By assuming that the mass-metallicity relation for star-forming galaxies at z \sim 2 is applicable for the GRB host, this large stellar mass suggests the high metallicity environment around GRB 080325, consistent with inferred large extinction. Comment: 22 pages, 10 figures, accepted for publication in The Astrophysical Journal
03/2010;
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ABSTRACT: The presence of the lithium-6 isotope in some metal-poor stars is a matter of surprise because of the high values observed. Non-standard models of Big Bang nucleosynthesis and pre-Galactic cosmic ray fusion and spallation have been proposed to explain these values. However, the observations of this light isotope are challenging which may make some detections disputable. The goal was to determine 6Li/7Li for a sample of metal-poor stars; three of them have been previously studied and the remaining two are new for this type of study. The purpose was to increase, if possible, the number of lithium-6 detections and to confirm previously published results. Spectra of the resonance doublet line of neutral lithium Li I 670.78nm were taken with the High Dispersion Spectrograph at the Subaru 8.2m-telescope for a sample of five metal-poor stars (-3.12 < [Fe/H] < -2.19). The contribution of lithium-6 to the total observed line profile was estimated from the 1D-LTE analysis of the line asymmetry. Observed asymmetries could be reproduced assuming isotopic abundance ratios 6Li/7Li of the order of: 0.004 for BD+26 3578, ~ 0.010 for BD+02 3375 and G 64-37, 0.025 for BD+20 3603 and 0.047 for BD-04 3208. We found that these results were very sensitive to several of the assumptions made in the analysis, in particular, the treatment of the residual structure in the analysed spectra. Our final estimates for the errors are respectively delta_6Li/7Li=+- 0.028, 0.029, 0.039, 0.025 and 0.039. The 6Li/7Li ratios for the sample are comparable to or even lower than these error values, so that detections of lithium-6 can not safely be claimed despite of the high resolving power (R ~ 95 000) and S/N (400-600). Comment: 10 pages, 9 figures
09/2009;
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[hide abstract]
ABSTRACT: 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 tables
09/2009;
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S. Kawanomoto,
T. Kajino, W. Aoki,
M. Bessell,
T. K. Suzuki,
H. Ando,
K. Noguchi,
S. Honda,
H. Izumiura,
E. Kambe,
K. Okita,
K. Sadakane,
B. Sato,
A. Tajitsu,
M. Takada-Hidai,
W. Tanaka,
E. Watanabe,
and M. Yoshida
[show abstract]
[hide abstract]
ABSTRACT: We have determined the isotopic abundance ratio of 7Li/6Li in the interstellar media (ISMs) along lines of sight to HD169454 and HD250290 using the High-Dispersion Spectrograph on the Subaru Telescope. We also observed ζ Oph for comparison with previous data. The observed abundance ratios were 7Li/6Li = 8.1+3.6 –1.8 and 6.3+3.0 –1.7 for HD169454 and HD250290, respectively. These values are in reasonable agreement with those observed previously in the solar neighborhood ISMs within ±2σ error bars and are also consistent with our measurement of 7Li/6Li = 7.1+2.9 –1.6 for a cloud along the line of sight to ζ Oph. This is good evidence for homogeneous mixing and instantaneous recycling of the gas component in the Galactic disk. We also discuss several source compositions of 7Li, Galactic cosmic-ray interactions, stellar nucleosynthesis, and big bang nucleosynthesis.
The Astrophysical Journal 08/2009; 701(2):1506. · 6.02 Impact Factor
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W. Aoki,
N. Arimoto,
K. Sadakane,
E. Tolstoy,
G. Battaglia,
P. Jablonka,
M. Shetrone,
B. Letarte,
M. Irwin,
V. Hill,
P. Francois,
K. Venn,
F. Primas,
A. Helmi,
A. Kaufer,
M. Tafelmeyer,
T. Szeifert,
C. Babusiaux
[show abstract]
[hide abstract]
ABSTRACT: Chemical abundances of six extremely metal-poor ([Fe/H]<-2.5) stars in the Sextans dwarf spheroidal galaxy are determined based on high resolution spectroscopy (R=40,000) with the Subaru Telescope High Dispersion Spectrograph. (1) The Fe abundances derived from the high resolution spectra are in good agreement with the metallicity estimated from the Ca triplet lines in low resolution spectra. The lack of stars with [Fe/H]=<-3 in Sextans, found by previous estimates from the Ca triplet, is confirmed by our measurements, although we note that high resolution spectroscopy for a larger sample of stars will be necessary to estimate the true fraction of stars with such low metallicity. (2) While one object shows an overabundance of Mg (similar to Galactic halo stars), the Mg/Fe ratios of the remaining five stars are similar to the solar value. This is the first time that low Mg/Fe ratios at such low metallicities have been found in a dwarf spheroidal galaxy. No evidence for over-abundances of Ca and Ti are found in these five stars, though the measurements for these elements are less certain. Possible mechanisms to produce low Mg/Fe ratios, with respect to that of Galactic halo stars, are discussed. (3) Ba is under-abundant in four objects, while the remaining two stars exhibit large and moderate excesses of this element. The abundance distribution of Ba in this galaxy is similar to that in the Galactic halo, indicating that the enrichment of heavy elements, probably by the r-process, started at metallicities [Fe/H] < -2.5, as found in the Galactic halo.
05/2009;
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[show abstract]
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ABSTRACT: Based on spectroscopic constraints derived from NLTE line formation, we
explore the likely range of stellar parameters (T_eff and log g) for the
hyper-metal-poor (HMP) star HE 1327-2326. Combining the constraints from Balmer
line profiles and the Ca I/II ionization equilibrium, a subgiant stage of
evolution is indicated. This result is further supported by spectrophotometric
observations of the Balmer jump. If a higher T_eff value was used (as favoured
by some photometric calibrations), the spectroscopic analysis would indicate a
turnoff-point stage of evolution. Using a stellar-structure code that treats
the effects of atomic diffusion throughout the star in detail, we evolve a
low-mass model star to reach the HR-diagram position of HE 1327-2326 after
roughly 13 Gyr. While the surface abundances are modified significantly (by
more than 1 dex for the case of uninhibited diffusion), such corrections can
not resolve the discrepancy between the abundance inferred from the
non-detection of the Li I resonance line at 6707 Angstroem and the WMAP-based
primordial lithium abundance. As there are numerous processes that can destroy
lithium, any cosmological interpretation of a lower-than-expected lithium
abundance at the lowest metallicities will have to await sample sizes of
unevolved hyper-metal-poor stars that are one order of magnitude larger. The
situation remains equally inconclusive concerning atomic-diffusion corrections.
Here, attempts have to be made to better constrain internal mixing processes,
both observationally and by means of sophisticated modelling. With constraints
on additional mixing processes taken from a recent globular-cluster study, the
likeliest scenario is that HE 1327-2326's surface abundances have undergone
mild depletion (of order 0.2 dex).
03/2009;
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W. Aoki,
A. Frebel,
N. Christlieb,
J. E. Norris,
T. C. Beers,
T. Minezaki,
P. S. Barklem,
S. Honda,
M. Takada-Hidai,
M. Asplund,
S. G. Ryan,
S. Tsangarides,
K. Eriksson,
A. Steinhauer,
C. P. Deliyannis,
K. Nomoto,
M. Y. Fujimoto,
H. Ando,
Y. Yoshii,
and T. Kajino
[show abstract]
[hide abstract]
ABSTRACT: We present the elemental abundances of HE 1327-2326, the most iron-deficient star known, determined from a comprehensive analysis of spectra obtained with the Subaru Telescope High Dispersion Spectrograph. HE 1327-2326 is either in its main-sequence or subgiant phase of evolution. Its non-LTE-corrected iron abundance is [Fe/H] = -5.45, 0.2 dex lower than that of HE 0107-5240, the previously most iron-poor object known, and more than 1 dex lower than those of all other metal-poor stars. Both HE 1327-2326 and HE 0107-5240 exhibit extremely large overabundances of carbon ([C/Fe] ~ +4). The combination of extremely high carbon abundance with outstandingly low iron abundance in these objects clearly distinguishes them from other metal-poor stars. The large carbon excesses in these two stars are not the result of a selection effect. There also exist important differences between HE 1327-2326 and HE 0107-5240. While the former shows remarkable overabundances of the light elements (N, Na, Mg, and Al), the latter shows only relatively small excesses of N and Na. The neutron-capture element Sr is detected in HE 1327-2326, but not in HE 0107-5240; its Sr abundance is significantly higher than the upper limit for HE 0107-5240. The Li I λ6707 line, which is detected in most metal-poor dwarfs and warm subgiants having the same temperature as HE 1327-2326, is not found in this object. The upper limit of its Li abundance [log < 1.5] is clearly lower than the Spite plateau value. These data provide new constraints on models of nucleosynthesis processes in the first-generation objects that were responsible for metal enrichment at the earliest times. We discuss possible scenarios to explain the observed abundance patterns.
The Astrophysical Journal 12/2008; 639(2):897. · 6.02 Impact Factor
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A E García Pérez,
N Christlieb,
S G Ryan,
T C Beers, W Aoki,
M Asplund,
P S Barklem,
M S Bessell,
K Eriksson,
A Frebel,
B Gustafsson,
A J Korn,
B Nordström,
J E Norris
[show abstract]
[hide abstract]
ABSTRACT: A sample of 30 very metal-poor stars from the Hamburg-European Southern Observatory (ESO) objective-prism survey have been observed at high spectral resolution at the Very Large Telescope (VLT) using the Ultraviolet and Visual Echelle Spectrograph (UVES). Two of the observed stars are very interesting not only because of their very low iron content, approximately four orders of magnitude lower than the solar value, but also because we detected the neutral lithium resonance line at 670.8 nm. Hydrogen lines suggest that the two observed stars have effective temperatures around 6000–6250 K and according to isochrones, they are either on the main–sequence or on the subgiant branch, in which case they would probably be the most metal-poor dwarfs or warm subgiants with lithium detections known. These detections would allow to determine more accurately the slope of the trend of the lithium abundance with [Fe/H] than was possible with samples of unevolved stars restricted to higher metallicities.
Physica Scripta 12/2008; 2008(T133):014036. · 1.20 Impact Factor
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K. S. Kawabata,
J. Deng,
L. Wang,
P. Mazzali,
K. Nomoto,
K. Maeda,
N. Tominaga,
H. Umeda,
M. Iye,
G. Kosugi, [......],
N. Kobayashi,
Y. Komiyama,
Y. Mizumoto,
J. Noumaru,
R. Ogasawara,
K. Sekiguchi,
Y. Shirasaki,
T. Totani,
J. Watanabe,
and T. Yamada
[show abstract]
[hide abstract]
ABSTRACT: Spectroscopic and spectropolarimetric observations of SN 2003dh/GRB 030329 obtained in 2003 May using the Subaru 8.2 m Telescope are presented. The properties of the supernova (SN) are investigated through a comparison with spectra of the Type Ic hypernovae SN 1997ef and SN 1998bw (hypernovae being a tentatively defined class of SNe with very broad absorption features: these features suggest a large velocity of the ejected material and possibly a large explosion kinetic energy). Comparison with spectra of other hypernovae shows that the spectrum of SN 2003dh obtained on 2003 May 8 and 9, i.e., 34-35 rest-frame days after the gamma-ray burst (GRB; for z = 0.1685), are similar to those of SN 1997ef obtained ~34-42 days after the fiducial time of explosion of that SN. The match with SN 1998bw spectra is not as good (at rest 7300-8000 Å), but again spectra obtained ~33-43 days after GRB 980425 are preferred. This indicates that the SN may have intermediate properties between SNe 1997ef and 1998bw. On the basis of the analogy with the other hypernovae, the time of explosion of SN 2003dh is then constrained to be between -8 and +2 days of the GRB. The Si and O P Cygni lines of SN 2003dh seem comparable to those of SN 1997ef, which suggests that the ejected mass in SN 2003dh may match that in SN 1997ef. Polarization was marginally detected at optical wavelengths. This is consistent with measurements of the late afterglow, implying that it originated mostly in the interstellar medium of the host galaxy.
The Astrophysical Journal 12/2008; 593(1):L19. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The Lithium (Li) abundances measured for very metal-poor turn-off (unevolved) stars have been interpreted as the result of Big Bang nucleosynthesis. However, the value is lower by a factor of two or three than the prediction of standard Big Bang nucleosynthesis models, adopting the cosmological parameters determined by the measurements of cosmic microwave background radiation with the WMAP satellite. Moreover, the recent measurements for extremely metal-poor stars (objects having iron abundances less than 1/1000th solar) suggest a scatter of the Li abundance, or a possible decreasing trend with decreasing metallicity. In order to further investigate the Li production and destruction processes in the very early universe, we have determined Li abundances for extremely metal-poor stars based on high-resolution spectra for the resonance line of neutral Li. The result of our analysis, combined with previous measurements, indicates that the Li abundances of extremely metal-poor stars are, on average, lower than those of stars with higher metallicity, while the scatter or trend of the Li abundance remains unclear. We discuss possible reasons for the lower Li abundances in extremely metal-poor stars, such as depletion of Li in low-mass unevolved stars, or destruction of Li by the first generations of massive progenitors.
AIP Conference Proceedings 05/2008; 1016(1).
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[show abstract]
[hide abstract]
ABSTRACT: We have detected the isotopic abundance ratio of {sup 7}Li/{sup 6}Li in the interstellar media (ISMs) along the line of sight to HD169454 and HD250290 using High Dispersion Spectrograph equipped with the Subaru Telescope. We also observed {zeta} Oph for the comparison purpose with the previous data. The first background star HD169454 is at the distance 1.7 kpc inward from the solar sysytem in the Galactic plane, and the absorbing clouds are estimated to locate at 0.125 kpc{<=}d{<=}0.7 kpc and/or 1 kpc{<=}d{<=}1.7 kpc. If the observed {sup 7}Li/{sup 6}Li ratio is for the gas cloud at the larger distance 1 kpc{<=}d{<=}1.7 kpc, this could be the first observation of the ISM {sup 7}Li/{sup 6}Li ratio beyond the solar neighborhood. As for the second background star HD250290, the distance from the solar system is uncertain, 2.2{+-}0.2 kpc or 0.439{sub -0.147}{sup +0.446} kpc. The observed abundance ratios turn out to be {sup 7}Li/{sup 6}Li = 8.1{sub -1.8}{sup +3.6}, and 6.3{sub -1.7}{sup +3.0} for HD169454 and HD250290, respectively. These values are in reasonable agreement with those observed previously in the ISMs of solar neighborhood within {+-}1{sigma} error bars and also consistent with our measurement of {sup 7}Li/{sup 6}Li 7.1{sub -1.6}{sup +2.9} for nearby cloud along the line of sight to {zeta} Oph. This suggests a piece of evidence for homogeneous mixing and instantaneous recycling of gas component in the Galactic disk. We cannot claim a slope of the {sup 7}Li/{sup 6}Li ratio within {+-}1{sigma} error bars as a function of Galactocentric distance R{sub G}. Decomposing the observed ISM {sup 7}Li/{sup 6}Li ratio into several possible source of {sup 7}Li, we discuss three different contributions to the {sup 7}Li from the Galactic cosmic-ray interactions, the stellar nucleosynthesis, and the Big-Bang nucleosynthesis.
AIP Conference Proceedings 05/2008; 1016(1).
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[show abstract]
[hide abstract]
ABSTRACT: CS 29497–030 and CS 31062–050 belong to a sample of C‐rich, s‐process rich and extremely metal‐poor stars (CEMP–s+r). To explain the s–process enrichment, we considered these stars to be extrinsic asymptotic giant branch (AGB) stars, belonging to binary systems where the more massive AGB companion polluted the observed star (of ∼ 0.8 M☉) with efficient stellar winds. To explain the r–process enrichment, we assumed that the parental cloud was already enriched in r‐process elements.
AIP Conference Proceedings. 03/2008; 990(1):330-332.
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C. Allende Prieto,
T. Sivarani,
T. C. Beers,
Y. S. Lee,
L. Koesterke,
M. Shetrone,
C. Sneden,
D. L. Lambert,
R. Wilhelm,
C. M. Rockosi,
D. Lai,
B. Yanny,
I. I. Ivans,
J. A. Johnson, W. Aoki,
C. A. L. Bailer-Jones,
P. Re Fiorentin
[show abstract]
[hide abstract]
ABSTRACT: We report high-resolution spectroscopy of 125 field stars previously observed as part of the Sloan Digital Sky Survey and its program for Galactic studies, the Sloan Extension for Galactic Understanding and Exploration (SEGUE). These spectra are used to measure radial velocities and to derive atmospheric parameters, which we compare with those reported by the SEGUE Stellar Parameter Pipeline (SSPP). The SSPP obtains estimates of these quantities based on SDSS ugriz photometry and low-resolution (R = 2000) spectroscopy. For F- and G-type stars observed with high signal-to-noise ratios (S/N), we empirically determine the typical random uncertainties in the radial velocities, effective temperatures, surface gravities, and metallicities delivered by the SSPP to be 2.4 km/s, 130 K (2.2%), 0.21 dex, and 0.11 dex, respectively, with systematic uncertainties of a similar magnitude in the effective temperatures and metallicities. We estimate random errors for lower S/N spectra based on numerical simulations.
11/2007;
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ABSTRACT: Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba, and Eu are determined for four very metal-poor stars (-2.66 < [Fe/H] < -2.15) based on non-LTE line formation and analysis of high-resolution (R ~60000 and 90000) high signal-to-noise (S/N > 200) observed spectra. A model atom for H I is presented. An effective temperature was obtained from the Balmer Halpha and Hbeta line wing fits, the surface gravity from the Hipparcos parallax if available and the non-LTE ionization balance between Ca I and Ca II. Based on the hyperfine structure affecting the Ba II resonance line, the fractional abundance of the odd isotopes of Ba was derived for HD 84937 and HD 122563 from a requirement that Ba abundances from the resonance line and subordinate lines of Ba II must be equal. For each star, non-LTE leads to a consistency of Teff from two Balmer lines and to a higher temperature compared to the LTE case, by up to 60 K. Non-LTE effects are important in spectroscopic determination of surface gravity from Ca I/Ca II. For each star with a known trigonometric gravity, non-LTE abundances from the lines of two ionization stages agree within the error bars, while a difference in the LTE abundances consists of 0.23 dex to 0.40 dex for different stars. Departures from LTE are found to be significant for the investigated atoms, and they strongly depend on stellar parameters. For HD 84937, the Eu/Ba ratio is consistent with the relative solar system r-process abundances, and the fraction of the odd isotopes of Ba, f_odd, equals 0.43+-0.14. The latter can serve as a constraint on r-process models. The lower Eu/Ba ratio and f_odd = 0.22+-0.15 found for HD 122563 suggest that the s-process or the unknown process has contributed significantly to the Ba abundance in this star. Comment: accepted for publication in A&A, November 16, 2007
11/2007;
