13,026 research outputs found
Chemical Abundances of the Outer Halo Stars in the Milky Way
We present chemical abundances of 57 metal-poor stars that are likely
constituents of the outer stellar halo in the Milky Way. Almost all of the
sample stars have an orbit reaching a maximum vertical distance (Z_max) of >5
kpc above and below the Galactic plane. High-resolution, high signal-to-noise
spectra for the sample stars obtained with Subaru/HDS are used to derive
chemical abundances of Na, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y and Ba with an LTE
abundance analysis code. The resulting abundance data are combined with those
presented in literature that mostly targeted at smaller Z_max stars, and both
data are used to investigate any systematic trends in detailed abundance
patterns depending on their kinematics. It is shown that, in the metallicity
range of -25 kpc are
systematically lower (~0.1 dex) than those with smaller Z_max. This result of
the lower [alpha/Fe] for the assumed outer halo stars is consistent with
previous studies that found a signature of lower [alpha/Fe] ratios for stars
with extreme kinematics. A distribution of the [Mg/Fe] ratios for the outer
halo stars partly overlaps with that for stars belonging to the Milky Way dwarf
satellites in the metallicity interval of -2<[Fe/H]<-1 and spans a range
intermediate between the distributions for the inner halo stars and the stars
belonging to the satellites. Our results confirm inhomogeneous nature of
chemical abundances within the Milky Way stellar halo depending on kinematic
properties of constituent stars as suggested by earlier studies. Possible
implications for the formation of the Milky Way halo and its relevance to the
suggested dual nature of the halo are discussed.Comment: 68 pages with 23 figures, Accepted for publication in PAS
Electronic structure of an electron on the gyroid surface, a helical labyrinth
Previously reported formulation for electrons on curved periodic surfaces is
used to analyze the band structure of an electron bound on the gyroid surface
(the only triply-periodic minimal surface that has screw axes). We find that an
effect of the helical structure appears as the bands multiply sticking together
on the Brillouin zone boundaries. We elaborate how the band sticking is lifted
when the helical and inversion symmetries of the structure are degraded. We
find from this that the symmetries give rise to prominent peaks in the density
of states.Comment: RevTeX, 4 pages, 6 figure
Chemical Abundances of the Milky Way Thick Disk and Stellar Halo I.: Implications of [alpha/Fe] for Star Formation Histories in Their Progenitors
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]. 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.Comment: Accepted for publication in Ap
Variations in the Abundance Pattern of Extremely Metal-poor Stars and Nucleosynthesis in Population III Supernovae
We calculate nucleosynthesis in Population (Pop) III supernovae (SNe) and
compare the yields with various abundance patterns of extremely metal-poor
(EMP) stars. We assume that the observed EMP stars are the second generation
stars, which have the metal-abundance patterns of Pop III SNe. Previous
theoretical yields of Pop III SNe cannot explain the trends in the abundance
ratios among iron-peak elements (Mn, Co, Ni, Zn)/Fe as well as the large C/Fe
ratio observed in certain EMP stars with [Fe/H] <~ -2.5. In the present paper,
we show that if we introduce higher explosion energies and mixing-fallback in
the core-collapse SN models of M ~ 20 - 130 Msun, the above abundance features
of both typical and C-rich EMP stars can be much better explained. We suggest
that the abundance patterns of the [Fe/H] ~ -2.5 stars correspond to supernova
yields with normal explosion energies, while those of the carbon un-enhanced
([C/Fe] < 1) stars with [Fe/H] =~ -4 ~ - 3 correspond to high-energy supernova
yields. The abundance patterns of the C-rich ([C/Fe]>~ 2) and low [Fe/H] (=~ -5
\~ -3.5) stars can be explained with the yields of faint SNe that eject little
56Ni as observed in SN1997D. In the supernova-induced star formation model, we
can qualitatively explain why the EMP stars formed by the faint or energetic
supernovae have lower [Fe/H] than the EMP stars formed by normal supernovae. We
also examine how the abundance ratios among iron-peak elements depend on the
electron mole fraction Ye, and conclude that a large explosion energy is still
needed to realize the large Co/Fe and Zn/Fe ratios observed in typical EMP
stars with [Fe/H] <~ -3.5.Comment: 33 pages, 17 figures, 7 tables, To appear in the Astrophysical
Journal 2005, January 1
Nucleon-nucleon interactions via Lattice QCD: Methodology --HAL QCD approach to extract hadronic interactions in lattice QCD--
We review the potential method in lattice QCD, which has recently been
proposed to extract nucleon-nucleon interactions via numerical simulations. We
focus on the methodology of this approach by emphasizing the strategy of the
potential method, the theoretical foundation behind it, and special numerical
techniques. We compare the potential method with the standard finite volume
method in lattice QCD, in order to make pros and cons of the approach clear. We
also present several numerical results for the nucleon-nucleon potentials.Comment: 12 pages, 10 figure
Dominance of a single topological sector in gauge theory on non-commutative geometry
We demonstrate a striking effect of non-commutative (NC) geometry on
topological properties of gauge theory by Monte Carlo simulations. We study 2d
U(1) NC gauge theory for various boundary conditions using a new finite-matrix
formulation proposed recently. We find that a single topological sector
dictated by the boundary condition dominates in the continuum limit. This is in
sharp contrast to the results in commutative space-time based on lattice gauge
theory, where all topological sectors appear with certain weights in the
continuum limit. We discuss possible implications of this effect in the context
of string theory compactifications and in field theory contexts.Comment: 16 pages, 27 figures, typos correcte
Phase diagram for the one-dimensional Hubbard-Holstein model: A density-matrix renormalization group study
Phase diagram of the Hubbard-Holstein model in the coexistence of
electron-electron and electron-phonon interactions has been theoretically
obtained with the density-matrix renormalization group method for
one-dimensional (1D) systems, where an improved warm-up (the recursive sweep)
procedure has enabled us to calculate various correlation functions. We have
examined the cases of (i) the systems half-filled by electrons for the full
parameter space spanned by the electron-electron and electron-phonon coupling
constants and the phonon frequency, (ii) non-half-filled system, and (iii)
trestle lattice. For (i), we have detected a region where both the charge and
on-site pairing correlations decay with power-laws in real space, which
suggests a metallic behavior. While pairing correlations are not dominant in
(i), we have found that they become dominant as the system is doped in (ii), or
as the electronic band structure is modified (with a broken electron-hole
symmetry) in (iii) in certain parameter regions.Comment: 12 pages, 14 figures, submitted to Phys. Rev.
Chiral Gauge Theory on Lattice with Domain Wall Fermions
We investigate a U(1) lattice chiral gauge theory with domain wall fermions
and compact gauge fixing. In the reduced model limit, our perturbative and
numerical investigations show that there exist no extra mirror chiral modes.
The longitudinal gauge degrees of freedom have no effect on the free domain
wall fermion spectrum consisting of opposite chiral modes at the domain wall
and at the anti-domain wall which have an exponentially damped overlap.Comment: 16 pages revtex, 5 postscript figures, PRD versio
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