8,391 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
Study of Field-Induced Magnetic Order in Singlet-Ground-State Magnet CsFeCl
The field-induced magnetic order in the singlet-ground-state system
CsFeCl has been studied by measuring magnetization and neutron diffraction.
The field dependence of intensity for the neutron magnetic reflection has
clearly demonstrated that the field-induced ordered phase is described by the
order parameter . A condensate growth of magnons is investigated through
the temperature dependence of and , and this ordering is
discussed in the context of a magnon Bose-Einstein condensation. Development of
the coherent state and the static correlation length has been observed in the
incommensurate phase in the field region of , a satellite peak was found in coexistence with the commensurate
peak at the phase boundary around 10 T, which indicates that the tilt of the
c-axis would be less than in the whole experiments.Comment: 5 pages, 5 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
Inversion doublets of reflection-asymmetric clustering in 28Si and their isoscalar monopole and dipole transitions
[Background] Various cluster states of astrophysical interest are expected to
exist in the excited states of . However, they have not been
identified firmly, because of the experimental and theoretical difficulties.
[Purpose] To establish the Mg+, O+C and
Ne+2 cluster bands, we theoretically search for the
negative-parity cluster bands that are paired with the positive-parity bands to
constitute the inversion doublets. We also offer the isoscalar monopole and
dipole transitions as a promising probe for the clustering. We numerically show
that these transition strengths from the ground state to the cluster states are
very enhanced. [Method] The antisymmetrized molecular dynamics with Gogny D1S
effective interaction is employed to calculate the excited states of . The isoscalar monopole and dipole transition strengths are directly
evaluated from wave functions of the ground and excited states. [Results]
Negative-parity bands having Mg+ and O+C cluster
configurations are obtained in addition to the newly calculated
Ne+2 cluster bands. All of them are paired with the
corresponding positive-parity bands to constitute the inversion doublets with
various cluster configurations. The calculation show that the band-head of the
Mg+ and Ne+2 cluster bands are strongly excited
by the isoscalar monopole and dipole transitions. [Conclusions] The present
calculation suggests the existence of the inversion doublets with the
Mg+, O+C and Ne+2
configurations.Because of the enhanced transition strengths, we offer the
isoscalar monopole and dipole transitions as good probe for the
Mg+ and Ne+2 cluster bands.Comment: 28 pages, 8 figure
3alpha clustering in the excited states of 16C
The alpha cluster states of 16C are investigated by using the antisymmetrized
molecular dynamics. It is shown that two different types of alpha cluster
states exist: triangular and linear-chain states. The former has an approximate
isosceles triangular configuration of alpha particles surrounded by four
valence neutrons occupying sd-shell, while the latter has the linearly aligned
alpha particles with two sd-shell neutrons and two pf-shell neutrons. It is
found that the structure of the linear-chain state is qualitatively understood
in terms of the 3/2 pi- and 1/2 sigma- molecular orbit as predicted by
molecular-orbital model, but there exists non-negligible Be+alpha+2n
correlation. The band-head energies of the triangular and linear-chain
rotational bands are 8.0 and 15.5 MeV, and the latter is close to the He+Be
threshold energy. It is also shown that the linear-chain state becomes the
yrast sstate at J=10 with excitation energy 27.8 MeV owing to its very large
moment-of-inertia comparable with hyperdeformation.Comment: 7 pages, 5 figure
Velocity of domain-wall motion induced by electrical current in a ferromagnetic semiconductor (Ga,Mn)As
Current-induced domain-wall motion with velocity spanning over five orders of
magnitude up to 22 m/s has been observed by magneto-optical Kerr effect in
(Ga,Mn)As with perpendicular magnetic anisotropy. The data are employed to
verify theories of spin-transfer by the Slonczewski-like mechanism as well as
by the torque resulting from spin-flip transitions in the domain-wall region.
Evidence for domain-wall creep at low currents is found.Comment: 5 pages, 3 figure
Instanton and Monopole in External Chromomagnetic Fields
We study properties of instanton and monopole in an external chromomagnetic
field. Generally, the 't Hooft ansatz is no longer a solution of the Yang-Mills
field equation in the presence of external fields. Therefore, we investigate a
stabilized instanton solution with minimal total Yang-Mills action in a
nontrivial topological sector. With this aim, we consider numerical
minimization of the action with respect to the global color orientation, the
anisotropic scale transformation and the local gauge-like transformation
starting from a simple superposed gauge field of the 't Hooft ansatz and the
external color field. Here, the external color field is, for simplicity, chosen
to be a constant Abelian magnetic field along a certain direction. Then, the
4-dimensional rotational symmetry O(4) of the instanton solution is reduced to
two 2-dimensional rotational symmetries due to the effect of
a homogeneous external field. In the space \mib{R}^{3} at fixed , we find
a quadrupole deformation of this instanton solution. In the presence of a
magnetic field , a prolate deformation occurs along the direction of
. Contrastingly, in the presence of an electric field an
oblate deformation occurs along the direction of . We further discuss
the local correlation between the instanton and the monopole in the external
field in the maximally Abelian gauge. The external field affects the appearance
of the monopole trajectory around the instanton. In fact, a monopole and
anti-monopole pair appears around the instanton center, and this monopole loop
seems to partially screen the external field.Comment: 15 pages,8 figure
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