1,913 research outputs found
Abundances of disk and bulge giants from hi-res optical spectra: II. O, Mg, Ca, and Ti in the bulge sample
Determining elemental abundances of bulge stars can, via chemical evolution
modeling, help to understand the formation and evolution of the bulge. Recently
there have been claims both for and against the bulge having a different
[/Fe] vs. [Fe/H]-trend as compared to the local thick disk possibly
meaning a faster, or at least different, formation time scale of the bulge as
compared to the local thick disk. We aim to determine the abundances of oxygen,
magnesium, calcium, and titanium in a sample of 46 bulge K-giants, 35 of which
have been analyzed for oxygen and magnesium in previous works, and compare them
to homogeneously determined elemental abundances of a local disk sample of 291
K-giants. We use spectral synthesis to determine both the stellar parameters as
well as the elemental abundances of the bulge stars analyzed here. The method
is exactly the same as was used for analyzing the comparison sample of 291
local K-giants in Paper I of this series. Compared to the previous analysis of
the 35 stars in our sample, we find lower [Mg/Fe] for [Fe/H]>-0.5, and
therefore contradict the conclusion about a declining [O/Mg] for increasing
[Fe/H]. We instead see a constant [O/Mg] over all the observed [Fe/H] in the
bulge. Furthermore, we find no evidence for a different behavior of the
alpha-iron trends in the bulge as compared to the local thick disk from our two
samples.Comment: Accepted for publication in A&
Fluorine in the solar neighborhood - is it all produced in AGB-stars?
The origin of 'cosmic' fluorine is uncertain, but there are three proposed
production sites/mechanisms: AGB stars, nucleosynthesis in Type II
supernovae, and/or the winds of Wolf-Rayet stars. The relative importance of
these production sites has not been established even for the solar
neighborhood, leading to uncertainties in stellar evolution models of these
stars as well as uncertainties in the chemical evolution models of stellar
populations.
We determine the fluorine and oxygen abundances in seven bright, nearby
giants with well-determined stellar parameters. We use the 2.3 m
vibrational-rotational HF line and explore a pure rotational HF line at 12.2
m. The latter has never been used before for an abundance analysis. To be
able to do this we have calculated a line list for pure rotational HF lines. We
find that the abundances derived from the two diagnostics agree.
Our derived abundances are well reproduced by chemical evolution models only
including fluorine production in AGB-stars and therefore we draw the conclusion
that this might be the main production site of fluorine in the solar
neighborhood. Furthermore, we highlight the advantages of using the 12 m
HF lines to determine the possible contribution of the -process to the
fluorine budget at low metallicities where the difference between models
including and excluding this process is dramatic
Detailed Abundances for the Old Population near the Galactic Center: I. Metallicity distribution of the Nuclear Star Cluster
We report the first high spectral resolution study of 17 M giants
kinematically confirmed to lie within a few parsecs of the Galactic Center,
using R=24,000 spectroscopy from Keck/NIRSPEC and a new linelist for the
infrared K band. We consider their luminosities and kinematics, which classify
these stars as members of the older stellar population and the central cluster.
We find a median metallicity of =-0.16 and a large spread from
approximately -0.3 to +0.3 (quartiles). We find that the highest metallicities
are [Fe/H]<+0.6, with most of the stars being at or below the Solar iron
abundance. The abundances and the abundance distribution strongly resembles
that of the Galactic bulge rather than disk or halo; in our small sample we
find no statistical evidence for a dependence of velocity dispersion on
metallicity.Comment: 18 pages, 14 figures, accepted for publication in A
Absence of Conventional Spin-Glass Transition in the Ising Dipolar System LiHo_xY_{1-x}F_4
The magnetic properties of single crystals of LiHo_xY_{1-x}F_4 with x=16.5%
and x=4.5% were recorded down to 35 mK using a micro-SQUID magnetometer. While
this system is considered as the archetypal quantum spin glass, the detailed
analysis of our magnetization data indicates the absence of a phase transition,
not only in a transverse applied magnetic field, but also without field. A
zero-Kelvin phase transition is also unlikely, as the magnetization seems to
follow a non-critical exponential dependence on the temperature. Our analysis
thus unmasks the true, short-ranged nature of the magnetic properties of the
LiHo_xY_{1-x}F_4 system, validating recent theoretical investigations
suggesting the lack of phase transition in this system.Comment: 5 pages, 4 figure
Evidence against anomalous compositions for giants in the Galactic Nuclear Star Cluster
Very strong Sc I lines have been found recently in cool M giants in the
Nuclear Star Cluster in the Galactic Center. Interpreting these as anomalously
high scandium abundances in the Galactic Center would imply a unique
enhancement signature and chemical evolution history for nuclear star clusters,
and a potential test for models of chemical enrichment in these objects. We
present high resolution K-band spectra (NIRSPEC/Keck II) of cool M giants
situated in the solar neighborhood and compare them with spectra of M giants in
the Nuclear Star Cluster. We clearly identify strong Sc I lines in our solar
neighborhood sample as well as in the Nuclear Star Cluster sample. The strong
Sc I lines in M giants are therefore not unique to stars in the Nuclear Star
Cluster and we argue that the strong lines are a property of the line formation
process that currently escapes accurate theoretical modeling. We further
conclude that for giant stars with effective temperatures below approximately
3800 K these Sc I lines should not be used for deriving the scandium abundances
in any astrophysical environment until we better understand how these lines are
formed. We also discuss the lines of vanadium, titanium, and yttrium identified
in the spectra, which demonstrate a similar striking increase in strength below
3500 K effective temperature.Comment: 11 pages, 6 figures, accepted for publication in Ap
Riqueza en nutrientes del mantillo de la madera en cavidades arbóreas donde está presente el escarabeideo Osmoderma eremita
Trunk hollows with wood mould harbour a rich invertebrate fauna with many threatened species, and it has been suggested that the beetle Osmoderma eremita (Coleoptera, Scarabaeidae) is a keystone species in this community. We estimated the amount of nitrogen and phosphorus in wood mould and compared the coarse fraction which constitutes frass of O. eremita with the finer fraction of wood mould, and found that the nutrient richness was higher in frass. O. eremita larvae have a fermentation chamber that harbours nitrogen fixing bacteria. As the levels of absorbable nitrogen are a limiting factor in insect growth, an increase in nutrient richness is one of several possible explanations why the species richness of saproxylic beetles is higher in hollow oaks where O. eremita is present in relation to similar trees where the beetle is absent.Las cavidades de los troncos con mantillo de la madera albergan una rica fauna de invertebrados, entre los que se incluyen numerosas especies amenazadas. Se ha sugerido que Osmoderma eremita (Coleoptera, Scarabaeoidea) constituye una especie clave de esta comunidad. Se estimó la cantidad de nitrógeno y fósforo presentes en el mantillo de la madera, y se comparó la fracción gruesa formada por las deyecciones de O. eremita con la fracción más fina del mantillo de la madera y se vió que la riqueza en nutrientes era más elevada en la fracción gruesa. Las larvas de O. eremita contienen una cámara de fermentación que alberga el nitrógeno fijado por las bacterias. Puesto que los niveles de nitrógeno absorbible constituyen un factor limitador del crecimiento de insectos, un aumento de la riqueza en nutrientes es una de las posibles explicaciones del porqué la riqueza en especies de los escarabajos saproxÃlicos es más elevada en los robles huecos donde O. eremita está presente que en otros árboles similares donde está ausente
Finite-size effects in amorphous Fe90Zr10/Al75Zr25 multilayers
The thickness dependence of the magnetic properties of amorphous Fe90Zr10
layers has been explored using Fe90Zr10/Al75Zr25 multilayers. The Al75Zr25
layer thickness is kept at 40 \AA, while the thickness of the Fe90Zr10 layers
is varied between 5 and 20 \AA. The thickness of the Al75Zr25 layers is
sufficiently large to suppress any significant interlayer coupling. Both the
Curie temperature and the spontaneous magnetization decrease non-linearly with
decreasing thickness of the Fe90Zr10 layers. No ferromagnetic order is observed
in the multilayer with 5 {\AA} Fe90Zr10 layers. The variation of the Curie
temperature with the Fe90Zr10 layer thickness is fitted with a
finite-size scaling formula [1-\Tc(t)/\Tc(\infty)]=[(t-t')/t_0]^{-\lambda},
yielding , and a critical thickness \AA, below which the
Curie temperature is zero.Comment: 8 pages, 8 figure
Abundances of disk and Bulge giants from high-resolution optical spectra III. Sc, V, Cr, Mn, Co, Ni
Context. Recent observations of the Bulge, e.g., its X-shape, cylindrical
stellar motions, and a potential fraction of young stars propose that it formed
through secular evolution of the disk and not through gas dissipation and/or
mergers, as thought previously. Aims. We measure abundances of six iron-peak
elements (Sc, V, Cr, Mn, Co and Ni) in the local thin and thick disks as well
as the Bulge to provide additional observational constraints for Galaxy
formation and chemical evolution models. Methods. We use high-resolution
optical spectra of 291 K giants in the local disk mostly obtained by the FIES
at NOT (signal-to-noise (S/N) ratio of 80-100) and 45 K giants in the Bulge
obtained by the UVES/FLAMES at VLT (S/N ratio of 10-80). We measure abundances
in SME and apply NLTE corrections to the [Mn/Fe] and [Co/Fe] ratios. To
discriminate between the thin and thick, we use stellar metallicity,
[Ti/Fe]-ratios, and kinematics from Gaia DR2 (proper motions and the radial
velocities). Results. The observed disk trend of V is more enhanced in the
thick disk, while the Co disk trend shows a minor enhancement in the thick
disk. The Bulge trends of V and Co appear even more enhanced w.r.t. the thick
disk, but within the uncertainties. The [Ni/Fe] ratio seems slightly
overabundant in the thick disk and the Bulge w.r.t. the thin disk, although the
difference is minor. The disk and Bulge trends of Sc, Cr and Mn overlap
strongly. Conclusions. The somewhat enhanced [(V,Co)/Fe] ratios observed in the
Bulge suggest that the local thick disk and the Bulge might have experienced
different chemical enrichment and evolutionary paths. However, we are unable to
predict the exact evolutionary path of the Bulge solely based on these
observations. Galactic chemical evolution models could, on the other hand,
provide that using these results.Comment: Accepted for publication in A&A; 25 pages, 15 figure
Existence of a phase transition under finite magnetic field in the long-range RKKY Ising spin glass DyYRuSi
A phase transition of a model compound of the long-range Ising spin glass
(SG) DyYRuSi, where spins interact via the RKKY
interaction, has been investigated. The static and the dynamic scaling analyses
reveal that the SG phase transition in the model magnet belongs to the
mean-field universality class. Moreover, the characteristic relaxation time in
finite magnetic fields exhibits a critical divergent behavior as well as in
zero field, indicating a stability of the SG phase in finite fields. The
presence of the SG phase transition in field in the model magnet strongly
syggests that the replica symmetry is broken in the long-range Ising SG.Comment: 4 pages, 4 figures, to be published in JPSJ (2010
Optical response of two-dimensional electron fluids beyond the Kohn regime: strong non-parabolic confinement and intense laser light
We investigate the linear and non-linear optical response of two-dimensional
(2D) interacting electron fluids confined by a strong non-parabolic potential.
We show that such fluids may exhibit higher-harmonic spectra under realistic
experimental conditions. Higher harmonics arise as the electrons explore
anharmonicities of the confinement potential (electron-electron interactions
reduce this non-linear effect). This opens the possibility of controlling the
optical functionality of such systems by engineering the confinement potential.
Our results were obtained within time-dependent density-functional theory,
employing the adiabatic local-density approximation. A classical hydrodynamical
model is in good agreement with the quantum-mechanical results.Comment: 4 pages, 4 figure
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