3,042 research outputs found
Non-LTE radiative transfer in cool stars. Theory and applications to the abundance analysis for 24 chemical elements
The interpretation of observed spectra of stars in terms of fundamental
stellar properties is a key problem in astrophysics. For FGK-type stars, the
radiative transfer models are often computed using the assumption of local
thermodynamic equilibrium (LTE). Its validity is often questionable and needs
to be supported by detailed studies, which build upon the consistent framework
of non-LTE. In this review, we outline the theory of non-LTE. The processes
causing departures from LTE are introduced qualitatively by their physical
interpretation, as well as quantitatively by their impact on the models of
stellar spectra and element abundances. We also compile and analyse the most
recent results from the literature. In particular, we examine the non-LTE
effects for 24 chemical elements for six late-studied FGK-type stars.Comment: 19 pages, accepted for publication as a chapter in "Determination of
Atmospheric Parameters of B, A, F and G Type Stars", Springer (2014), eds. E.
Niemczura, B. Smalley, W. Pyc
Electronic structure and optical properties of metallic nanoshells
The electronic structure and optical properties of metallic nanoshells are
investigated using a jellium model and the Time Dependent Local Density
Approximation (TDLDA). An efficient numerical implementation enables
applications to nanoshells of realistic size with up to a million electrons. We
demonstrate how a frequency dependent background polarizability of the jellium
shell can be included in the TDLDA formalism. The energies of the plasmon
resonances are calculated for nanoshells of different sizes and with different
dielectric cores, dielectric embedding media, and dielectric shell backgrounds.
The plasmon energies are found to be in good agreement with the results from
classical Mie scattering theory using a Drude dielectric function. A comparison
with experimental data shows excellent agreement between theory and the
measured frequency dependent absorption spectra
Non-LTE aluminium abundances in late-type stars
Aluminium plays a key role in studies of the chemical enrichment of the
Galaxy and of globular clusters. However, strong deviations from LTE (non-LTE)
are known to significantly affect the inferred abundances in giant and
metal-poor stars. We present NLTE modeling of aluminium using recent and
accurate atomic data, in particular utilizing new transition rates for
collisions with hydrogen atoms, without the need for any astrophysically
calibrated parameters. For the first time, we perform 3D NLTE modeling of
aluminium lines in the solar spectrum. We also compute and make available
extensive grids of abundance corrections for lines in the optical and
near-infrared using one-dimensional model atmospheres, and apply grids of
precomputed departure coefficients to direct line synthesis for a set of
benchmark stars with accurately known stellar parameters. Our 3D NLTE modeling
of the solar spectrum reproduces observed center-to-limb variations in the
solar spectrum of the 7835 {\AA} line as well as the mid-infrared photospheric
emission line at 12.33 micron. We infer a 3D NLTE solar photospheric abundance
of A(Al) = 6.43+-0.03, in exact agreement with the meteoritic abundance. We
find that abundance corrections vary rapidly with stellar parameters; for the
3961 {\AA} resonance line, corrections are positive and may be as large as +1
dex, while corrections for subordinate lines generally have positive sign for
warm stars but negative for cool stars. Our modeling reproduces the observed
line profiles of benchmark K-giants, and we find abundance corrections as large
as -0.3 dex for Arcturus. Our analyses of four metal-poor benchmark stars yield
consistent abundances between the 3961 {\AA} resonance line and lines in the
UV, optical and near-infrared regions. Finally, we discuss implications for the
galactic chemical evolution of aluminium.Comment: 20 pages, 15 figures. Accepted for publication in A&
Atomic Diffusion and Mixing in Old Stars V: A deeper look into the Globular Cluster NGC 6752
Abundance trends in heavier elements with evolutionary phase have been shown
to exist in the globular cluster NGC 6752 [Fe/H]=-1.6. These trends are a
result of atomic diffusion and additional (non-convective) mixing. Studying
such trends can provide us with important constraints on the extent to which
diffusion modifies the internal structure and surface abundances of solar-type,
metal-poor stars. Taking advantage of a larger data sample, we investigate the
reality and the size of these abundance trends and address questions and
potential biases associated with the various stellar populations that make up
NGC6752. Based on uvby Str\"omgren photometry, we are able to separate three
stellar populations in NGC 6752 along the evolutionary sequence from the base
of the red giant branch down to the turnoff point. We find weak systematic
abundance trends with evolutionary phase for Ca, Ti, and Fe which are best
explained by stellar-structure models including atomic diffusion with efficient
additional mixing. We derive a new value for the initial lithium abundance of
NGC 6752 after correcting for the effect of atomic diffusion and additional
mixing which falls slightly below the predicted standard BBN value. We find
three stellar populations by combining photometric and spectroscopic data of
194 stars in the globular cluster NGC 6752. Abundance trends for groups of
elements, differently affected by atomic diffusion and additional mixing, are
identified. Although the statistical significance of the individual trends is
weak, they all support the notion that atomic diffusion is operational along
the evolutionary sequence of NGC 6752.Comment: 15 pages, 11 figures, 2 online table
Emergence of ferroelectricity at the morphotropic phase boundary of ultrathin BiFeO
We demonstrate the robustness of polarization in ultrathin compressive
strained BiFeO single layers and heterostructures during epitaxial
thin-film growth. Using in-situ optical second harmonic generation (ISHG), we
explore the emergence of ferroelectric phases at the strain-driven morphotropic
phase boundary in the ultrathin regime. We find that the epitaxial films grow
in the ferroelectric tetragonal (T-) phase without exhibition of a critical
thickness. The robustness of this high-temperature T-phase against
depolarizing-field effects is further demonstrated during the growth of
capacitor-like (metal|ferroelectric|metal) heterostructures. Using
temperature-dependent ISHG post-deposition, we identify the thickness-dependent
onset of the monoclinic distortion in the T-matrix and trace the signature of
the subsequent emergence of the strain-relaxed rhombohedral-like monoclinic
phase. Our results show that strain-driven T-phase stabilization in BiFeO
yields a prominent candidate material for realizing ultrathin ferroelectric
devices.Comment: 5 pages, 3 figure
Dynamics of passing-stars-perturbed binary star systems
In this work, we investigate the dynamical effects of a sequence of close
encounters over 200 Myr varying in the interval of 10000 -- 100000 au between a
binary star system and passing stars with masses ranging from 0.1 to
10. We focus on binaries consisting of two Sun-like stars with
various orbital separations from 50 au to 200 au
initially on circular-planar orbits. We treat the problem statistically since
each sequence is cloned 1000 times. Our study shows that orbits of binaries
initially at = 50 au will slightly be perturbed by
each close encounter and exhibit a small deviation in eccentricity (+0.03) and
in periapsis distance (+1 and -2 au) around the mean value. However increasing
will drastically increase these variances: up to
+0.45 in eccentricity and between +63 au and -106 au in periapsis, leading to a
higher rate of disrupted binaries up to 50% after the sequence of close
encounters. Even though the secondary star can remain bound to the primary,
20% of the final orbits will have inclinations greater than 10.
As planetary formation already takes place when stars are still members of
their birth cluster, we show that the variances in eccentricity and periapsis
distance of Jupiter- and Saturn-like planets will inversely decrease with
after successive fly-bys. This leads to higher
ejection rate at = 50 au but to a higher extent for
Saturn-likes (60%) as those planets' apoapsis distances cross the critical
stability distance for such binary separation.Comment: Accepted for publication (MNRAS
Atomic Diffusion and Mixing in Old Stars. III. Analysis of NGC 6397 Stars under New Constraints
We have previously reported on chemical abundance trends with evolutionary
state in the globular cluster NGC 6397 discovered in analyses of spectra taken
with FLAMES at the VLT. Here, we reinvestigate the FLAMES-UVES sample of 18
stars, ranging from just above the turnoff point (TOP) to the red giant branch
below the bump. Inspired by new calibrations of the infrared flux method, we
adopt a set of hotter temperature scales. Chemical abundances are determined
for six elements (Li, Mg, Ca, Ti, Cr, and Fe). Signatures of cluster-internal
pollution are identified and corrected for in the analysis of Mg.
On the modified temperature scales, evolutionary trends in the abundances of
Mg and Fe are found to be significant at the 2{\sigma} and 3{\sigma} levels,
respectively. The detailed evolution of abundances for all six elements agrees
with theoretical isochrones, calculated with effects of atomic diffusion and a
weak to moderately strong efficiency of turbulent mixing. The age of these
models is compatible with the external determination from the white dwarf
cooling sequence. We find that the abundance analysis cannot be reconciled with
the strong turbulent-mixing efficiency inferred elsewhere for halo field stars.
A weak mixing efficiency reproduces observations best, indicating a
diffusion-corrected primordial lithium abundance of log {\epsilon}(Li) = 2.57
+- 0.10. At 1.2{\sigma}, this value agrees well with WMAP-calibrated Big-Bang
nucleosynthesis predictions.Comment: 14 pages, 5 figures, accepted by Ap
Stereospecific Synthesis of 2,3-Dimethoxynaphtho[1,2-b] indolizidine
(11aS)- and (11aR)-2,3-dimethoxy-naphtho[1,2-b]indolizidine (9a and 9b)
were synthesized from optically pure L- and D-glutamic acid through several
steps (scheme 1). All the intermediates of the route to the optical antipodes
of 9 exhibit identical physical and spectral properties except the sign of the
optical rotation values. The optical purity of the enantiomers of 6 was
checked by 1H-NMR spectra using Eu(tfc)3, that of the enantiomers of 9 by
HPLC-separation on a chiral column; the amount of racemization was less
than 3% in 9a and 9b, respectively.
Die (11aS)- und (11aR)-2,3-Dimethoxy-naphtho[1,2-b]indolizidine (9a) und
(9b) wurden, ausgehend von optisch reiner L- bzw. D-Glutaminsäure, synthetisiert
(Schema 1). Alle Zwischenprodukte auf dem Weg zu 9 zeigen
identische physikalische und spektrale Eigenschaften mit Ausnahme des
Drehsinns. Die optische Reinheit der 6-Enantiomere wurde durch 1H-NMRSpektroskopie
mit Eu(tfc)3 bestimmt, die der 9-Enantiomere durch HPLCTrennung
auf einer chiralen Säule: die Razemisierungsrate war in 9a und 9b
<3%
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