673 research outputs found
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
Effective temperature determinations of late-type stars based on 3D non-LTE Balmer line formation
Hydrogen Balmer lines are commonly used as spectroscopic effective
temperature diagnostics of late-type stars. However, the absolute accuracy of
classical methods that are based on one-dimensional (1D) hydrostatic model
atmospheres and local thermodynamic equilibrium (LTE) is still unclear. To
investigate this, we carry out 3D non-LTE calculations for the Balmer lines,
performed, for the first time, over an extensive grid of 3D hydrodynamic
STAGGER model atmospheres. For H, H, and H, we find
significant 1D non-LTE versus 3D non-LTE differences (3D effects): the outer
wings tend to be stronger in 3D models, particularly for H, while the
inner wings can be weaker in 3D models, particularly for H. For
H, we also find significant 3D LTE versus 3D non-LTE differences
(non-LTE effects): in warmer stars (K) the inner
wings tend to be weaker in non-LTE models, while at lower effective
temperatures (K) the inner wings can be stronger in
non-LTE models; the non-LTE effects are more severe at lower metallicities. We
test our 3D non-LTE models against observations of well-studied benchmark
stars. For the Sun, we infer concordant effective temperatures from H,
H, and H; however the value is too low by around 50K which could
signal residual modelling shortcomings. For other benchmark stars, our 3D
non-LTE models generally reproduce the effective temperatures to within
uncertainties. For H, the absolute 3D effects and non-LTE
effects can separately reach around 100K, in terms of inferred effective
temperatures. For metal-poor turn-off stars, 1D LTE models of H can
underestimate effective temperatures by around 150K. Our 3D non-LTE model
spectra are publicly available, and can be used for more reliable spectroscopic
effective temperature determinations.Comment: 19 pages, 10 figures, abstract abridged; accepted for publication in
Astronomy & Astrophysic
Kondo time scales for quantum dots - response to pulsed bias potentials
The response of a quantum dot in the Kondo regime to rectangular pulsed bias
potentials of various strengths and durations is studied theoretically. It is
found that the rise time is faster than the fall time, and also faster than
time scales normally associated with the Kondo problem. For larger values of
the pulsed bias, one can induce dramatic oscillations in the induced current
with a frequency approximating the splitting between the Kondo peaks that would
be present in steady state. The effect persists in the total charge transported
per pulse, which should facilitate the experimental observation of the
phenomenon.Comment: 5 pages with 4 encapsulated figures which come in separate postscript
files: latex file: text.tex figures: fig1.eps, fig2.eps, fig3.eps, fig4.ep
Activation of Tri(2-Furyl)Phosphine at a Dirhenium Centre: Formation of Phosphido-Bridged Dirhenium Complexes
Reaction of tri(2-furyl)phosphine (PFu3) with [Re2(CO)10−n(NCMe)n] (n = 1, 2) at 40 °C gave the substituted complexes [Re2(CO)10−n(PFu3)n] (1 and 2), the phosphines occupying axial position in all cases. Heating [Re2(CO)10] and PFu3 in refluxing xylene also gives 1 and 2 together with four phosphido-bridged complexes; [Re2(CO)8−n(PFu3)n(μ-PFu2)(μ-H)] (n = 0, 1, 2) (3–5) and [Re2(CO)6(PFu3)2(μ-PFu2)(μ-Cl)] (6) resulting from phosphorus–carbon bond cleavage. A series of separate thermolysis experiments has allowed a detailed reaction pathway to be unambiguously established. A similar reaction between [Re2(CO)10] and PFu3 in refluxing chlorobenzene furnishes four complexes which include 1, 2, 6 and the new binuclear complex [Re2(CO)6(η1-C4H3O)2(μ-PFu2)2] (7). All new complexes have been characterized by a combination of spectroscopic data and single crystal X-ray diffraction studies
Fundamental stellar parameters of benchmark stars from CHARA interferometry. I. Metal-poor stars
Benchmark stars are crucial as validating standards for current as well as
future large stellar surveys of the Milky Way. However, the number of suitable
metal-poor benchmarks is currently limited. We aim to construct a new set of
metal-poor benchmarks, based on reliable interferometric effective temperature
() determinations and a homogeneous analysis with a desired
precision of in . We observed ten late-type metal-poor
dwarf and giants: HD2665, HD6755, HD6833, HD103095, HD122563, HD127243,
HD140283, HD175305, HD221170, and HD224930. Only three of the ten stars
(HD103095, HD122563, and HD140283) have previously been used as benchmarks. For
the observations, we used the high angular resolution optical interferometric
instrument PAVO at the CHARA array. We modelled angular diameters using 3D limb
darkening models and determined directly from the
Stefan-Boltzmann relation, with an iterative procedure to interpolate over
tables of bolometric corrections. Surface gravities () were estimated
from comparisons to Dartmouth stellar evolution model tracks. We collected
spectroscopic observations from the ELODIE and FIES spectrographs and estimated
metallicities () from a 1D non-LTE abundance analysis of
unblended lines of neutral and singly ionized iron. We inferred
to better than for five of the stars (HD103095, HD122563, HD127243,
HD140283, and HD224930). The of the other five stars are
reliable to between ; the higher uncertainty on the for
those stars is mainly due to their having a larger uncertainty in the
bolometric fluxes. We also determined and with
median uncertainties of and ,
respectively. These ten stars can, therefore, be adopted as a new, reliable set
of metal-poor benchmarks.Comment: 13 pages, 7 figures, 8 tables + 10 online tables, abstract shortened
to meet arXiv requirements, accepted in A&
3D NLTE spectral line formation of lithium in late-type stars
Accurately known stellar lithium abundances may be used to shed light on a
variety of astrophysical phenomena such as Big Bang nucleosynthesis, radial
migration, ages of stars and stellar clusters, and planet engulfment events. We
present a grid of synthetic lithium spectra that are computed in non-local
thermodynamic equilibrium (NLTE) across the STAGGER grid of three-dimensional
(3D) hydrodynamic stellar atmosphere models. This grid covers three Li lines at
610.4 nm, 670.8 nm, and 812.6 nm for stellar parameters representative of
FGK-type dwarfs and giants, spanning -7000 K, -5.0, -0.5, and -4.0. We
find that our abundance corrections are up to 0.15 dex more negative than in
previous work, due to a previously overlooked NLTE effect of blocking of UV
lithium lines by background opacities, which has important implications for a
wide range of science cases. We derive a new 3D NLTE solar abundance of
, which is 0.09 dex lower than the commonly
used value. We make our grids of synthetic spectra and abundance corrections
publicly available through the Breidablik package. This package includes
methods for accurately interpolating our grid to arbitrary stellar parameters
through methods based on Kriging (Gaussian process regression) for line
profiles, and MLP (Multi-Layer Perceptrons, a class of fully connected
feedforward neural networks) for NLTE corrections and 3D NLTE abundances from
equivalent widths, achieving interpolation errors of the order 0.01 dex.Comment: 20 pages, 12 figures, accepted for publication in MNRA
Carbon−Phosphorus Bond Activation of Tri(2-thienyl)phosphine at Dirhenium and Dimanganese Centers
Reaction of [Re2(CO)9(NCMe)] with tri(2-thienyl)phosphine (PTh3) in refluxing cyclohexane affords three substituted dirhenium complexes: [Re2(CO)9(PTh3)] (1), [Re2(CO)8(NCMe)(PTh3)] (2), and [Re2(CO)8(PTh3)2] (3). Complex 2 was also obtained from the room-temperature reaction of [Re2(CO)8(NCMe)2] with PTh3 and is an unusual example in which the acetonitrile and phosphine ligands are coordinated to the same rhenium atom. Thermolysis of 1 and 3 in refluxing xylene affords [Re2(CO)8(μ-PTh2)(μ-η1:κ1-C4H3S)] (4) and [Re2(CO)7(PTh3)(μ-PTh2)(μ-H)] (5), respectively, both resulting from carbon−phosphorus bond cleavage of a coordinated PTh3 ligand. Reaction of [Re2(CO)10] and PTh3 in refluxing xylene gives a complex mixture of products. These products include 3−5, two further binuclear products, [Re2(CO)7(PTh3)(μ-PTh2)(μ-η1:κ1-C4H3S)] (6) and [Re2(CO)7(μ-κ1:κ2-Th2PC4H2SPTh)(μ-η1:κ1-C4H3S)] (7), and the mononuclear hydrides [ReH(CO)4(PTh3)] (8) and trans-[ReH(CO)3(PTh3)2] (9). Binuclear 6 is structurally similar to 4 and can be obtained from reaction of the latter with 1 equiv of PTh3. Formation of 7 involves a series of rearrangements resulting in the formation of a unique new diphosphine ligand, Th2PC4H2SPTh. Reaction of [Mn2(CO)10] with PTh3 in refluxing toluene affords the phosphine-substituted product [Mn2(CO)9(PTh3)] (10) and two carbon−phosphorus bond cleavage products, [Mn2(CO)6(μ-PTh2)(μ-η1:η5-C4H3S)] (11) and [Mn2(CO)5(PTh3)(μ-PTh2)(μ-η1:η5-C4H3S)] (12). Both 11 and 12 contain a bridging thienyl ligand that is bonded to one manganese atom in a η5-fashion. The molecular structures of eight of these new complexes were established by single-crystal X-ray diffraction studies, allowing a detailed analysis of the disposition of the coordinated ligands
- …