880 research outputs found
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
Transient currents and universal timescales for a fully time-dependent quantum dot in the Kondo regime
Using the time-dependent non-crossing approximation, we calculate the
transient response of the current through a quantum dot subject to a finite
bias when the dot level is moved suddenly into a regime where the Kondo effect
is present. After an initial small but rapid response, the time-dependent
conductance is a universal function of the temperature, bias, and inverse time,
all expressed in units of the Kondo temperature. Two timescales emerge: the
first is the time to reach a quasi-metastable point where the Kondo resonance
is formed as a broad structure of half-width of the order of the bias; the
second is the longer time required for the narrower split peak structure to
emerge from the previous structure and to become fully formed. The first time
can be measured by the gross rise time of the conductance, which does not
substantially change later while the split peaks are forming. The second time
characterizes the decay rate of the small split Kondo peak (SKP) oscillations
in the conductance, which may provide a method of experimental access to it.
This latter timescale is accessible via linear response from the steady
stateand appears to be related to the scale identified in that manner [A.
Rosch, J. Kroha, and P. Wolfle, Phys. Rev. Lett. 87, 156802 (2001)].Comment: Revtex with 15 eps figures. Compiles to 11 page
High-resolution spectroscopic study of extremely metal-poor stars in the Large Magellanic Cloud
We present detailed abundance results based on UVES high dispersion spectra
for 7 very and extremely metal-poor stars in the Large Magellanic Cloud. We
confirm that all 7 stars, two of which have [Fe/H] --3.0, are the most
metal-poor stars discovered so far in the Magellanic Clouds. The element
abundance ratios are generally consistent with Milky Way halo stars of similar
[Fe/H] values. We find that 2 of the more metal-rich stars in our sample are
enhanced in r-process elements. This result contrasts with the literature,
where all nine metal-poor LMC stars with higher [Fe/H] values than our sample
were found to be rich in r-process elements. The absence of r-process
enrichment in stars with lower [Fe/H] values is consistent with a minimum delay
timescale of 100 Myr for the neutron star binary merger process to
generate substantial r-process enhancements in the LMC. We find that the
occurrence rate of r-process enhancement (r-I or r-II) in our sample of very
and extremely metal-poor stars is statistically indistinguishable from that
found in the Milky Way's halo, although including stars from the literature
sample hints at a larger r-II frequency the LMC. Overall, our results shed
light on the earliest epochs of star formation in the LMC that may be
applicable to other galaxies of LMC-like mass
The SkyMapper search for extremely metal-poor stars in the Large Magellanic Cloud
We present results of a search for extremely metal-poor (EMP) stars in the
Large Magellanic Cloud, which can provide crucial information about the
properties of the first stars as well as on the formation conditions prevalent
during the earliest stages of star formation in dwarf galaxies. Our search
utilised SkyMapper photometry, together with parallax and proper motion cuts
(from Gaia), colour-magnitude cuts (by selecting the red giant branch region)
and finally a metallicity-sensitive cut. Low-resolution spectra of a sample of
photometric candidates were taken using the ANU 2.3m telescope/WiFeS
spectrograph, from which 7 stars with [Fe/H] -2.75 were identified, two
of which have [Fe/H] -3. Radial velocities, derived from the CaII
triplet lines, closely match the outer rotation curve of the LMC for the
majority of the candidates in our sample. Therefore, our targets are robustly
members of the LMC based on their 6D phase-space information (coordinates,
spectrophotometric distance, proper motions and radial velocities), and they
constitute the most metal-poor stars so far discovered in this galaxy.Comment: Accepted for publication in MNRA
Resonance Lifetimes from Complex Densities
The ab-initio calculation of resonance lifetimes of metastable anions
challenges modern quantum-chemical methods. The exact lifetime of the
lowest-energy resonance is encoded into a complex "density" that can be
obtained via complex-coordinate scaling. We illustrate this with one-electron
examples and show how the lifetime can be extracted from the complex density in
much the same way as the ground-state energy of bound systems is extracted from
its ground-state density
Host-plant acceptance on mineral soil and humus by the pine weevil Hylobius abietis (L.)
1 The pine weevil Hylobius abietis (L.) (Coleoptera, Curculionidae) is an economically important pest of conifer forest regeneration in Europe and Asia.
2 Soil scarification, which usually exposes mineral soil, is widely used to protect seedlings from weevil attack. However, the mechanism behind this protective effect is not yet fully understood.
3 Field experiments were conducted to determine the pine weevil's responses to visual and odour stimuli from seedlings when moving on mineral soil and on undisturbed humus surface.
4 One experiment measured the number of pine weevils approaching seedlings, with and without added host odour, on mineral soil and undisturbed humus. Seedlings with added host odour attracted more weevils on both soil types. Unexpectedly, somewhat more weevils approached seedlings surrounded by mineral soil.
5 In a similar experiment, feeding attacks on seedlings planted directly in the soil were recorded. Only half as many seedlings were attacked on mineral soil as on undisturbed humus.
6 In the first experiment, the weevils were trapped 2.5 cm from the bases of the seedlings' stems, whereas they could reach the seedlings in the experiment where seedlings were planted directly in the soil. We conclude that the pine weevils' decision on whether or not to feed on a seedling is strongly influenced by the surrounding soil type and that this decision is taken in the close vicinity of the seedling. The presence of pure mineral soil around the seedling strongly reduces the likelihood that an approaching pine weevil will feed on it
Hydrogenase biomimetics with redox-active ligands: Electrocatalytic proton reduction by [Fe2(CO)4(κ2-diamine)(μ-edt)] (diamine = 2,2′-bipy, 1,10-phen)
Diiron complexes bearing redox active diamine ligands have been studied as models of the active site of [FeFe]-hydrogenases. Heating [Fe2(CO)6(μ-edt)] (edt = 1,2-ethanedithiolate) with 2,2′-bipyridine (2,2′-bipy) or 1,10-phenanthroline (1,10-phen) in MeCN in the presence of Me3NO leads to the formation of [Fe2(CO)4(κ2-2,2′-bipy)(μ-edt)] (1-edt) and [Fe2(CO)4(κ2-1,10-phen)(μ-edt)] (2-edt), respectively, in moderate yields. In the solid state the diamine resides in dibasal sites, while both dibasal and apical–basal isomers are present in solution. Both stereoisomers protonate readily upon addition of strong acids. Cyclic voltammetry in MeCN shows that both complexes undergo irreversible oxidation and reduction, proposed to be a one- and two-electron process, respectively. The structures of neutral 2-edt and its corresponding one- and two-electron reduced species have been investigated by DFT calculations. In 2-edt− the added electron occupies a predominantly ligand-based orbital, and the iron–iron bond is maintained, being only slightly elongated. Addition of the second electron affords an open-shell triplet dianion where the second electron populates an Fe–Fe σ* antibonding orbital, resulting in effective scission of the iron–iron bond. The triplet state lies 4.2 kcal mol−1 lower in energy than the closed-shell singlet dianion whose HOMO correlates nicely with the LUMO of the neutral species 2-edt. Electrocatalytic proton reduction by both complexes has been studied in MeCN using CF3CO2H as the proton source. These catalysis studies reveal that while at high acid concentrations the active catalytic species is [Fe2(CO)4(μ-H)(κ2-diamine)(μ-edt)]+, at low acid concentrations the two complexes follow different catalytic mechanisms being associated with differences in their relative rates of protonation
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