993 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
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
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
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
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
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