2,287 research outputs found
Weather on Other Worlds. IV. H emission and photometric variability are not correlated in L0T8 dwarfs
Recent photometric studies have revealed that surface spots that produce flux
variations are present on virtually all L and T dwarfs. Their likely magnetic
or dusty nature has been a much-debated problem, the resolution to which has
been hindered by paucity of diagnostic multi-wavelength observations. To test
for a correlation between magnetic activity and photometric variability, we
searched for H emission among eight L3T2 ultra-cool dwarfs with
extensive previous photometric monitoring, some of which are known to be
variable at 3.6 m or 4.5 m. We detected H only in the
non-variable T2 dwarf 2MASS J125453930122474. The remaining seven objects do
not show H emission, even though six of them are known to vary
photometrically. Combining our results with those for 86 other L and T dwarfs
from the literature show that the detection rate of H emission is very
high (94) for spectral types between L0 and L3.5 and much smaller (20)
for spectral types L4, while the detection rate of photometric variability
is approximately constant (3055) from L0 to T8 dwarfs. We conclude
that chromospheric activity, as evidenced by H emission, and
large-amplitude photometric variability are not correlated. Consequently, dust
clouds are the dominant driver of the observed variability of ultra-cool dwarfs
at spectral types at least as early as L0.Comment: 12 pages, 4 figures, accepted for publication in Ap
Maximally Localized Wannier Functions within the FLAPW formalism
We report on the implementation of the Wannier Functions (WFs) formalism
within the full-potential linearized augmented plane wave method (FLAPW),
suitable for bulk, film and one-dimensional geometries. The details of the
implementation, as well as results for the metallic SrVO3, ferroelectric BaTiO3
grown on SrTiO3, covalently bonded graphene and a one-dimensional Pt-chain are
given. We discuss the effect of spin-orbit coupling on the Wannier Functions
for the cases of SrVO3 and platinum. The dependency of the WFs on the choice of
the localized trial orbitals as well as the difference between the maximally
localized and "first-guess" WFs are discussed. Our results on SrVO3 and BaTiO3,
e.g. the ferroelectric polarization of BaTiO3, are compared to results
published elsewhere and found to be in excellent agreement.Comment: 13 pages, 9 figures, accepted for publication in Phys. Rev.
Interplay of structure and spin-orbit strength in magnetism of metal-benzene sandwiches: from single molecules to infinite wires
Based on first-principles density functional theory calculations we explore
electronic and magnetic properties of experimentally producible sandwiches and
infinite wires made of repeating benzene molecules and transition-metal atoms
of V, Nb, and Ta. We describe the bonding mechanism in the molecules and in
particular concentrate on the origin of magnetism in these structures. We find
that all the considered systems have sizable magnetic moments and ferromagnetic
spin-ordering, with the single exception of the V3-Bz4 molecule. By including
the spin-orbit coupling into our calculations we determine the easy and hard
axes of the magnetic moment, the strength of the uniaxial magnetic anisotropy
energy (MAE), relevant for the thermal stability of magnetic orientation, and
the change of the electronic structure with respect to the direction of the
magnetic moment, important for spin-transport properties. While for the V-based
compounds the values of the MAE are only of the order of 0.05-0.5 meV per metal
atom, increasing the spin-orbit strength by substituting V with heavier Nb and
Ta allows to achieve an increase in anisotropy values by one to two orders of
magnitude. The rigid stability of magnetism in these compounds together with
the strong ferromagnetic ordering makes them attractive candidates for
spin-polarized transport applications. For a Nb-benzene infinite wire the
occurrence of ballistic anisotropic magnetoresistance is demonstrated.Comment: 23 pages, 8 figure
Relationship between hippocampal structure and memory function in elderly humans
With progressing age, the ability to recollect personal events declines, whereas familiarity-based memory remains relatively intact. It has been hypothesized that age-related hippocampal atrophy may contribute to this pattern because of its critical role for recollection in younger humans and after acute injury. Here, we show that hippocampal volume loss in healthy older persons correlates with gray matter loss (estimated with voxel-based morphometry) of the entire limbic system and shows no correlation with an electrophysiological (event-related potential [ERP]) index of recollection. Instead, it covaries with more substantial and less specific electrophysiological changes of stimulus processing. Age-related changes in another complementary structural measure, hippocampal diffusion, on the other hand, seemed to be more regionally selective and showed the expected correlation with the ERP index of recollection. Thus, hippocampal atrophy in older persons accompanies limbic atrophy, and its functional impact on memory is more fundamental than merely affecting recollection
A Variational Principle Based Study of KPP Minimal Front Speeds in Random Shears
Variational principle for Kolmogorov-Petrovsky-Piskunov (KPP) minimal front
speeds provides an efficient tool for statistical speed analysis, as well as a
fast and accurate method for speed computation. A variational principle based
analysis is carried out on the ensemble of KPP speeds through spatially
stationary random shear flows inside infinite channel domains. In the regime of
small root mean square (rms) shear amplitude, the enhancement of the ensemble
averaged KPP front speeds is proved to obey the quadratic law under certain
shear moment conditions. Similarly, in the large rms amplitude regime, the
enhancement follows the linear law. In particular, both laws hold for the
Ornstein-Uhlenbeck process in case of two dimensional channels. An asymptotic
ensemble averaged speed formula is derived in the small rms regime and is
explicit in case of the Ornstein-Uhlenbeck process of the shear. Variational
principle based computation agrees with these analytical findings, and allows
further study on the speed enhancement distributions as well as the dependence
of enhancement on the shear covariance. Direct simulations in the small rms
regime suggest quadratic speed enhancement law for non-KPP nonlinearities.Comment: 28 pages, 14 figures update: fixed typos, refined estimates in
section
Direct imaging constraints on planet populations detected by microlensing
Results from gravitational microlensing suggested the existence of a large
population of free-floating planetary mass objects. The main conclusion from
this work was partly based on constraints from a direct imaging survey. This
survey determined upper limits for the frequency of stars that harbor giant
exoplanets at large orbital separations. Aims. We want to verify to what extent
upper limits from direct imaging do indeed constrain the microlensing results.
We examine the current derivation of the upper limits used in the microlensing
study and re-analyze the data from the corresponding imaging survey. We focus
on the mass and semi-major axis ranges that are most relevant in context of the
microlensing results. We also consider new results from a recent M-dwarf
imaging survey as these objects are typically the host stars for planets
detected by microlensing. We find that the upper limits currently applied in
context of the microlensing results are probably underestimated. This means
that a larger fraction of stars than assumed may harbor gas giant planets at
larger orbital separations. Also, the way the upper limit is currently used to
estimate the fraction of free-floating objects is not strictly correct. If the
planetary surface density of giant planets around M-dwarfs is described as
df_Planet ~ a^beta da, we find that beta ~ 0.5 - 0.6 is consistent with results
from different observational studies probing semi-major axes between ~0.03 - 30
AU. Having a higher upper limit on the fraction of stars that may have gas
giant planets at orbital separations probed by the microlensing data implies
that more of the planets detected in the microlensing study are potentially
bound to stars rather than free-floating. The current observational data are
consistent with a rising planetary surface density for giant exoplanets around
M-dwarfs out to ~30 AU.Comment: Accepted for publication in A&A as Research Note, 3 page
Can Frequency Domain Heart Rate Measures Detect Impaired Driver Performance?
An overnight driving simulation scenario with partial sleep deprivation was utilized to induce driver performance impairment. Heart rate (HR) was recorded over the entire experiment; frequency domain HR measures were derived and correlated to variation of lane deviation (VLD), a driving performance measure, and to the driver\u27s state, which was estimated by the Karolinska Sleepiness Scale (KSS). The aim of this study is to evaluate whether frequency domain heart rate measures can be used to detect impaired driver performance as well as reduced driver state. We generalize the concept of the conventional frequency domain HR measures – namely the very-low frequency (VLF), low frequency (LF) band and high frequency (HF) band – into finer-grained frequency bands of 0.02 Hz width. These newly defined frequency bands show a more detailed correlation to driving performance and to driver sleepiness state, taking subjectspecific differences into account
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