2,387 research outputs found
Calibration of White Dwarf cooling sequences: theoretical uncertainty
White Dwarf luminosities are powerful age indicators, whose calibration
should be based on reliable models. We discuss the uncertainty of some chemical
and physical parameters and their influence on the age estimated by means of
white dwarf cooling sequences. Models at the beginning of the white dwarf
sequence have been obtained on the base of progenitor evolutionary tracks
computed starting from the zero age horizontal branch and for a typical halo
chemical composition (Z=0.0001, Y=0.23). The uncertainties due to nuclear
reaction rates, convection, mass loss and initial chemical composition are
discussed. Then, various cooling sequences for a typical white dwarf mass
(M=0.6 Mo) have been calculated under different assumptions on some input
physics, namely: conductive opacity, contribution of the ion-electron
interaction to the free energy and microscopic diffusion. Finally we present
the evolution of white dwarfs having mass ranging between 0.5 and 0.9 Mo. Much
effort has been spent to extend the equation of state down to the low
temperature and high density regime. An analysis of the latest improvement in
the physics of white dwarf interiors is presented. We conclude that at the
faint end of the cooling sequence (log L/Lo=-5.5) the present overall
uncertainty on the age is of the order of 20%, which correspond to about 3 Gyr.
We suggest that this uncertainty could be substantially reduced by improving
our knowledge of the conductive opacity (especially in the partially degenerate
regime) and by fixing the internal stratification of C and O.Comment: 14 figures, accepted by Ap
Single-leg squats can predict leg alignment in dancers performing ballet movements in turnout
The physical assessments used in dance injury surveillance programs are often adapted from the sports and exercise domain. Bespoke physical assessments may be required for dance, particularly when ballet movements involve “turning out” or external rotation of the legs beyond that typically used in sports. This study evaluated the ability of the traditional single-leg squat to predict the leg alignment of dancers performing ballet movements with turnout. Three-dimensional kinematic data of dancers performing the single-leg squat and five ballet movements were recorded and analyzed. Reduction of the three-dimensional data into a one-dimensional variable incorporating the ankle, knee, and hip joint center positions provided the strongest predictive model between the single-leg squat and the ballet movements. The single-leg squat can predict leg alignment in dancers performing ballet movements, even in “turned out” postures. Clinicians should pay careful attention to observational positioning and rating criteria when assessing dancers performing the single-leg squat
Neuronal Activity in the Human Subthalamic Nucleus Encodes Decision Conflict during Action Selection
The subthalamic nucleus (STN), which receives excitatory inputs from the cortex and has direct connections with the inhibitory pathways\ud
of the basal ganglia, is well positioned to efficiently mediate action selection. Here, we use microelectrode recordings captured during\ud
deep brain stimulation surgery as participants engage in a decision task to examine the role of the human STN in action selection. We\ud
demonstrate that spiking activity in the STN increases when participants engage in a decision and that the level of spiking activity\ud
increases with the degree of decision conflict. These data implicate the STN as an important mediator of action selection during decision\ud
processes.\u
The Phosphidosilicates AE2Li4SiP4 (AE=Ca, Sr, Eu) Ba4Li16Si3P12
The quaternary phosphidosilicates AE2Li4SiP4 (AE=Ca, Sr, Eu) and Ba4Li16Si3P12 were synthesized by heating the elements and Li3P under argon atmosphere. Their crystal structures were determined by single crystal X-ray diffraction. AE2Li4SiP4 crystallize in a new layered structure type (P21/m, Z=2) with CdI2-analoguos layers. Edge sharing CaP6 octahedra are separated by layers of vertex-sharing SiP4 and LiP4 tetrahedra, which contain additional chains of LiP6 octahedra. Ba4Li16Si3P12 forms likewise a new structure type (P21/c, Z=16) with a three-dimensional network of SiP4, Si2P6 and LiP4 entities as well as one phosphorus site not bonded to silicon. Barium is located in capped trigonal prisms of phosphorus which form strongly corrugated layers. 31P and 29Si solid-state NMR spectra confirm the crystal structures of the compounds AE2Li4SiP4. 7Li spectra show only one signal in spite of quite different crystallographic positions, which indicate possible Li+ mobility. However, this signal is much broader compared to the known Li+ conducting phosphidosilicates. Accordingly, electrochemical impedance measurements show low Li+ conductivities
Optical Properties of the Kara Sea
This study was motivated by the need to understand dispersion processes which affect the redistribution of nuclear wastes in the Arctic from dump sites in the Kara Sea and in the rivers which flow into the Kara Sea. We focus on vertical profiles of light beam transmission and fluorometry made over the delta region fronting the Ob and Yenisey Rivers and over the East Novaya Zemlya Trough (ENZT). The delta region fronting the Ob River Estuary contains a large repository of particles in a dense bottom nepheloid layer with a maximum centered similar to 100 km in front of the estuary entrance and covering an area of roughly 200 km diameter. This suspended particle mass repository appears to contain both sediments and detritus and lends credence to the Lisitsyn [1995] concept of the marginal filter zone. In the deep water of the ENZT we found a strong increase of beam attenuation with depth, indicating a relatively large increase of particle mass concentration from similar to 50 m to the bottom (depths in excess of 300 m). The strongest concentration was adjacent to the southeast coast of Novaya Zemlya. We suggest that a type of hyperpycnical flow occurs from accumulation of sediments in the bottom waters of Novaya Zemlya fjords which then cascades down the steep slopes adjacent to the island, producing the particle mass distribution as observed by the transmissometer. The accumulation of these repositories of high particle mass concentrations in suspension would suggest that the residence time is high but that storm-driven events could act to disperse the material
Mixed convection in a rotating porous cavity having local heater
Numerical simulation of convective heat transfer inside a rotating porous square cavity with local heater of constant temperature has been performed. Governing equations formulated on the basis of mass, momentum and energy conservation laws written using the dimensionless stream function, vorticity and temperature have been solved by the finite difference method. The effects of Rayleigh and Taylor numbers on periodic flow and heat transfer have been studied
From Young and Hot to Old and Cold: Comparing White Dwarf Cooling Theory to Main Sequence Stellar Evolution in Open Clusters
I explore the current ability of both white dwarf cooling theory and main
sequence stellar evolution theory to accurately determine stellar population
ages by comparing ages derived using both techniques for open clusters ranging
from 0.1 to 4 Gyr. I find good agreement between white dwarf and main sequence
evolutionary ages over the entire age range currently available for study. I
also find that directly comparing main sequence turn-off ages to white dwarf
ages is only weakly sensitive to realistic levels of errors in cluster
distance, metallicity, and reddening. Additional detailed comparisons between
white dwarf and main sequence ages have tremendous potential to refine and
calibrate both of these important clocks, and I present new simulations of
promising open cluster targets. The most demanding requirement for these white
dwarf studies are very deep (V > 25-28) cluster observations made necessary by
the faintness of the oldest white dwarfs.Comment: 25 pages, incl. 10 figures, ApJ accepted for April, 200
WIYN Open Cluster Study XI: WIYN 3.5m Deep Photometry of M35 (NGC 2168)
We present deep BVI observations of the core of M35 and a nearby comparison
field obtained at the WIYN 3.5m telescope under excellent seeing. These
observations display the lower main sequence in BV and VI CMDs down to V = 23.3
and 24.6, respectively. At these faint magnitudes background field stars are
far more numerous than the cluster stars, yet by using a smoothing technique
and CMD density distribution subtraction we recover the cluster fiducial main
sequence and luminosity function to V = 24.6. We find the location of the main
sequence in these CMDs to be consistent with earlier work on other open
clusters, specifically NGC 188, NGC 2420, and NGC 2477. We compare these open
cluster fiducial sequences to stellar models by Baraffe et al. (1998), Siess et
al. (2000), Girardi et al. (2000), and Yi et al. (2001) and find that the
models are too blue in both B-V and V-I for stars below ~0.4 Mo. M35 contains
stars to the limit of the extracted main sequence, at M ~ 0.10-0.15 Mo,
suggesting that M35 may harbor a large number of brown dwarfs, which should be
easy targets for near-IR instrumentation on 8-10m telescopes. We also identify
a new candidate white dwarf in M35 at V = 21.36 +- 0.01. Depending on which WD
models are used to interpret this cluster candidate, it is either a very high
mass WD (1.05 +- 0.05 Mo) somewhat older (0.19-0.26 Gyr, 3-4 sigma) than our
best isochrone age (150 Myr), or it is a modestly massive WD (0.67-0.78 Mo)
much too old (0.42-0.83 Gyr) to belong to the cluster.Comment: 28 pages + 24 figures; to be published in the Sept, 2002 A
The Chromospheric Activity and Ages of M Dwarf Stars in Wide Binary Systems
We investigate the relationship between age and chromospheric activity for
139 M dwarf stars in wide binary systems with white dwarf companions. The age
of each system is determined from the cooling age of its white dwarf component.
The current limit for activity-age relations found for M dwarfs in open
clusters is 4 Gyr. Our unique approach to finding ages for M stars allows for
the exploration of this relationship at ages older than 4 Gyr. The general
trend of stars remaining active for a longer time at later spectral type is
confirmed. However, our larger sample and greater age range reveals additional
complexity in assigning age based on activity alone. We find that M dwarfs in
wide binaries older than 4 Gyr depart from the log-linear relation for clusters
and are found to have activity at magnitudes, colors and masses which are
brighter, bluer and more massive than predicted by the cluster relation. In
addition to our activity-age results, we present the measured radial velocities
and complete space motions for 161 white dwarf stars in wide binaries.Comment: 22 pages including 9 figures and 5 tables. Accepted for publication
in The Astronomical Journa
Electrophysiological correlates of high-level perception during spatial navigation
We studied the electrophysiological basis of object recognition by recording scalp\ud
electroencephalograms while participants played a virtual-reality taxi driver game.\ud
Participants searched for passengers and stores during virtual navigation in simulated\ud
towns. We compared oscillatory brain activity in response to store views that were targets or\ud
nontargets (during store search) or neutral (during passenger search). Even though store\ud
category was solely defined by task context (rather than by sensory cues), frontal ...\ud
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