5,504 research outputs found
The asteroseismological potential of the pulsating DB white dwarf stars CBS 114 and PG 1456+103
We have acquired 65 h of single-site time-resolved CCD photometry of the
pulsating DB white dwarf star CBS 114 and 62 h of two-site high-speed CCD
photometry of another DBV, PG 1456+103. The pulsation spectrum of PG 1456+103
is complicated and variable on time scales of about one week and could only
partly be deciphered with our measurements. The modes of CBS 114 are more
stable in time and we were able to arrive at a frequency solution somewhat
affected by aliasing, but still satisfactory, involving seven independent modes
and two combination frequencies. These frequencies also explain the discovery
data of the star, taken 13 years earlier. We find a mean period spacing of 37.1
+/- 0.7 s significant at the 98% level between the independent modes of CBS 114
and argue that they are due to nonradial g-mode pulsations of spherical degree
l=1. We performed a global search for asteroseismological models of CBS 114
using a genetic algorithm, and we examined the susceptibility of the results to
the uncertainties of the observational frequency determinations and mode
identifications (we could not provide m values). The families of possible
solutions are identified correctly even without knowledge of m. Our optimal
model suggests Teff = 21,000 K and M_* = 0.730 M_sun as well as log(M_He/M_*) =
-6.66, X_O = 0.61. This measurement of the central oxygen mass fraction implies
a rate for the ^12C(alpha,gamma)^16O nuclear reaction near S_300=180 keV b,
consistent with laboratory measurements.Comment: 10 pages, 10 embedded figures, 3 embedded tables. Accepted for
publication in MNRA
Multiwavelength Observations of the Hot DB Star PG 0112+104
We present a comprehensive multiwavelength analysis of the hot DB white dwarf
PG 0112+104. Our analysis relies on newly-acquired FUSE observations, on
medium-resolution FOS and GHRS data, on archival high-resolution GHRS
observations, on optical spectrophotometry both in the blue and around Halpha,
as well as on time-resolved photometry. From the optical data, we derive a
self-consistent effective temperature of 31,300+-500 K, a surface gravity of
log g = 7.8 +- 0.1 (M=0.52 Msun), and a hydrogen abundance of log N(H)/N(He) <
-4.0. The FUSE spectra reveal the presence of CII and CIII lines that
complement the previous detection of CII transitions with the GHRS. The
improved carbon abundance in this hot object is log N(C)/N(He) = -6.15 +- 0.23.
No photospheric features associated with other heavy elements are detected. We
reconsider the role of PG 0112+104 in the definition of the blue edge of the
V777 Her instability strip in light of our high-speed photometry, and contrast
our results with those of previous observations carried out at the McDonald
Observatory.Comment: 10 pages in emulateapj, 9 figures, accepted for publication in Ap
Infrared photochemistry of ethylene clusters
Infrared irradiation of ethylene clusters formed in supersonic molecular beams, using a low power cw CO2 laser, results in the photodissociation of a large fraction of the van der Waals molecules. Under such conditions, infrared absorption intensity exhibits first-order power dependence and is readily detected as loss in molecular beam intensity. Intramolecular energy transfer rates, determined by measuring spectral linewidths, are shown to vary with the vibrational mode initially excited. Ethylene clusters containing one quantum of vibrational energy corresponding to the nun7 fundamental in the monomer (949 cm^–1) have a vibrationally predissociative lifetime of 0.33 psec. In comparison, the relaxation rate of ethylene-d4 clusters with one quantum of excitation corresponding to the nu12 (1078 cm^–1) mode of C2D4 is 0.7 psec
Towards Automated Benchmarking of Atomistic Forcefields: Neat Liquid Densities and Static Dielectric Constants from the ThermoML Data Archive
Atomistic molecular simulations are a powerful way to make quantitative
predictions, but the accuracy of these predictions depends entirely on the
quality of the forcefield employed. While experimental measurements of
fundamental physical properties offer a straightforward approach for evaluating
forcefield quality, the bulk of this information has been tied up in formats
that are not machine-readable. Compiling benchmark datasets of physical
properties from non-machine-readable sources require substantial human effort
and is prone to accumulation of human errors, hindering the development of
reproducible benchmarks of forcefield accuracy. Here, we examine the
feasibility of benchmarking atomistic forcefields against the NIST ThermoML
data archive of physicochemical measurements, which aggregates thousands of
experimental measurements in a portable, machine-readable, self-annotating
format. As a proof of concept, we present a detailed benchmark of the
generalized Amber small molecule forcefield (GAFF) using the AM1-BCC charge
model against measurements (specifically bulk liquid densities and static
dielectric constants at ambient pressure) automatically extracted from the
archive, and discuss the extent of available data. The results of this
benchmark highlight a general problem with fixed-charge forcefields in the
representation low dielectric environments such as those seen in binding
cavities or biological membranes
Coherent Error Suppression in Multi-Qubit Entangling Gates
We demonstrate a simple pulse shaping technique designed to improve the
fidelity of spin-dependent force operations commonly used to implement
entangling gates in trapped-ion systems. This extension of the
M{\o}lmer-S{\o}rensen gate can theoretically suppress the effects of certain
frequency and timing errors to any desired order and is demonstrated through
Walsh modulation of a two-qubit entangling gate on trapped atomic ions. The
technique is applicable to any system of qubits coupled through collective
harmonic oscillator modes
Neural Substrates of Reliability-Weighted Visual-Tactile Multisensory Integration
As sensory systems deteriorate in aging or disease, the brain must relearn the appropriate weights to assign each modality during multisensory integration. Using blood-oxygen level dependent functional magnetic resonance imaging of human subjects, we tested a model for the neural mechanisms of sensory weighting, termed “weighted connections.” This model holds that the connection weights between early and late areas vary depending on the reliability of the modality, independent of the level of early sensory cortex activity. When subjects detected viewed and felt touches to the hand, a network of brain areas was active, including visual areas in lateral occipital cortex, somatosensory areas in inferior parietal lobe, and multisensory areas in the intraparietal sulcus (IPS). In agreement with the weighted connection model, the connection weight measured with structural equation modeling between somatosensory cortex and IPS increased for somatosensory-reliable stimuli, and the connection weight between visual cortex and IPS increased for visual-reliable stimuli. This double dissociation of connection strengths was similar to the pattern of behavioral responses during incongruent multisensory stimulation, suggesting that weighted connections may be a neural mechanism for behavioral reliability weighting
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