4,078 research outputs found
Unexpected Effect of Internal Degrees of Freedom on Transverse Phonons in Supercooled Liquids
We show experimentally that in a supercooled liquid composed of molecules
with internal degrees of freedom the internal modes contribute to the frequency
dependent shear viscosity and damping of transverse phonons, which results in
an additional broadening of the transverse Brillouin lines. Earlier, only the
effect of internal modes on the frequency dependent bulk viscosity and damping
of longitudinal phonons was observed and explained theoretically in the limit
of weak coupling of internal degrees of freedom to translational motion. A new
theory is needed to describe this new effect. We also demonstrate, that the
contributions of structural relaxation and internal processes to the width of
the Brillouin lines can be separated by measurements under high pressure
Discovery of a High Proper Motion L Dwarf Binary: 2MASS J15200224-4422419AB
We report the discovery of the wide L1.5+L4.5 binary 2MASS
J15200224-4422419AB, identified during spectroscopic followup of high proper
motion sources selected from the Two Micron All Sky Survey. This source was
independently identified by Kendall et al. in the SuperCOSMOS Sky Survey.
Resolved JHK photometry and low resolution near-infrared spectroscopy
demonstrate that this system is composed of two well-separated (1"174+/-0"016)
L dwarfs. Component classifications are derived using both spectral ratios and
comparison to near-infrared spectra of previously classified field L dwarfs.
Physical association for the pair is deduced from the large (mu = 0"73+/-0"03
/yr) common proper motion of the components and their similar
spectrophotometric distances (19+/-2 pc). The projected separation of the
binary, 22+/-2 AU, is consistent with maximum separation/total system mass
trends for very low mass binaries. The 2MASS J1520-4422 system exhibits both
large tangential (66+/-7 km/s) and radial velocities (-70+/-18 km/s), and its
motion in the local standard of rest suggests that it is an old member of the
Galactic disk population. This system joins a growing list of well-separated
(>0"5), very low mass binaries, and is an excellent target for resolved optical
spectroscopy to constrain its age as well as trace activity/rotation trends
near the hydrogen-burning limit.Comment: 35 pages, 8 figures; accepted for publication to ApJ; see also
Kendall et al. astro-ph/060939
JHK Magnitudes for L and T Dwarfs and Infrared Photometric Systems
L and T dwarfs emit most of their radiation in the near infrared and their
spectral energy distributions are dominated by strong molecular absorption
bands. These highly structured energy distributions lead to JHK magnitudes that
are extremely sensitive to the exact filter bandpass used. In the case of the T
dwarfs, the differences between commonly used photometric systems can be as
large as 0.4 mag at J and 0.5 mag at J-K. To address this problem, we have
synthesized J,H and K magnitudes for some of the common photometric systems and
present transformation equations. If the spectral type of the dwarf is known,
our transformations allow data to be converted between systems to 0.01 mag,
which is better than the typical measurement uncertainty. Transforming on the
basis of color alone is more difficult because of the degeneracy and intrinsic
scatter in near-infrared colors of L and T dwarfs; in this case J magnitudes
can only be transformed to < 0.05 mag and H and K to < 0.02 mag.Comment: 26 pages including 9 figures, uses aastex, to be published in PASP
Jan 200
A Global Photometric Analysis of 2MASS Calibration Data
We present results from the application of a global photometric calibration
(GPC) procedure to calibration data from the first 2 years of The Two Micron
All Sky Survey (2MASS). The GPC algorithm uses photometry of both primary
standards and moderately bright `tracer' stars in 35 2MASS calibration fields.
During the first two years of the Survey, each standard was observed on
approximately 50 nights, with about 900 individual measurements. Based on the
photometry of primary standard stars and secondary tracer stars and under the
assumption that the nightly zeropoint drift is linear, GPC ties together all
calibration fields and all survey nights simultaneously, producing a globally
optimized solution. Calibration solutions for the Northern and Southern
hemisphere observatories are found separately, and are tested for global
consistency based on common fields near the celestial equator.
Several results from the GPC are presented, including establishing candidate
secondary standards, monitoring of near-infrared atmospheric extinction
coefficients, and verification of global validity of the standards. The
solution gives long-term averages of the atmospheric extinction coefficients,
A_J=0.096, A_H=0.026, A_{K_s}=0.066 (North) and A_J=0.092, A_H=0.031,
A_{K_s}=0.065 (South), with formal error of 0.001. The residuals show small
seasonal variations, most likely due to changing atmospheric content of water
vapor. Extension of the GPC to approximately 100 field stars in each of the 35
calibration fields yields a catalog of more than two thousand photometric
standards ranging from 10th to 14th magnitude, with photometry that is globally
consistent to .Comment: 19 pages, 10 figures; Submitted to AJ. The table of secondary
standards is available from ftp://nova.astro.umass.edu/pub/nikolaev/ or
ftp://anon-ftp.ipac.caltech.edu/pub/2mass/globalcal
Dynamic structure factors of a dense mixture
We compute the dynamic structure factors of a dense binary liquid mixture.
These describe dynamics on molecular length scales, where structural relaxation
is important. We find that the presence of a few large particles in a dense
fluid of small particles slows down the dynamics considerably. We also observe
a deep narrowing of the spectrum for a disordered mixture composed of a nearly
equal packing of the two species. In contrast, a few small particles diffuse
easily in the background of a dense fluid of large particles. We expect our
results to describe neutron scattering from a dense mixture
The Magnetic Distortion Calibration System of the LHCb RICH1 Detector
The LHCb RICH1 detector uses hybrid photon detectors (HPDs) as its optical
sensors. A calibration system has been constructed to provide corrections for
distortions that are primarily due to external magnetic fields. We describe
here the system design, construction, operation and performance.Comment: 9 pages, 14 figure
Light scattering spectra of supercooled molecular liquids
The light scattering spectra of molecular liquids are derived within a
generalized hydrodynamics. The wave vector and scattering angle dependences are
given in the most general case and the change of the spectral features from
liquid to solidlike is discussed without phenomenological model assumptions for
(general) dielectric systems without long-ranged order. Exact microscopic
expressions are derived for the frequency-dependent transport kernels,
generalized thermodynamic derivatives and the background spectra.Comment: 12 page
Solidity of viscous liquids. V. Long-wavelength dominance of the dynamics
This paper is the fifth in a series exploring the physical consequences of
the solidity of glass-forming liquids. Paper IV proposed a model where the
density field is described by a time-dependent Ginzburg-Landau equation of the
nonconserved type with rates in space of the form . The
model assumes that where is the average intermolecular
distance; this inequality expresses a long-wavelength dominance of the dynamics
which implies that the Hamiltonian (free energy) to a good approximation may be
taken to be ultralocal. In the present paper we argue that this is the simplest
model consistent with the following three experimental facts: 1) Viscous
liquids approaching the glass transition do not develop long-range order; 2)
The glass has lower compressibility than the liquid; 3) The alpha process
involves several decades of relaxation times shorter than the mean relaxation
time. The paper proceeds to list six further experimental facts characterizing
equilibrium viscous liquid dynamics and shows that these are readily understood
in terms of the model; some are direct consequences, others are quite natural
when viewed in light of the model
Growing spatial correlations of particle displacements in a simulated liquid on cooling toward the glass transition
We define a correlation function that quantifies the spatial correlation of
single-particle displacements in liquids and amorphous materials. We show for
an equilibrium liquid that this function is related to fluctuations in a bulk
dynamical variable. We evaluate this function using computer simulations of an
equilibrium glass-forming liquid, and show that long range spatial correlations
of displacements emerge and grow on cooling toward the mode coupling critical
temperature
Simulation of thermal conductivity and heat transport in solids
Using molecular dynamics (MD) with classical interaction potentials we
present calculations of thermal conductivity and heat transport in crystals and
glasses. Inducing shock waves and heat pulses into the systems we study the
spreading of energy and temperature over the configurations. Phonon decay is
investigated by exciting single modes in the structures and monitoring the time
evolution of the amplitude using MD in a microcanonical ensemble. As examples,
crystalline and amorphous modifications of Selenium and are
considered.Comment: Revtex, 8 pages, 11 postscript figures, accepted for publication in
PR
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