3,794 research outputs found
A Long-Lived Accretion Disk Around a Lithium-Depleted Binary T Tauri Star
We present a high dispersion optical spectrum of St 34 and identify the
system as a spectroscopic binary with components of similar luminosity and
temperature (both M3+/-0.5). Based on kinematics, signatures of accretion, and
location on an H-R diagram, we conclude that St 34 is a classical T Tauri star
belonging to the Taurus-Auriga T Association. Surprisingly, however, neither
component of the binary shows LiI 6708 A, absorption, the most universally
accepted criterion for establishing stellar youth. In this uniquely known
instance, the accretion disk appears to have survived longer than the lithium
depletion timescale. We speculate that the long-lived accretion disk is a
consequence of the sub-AU separation companion tidally inhibiting, though not
preventing, circumstellar accretion. Comparisons with pre-main sequence
evolutionary models imply, for each component of St 34, a mass of 0.37+/-0.08
Msun and an isochronal age of 8+/-3 Myr, which is much younger than the
predicted lithium depletion timescale of ~ 25 Myr. Although a distance 38%
closer than that of Taurus-Auriga or a hotter temperature scale could reconcile
this discrepancy at 21-25 Myr, similar discrepancies in other systems and the
implications of an extremely old accreting Taurus-Auriga member suggest instead
a possible problem with evolutionary models. Regardless, the older age implied
by St 34's depleted lithium abundance is the first compelling evidence for a
substantial age spread in this region. Additionally, since St 34's coeval
co-members with early M spectral types would likewise fail the lithium test for
youth, current membership lists may be incomplete.Comment: 4 pages, including 2 figures. Accepted for publication in ApJ Let
Remote sensing applications in forestry. The development of an earth resources information system using aerial photographs and digital computers photographs and digital computers
Remote aerial sensing and automatic mapping for forest resources information syste
Multiplicity at the Stellar/Substellar Boundary in Upper Scorpius
We present the results of a high-resolution imaging survey of 12 brown dwarfs
and very low mass stars in the closest (~145 pc) young (~5 Myr) OB association,
Upper Scorpius. We obtained images with the Advanced Camera for Surveys/High
Resolution Camera on HST through the F555W (V), F775W (i'), and F850LP (z')
filters. This survey discovered three new binary systems, including one
marginally resolved pair with a projected separation of only 4.9 AU, resulting
in an observed binary fraction of 25+/-14% at separations >4 AU. After
correcting for detection biases assuming a uniform distribution of mass ratios
for q>0.6, the estimated binary fraction is 33+/-17%. The binary fraction is
consistent with that inferred for higher-mass stars in Upper Sco, but the
separation and mass ratio distributions appear to be different. All three
low-mass binary systems in Upper Sco are tight (<18 AU) and of similar mass
(q>0.6), consistent with expectations based on previous multiplicity studies of
brown dwarfs and very low mass stars in the field and in open clusters. The
implication is that the distinct separation and mass ratio distributions of
low-mass systems are set in the formation process or at very young ages, rather
than by dynamical disruption of wide systems at ages >5 Myr. Finally, we
combine the survey detection limits with the models of Burrows et al. (1997) to
show that there are no planets or very low-mass brown dwarfs with masses >10
M_J at projected separations >20 AU, or masses >5 M_J at projected separations
>40 AU orbiting any of the low-mass (0.04-0.10 M_sun) objects in our sample.Comment: Accepted for publication in ApJ; 10 pages, 4 figures in emulateapj
forma
The van Hove distribution function for Brownian hard spheres: dynamical test particle theory and computer simulations for bulk dynamics
We describe a test particle approach based on dynamical density functional
theory (DDFT) for studying the correlated time evolution of the particles that
constitute a fluid. Our theory provides a means of calculating the van Hove
distribution function by treating its self and distinct parts as the two
components of a binary fluid mixture, with the `self' component having only one
particle, the `distinct' component consisting of all the other particles, and
using DDFT to calculate the time evolution of the density profiles for the two
components. We apply this approach to a bulk fluid of Brownian hard spheres and
compare to results for the van Hove function and the intermediate scattering
function from Brownian dynamics computer simulations. We find good agreement at
low and intermediate densities using the very simple Ramakrishnan-Yussouff
[Phys. Rev. B 19, 2775 (1979)] approximation for the excess free energy
functional. Since the DDFT is based on the equilibrium Helmholtz free energy
functional, we can probe a free energy landscape that underlies the dynamics.
Within the mean-field approximation we find that as the particle density
increases, this landscape develops a minimum, while an exact treatment of a
model confined situation shows that for an ergodic fluid this landscape should
be monotonic. We discuss possible implications for slow, glassy and arrested
dynamics at high densities.Comment: Submitted to Journal of Chemical Physic
Electrophoresis of colloidal dispersions in the low-salt regime
We study the electrophoretic mobility of spherical charged colloids in a
low-salt suspension as a function of the colloidal concentration. Using an
effective particle charge and a reduced screening parameter, we map the data
for systems with different particle charges and sizes, including numerical
simulation data with full electrostatics and hydrodynamics and experimental
data for latex dispersions, on a single master curve. We observe two different
volume fraction-dependent regimes for the electrophoretic mobility that can be
explained in terms of the static properties of the ionic double layer.Comment: Substantially revised versio
Nonlinearities and Effects of Transverse Beam Size in Beam Position Monitors (revised)
The fields produced by a long beam with a given transverse charge
distribution in a homogeneous vacuum chamber are studied. Signals induced by a
displaced finite-size beam on electrodes of a beam position monitor (BPM) are
calculated and compared to those produced by a pencil beam. The non-linearities
and corrections to BPM signals due to a finite transverse beam size are
calculated for an arbitrary chamber cross section. Simple analytical
expressions are given for a few particular transverse distributions of the beam
current in a circular or rectangular chamber. Of particular interest is a
general proof that in an arbitrary homogeneous chamber the beam-size
corrections vanish for any axisymmetric beam current distribution.Comment: REVTeX, 8 pages, 9 figures. Corrected Eqs. (7),(22),(25) and Figs.
2-9. Expande
Self-assembly of the simple cubic lattice with an isotropic potential
Conventional wisdom presumes that low-coordinated crystal ground states
require directional interactions. Using our recently introduced optimization
procedure to achieve self-assembly of targeted structures (Phys. Rev. Lett. 95,
228301 (2005), Phys. Rev. E 73, 011406 (2006)), we present an isotropic pair
potential for a three-dimensional many-particle system whose classical
ground state is the low-coordinated simple cubic (SC) lattice. This result is
part of an ongoing pursuit by the authors to develop analytical and
computational tools to solve statistical-mechanical inverse problems for the
purpose of achieving targeted self-assembly. The purpose of these methods is to
design interparticle interactions that cause self-assembly of technologically
important target structures for applications in photonics, catalysis,
separation, sensors and electronics. We also show that standard approximate
integral-equation theories of the liquid state that utilize pair correlation
function information cannot be used in the reverse mode to predict the correct
simple cubic potential. We report in passing optimized isotropic potentials
that yield the body-centered cubic and simple hexagonal lattices, which provide
other examples of non-close-packed structures that can be assembled using
isotropic pair interactions.Comment: 16 pages, 12 figures. Accepted for publication in Physical Review
Structure and dynamics of colloidal depletion gels: coincidence of transitions and heterogeneity
Transitions in structural heterogeneity of colloidal depletion gels formed
through short-range attractive interactions are correlated with their dynamical
arrest. The system is a density and refractive index matched suspension of 0.20
volume fraction poly(methyl methacyrlate) colloids with the non-adsorbing
depletant polystyrene added at a size ratio of depletant to colloid of 0.043.
As the strength of the short-range attractive interaction is increased,
clusters become increasingly structurally heterogeneous, as characterized by
number-density fluctuations, and dynamically immobilized, as characterized by
the single-particle mean-squared displacement. The number of free colloids in
the suspension also progressively declines. As an immobile cluster to gel
transition is traversed, structural heterogeneity abruptly decreases.
Simultaneously, the mean single-particle dynamics saturates at a localization
length on the order of the short-range attractive potential range. Both
immobile cluster and gel regimes show dynamical heterogeneity. Non-Gaussian
distributions of single particle displacements reveal enhanced populations of
dynamical trajectories localized on two different length scales. Similar
dependencies of number density fluctuations, free particle number and dynamical
length scales on the order of the range of short-range attraction suggests a
collective structural origin of dynamic heterogeneity in colloidal gels.Comment: 14 pages, 10 figure
Quantifying the Reversible Association of Thermosensitive Nanoparticles
Under many conditions, biomolecules and nanoparticles associate by means of
attractive bonds, due to hydrophobic attraction. Extracting the microscopic
association or dissociation rates from experimental data is complicated by the
dissociation events and by the sensitivity of the binding force to temperature
(T). Here we introduce a theoretical model that combined with light-scattering
experiments allows us to quantify these rates and the reversible binding energy
as a function of T. We apply this method to the reversible aggregation of
thermoresponsive polystyrene/poly(N-isopropylacrylamide) core-shell
nanoparticles, as a model system for biomolecules. We find that the binding
energy changes sharply with T, and relate this remarkable switchable behavior
to the hydrophobic-hydrophilic transition of the thermosensitive nanoparticles
LONTalk as a Standard Protocol For Underwater Sensor Platforms
Proceedings IEEE, Oceans 97, Halifax, Oct. 1997 IEEE CD-ROM 0-7803-4111-
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