13,998 research outputs found
Direct Observation of the Fourth Star in the Zeta Cancri System
Direct imaging of the zeta Cnc system has resolved the fourth star in the
system, which is in orbit around zeta Cnc C. The presence of the fourth star
has been inferred for many years from irregularities in the motion of star C,
and recently from C's spectroscopic orbit. However, its mass is close to that
of C, making its non-detection puzzling. Observing at wavelengths of 1.2, 1.7,
and 2.2 microns with the adaptive-optics system of the CFHT, we have obtained
images which very clearly reveal star D and show it to have the color of an M2
star. Its brightness is consonant with its being two M stars, which are not
resolved in our observations but are likely to be in a short-period orbit,
thereby accounting for the large mass and the difficulty of detection at
optical wavelengths, where the magnitude difference is much larger. The
positions and colors of all four stars in the system are reported and are
consistent with the most recent astrometric observations.Comment: 7 pages including 3 tables, 1 figure; To appear in PAS
Behavior of materials in vacuum Final report, Jun. 1967 - Sep. 1968
Outgassed materials condensed on magnesium overcoated aluminum mirrors irradiated with ultraviolet radiation in vacuu
Proof of the Umbral Moonshine Conjecture
The Umbral Moonshine Conjectures assert that there are infinite-dimensional
graded modules, for prescribed finite groups, whose McKay-Thompson series are
certain distinguished mock modular forms. Gannon has proved this for the
special case involving the largest sporadic simple Mathieu group. Here we
establish the existence of the umbral moonshine modules in the remaining 22
cases.Comment: 56 pages, to appear in Research in the Mathematical Science
On Bayesian nonparametric modelling of two correlated distributions
In this paper, we consider the problem of modelling a pair of related distributions
using Bayesian nonparametric methods. A representation of the distributions as
weighted sums of distributions is derived through normalisation. This allows us to
define several classes of nonparametric priors. The properties of these distributions
are explored and efficient Markov chain Monte Carlo methods are developed. The
methodology is illustrated on simulated data and an example concerning hospital efficiency
measurement
Constructing a Social Problem: The Press and the Environment
The U. S. daily press might seem to be in a strategic position to function as a claims-maker in the early construction of a social problem. But in the case of the manufacture of environmentalism as a social reality in the 1960\u27s and 70\u27s, the press was fairly slow to adopt a holistic environmental lexicon. Its reporting of environmental news even now only partially reflects concepts promoted by positive environmental claims-makers, such as planet-wide interdependence, and the threats to it by destructive technologies. The movement of environmental claims seems to have started with interest-group entrepreneurship using interpersonal communication and independent publication, gone on to attention in government, then finally--and incompletely--been put on the agenda of the daily press. Once on the press agenda, coverage of environmental issues may have improved. But there are some constraints, possibly inherent in the press as an institution, that limit its role in the incipient construction of some social problems
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Adverse Childhood Experiences in Medical Students: Implications for Wellness.
ObjectiveThe primary purpose of the study was to assess the prevalence of adverse childhood experiences (ACEs) in a cohort of third-year medical students and characterize their childhood protective factors.MethodsThe authors developed a web-based anonymous survey distributed to all third-year medical students in one school (N = 98). The survey included the 10-item ACE Study questionnaire, a list of childhood protective factors (CPF) and questions to assess students' perception of the impact of ACEs on their physical and mental health. The medical school's IRB approved the student survey as an exempt study. The authors computed descriptive and comparative statistical analyses.ResultsEighty-six of 98 students responded (88% response rate). Forty-four students (51%) reported at least one ACE exposure and 10 (12%) reported ≥ 4 exposures. The latter were all female. The average difference in the ACE score between male and female medical students was - 1.1 (independent t test with unequal variances t(57.7) = - 2.82, P = .007). Students with an ACE score of ≥ 4 were significantly more likely to report a moderate or significant effect on their mental health, compared with students with scores ≤ 3 (chi-square test, P = < .0001). Most students reported high levels of CPF (median score = 13 of a maximum score = 14). ACEs and CPF were inversely associated (Pearson correlation = - 0.32, P = .003).ConclusionsA sizeable minority of medical students reported exposure to multiple ACEs. If replicated, findings suggest a significant vulnerability of these medical students to health risk behaviors and physical and mental health problems during training and future medical practice
Adaptive MC^3 and Gibbs algorithms for Bayesian Model Averaging in Linear Regression Models
The MC (Madigan and York, 1995) and Gibbs (George and McCulloch, 1997)
samplers are the most widely implemented algorithms for Bayesian Model
Averaging (BMA) in linear regression models. These samplers draw a variable at
random in each iteration using uniform selection probabilities and then propose
to update that variable. This may be computationally inefficient if the number
of variables is large and many variables are redundant. In this work, we
introduce adaptive versions of these samplers that retain their simplicity in
implementation and reduce the selection probabilities of the many redundant
variables. The improvements in efficiency for the adaptive samplers are
illustrated in real and simulated datasets
Landau damping of Bogoliubov excitations in optical lattices at finite temperature
We study the damping of Bogoliubov excitations in an optical lattice at
finite temperatures. For simplicity, we consider a Bose-Hubbard tight-binding
model and limit our analysis to the lowest excitation band. We use the Popov
approximation to calculate the temperature dependence of the number of
condensate atoms in each lattice well. We calculate the Landau
damping of a Bogoliubov excitation in an optical lattice due to coupling to a
thermal cloud of excitations. While most of the paper concentrates on 1D
optical lattices, we also briefly present results for 2D and 3D lattices. For
energy conservation to be satisfied, we find that the excitations in the
collision process must exhibit anomalous dispersion ({\it i.e.} the excitation
energy must bend upward at low momentum), as also exhibited by phonons in
superfluid . This leads to the sudden disappearance of all damping
processes in -dimensional simple cubic optical lattice when , where is the on-site interaction, and is the hopping matrix
element. Beliaev damping in a 1D optical lattice is briefly discussed.Comment: 28 pages, 9 figure
Damping of Bogoliubov Excitations in Optical Lattices
Extending recent work to finite temperatures, we calculate the Landau damping
of a Bogoliubov excitation in an optical lattice, due to coupling to a thermal
cloud of such excitations. For simplicity, we consider a 1D Bose-Hubbard model
and restrict ourselves to the first energy band. For energy conservation to be
satisfied, the excitations in the collision processes must exhibit ``anomalous
dispersion'', analogous to phonons in superfluid . This leads to the
disappearance of all damping processes when , where is
the on-site interaction, is the hopping matrix element and
is the number of condensate atoms at a lattice site. This phenomenon also
occurs in 2D and 3D optical lattices. The disappearance of Beliaev damping
above a threshold wavevector is noted.Comment: 4pages, 5figures, submitted to Phys. Rev. Let
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