11,291 research outputs found
The Field White Dwarf Mass Distribution
We revisit the properties and astrophysical implications of the field white
dwarf mass distribution in preparation of Gaia applications. Our study is based
on the two samples with the best established completeness and most precise
atmospheric parameters, the volume-complete survey within 20 pc and the Sloan
Digital Sky Survey (SDSS) magnitude-limited sample. We explore the modelling of
the observed mass distributions with Monte Carlo simulations, but find that it
is difficult to constrain independently the initial mass function (IMF), the
initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the
variation of the Galactic disk vertical scale height as a function of stellar
age, and binary evolution. Each of these input ingredients has a moderate
effect on the predicted mass distributions, and we must also take into account
biases owing to unidentified faint objects (20 pc sample), as well as unknown
masses for magnetic white dwarfs and spectroscopic calibration issues (SDSS
sample). Nevertheless, we find that fixed standard assumptions for the above
parameters result in predicted mean masses that are in good qualitative
agreement with the observed values. It suggests that derived masses for both
studied samples are consistent with our current knowledge of stellar and
Galactic evolution. Our simulations overpredict by 40-50% the number of massive
white dwarfs (M > 0.75 Msun) for both surveys, although we can not exclude a
Salpeter IMF when we account for all biases. Furthermore, we find no evidence
of a population of double white dwarf mergers in the observed mass
distributions.Comment: 15 pages, 16 figures, accepted for publication in MNRA
Mitigation of Human Supervisory Control Wait Times through Automation Strategies
The application of network centric operations principles to human supervisory control
(HSC) domains means that humans are increasingly being asked to manage multiple
simultaneous HSC processes. However, increases in the number of available information
sources, volume of information and operational tempo, all which place higher
cognitive demands on operators, could become constraints limiting the success of network
centric processes. In time-pressured scenarios typical of networked command
and control scenarios, efficiently allocating attention between a set of dynamic tasks
is crucial for mission success. Inefficient attention allocation leads to system wait
times, which could eventually lead to critical events such as missed times on targets
and degraded overall mission success. One potential solution to mitigating wait times
is the introduction of automated decision support in order to relieve operator workload.
However, it is not obvious what automated decision support is appropriate, as
higher levels of automation may result in a situation awareness decrement and other
problems typically associated with excessive automation such as automation bias.
To assess the impact of increasing levels of automation on human and system performance
in a time-critical HSC multiple task management context, an experiment
was run in which an operator simultaneously managed four highly autonomous unmanned
aerial vehicles (UAVs) executing an air tasking order, with the overall goal
of destroying a pre-determined set of targets within a limited time period. Four increasing
levels automated decision support were investigated as well as high and low
operational replanning tempos. The highest level of automation, management-byexception,
had the best performance across several metrics but had a greater number
of catastrophic events during which a UAV erroneously destroyed a friendly target.
Contrary to expectations, the collaborative level of decision support, which provided
predictions for possible periods of task overload as well as possible courses of action
to relieve the high workload, produced the worst performance. This is attributable
to an unintended consequence of the automation where the graphical visualization of
the computer’s predictions caused users to try to globally optimize the schedules for
all UAVs instead of locally optimizing schedules in the immediate future, resulting in
them being overwhelmed. Total system wait time across both experimental factors
was dominated by wait time caused by lack of situation awareness, which is difficult
to eliminate, implying that there will be a clear upper limit on the number of vehicles
that any one person can supervise because of the need to stay cognitively aware of
unfolding events.Prepared for Boeing, Phantom Work
The early X-ray afterglows of optically bright and dark Gamma-Ray Bursts
A systematical study on the early X-ray afterglows of both optically bright
and dark gamma-ray bursts (B-GRBs and D-GRBs) observed by Swift has been
presented. Our sample includes 25 GRBs. Among them 13 are B-GRBs and 12 are
D-GRBs. Our results show that the distributions of the X-ray afterglow fluxes
(), the gamma-ray fluxes (), and the ratio ()
for both the D-GRBs and B-GRBs are similar. The differences of these
distributions for the two kinds of GRBs should be statistical fluctuation.
These results indicate that the progenitors of the two kinds of GRBs are the
same population. Their total energy explosions are comparable. The suppression
of the optical emissions from D-GRBs should results from circumburst but not
their central engine.Comment: 10 pages, 3 figures, 1 table; accepted by ChJA
Improving health literacy about Tuberculosis among drug users. A pilot randomized controlled trial
Introduction: Despite effective treatment, tuberculosis remains among the top-10 causes of death causing ~1.3 million deaths in 2017. Furthermore, tuberculosis infection rates have increased amongst excluded populations such as people misusing substances.
Objectives and design: We conducted a two London sites pilot randomized controlled trial to test interventions, recruitment, attrition rates and assessment procedures of a parallel, three-arms controlled trial to assess the effectiveness of tuberculosis health literacy interventions among drug dependent (heroin, crack cocaine or heroin and crack cocaine) population in treatment.
Results: Forty-two subjects were recruited to the pilot trial (response rate = 26%) and randomized to three interventions (1st: Information booklet; 2nd: Interactive seminar; 3rd: Interactive seminar + contingency management targeting tuberculosis-health-related action). Baseline and post-intervention tuberculosis knowledge scores were obtained and re-assessed at 2-months follow up. The overall attrition rate was 43%. The knowledge scale had good internal reliability (Cronbach’s α = 0.7). Statistically significant increases in knowledge scores (baseline to post-intervention = 5.9 points, baseline to follow-up = 4.3) were recorded for the whole sample (CI = 99%; p < 0.001 for both analysis), but no statistically significant differences between-groups were observed (p = 0.7). Half of participants in the contingency management group achieved their health-action targets.
Conclusion: Health literacy interventions to increase knowledge about tuberculosis among drug users are feasible and achieve promising increases in knowledge and health-related actions but measures to prevent a high attrition rate in a large-scale trial must be introduced. The absence of difference between trial-group outcomes suggests low-intensity interventions may achieve knowledge gain too. Further investigation of contingency management to promote tuberculosis-related health behaviours is needed
Fluids of hard ellipsoids: Phase diagram including a nematic instability from Percus-Yevick theory
An important aspect of molecular fluids is the relation between orientation
and translation parts of the two-particle correlations. Especially the detailed
knowledge of the influence of orientation correlations is needed to explain and
calculate in detail the occurrence of a nematic phase.
The simplest model system which shows both orientation and translation
correlations is a system of hard ellipsoids. We investigate an isotropic fluid
formed of hard ellipsoids with Percus-Yevick theory.
Solving the Percus-Yevick equations self-consistently in the high density
regime gives a clear criterion for a nematic instability. We calculate in
detail the equilibrium phase diagram for a fluid of hard ellipsoids of
revolution. Our results compare well with Monte Carlo Simulations and density
functional theory.Comment: 7 pages including 4 figure
High Flux Femtosecond X-ray Emission from the Electron-Hose Instability in Laser Wakefield Accelerators
Bright and ultrashort duration X-ray pulses can be produced by through
betatron oscillations of electrons during Laser Wakefield Acceleration (LWFA).
Our experimental measurements using the \textsc{Hercules} laser system
demonstrate a dramatic increase in X-ray flux for interaction distances beyond
the depletion/dephasing lengths, where the initial electron bunch injected into
the first wake bucket catches up with the laser pulse front and the laser pulse
depletes. A transition from an LWFA regime to a beam-driven plasma wakefield
acceleration (PWFA) regime consequently occurs. The drive electron bunch is
susceptible to the electron-hose instability and rapidly develops large
amplitude oscillations in its tail, which leads to greatly enhanced X-ray
radiation emission. We measure the X-ray flux as a function of acceleration
length using a variable length gas cell. 3D particle-in-cell (PIC) simulations
using a Monte Carlo synchrotron X-ray emission algorithm elucidate the
time-dependent variations in the radiation emission processes.Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev. Accel.
Beam
The Solar Energetic Particle Event of 6 May 1998
The abundances of elements from helium to iron have been measured in more than a dozen moderate to large solar energetic particle (SEP) events using the Solar Isotope Spectrometer (SIS) on-board the Advanced Composition Explorer (ACE). Time variations within some of these events and from event to event have been reported previously. This paper presents an analysis of the event of 6 May 1998, for which relatively time-independent abundance ratios are found. This event has been considered to be an example of an impulsive event, a gradual event, and as a hybrid of the two. Difficulties with classifying this event are discussed
Observations of Anomalous Cosmic Rays at 1 AU
Anomalous cosmic rays (ACRs) provide a sensitive probe of the access of energetic particles to the inner heliosphere, varying in intensity by more than two orders of magnitude during the course of the solar cycle. New data which are becoming available from the Advanced Composition Explorer (ACE) can provide a detailed record of ACR intensity and spectral changes on short (~ 1 day) time scales during the approach to solar maximum, which will help address issues of ACR modulation and transport. The elemental and isotopic composition of ACRs provides important information on the source or sources of these particles, while their ionic charge state composition and its energy dependence
serves as a diagnostic of their acceleration time scale. We review measurements of the ACR elemental, isotopic,
and charge state composition and spectra as determined at 1 AU by SAMPEX, ACE, Wind, and other spacecraft. These
results are important input to models of the acceleration, modulation, and transport of ACRs
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