2,463 research outputs found
Experimentation and Modeling of the Effects of Along-Wind Dispersion on Cloud Characteristics of Finite-Duration Contaminant Releases in the Atmosphere
Along-wind dispersion, or stretching of the cloud in the direction of the wind, plays an important role in the concentration and modeling of contaminant releases in the atmosphere. Theoretical and empirical derivations were compared for appropriate parameterization of along-wind dispersion. Available field data were analyzed to evaluate and improve previous parameterizations of the along-wind dispersion coefficient, σ_x. An experimental test program was developed and executed in an ultra-low speed wind tunnel at the Chemical Hazards Research Center to determine the effects of the time distribution coefficient on true finite-duration releases from an original area source. Multiple wind speeds, release durations, and downwind distances were investigated with ensemble averages for improved quality of the cloud characteristics for each set of test conditions. An overall relationship between the time of peak arrival (TOPa) and the time distribution coefficient σ_t= 0.23*TOPa was demonstrated across all parameters. From this relationship, the along-wind dispersion coefficient can be accurately predicted and scaled appropriately. Additional relationships for the leading and trailing edges of the cloud are appropriately modeled
The orbital motion of the Arches cluster — clues on cluster formation near the galactic center
The Arches cluster is one of the most massive, young clusters in the Milky Way. Located inside the central molecular zone in the inner 200 pc of the Galactic center, it formed in one of the most extreme star-forming environments in the present-day Galaxy. Its young age of only 2.5 Myr allows us to observe the cluster despite the strong tidal shear forces in the inner Galaxy. The orbit of the cluster determines its dynamical evolution, tidal stripping, and hence its fate. We have measured the proper motion of the Arches cluster relative to the ambient field from Keck/NIRC2 LGS-AO and VLT/NAOS-CONICA NGS-AO observations taken 4.3 years earlier. When combined with the radial velocity, we derive a 3D space motion of 232 ± 30 km/s for the Arches. This motion is exceptionally large when compared to molecular cloud orbits in the GC, and places stringent constraints on the formation scenarios for starburst clusters in dense, nuclear environments
Discovery of low-metallicity stars in the central parsec of the Milky Way
We present a metallicity analysis of 83 late-type giants within the central 1
pc of the Milky Way. K-band spectroscopy of these stars were obtained with the
medium-spectral resolution integral-field spectrograph NIFS on Gemini North
using laser-guide star adaptive optics. Using spectral template fitting with
the MARCS synthetic spectral grid, we find that there is large variation in
metallicity, with stars ranging from [M/H] -1.0 to above solar metallicity.
About 6\% of the stars have [M/H] -0.5. This result is in contrast to
previous observations, with smaller samples, that show stars at the Galactic
center have approximately solar metallicity with only small variations. Our
current measurement uncertainties are dominated by systematics in the model,
especially at [M/H] 0, where there are stellar lines not represented in the
model. However, the conclusion that there are low metallicity stars, as well as
large variations in metallicity is robust. The metallicity may be an indicator
of the origin of these stars. The low-metallicity population is consistent with
that of globular clusters in the Milky Way, but their small fraction likely
means that globular cluster infall is not the dominant mechanism for forming
the Milky Way nuclear star cluster. The majority of stars are at or above solar
metallicity, which suggests they were formed closer to the Galactic center or
from the disk. In addition, our results indicate that it will be important for
star formation history analyses using red giants at the Galactic center to
consider the effect of varying metallicity.Comment: 11 pages, 10 figures, ApJ Accepte
Properties of the Remnant Clockwise Disk of Young Stars in the Galactic Center
We present new kinematic measurements and modeling of a sample of 116 young
stars in the central parsec of the Galaxy in order to investigate the
properties of the young stellar disk. The measurements were derived from a
combination of speckle and laser guide star adaptive optics imaging and
integral field spectroscopy from the Keck telescopes. Compared to earlier disk
studies, the most important kinematic measurement improvement is in the
precision of the accelerations in the plane of the sky, which have a factor of
six smaller uncertainties (~10 uas/yr/yr). We have also added the first radial
velocity measurements for 8 young stars, increasing the sample at the largest
radii (6"-12") by 25%. We derive the ensemble properties of the observed stars
using Monte-Carlo simulations of mock data. There is one highly significant
kinematic feature (~20 sigma), corresponding to the well-known clockwise disk,
and no significant feature is detected at the location of the previously
claimed counterclockwise disk. The true disk fraction is estimated to be ~20%,
a factor of ~2.5 lower than previous claims, suggesting that we may be
observing the remnant of what used to be a more densely populated stellar disk.
The similarity in the kinematic properties of the B stars and the O/WR stars
suggests a common star formation event. The intrinsic eccentricity distribution
of the disk stars is unimodal, with an average value of = 0.27 +/- 0.07,
which we show can be achieved through dynamical relaxation in an initially
circular disk with a moderately top-heavy mass function.Comment: 65 pages, 22 figures, 8 tables, submitted to Ap
Recent Results and Perspectives for Precision Astrometry and Photometry with Adaptive Optics
Large ground-based telescopes equipped with adaptive optics (AO) systems have
ushered in a new era of high-resolution infrared photometry and astrometry.
Relative astrometric accuracies of <0.2 mas have already been demonstrated from
infrared images with spatial resolutions of 55-95 mas resolution over 10-20''
fields of view. Relative photometric accuracies of 3% and absolute photometric
accuracies of 5%-20% are also possible. I will review improvements and current
limitations in astrometry and photometry with adaptive optics of crowded
stellar fields. These capabilities enable experiments such as measuring orbits
for brown dwarfs and exoplanets, studying our Galaxy's supermassive black hole
and its environment, and identifying individual stars in young star clusters,
which can be used test the universality of the initial mass function.Comment: SPIE Conference Proceedin
The Arches Cluster: Extended Structure and Tidal Radius
At a projected distance of ~26 pc from Sgr A*, the Arches cluster provides
insight to star formation in the extreme Galactic Center (GC) environment.
Despite its importance, many key properties such as the cluster's internal
structure and orbital history are not well known. We present an astrometric and
photometric study of the outer region of the Arches cluster (R > 6.25") using
HST WFC3IR. Using proper motions we calculate membership probabilities for
stars down to F153M = 20 mag (~2.5 M_sun) over a 120" x 120" field of view, an
area 144 times larger than previous astrometric studies of the cluster. We
construct the radial profile of the Arches to a radius of 75" (~3 pc at 8 kpc),
which can be well described by a single power law. From this profile we place a
3-sigma lower limit of 2.8 pc on the observed tidal radius, which is larger
than the predicted tidal radius (1 - 2.5 pc). Evidence of mass segregation is
observed throughout the cluster and no tidal tail structures are apparent along
the orbital path. The absence of breaks in the profile suggests that the Arches
has not likely experienced its closest approach to the GC between ~0.2 - 1 Myr
ago. If accurate, this constraint indicates that the cluster is on a prograde
orbit and is located front of the sky plane that intersects Sgr A*. However,
further simulations of clusters in the GC potential are required to interpret
the observed profile with more confidence.Comment: 24 pages (17-page main text, 7-page appendix), 24 figures, accepted
to Ap
The Quintuplet Cluster: Extended Structure and Tidal Radius
The Quintuplet star cluster is one of only three known young ( Myr)
massive (M M) clusters within pc of the Galactic
Center. In order to explore star cluster formation and evolution in this
extreme environment, we analyze the Quintuplet's dynamical structure. Using the
HST WFC3-IR instrument, we take astrometric and photometric observations of the
Quintuplet covering a field-of-view, which is times
larger than those of previous proper motion studies of the Quintuplet. We
generate a catalog of the Quintuplet region with multi-band, near-infrared
photometry, proper motions, and cluster membership probabilities for
stars. We present the radial density profile of candidate Quintuplet
cluster members with M out to pc from the cluster
center. A lower limit of pc is placed on the tidal radius,
indicating the lack of a tidal truncation within this radius range. Only weak
evidence for mass segregation is found, in contrast to the strong mass
segregation found in the Arches cluster, a second and slightly younger massive
cluster near the Galactic Center. It is possible that tidal stripping hampers a
mass segregation signature, though we find no evidence of spatial asymmetry.
Assuming that the Arches and Quintuplet formed with comparable extent, our
measurement of the Quintuplet's comparatively large core radius of
pc provides strong empirical evidence that young massive
clusters in the Galactic Center dissolve on a several Myr timescale.Comment: 25 pages (21-page main text, 4-page appendix), 18 figures, submitted
to Ap
The orbital motion of the Quintuplet cluster - a common origin for the Arches and Quintuplet clusters?
We investigate the orbital motion of the Quintuplet cluster near the Galactic
center with the aim of constraining formation scenarios of young, massive star
clusters in nuclear environments. Three epochs of adaptive optics high-angular
resolution imaging with Keck/NIRC2 and VLT/NACO were obtained over a time
baseline of 5.8 years, delivering an astrometric accuracy of 0.5-1 mas/yr.
Proper motions were derived in the cluster reference frame and were used to
distinguish cluster members from the majority of field stars. Fitting the
cluster and field proper motion distributions with 2D gaussian models, we
derive the orbital motion of the cluster for the first time. The Quintuplet is
moving with a 2D velocity of 132 +/- 15 km/s with respect to the field along
the Galactic plane, which yields a 3D orbital velocity of 167 +/- 15 km/s when
combined with the previously known radial velocity. From a sample of 119 stars
measured in three epochs, we derive an upper limit to the velocity dispersion
in the core of the Quintuplet cluster of sigma_1D < 10 km/s. Knowledge of the
three velocity components of the Quintuplet allows us to model the cluster
orbit in the potential of the inner Galaxy. Comparing the Quintuplet's orbit
with the Arches orbit, we discuss the possibility that both clusters originated
in the same area of the central molecular zone. [abridged]Comment: 40 pages, 12 figures, accepted for publication in Ap
Improvements in the South African HIV care cascade: findings on 90-90-90 targets from successive population-representative surveys in North West Province.
IntroductionTo achieve epidemic control of HIV by 2030, countries aim to meet 90-90-90 targets to increase knowledge of HIV-positive status, initiation of antiretroviral therapy (ART) and viral suppression by 2020. We assessed the progress towards these targets from 2014 to 2016 in South Africa as expanded treatment policies were introduced using population-representative surveys.MethodsData were collected in January to March 2014 and August to November 2016 in Dr. Ruth Segomotsi Mompati District, North West Province. Each multi-stage cluster sample included 46 enumeration areas (EA), a target of 36 dwelling units (DU) per EA, and a single resident aged 18 to 49 per DU. Data collection included behavioural surveys, rapid HIV antibody testing and dried blood spot collection. We used weighted general linear regression to evaluate differences in the HIV care continuum over time.ResultsOverall, 1044 and 971 participants enrolled in 2014 and 2016 respectively with approximately 77% undergoing HIV testing. Despite increases in reported testing, known status among people living with HIV (PLHIV) remained similar at 68.7% (95% Confidence Interval (CI) = 60.9-75.6) in 2014 and 72.8% (95% CI = 63.6-80.4) in 2016. Men were consistently less likely than women to know their status. Among those with known status, PLHIV on ART increased significantly from 80.9% (95% CI = 71.9-87.4) to 91.5% (95% CI = 84.4-95.5). Viral suppression (<5000 copies/mL using DBS) among those on ART increased significantly from 55.0% (95% CI = 39.6-70.4) in 2014 to 81.4% (95% CI = 72.0-90.8) in 2016. Among all PLHIV an estimated 72.0% (95% CI = 63.8-80.1) of women and 45.8% (95% CI = 27.0-64.7) of men achieved viral suppression by 2016.ConclusionsOver a period during which fixed-dose combination was introduced, ART eligibility expanded, and efforts to streamline treatment were implemented, major improvements in the second and third 90-90-90 targets were achieved. Achieving the first 90 target will require targeted and improved testing models for men
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