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The LTHE LUMINOSITY, MASS, AND AGE DISTRIBUTIONS OF COMPACT STAR CLUSTERS IN M83 BASED ON HUBBLE SPACE TELESCOPE/WIDE FIELD CAMERA 3 OBSERVATIONS
The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multi-band images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near-ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function (LF) for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL vprop L α, with α = –2.04 ± 0.08, down to MV ≈ –5.5. We test the sensitivity of the LF to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in α. We estimate ages and masses for the clusters by comparing their measured UBVI, Hα colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power law, dN/dτ vprop τγ, with γ = –0.9 ± 0.2, for M gsim few × 103 M sun and τ lsim 4 × 108 yr. This indicates that clusters are disrupted quickly, with ≈80%-90% disrupted each decade in age over this time. The mass function of clusters over the same M-τ range is a power law, dN/dM vprop M β, with β = –1.94 ± 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, MC , or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e., mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters
Age distributions of star clusters in spiral and barred galaxies as a test for theories of spiral structure
We consider models of gas flow in spiral galaxies in which the spiral
structure has been excited by various possible mechanisms: a global steady
density wave, self-gravity of the stellar disc and an external tidal
interaction, as well as the case of a galaxy with a central rotating bar. In
each model we estimate in a simple manner the likely current positions of star
clusters of a variety of ages, ranging from ~ 2 Myr to around 130 Myr,
depending on the model. We find that the spatial distribution of cluster of
different ages varies markedly depending on the model, and propose that
observations of the locations of age-dated stellar clusters is a possible
discriminant between excitation mechanisms for spiral structure in an
individual galaxy.Comment: 10 pages, 4 figures, accepted for publication in MNRA
The Luminosity, Mass, and Age Distributions of Compact Star Clusters in M83 Based on HST/WFC3 Observations
The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope
has been used to obtain multi-band images of the nearby spiral galaxy M83.
These new observations are the deepest and highest resolution images ever taken
of a grand-design spiral, particularly in the near ultraviolet, and allow us to
better differentiate compact star clusters from individual stars and to measure
the luminosities of even faint clusters in the U band. We find that the
luminosity function for clusters outside of the very crowded starburst nucleus
can be approximated by a power law, dN/dL \propto L^{alpha}, with alpha = -2.04
+/- 0.08, down to M_V ~ -5.5. We test the sensitivity of the luminosity
function to different selection techniques, filters, binning, and aperture
correction determinations, and find that none of these contribute significantly
to uncertainties in alpha. We estimate ages and masses for the clusters by
comparing their measured UBVI,Halpha colors with predictions from single
stellar population models. The age distribution of the clusters can be
approximated by a power-law, dN/dt propto t^{gamma}, with gamma=-0.9 +/- 0.2,
for M > few x 10^3 Msun and t < 4x10^8 yr. This indicates that clusters are
disrupted quickly, with ~80-90% disrupted each decade in age over this time.
The mass function of clusters over the same M-t range is a power law, dN/dM
propto M^{beta}, with beta=-1.94 +/- 0.16, and does not have bends or show
curvature at either high or low masses. Therefore, we do not find evidence for
a physical upper mass limit, M_C, or for the earlier disruption of lower mass
clusters when compared with higher mass clusters, i.e. mass-dependent
disruption. We briefly discuss these implications for the formation and
disruption of the clusters.Comment: 36 pages, 13 figures, 1 table; accepted for publication in the
Astrophysical Journa
Using H-alpha Morphology and Surface Brightness Fluctuations to Age-Date Star Clusters in M83
We use new WFC3 observations of the nearby grand design spiral galaxy M83 to
develop two independent methods for estimating the ages of young star clusters.
The first method uses the physical extent and morphology of Halpha emission to
estimate the ages of clusters younger than tau ~10 Myr. It is based on the
simple premise that the gas in very young (tau < few Myr) clusters is largely
coincident with the cluster stars, is in a small, ring-like structure
surrounding the stars in slightly older clusters (e.g., tau ~5 Myr), and is in
a larger ring-like bubble for still older clusters (i.e., ~5-10 Myr). The
second method is based on an observed relation between pixel-to-pixel flux
variations within clusters and their ages. This method relies on the fact that
the brightest individual stars in a cluster are most prominent at ages around
10 Myr, and fall below the detection limit (i.e., M_V < -3.5) for ages older
than about 100 Myr. These two methods are the basis for a new morphological
classification system which can be used to estimate the ages of star clusters
based on their appearance. We compare previous age estimates of clusters in M83
determined from fitting UBVI Halpha measurements using predictions from stellar
evolutionary models with our new morphological categories and find good
agreement at the ~95% level. The scatter within categories is ~0.1 dex in log
tau for young clusters (10 Myr) clusters. A
by-product of this study is the identification of 22 "single-star" HII regions
in M83, with central stars having ages ~4 Myr.Comment: 33 pages, 10 figures, 3 tables; published in March Ap
The clinical course of acute otitis media in high-risk Australian Aboriginal children: a longitudinal study
BACKGROUND: It is unclear why some children with acute otitis media (AOM) have poor outcomes. Our aim was to describe the clinical course of AOM and the associated bacterial nasopharyngeal colonisation in a high-risk population of Australian Aboriginal children. METHODS: We examined Aboriginal children younger than eight years who had a clinical diagnosis of AOM. Pneumatic otoscopy and video-otoscopy of the tympanic membrane (TM) and tympanometry was done every weekday if possible. We followed children for either two weeks (AOM without perforation), or three weeks (AOM with perforation), or for longer periods if the infection persisted. Nasopharyngeal swabs were taken at study entry and then weekly. RESULTS: We enrolled 31 children and conducted a total of 219 assessments. Most children had bulging of the TM or recent middle ear discharge at diagnosis. Persistent signs of suppurative OM (without ear pain) were present in most children 7 days (23/30, 77%), and 14 days (20/26, 77%) later. Episodes of AOM did not usually have a sudden onset or short duration. Six of the 14 children with fresh discharge in their ear canal had an intact or functionally intact TM. Perforation size generally remained very small (<2% of the TM). Healing followed by re-perforation was common. Ninety-three nasophyngeal swabs were taken. Most swabs cultured Streptococcus pneumoniae (82%), Haemophilus influenzae (71%), and Moraxella catarrhalis (95%); 63% of swabs cultured all three pathogens. CONCLUSION: In this high-risk population, AOM was generally painless and persistent. These infections were associated with persistent bacterial colonisation of the nasopharynx and any benefits of antibiotics were modest at best. Systematic follow up with careful examination and review of treatment are required and clinical resolution cannot be assumed
The Science Performance of JWST as Characterized in Commissioning
This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies
The James Webb Space Telescope Mission
Twenty-six years ago a small committee report, building on earlier studies,
expounded a compelling and poetic vision for the future of astronomy, calling
for an infrared-optimized space telescope with an aperture of at least .
With the support of their governments in the US, Europe, and Canada, 20,000
people realized that vision as the James Webb Space Telescope. A
generation of astronomers will celebrate their accomplishments for the life of
the mission, potentially as long as 20 years, and beyond. This report and the
scientific discoveries that follow are extended thank-you notes to the 20,000
team members. The telescope is working perfectly, with much better image
quality than expected. In this and accompanying papers, we give a brief
history, describe the observatory, outline its objectives and current observing
program, and discuss the inventions and people who made it possible. We cite
detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space
Telescope Overview, 29 pages, 4 figure