2,430 research outputs found
Structure and Mass of a Young Globular Cluster in NGC 6946
Using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope,
we have imaged a luminous young star cluster in the nearby spiral galaxy NGC
6946. The cluster has an absolute visual magnitude M(V)=-13.2, comparable to
the brightest young `super-star clusters' in the Antennae merger galaxy. UBV
colors indicate an age of about 15 Myr. The cluster has a compact core (core
radius = 1.3 pc), surrounded by an extended envelope. We estimate that the
effective radius (Reff) = 13 pc, but this number is uncertain because the outer
parts of the cluster profile gradually merge with the general field. Combined
with population synthesis models, the luminosity and age of the cluster imply a
mass of 8.2x10^5 Msun for a Salpeter IMF extending down to 0.1 Msun, or
5.5x10^5 Msun if the IMF is log-normal below 0.4 Msun. Depending on model
assumptions, the central density of the cluster is between 5300 Msun pc^-3 and
17000 Msun pc^-3, comparable to other high-density star forming regions. We
also estimate a dynamical mass for the cluster, using high-dispersion spectra
from the HIRES spectrograph on the Keck I telescope. The velocity dispersion is
10.0 +/- 2.7 km/s, implying a total cluster mass within 65 pc of (1.7 +/- 0.9)
x 10^6 Msun. Comparing the dynamical mass with the mass estimates based on the
photometry and population synthesis models, the mass-to-light ratio is at least
as high as for a Salpeter IMF extending down to 0.1 Msun, although a turn-over
in the IMF at 0.4 Msun is still possible within the errors. The cluster will
presumably remain bound, evolving into a globular cluster-like object.Comment: 33 pages, including 10 figures and 3 tables. Accepted for publication
in the Astrophysical Journa
BEACH HAZARDS: RIU FUNANA, CAPE VERDE
Coastal Marine Applied Research (CMAR) have been commissioned by Adam Wooler to assess physical bathing hazards at Clubhotel Riu Funana in the Santa Maria resort, Sal, Cape Verde. A bathing injury has occurred at the beach adjacent to the Clubhotel Riu Funana, and this report seeks to estimate the wave and beach morphology conditions at the time of the bathing incident, as well as to identify typical bathing conditions that occur throughout the year at the beach. Through a process of expert judgement using a combination of wave analysis, and assessment of in-situ and satellite imagery, the beach at Riu Funana beach is expected to sit within the ‘reflective’ or ‘low tide terrace’ end of the beach morphology spectrum. The likely beach profile gradient in the area of wave breaking is expected to be 0.1 (slope of 1-in-10), which represents a steep beach profile. Steep beaches exposed to small wave heights with long wavelengths experience plunging or collapsing/surging wave breakers. Such waves break intensely across a narrow region of beach, with collapsing/surging waves breaking right at the shoreline. Given the expected beach morphology and wave breaker types, shore-break impact injuries are expected to be the primary beach hazard type at Riu Funana beach. From processed wave model data, maximum breaking wave heights (defined here as the largest individual wave occurring in a given period of time) at the site are predicted to vary between 0.5 and 7.0 m, but only exceed 2.3 m 10% of the time. Wave breaking is predicted to be predominantly within the ‘plunging’ regime, but periods characterised by ‘collapsing’ and ‘surging’ breakers are also evident. These represent the most powerful of the wave breaker types. In-situ photographs of waves breaking at Clubhotel Riu Funana confirm that waves typically break very close to the beach, and even the larger waves (breaking wave height, Hb > 1 m) break as plunging breakers with considerable breaking intensity and power at the shoreline. On the day of the bathing incident on the 31st March 2018, wave heights are predicted to have been similar to the annual-average significant breaking wave height but lower than the seasonal-average breaking wave height for the season in which the incident occurred (winter). Accounting for uncertainties in the wave conditions and beach slope, there is 95% confidence that the breaker type was plunging, collapsing, or surging on the day of the incident, with the largest individual wave on the day predicted to have been either plunging or collapsing as it broke
The cultural capitalists: notes on the ongoing reconfiguration of trafficking culture in Asia
Most analysis of the international flows of the illicit art market has described a global situation in which a postcolonial legacy of acquisition and collection exploits cultural heritage by pulling it westwards towards major international trade nodes in the USA and Europe. As the locus of consumptive global economic power shifts, however, these traditional flows are pulled in other directions: notably for the present commentary, towards and within Asia
Fine-suspended sediment and water budgets for a large, seasonally dry tropical catchment: Burdekin River catchment, Queensland, Australia
The Burdekin River catchment (~130,400 km2) is a seasonally dry tropical catchment located in north-east Queensland, Australia. It is the single largest source of suspended sediment to the Great Barrier Reef (GBR). Fine sediments are a threat to ecosystems on the GBR where they contribute to elevated turbidity (reduced light), sedimentation stress, and potential impacts from the associated nutrients. Suspended sediment data collected over a 5 year period were used to construct a catchment-wide sediment source and transport budget. The Bowen River tributary was identified as the major source of end-of-river suspended sediment export, yielding an average of 530 t km−2 yr−1 during the study period. Sediment trapping within a large reservoir (1.86 million ML) and the preferential transport of clays and fine silts downstream of the structure were also examined. The data reveal that the highest clay and fine silt loads—which are of most interest to environmental managers of the GBR—are not always sourced from areas that yield the largest total suspended sediment load (i.e., all size fractions). Our results demonstrate the importance of incorporating particle size into catchment sediment budget studies undertaken to inform management decisions to reduce downstream turbidity and sedimentation. Our data on sediment source, reservoir influence, and subcatchment and catchment yields will improve understandings of sediment dynamics in other tropical catchments, particularly those located in seasonally wet-dry tropical savannah/semiarid climates. The influence of climatic variability (e.g., drought/wetter periods) on annual sediment loads within large seasonally dry tropical catchments is also demonstrated by our data
The M31 Globular Cluster Luminosity Function
We combine our compilation of photometry of M31 globular cluster and probable
cluster candidates with new near-infrared photometry for 30 objects. Using
these data we determine the globular cluster luminosity function (GCLF) in
multiple filters for the M31 halo clusters. We find a GCLF peak and dispersion
of V_0^0=16.84 +/-0.11, sigma_t=0.93 +/- 0.13 (Gaussian sigma = 1.20 +/- 0.14),
consistent with previous results. The halo GCLF peak colors (e.g., B^0_0 -
V^0_0) are consistent with the average cluster colors. We also measure V-band
GCLF parameters for several other subsamples of the M31 globular cluster
population. The inner third of the clusters have a GCLF peak significantly
brigher than that of the outer clusters (delta V =~ 0.5mag). Dividing the
sample by both galacticentric distance and metallicity, we find that the GCLF
also varies with metallicity, as the metal-poor clusters are on average 0.36
mag fainter than the metal-rich clusters. Our modeling of the catalog selection
effects suggests that they are not the cause of the measured differences, but a
more complete, less-contaminated M31 cluster catalog is required for
confirmation. Our results imply that dynamical destruction is not the only
factor causing variation in the M31 GCLF: metallicity, age, and cluster initial
mass function may also be important.Comment: AJ, in press. 36 pages, including 7 figure
An HST/WFPC2 survey of bright young clusters in M31. I. VdB0, a massive star cluster seen at t ≃ 25 Myr
Aims. We introduce our imaging survey of possible young massive globular clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). We obtained shallow (to B ~ 25) photometry of individual stars in 20 candidate clusters. We present here details of the data reduction pipeline that is being applied to all the survey data and describe its application to the brightest among our targets, van den Bergh 0 (VdB0), taken as a test case.
Methods. Point spread function fitting photometry of individual stars was obtained for all the WFPC2 images of VdB0 and the completeness of the final samples was estimated using an extensive set of artificial stars experiments. The reddening, the age and the metallicity of the cluster were estimated by comparing the observed color magnitude diagram (CMD) with theoretical isochrones. Structural parameters were obtained from model-fitting to the intensity profiles measured within circular apertures on the WFPC2 images.
Results. Under the most conservative assumptions, the stellar mass of VdB0 is M> 2.4 x 10^4 M_☉ , but our best estimates lie in the range ≃4-9 x 10^4 M_☉. The CMD of VdB0 is best reproduced by models having solar metallicity and age ≃25 Myr. Ages less than ≃12 Myr and greater than ≃60 Myr are clearly ruled out by the available data. The cluster has a remarkable number of red super giants (≳18) and a CMD very similar to Large Magellanic Cloud clusters usually classified as young globulars such as NGC 1850, for example.
Conclusions. VdB0 is significantly brighter (≳1 mag) than Galactic open clusters of similar age. Its present-day mass and half-light radius ((r_h = 7.4 pc) are more typical of faint globular clusters than of open clusters. However, given its position within the disk of M31, it is expected to be destroyed by dynamical effects, in particular by encounters with giant molecular clouds, within the next ~4 Gyr
Star cluster dynamics
Dynamical evolution plays a key role in shaping the current properties of
star clusters and star cluster systems. A detailed understanding of the effects
of evolutionary processes is essential to be able to disentangle the properties
which result from dynamical evolution from those imprinted at the time of
cluster formation. In this review, we focus our attention on globular clusters
and review the main physical ingredients driving their early and long-term
evolution, describe the possible evolutionary routes and show how cluster
structure and stellar content are affected by dynamical evolution.Comment: 20 pages, 2 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 7 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
LaTeX, requires rspublic.cls style fil
More on Chiral-Nonchiral Dual Pairs
Expanding upon earlier work of Pouliot and Strassler, we construct chiral
magnetic duals to nonchiral supersymmetric electric theories based upon SO(7),
SO(8) and SO(9) gauge groups with various numbers of vector and spinor matter
superfields. Anomalies are matched and gauge invariant operators are mapped
within each dual pair. Renormalization group flows along flat directions are
also examined. We find that confining phase quantum constraints in the electric
theories are recovered from semiclassical equations of motion in their magnetic
counterparts when the dual gauge groups are completely Higgsed.Comment: 25 pages, harvmac and tables macros, 1 figur
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