Analyzing Starbursts Using Magellanic Cloud Star Clusters as Simple Stellar Populations

Integrated spectra have been obtained of 31 star clusters in the Magellanic Clouds (MC) and of four Galactic globular clusters. The spectra cover the wavelength range 3500–4700 Å at a resolution of 3.2 Å FWHM. The MC clusters primarily cover the age range from less than 108 to about 3 Gyr and hence are well-suited to an empirical study of aging poststarburst stellar populations. An age-dating method is presented that relies on two spectral absorption feature indices, Hδ/Fe I λ4045 and Ca II, as well as an index measuring the strength of the Balmer discontinuity. We compare the behavior of the spectral indices in the observed integrated spectra of the MC clusters with that of indices generated from theoretical evolutionary synthesis models of varying age and metal abundance. The synthesis models are based on those of Worthey, when coupled with the combination of an empirical library of stellar spectra by Jones for the cooler stars and synthetic spectra, generated from Kurucz model atmospheres, for the hotter stars. Overall, we find good agreement between the ages of the MC clusters derived from our integrated spectra (and the evolutionary synthesis modelling of the spectral indices) and ages derived from analyses of the cluster color-magnitude diagrams, as found in the literature. Hence, the principal conclusion of this study is that ages of young stellar populations can be reliably measured from modelling of their integrated spectra

The Ages of Starbursts in Post-Starburst Galaxies

We present a new technique for accurately determining the ages of starbursts in post-starburst galaxies. In particular, it uses the strength of the Ca II H + H$\epsilon$ absorption feature relative to that of Ca II K to separate the effects of burst strength from burst age in a spectrum comprised of a post-starburst population and an underlying old galaxy population. To test the technique we have measured spectral indices in the observed spectra of three post-starburst early-type galaxies. The three galaxies are shown to have different post-starburst ages, ranging from 0 Gyr to 1.5 Gyr after completion of the burst. We also demonstrate that if the burst contributes greater than 50\% of the light at 4000 \AA \ then the technique can distinguish between a post-starburst systems and models in which a constant star formation history is suddenly truncated, such as that which could occur in a spiral that is rapidly stripped of its interstellar medium. Finally, we demonstrate that while the technique is well-suited for determining the time elapsed since the termination of a starburst (up until 2 Gyr after the end of the burst), the derived termination times are relatively insensitive to the duration of the burst.Comment: 14 pages + 7 figures, uuencoded gzipped postscript. To appear in January A

Quantification of the uncertainty in coastal storm hazard predictions due to wave‐current interaction and wind forcing

Coastal flood warning and design of coastal protection schemes rely on accurate estimations of water level and waves during hurricanes and violent storms. These estimations frequently use numerical models, which, for computational reasons, neglect the interaction between the hydrodynamic and wave fields. Here, we show that neglecting such interactions, or local effects of atmospheric forcing, causes large uncertainties, which could have financial and operational consequences because flood warnings are potentially missed or protection schemes underdesigned. Using the Severn Estuary, SW England, we show that exclusion of locally generated winds underestimates high water significant wave height by up to 90.1%, high water level by 1.5%, and hazard proxy (water level + 1/2 significant wave height) by 9.1%. The uncertainty in water level and waves is quantified using a system to model tide‐surge‐wave conditions, Delft3D‐FLOW‐WAVE in a series of eight model simulations for four historic storm events

Evidence for a Young Stellar Population in NGC 5018

Two absorption line indices, Ca II and Hdelta/FeI4045, measured from high resolution spectra are used with evolutionary synthesis models to verify the presence of a young stellar population in NGC 5018. The derived age of this population is about ~2.8 Gyr with a metallicity roughly solar and it completely dominates the integrated light of the galaxy near 4000 A.Comment: 13 pages, 7 figures (figs 3-7 are color figures), to be published in the May 2000 issue of the Astrophysical Journa

Aerodynamics of a Supersonic Projectile in Proximity to a Solid Surface

Flow around a Mach 2.4 NATO 5.56 mm projectile in close proximity to a ground plane was investigated using computational fluid dynamics for a direct numerical reproduction of live-range experiments. The numerical approach was validated against both the live-range tests and subsequent wind-tunnel experiments. A nonspinning half-model and a full, spinning projectile were examined to clarify the influence of rotation. Multiple ground clearances were tested to obtain clear trends in changes to the aerodynamic coefficients, and the three-dimensional propagation and reflection of the shock waves were considered in detail. The behavior of the flow in the near wake was also studied as ground clearance was reduced. Ground proximity was found to significantly increase the drag force acting on the projectile, as well as generate a force normal to the ground and an increased side force, when ground clearance was less than one diameter. For clearances between approximately 0.4 and 1 diameter, the pitching moment produced was nose-down. For lower clearances, a more distinct nose-up trend was produced. The generated side force was orders of magnitude lower than the normal and drag forces

Sensitivity of flood hazard and damage to modelling approaches

Combination of uncertainties in water level and wave height predictions for extreme storms can result in unacceptable levels of error, rendering flood hazard assessment frameworks less useful. A 2D inundation model, LISFLOOD-FP, was used to quantify sensitivity of flooding to uncertainty in coastal hazard conditions and method used to force the coastal boundary of the model. It is shown that flood inundation is more sensitive to small changes in coastal hazard conditions due to the setup of the regional model, than the approach used to apply these conditions as boundary forcing. Once the threshold for flooding is exceeded, a few centimetres increase in combined water level and wave height increases both the inundation and consequent damage costs. Improved quantification of uncertainty in inundation assessments can aid long-term coastal flood hazard mitigation and adaptation strategies, to increase confidence in knowledge of how coastlines will respond to future changes in sea-level

Flood hazard assessment for a hyper-tidal estuary as a function of tide-surge-morphology interaction

Astronomical high tides and meteorological storm surges present a combined flood hazard to communities and infrastructure. There is a need to incorporate the impact of tide-surge interaction and the spatial and temporal variability of the combined flood hazard in flood risk assessments, especially in hyper-tidal estuaries where the consequences of tide and storm surge concurrence can be catastrophic. Delft3D-FLOW is used to assess up-estuary variability in extreme water levels for a range of historical events of different severity within the Severn Estuary, southwest England as an example. The influence of the following on flood hazard is investigated: (i) event severity, (ii) timing of the peak of a storm surge relative to tidal high water and (iii) the temporal distribution of the storm surge component (here in termed the surge skewness). Results show when modelling a local area event severity is most important control on flood hazard. Tide-surge concurrence increases flood hazard throughout the estuary. Positive surge skewness can result in a greater variability of extreme water levels and residual surge component, the effects of which are magnified up-estuary by estuarine geometry to exacerbate flood hazard. The concepts and methodology shown here can be applied to other estuaries worldwide