The Formation and Evolution of Massive Stellar Clusters in IC 4662

We present a multiwavelength study of the formation of massive stellar clusters, their emergence from cocoons of gas and dust, and their feedback on surrounding matter. Using data that span from radio to optical wavelengths, including Spitzer and Hubble ACS observations, we examine the population of young star clusters in the central starburst region of the irregular Wolf-Rayet galaxy IC 4662. We model the radio-to-IR spectral energy distributions of embedded clusters to determine the properties of their HII regions and dust cocoons (sizes, masses, densities, temperatures), and use near-IR and optical data with mid-IR spectroscopy to constrain the properties of the embedded clusters themselves (mass, age, extinction, excitation, abundance). The two massive star-formation regions in IC 4662 are excited by stellar populations with ages of ~ 4 million years and masses of ~ 3 x 10^5 M_sun (assuming a Kroupa IMF). They have high excitation and sub-solar abundances, and they may actually be comprised of several massive clusters rather than the single monolithic massive compact objects known as Super Star Clusters (SSCs). Mid-IR spectra reveal that these clusters have very high extinctions, A_V ~ 20-25 mag, and that the dust in IC 4662 is well-mixed with the emitting gas, not in a foreground screen.Comment: 7 pages, 11 figures, to appear in proceedings of the conference "Young Massive Star Clusters: Initial Conditions and Environments ", held in Granada, Spain, September 200

Motor-Driven Bacterial Flagella and Buckling Instabilities

Many types of bacteria swim by rotating a bundle of helical filaments also called flagella. Each filament is driven by a rotary motor and a very flexible hook transmits the motor torque to the filament. We model it by discretizing Kirchhoff's elastic-rod theory and develop a coarse-grained approach for driving the helical filament by a motor torque. A rotating flagellum generates a thrust force, which pushes the cell body forward and which increases with the motor torque. We fix the rotating flagellum in space and show that it buckles under the thrust force at a critical motor torque. Buckling becomes visible as a supercritical Hopf bifurcation in the thrust force. A second buckling transition occurs at an even higher motor torque. We attach the flagellum to a spherical cell body and also observe the first buckling transition during locomotion. By changing the size of the cell body, we vary the necessary thrust force and thereby obtain a characteristic relation between the critical thrust force and motor torque. We present a sophisticated analytical model for the buckling transition based on a helical rod which quantitatively reproduces the critical force-torque relation. Real values for motor torque, cell body size, and the geometry of the helical filament suggest that buckling should occur in single bacterial flagella. We also find that the orientation of pulling flagella along the driving torque is not stable and comment on the biological relevance for marine bacteria.Comment: 15 pages, 11 figure

Orbital Observations of Dust Lofted by Daytime Convective Turbulence

Over the past several decades, orbital observations of lofted dust have revealed the importance of mineral aerosols as a climate forcing mechanism on both Earth and Mars. Increasingly detailed and diverse data sets have provided an ever-improving understanding of dust sources, transport pathways, and sinks on both planets, but the role of dust in modulating atmospheric processes is complex and not always well understood. We present a review of orbital observations of entrained dust on Earth and Mars, particularly that produced by the dust-laden structures produced by daytime convective turbulence called “dust devils”. On Earth, dust devils are thought to contribute only a small fraction of the atmospheric dust budget; accordingly, there are not yet any published accounts of their occurrence from orbit. In contrast, dust devils on Mars are thought to account for several tens of percent of the planet’s atmospheric dust budget; the literature regarding martian dust devils is quite rich. Because terrestrial dust devils may temporarily contribute significantly to local dust loading and lowered air quality, we suggest that martian dust devil studies may inform future studies of convectively-lofted dust on Earth

Mineral deficiency and the presence of Pinus sylvestris on mires during the mid- to late Holocene: Palaeoecological data from Cadogan's Bog, Mizen Peninsula, Co. Cork, southwest Ireland

Pollen records across parts of Ireland, England and northern Scotland show a dramatic collapse in Pinus pollen percentages at approximately 4000 radiocarbon years BP. This phenomenon has attracted much palaeoecological interest and several hypotheses have been put forward to account for this often synchronous and rapid reduction in pine from mid-Holocene woodland. Explanations for the 'pine decline' include prehistoric human activity, climatic change, in particular a substantial increase in precipitation resulting in increased mire wetness, and airborne pollution associated with the deposition of tephra. Hitherto, one largely untested hypothesis is that mineral deficiency could adversely affect pine growth and regeneration on mire surfaces. The discovery of pine-tree remains (wood pieces, stumps and trunks) within a peat located at Cadogan's Bog on the Mizen Peninsula, southwest Ireland, provided an opportunity to investigate the history of Pinus sylvestris and also to assess the importance of mineral nutrition in maintaining pine growth on mires. Pollen, plant macrofossils, microscopic charcoal and geochemical data are presented from a radiocarbon dated monolith extracted from this peat together with tree ring-width data and radiocarbon dated age estimates from subfossil wood. Analyses of these data suggest that peat accumulation commenced at the site around 6000 years BP when pine was the dominant local tree. Thereafter Pinus pollen percentages diminish in two stages, with the second decline taking place around 4160 ± 50 years BP. Concomitant with this decline in Pinus pollen, there is a noticeable, short-lived increase in wet-loving mire taxa and a decrease in the concentration of phosphorus, potassium, magnesium, calcium, sodium, iron and zinc. These results suggest that increased mire surface wetness, possibly the result of a change in climate, created conditions unsuitable for pine growth c. 4000 years BP. Mire surface wetness, coupled with a period of associated nutrient deficiency, appears to be a possible explanation for a lack of subsequent pine-seedling establishment for most of the later Holocene

On the use of spectrally constrained ICA applied to single-channel ictal EEG recordings within a dynamical embedding framework

Within a dynamical embedding (DE) framework it is possible to extract information on multiple-sources underlying just a single channel recording of electromagnetic brain activity. Independent Component Analysis (ICA) is a technique which, when used in conjunction with DE, can identify and extract statistically independent sources underlying these single channel recordings. However, these powerful techniques still generally require subjective a posteriori analysis in order to visualise neurophysiologically meaningful components in the outputs. For this reason we introduce a variant of ICA known as constrained ICA (cICA) which allows for the extraction of one of many sources underlying the measurement signal, through the provision of a basic reference signal. This constraint can be chosen to reflect neurophysiological prior knowledge of the sources in question given the measured signal. Here we present a technique which allows for the application of spectral constraints on single channel recordings of epileptic EEG data. We show that through a combination of DE and cICA it is possible to extract meaningful information on epileptic seizures and other rhythmic activity from just a single channel of EEG. We further show that accurate extraction of the sources of interest is not critically dependent on the closeness of the measurement channel to the location of the source activity

The fast ICA algorithm with spatial constraints

In many blind source separation (BSS) applications, especially for biomedical signal processing, there are specific expectations regarding the spatial and temporal characteristics of some sources, but post-hoc comparisons between source estimates and anticipated outcomes can be complicated and unreliable. One alternative is to incorporate additional prior knowledge, e.g., about the spatial topography of selected source sensor projections, into the BSS approach by means of constraints. This letter describes a modified version of the FastICA algorithm for spatially constrained BSS, where the estimates of selected columns of the mixing matrix are constrained with reference to predetermined source sensor projections