Star formation, massive stars, and super star clusters in nearby galaxies

The Square Kilometer Array (SKA) will enable studies of star formation in nearby galaxies with a level of detail never before possible outside of the Milky Way. Because the earliest stages of stellar evolution are often inaccessible at optical and near-infrared wavelengths, high spatial resolution radio observations are necessary to explore extragalactic star formation. The SKA will have the sensitivity to detect individual ultracompact HII regions out to the distance of nearly 50 Mpc, allowing us to study their spatial distributions, morphologies, and populations statistics in a wide range of environments. Radio observations of Wolf-Rayet stars outside of the Milky Way will also be possible for the first time, greatly expanding the range of conditions in which their mass loss rates can be determined from free-free emission. On a vastly larger scale, natal of super star clusters will be accessible to the SKA out to redshifts of nearly z 0.1. The unprecedented sensitivity of radio observations with the SKA will also place tight constraints on the star formation rates as low as 1M yr⁻¹ in galaxies out to a redshift of z 1 by directly measuring the thermal radio flux density without assumptions about a galaxy’s magnetic field strength, cosmic ray production rate, or extinction.     

Dynamical and compositional assessment of near‐Earth object mission targets

Abstract— Using an H‐plot analysis, we identify 234 currently known near‐Earth objects that are accessible for rendezvous with a “best case” delta‐V of less than 7 km/s. We provide a preliminary compositional interpretation and assessment of these potential targets by summarizing the taxonomic properties for 44 objects. Results for one‐half (22) of this sample are based on new spectroscopic measurements presented here. Our approach provides an easy‐to‐update method for giving guidelines to both observers and mission analysts for focusing on objects for which actual mission opportunities are most likely to be found. Observing prospects are presented for categorizing the taxonomic properties of the most accessible targets that are not yet measured.     

An Historical Overview

Studies in extragalactic astronomy, galactic structure and the late stages of stellar evolution provide ample motivation for surveys of fields in the Galactic Halo. Apart from white dwarfs, blue stars had been regarded as luminous objects confined to star-forming regions in the Galactic Plane; finding them at high galactic latitudes attracted immediate interest, because their luminosities were intermediate between those of white dwarfs and blue Main Sequence stars. The study of blue stars away from the Galactic Plane was initiated by Greenstein; in due course effective temperatures (T _e ff), surface gravities (log g) and abundances showed these stars form what appeared to be a blue extension of the known Horizontal Branch (HB) in the Hertzsprung–Russell Diagram. Extended Horizontal Branch (EHB) stars were identified with Extreme Horizontal Branch stars in globular clusters. It was realised that HB and EHB stars must have formed as a consequence of mass-loss on the Giant Branch, either at or before the helium flash. Mass-loss on the Giant Branch leading to the formation of EHB stars was considered more likely for stars in binary systems. The scene was then set for three decades of EHB star research.     

Depositional and post‐depositional history of warm stage deposits at Knocknacran,Co. Monaghan,Ireland: implications for preservation of Irish last interglacial deposits

Organic‐rich deposits, uncovered during overburden removal from mantled gypsum karst at Knocknacran opencast gypsum mine, Co. Monaghan, are the best candidate to date for a last interglacial record in Ireland. The two till and organic‐rich deposits (preserved at different quarry elevations) were emplaced on to a Tertiary dolerite surface during high‐energy flood events and subsequently folded and faulted by movement towards sinkholes in underlying gypsum. Uranium–thorium disequilibrium dating suggests that the organic‐rich deposits in the upper section were hydrologically isolated at ca. 41 ka and those in the lower section at ca. 86 ka. Interpretation of the pollen content, although tentative because of the depositional and post‐depositional history of the material, suggests that the organic material originated in a warm stage possibly warmer than the post‐Eemian interstadials. The unusual setting of preservation may indicate that in situ, last interglacial deposits have generally been removed by erosion in Ireland. Copyright © 2004 John Wiley & Sons, Ltd.     

Is the Kolmogoroff model applicable to large-scale turbulence?

We discuss the difficulties encountered when the Heisenberg-Kolmogoroff model for turbulence is applied to the large-scale turbulence in: (A) molecular clouds (specifically the velocity vs size relationship) and (B) stars (specifically, the estimate of convective fluxes).A new model for large-scale turbulence is, therefore, needed.     

Slumping and shallow faulting related to the presence of salt on the Continental Slope and Rise off North Carolina

Seismic reflection profiles and long- and medium-range sidescan sonar were used to investigate a salt diapir complex and area of slope instability near the base of the Continental Slope off North Carolina. Within the area of investigation three diapirs are bounded on their upslope side by a scarp 60 m high and 50 km long. The slope above the scarp is characterized by a series of shallow rotational normal faults. The bottom below the scarp is furrowed by slide tracks, which were probably carved by large blocks that broke off the scarp face and slid downslope leaving rubble and scree lobes.Extensive slumping in this area appears to be a result of uplift and faulting associated with salt intrusion, which has fractured and oversteepened the slope leading to instability and failure. Sharply defined slide tracks suggest that slope failure above the breached diapir complex is a continuing process, in contrast to much of the surrounding slope area where few instability features were observed.     

A comparative study of the zooplankton in two adjacent embayments,Port Phillip and Westernport Bays,Australia

We report on a year's study of spatial and seasonal patterns of zooplankton abundance in Port Phillip and Westernport Bays, July 1982 to August 1983. These two bays, closely adjacent on the southern coast of Victoria, Australia, differ in several respects: Port Phillip is a nearly landlocked bay with a broad basin, while Westernport is an open tidal embayment with extensive mud and seagrass banks. Both bays have a resident zooplankton fauna distinct from that of Bass Strait. Although these resident communities have many species in common, patterns of abundance and dominance are quite different. We found that the holoplankton of Port Phillip was about half copepods, mostly Paracalanus indicus, with 23% Caldocera and 21% larvaceans. Westernport Bay zooplankton was dominated by Acartia tranteri, with no resident cladoceran fauna. Bass Strait species were more often found in Westernport than in Port Phillip Bay, but the resident community of Port Phillip Bay was more similar to that of Bass Strait than to that of Westernport.Although this study was undertaken in an exceptionally dry year, the available historical data show that the overall patterns found in 1982–1983 are typical for these bays. The differences in community composition probably relate to differences in depth profile, predator abundance, and suspended matter between the bays.     

Gravitational collapse of a dust ball — The external observer's point of view

The problem of collapse of a dust ball has been studied in detail from the point of view of an external observer (O). It is seen that although there is no material pressure from the point of view of the comoving observer, there is a non-vanishing material pressure in the ball from the point of view of O. In the early stages of collapse (w.r.t. O), this pressure is positive everywhere inside the ball, while during the later stages, if one goes outwards from the centre towards the surface of the ball, this pressure is negative up to a certain value of the radial coordinate, and then changes its sign thereafter. On the basis of this pressure, one can understand the physics of the whole scenario of collapse w.r.t. O; in particular, the important feature is that in the ultimate stages an event horizon is formed asymptotically and there is no collapse to a point (as takes place from the point of view of the comoving observer).     

An analytical model for the HF backscattered Doppler spectrum for the ocean surface

An analysis previously developed for rough surface scattering with narrow-beam reception is extended to wide-beam or omnidirectional reception. The source is considered to be a pulsed electric dipole. The analysis includes multipathing effects on the received signal. The results are used to develop a model for the backscattered radar cross section of the ocean surface, up to a second-order approximation.