On the use of collocation methods for the construction of stellar models

Collocation with piecewise continuous polynomials is studied for use in the numerical modelling of stellar evolution. Accuracy and convergence of the method are demonstrated for a 5M star with a convective core. Collocation should be further studied since it is likely to lead to significant gains in computational efficiency for the construction of stellar models.     

Progress report on rhenish massif seismic experiment

This paper presents some data and results from a seismic refraction experiment, completed mainly in 1979 in the Rhenish Massif, Federal Republic of Germany and extending through Luxembourg and Belgium into the Paris Basin in France.Velocity-depth functions have been derived for each record section independently, based on the assumption that velocity varies only with depth: these models are being improved upon by time-term and ray-tracing methods capable of handling laterally varying velocity structures and by calculating synthetic seismograms.The P_g phase which is observed very clearly on all record sections represents a refracted wave, with velocity generally > 6 km/s, from depths below 1.5–5.5 km. Along the 600 km long main profile one intracrustal reflection can usually be recognized, while from the three shorter crossing profiles in the massif two intracrustal reflectors can always be seen. Beneath much of the main profile the crust-mantle boundary is either a first order discontinuity or thin (< 1 km) transition zone at ~30 km depth. However, beneath the Ardennes and West Eifel there is a 6–12 km thick transition zone before a velocity of 8.1 km/s is reached at ~36 km depth. Beneath the crossing profiles, there is generally a transition zone < 3 km thick between crust and mantle. In some cases, there can be recognized at the top of the mantle a thin high velocity layer which is underlain by a low velocity layer which, in turn, is underlain by a reflector 4–11 km below the crust-mantle boundary.     

The search for the origin of the Local Bubble redivivus

We present a new unbiased search and analysis of all B stars in the solar neighbourhood (within a volume of 400 pc diameter) using the Arivel data base to track down the remains of the OB associations, which hosted the supernovae (SNe) responsible for the Local Bubble (LB) in the interstellar gas. We find after careful dereddening and by comparison with theoretical isochrones, that besides the Upper Scorpius the Upper Centaurus Lupus and Lower Centaurus Crux subgroups are the youngest stellar associations in the solar neighbourhood with ages of 20–30 Myr, in agreement with previous work. In search for the 'smoking gun' of the origin of the LB, we have traced the paths of the associations back into the past and found that they entered the present bubble region 10–15 Myr ago. We argue that the LB began to form then and estimate that 14–20 SNe have gone off since. It is shown that the implied energy input is sufficient to excavate a bubble of the presently observed size.     

Paleolimnological features of a mega-lake phase in the Makgadikgadi Basin (Kalahari,Botswana) during Marine Isotope Stage 5 inferred from diatoms

The Makgadikgadi–Okavango–Zambezi basin (MOZB) is a structural depression in the south-western branch of the East African Rift System of the northern and middle Kalahari, central southern Africa. In the present day, the mainly dry subbasins of the MOZB are part of a long-lived lacustrine system that has likely existed since Early Pleistocene and from which an extant freshwater fish radiation emerged seeding all major river systems of southern Africa. During hydrologically favourable periods the subbasins were connected as a single mega-lake termed Lake Palaeo-Makgadikgadi. Previous geomorphological studies and OSL dates have provided evidence for repeated mega-lake periods since approximately 300 ka. The environmental and climatic implications of such large scale late Quaternary lake-level fluctuations are controversial, with the duration of mega-lake phases poorly constrained. Here, we present the first evidence for a Marine Isotope Stage (MIS) 5 mega-lake period (about 935–940 m a.s.l.) reconstructed from a diatom-rich, 30-cm-thick lacustrine sediment section, exposed close to a palaeo-shoreline of the Makgadikgadi Basin. Based upon the environmental setting and in comparison with sedimentation rates of other similar lake environments, we tentatively estimated that the highstand lasted approximately 1 ka during MIS 5d–b. The 30-cm section was sampled in 0.5-cm steps. Diatom species diversity ranges from 19 to 30 through the section. The dominant species are Pseudostaurosira brevistriata, Rhopalodia gibberula, Cyclotella meneghiniana and Epithemia sorex. The total of 60 sediment samples provide us with a record at decadal to bi-decadal resolution. Based on diatom assemblages and their oxygen isotope composition (δ¹⁸O) we infer an alkaline and mostly oligohaline lake with shallow water conditions prevailing in MIS 5, and is potentially analogous to a Heinrich event. The climate over southern Africa during MIS 5 has been considered very arid but the hydromorphological context of our sediment section indicates that we captured a mega-lake period providing evidence that short-term excursions to significantly higher humidity existed. A hydrologically more favourable environment during MIS 5 than formerly presumed is in line with the early human occupation of the Kalahari.     

Brittle–ductile deformation in the Glarus thrust Lochseiten (LK) calc-mylonite

The Glarus thrust accommodated at least 30 km of northward displacement strongly localized within a 1-m layer of 'Lochseiten' (LK) calc-mylonite. This layer displays veins in various states of plastic deformation and a wildly refolded foliated gouge texture. Lattice- and shape-preferred orientations are observed within the fine-grained, recrystallized matrix. These features indicate the alternate activity of brittle and ductile deformation mechanisms. In contrast to the classical view that grain boundary sliding (superplasticity) is the dominant deformation mechanism, it is advocated that fluids, derived from the footwall and expelled along the thrust, are responsible for hydrofracturing and cataclastic deformation. In periods between fracture events, deformation was ductile. In this new interpretation, a substantial amount of the total thrust displacement was accommodated by numerous short-lived and strongly localized fracture events at the base of the Verrucano thrust sheet, rather than a permanently weak décollement lithology.