The effect of magnetic fields on the formation of circumstellar discs around young stars

We present first results of our simulations of magnetic fields in the formation of single and binary stars using a recently developed method for incorporating Magnetohydrodynamics (MHD) into the Smoothed Particle Hydrodynamics (SPH) method. An overview of the method is presented before discussing the effect of magnetic fields on the formation of circumstellar discs around young stars. We find that the presence of magnetic fields during the disc formation process can lead to significantly smaller and less massive discs which are much less prone to gravitational instability. Similarly in the case of binary star formation we find that magnetic fields, overall, suppress fragmentation. However these effects are found to be largely driven by magnetic pressure. The relative importance of magnetic tension is dependent on the orientation of the field with respect to the rotation axis, but can, with the right orientation, lead to a dilution of the magnetic pressure-driven suppression of fragmentation.     

Multiwavelength study of X-ray selected star-forming galaxies within the Chandra Deep Field-South

We have combined multiwavelength observations of a selected sample of star-forming galaxies with galaxy evolution models in order to compare the results obtained for different star formation rate (SFR) tracers and to study the effect that the evolution of the star-forming regions has on them. We also aimed at obtaining a better understanding of the corrections due to extinction and nuclear activity on the derivation of the SFR. We selected the sample from Chandra data for the well studied region Chandra Deep Field -South (CDFS) and chose the objects that also have ultraviolet (UV) and infrared (IR) data from Galaxy Evolution Explorer ( GALEX ) and Great Observatories Origins Deep Survey (GOODS) Spitzer , respectively.
Our main finding is that there is good agreement between the extinction corrected SFR(UV) and the SFR(X), and we confirm the use of X-ray luminosities as a trustful tracer of recent star formation activity. Nevertheless, at SFR(UV) larger than about  5 M_⊙ yr⁻¹  there are several galaxies with an excess of SFR(X) suggesting the presence of an obscured active galactic nucleus (AGN) not detected in the optical spectra. We conclude that the IR luminosity is driven by recent star formation even in those galaxies where the SFR(X) is an order of magnitude higher than the SFR(UV) and therefore may harbour an AGN. One object shows SFR(X) much lower than expected based on the SFR(UV); this SFR(X) 'deficit' may be due to an early transient phase before most of the massive X-ray binaries were formed. An X-ray deficit could be used to select extremely young bursts in an early phase just after the explosion of the first supernovae associated with massive stars and before the onset of massive X-ray binaries.     

Unstable flow in layered soils: A review

Release of water from the soil in the process of internal drainage, and its continued downward movement through the vadose zone, constitute the main mechanism of groundwater recharge. Water released from the soil generally contains solutes, and these are conveyed to the groundwater via the same pathways as the drained water. Knowledge of those pathways is essential in any attempt to minimize the likelihood of groundwater pollution. Solutes generally interact with the medium in which they reside or travel, and the spatial and temporal pattern of their movement influences the nature and extent of their interactions. For many years, the assumption had prevailed that flow in the vadose zone is a steady-state, uniform process. Hence the vadose zone can serve to filter, attenuate, as well as degrade, potential pollutants. Recently, however, the existence of preferred pathways has come to light. Such pathways might connect the soil's upper zone directly to the water-table, thus bypassing the greater volume of the vadose zone and evading its filtering mechanisms. Groundwater recharge models that ignore the possibility of such spurts of contamination may be highly misleading. Preferred flow path may be cracks, animal burrows, or decayed root channels. Less easily discernible are transient and random paths associated with the phenomenon of ‘unstable flow’, which is most likely to occur in layered soils during infiltration. The wetting front, instead of remaining horizontal and advancing continuously from one layer to the next, may begin (particularly in transition from a fine-textured to a coarse-textured layer) to form bulges, called ‘fingers’, which propagate downwards and may become, in effect, vertical pipes. At present we are aware only of the occasional occurrence and potential importance of such phenomena, but as yet have neither the systematic empirical data, nor a proven comprehensive theoretical framework, by which to assess where, when, and according to what pattern, they are likely to occur.     

Comparison of British and American Carboniferous cyclic rock sequences

Classification is of interest to geologists as a convenient means of expressing ideas and concepts. Most classification schemes categorize a continuum into discrete classes or states based on some prominent character of the objects being classified. Unknowns then are identified as to their position within the classification scheme. Until recently most geologic classification schemes have been qualitative. With advent of the computer, applications of many quantitative statistical techniques have become practical. These techniques offer the advantages of repeatability and objectivity. This report gives results of the applications of several techniques for classifying Carboniferous cyclic rock sequences. Twenty sections were measured in detail in Great Britain and the United States. Particular importance was placed on noting transition from one lithology to another. Seven lithologic types were distinguished: (a) sandstone, (b) siltsone, (c) nonfossiliferous shale, (d) seatearth or underclay, (e) coal, (f) fossiliferous shale, and (g) limestone. It was noted also which part of the sequence was marine and which nonmarine. From the original data, the number of changes per 100 ft were calculated as well as an entropy index indicating the “orderliness” of the sequence and a matching index obtained by comparing the similarity in sequences of lithology between pairs of sections. The matching index is based on qualitative characters and in this regard belongs to a type of sequential analysis of scaleless nonnumeric data. The matching coefficients were clustered and displayed as dendrograms. A cluster analysis also was performed using nine variables (number of changes per 100 ft, entropy index, percentage thickness of sandstone, siltstone, nonfossiliferous shale, seatearth and coal, fossiliferous shale, and limestone, and percentage of nonmarine units) and the results displayed as dendrograms. In addition principal components analysis utilized the nine variables to determine if groups were present in the data. The first three principal components were interpreted geologically and a three-dimensional model constructed. Three loosely grouped clusters could be recognized in this display: (1) cyclic sequences associated with deltaic complexes, (2) sequences characteristic of deposition farther offshore, and (3) those composed mainly of marine sequences formed in an offshore open-marine environment.     

Is there a black hole among the black-hole candidates?

None of the tentative black-hole identifications has remained without serious difficulties. As a class, the black-hole candidates differ in no obvious property from neutron-star binaries other than in their estimated high mass. We favour an interpretation of the black-hole candidates as binary neutron stars surrounded by a more or less massive accretion disc