The initial stages of cave formation: Beyond the one-dimensional paradigm

The solutional origin of limestone caves was recognized over a century ago, but the short penetration length of an undersaturated solution made it seem impossible for long conduits to develop. This is contradicted by field observations, where extended conduits, sometimes several kilometers long, are found in karst environments. However, a sharp drop in the dissolution rate of CaCO_3 near saturation provides a mechanism for much deeper penetration of reactant. The notion of a "kinetic trigger" - a sudden change in rate constant over a narrow concentration range - has become a widely accepted paradigm in speleogenesis modeling. However, it is based on one-dimensional models for the fluid and solute transport inside the fracture, assuming that the dissolution front is planar in the direction perpendicular to the flow. Here we show that this assumption is incorrect; a planar dissolution front in an entirely uniform fracture is unstable to infinitesimal perturbations and inevitably breaks up into highly localized regions of dissolution. This provides an alternative mechanism for cave formation, even in the absence of a kinetic trigger. Our results suggest that there is an inherent wavelength to the erosion pattern in dissolving fractures, which depends on the reaction rate and flow rate, but is independent of the initial roughness. In contrast to one-dimensional models, two-dimensional simulations indicate that there is only a weak dependence of the breakthrough time on kinetic order; localization of the flow tends to keep the undersaturation in the dissolution front above the threshold for non-linear kinetics.Comment: to be published in Earth and Planetary Science Letter

Spin Waves in the Ferromagnetic Ground State of the Kagome Staircase System Co3V2O8

Inelastic neutron scattering measurements were performed on single crystal Co3V2O8 wherein magnetic cobalt ions reside on distinct spine and cross-tie sites within kagome staircase planes. This system displays a rich magnetic phase diagram which culminates in a ferromagnetic ground state below Tc~6 K. We have studied the low-lying magnetic excitations in this phase within the kagome plane. Despite the complexity of the system at higher temperatures, linear spin-wave theory describes most of the quantitative detail of the inelastic neutron measurements. Our results show two spin-wave branches, the higher energy of which displays finite spin-wave lifetimes well below Tc, and negligible magnetic exchange coupling between Co moments on the spine sites.Comment: 4 pages and 4 figure

Milliarcsecond structure of water maser emission in two young high-mass stellar objects associated with methanol masers

The 22.2 GHz water masers are often associated with the 6.7 GHz methanol masers but owing to the different excitation conditions they likely probe independent spatial and kinematic regions around the powering young massive star. We compared the emission of these two maser species on milliarcsecond scales to determine in which structures the masers arise and to test a disc-outflow scenario where the methanol emission arises in a circumstellar disc while the water emission comes from an outflow. We obtained high-angular and spectral resolution 22.2 GHz water maser observations of the two sources G31.581+00.077 and G33.641-00.228 using the EVN. In both objects the water maser spots form complex and filamentary structures of sizes 18-160 AU. The emission towards the source G31.581+00.077 comes from two distinct regions of which one is related to the methanol maser source of ring-like shape. In both targets the main axis of methanol distribution is orthogonal to the water maser distribution. Most of water masers appear to trace shocks on a working surface between an outflow/jet and a dense envelope. Some spots are possibly related to the disc-wind interface which is as close as 100-150 AU to the regions of methanol emission.Comment: 10 pages, accepted to Astronomy and Astrophysic

Magnetic field in Cepheus A as deduced from OH maser polarimetric observations

We present the results of MERLIN polarization mapping of OH masers at 1665 and 1667 MHz towards the Cepheus A star-forming region. The maser emission is spread over a region of 6 arcsec by 10 arcsec, twice the extent previously detected. In contrast to the 22 GHz water masers, the OH masers associated with H II regions show neither clear velocity gradients nor regular structures. We identified ten Zeeman pairs which imply a magnetic field strength along the line-of-sight from -17.3 to +12.7 mG. The magnetic field is organised on the arcsecond scale, pointing towards us in the west and away from us in the east side. The linearly polarized components, detected for the first time, show regularities in the polarization position angles depending on their position. The electric vectors of OH masers observed towards the outer parts of H II regions are consistent with the interstellar magnetic field orientation, while those seen towards the centres of H II regions are parallel to the radio-jets. A Zeeman quartet inside a southern H II region has now been monitored for 25 years; we confirm that the magnetic field decays monotonically over that period.Comment: 10 pages, 6 figures,accepted for publication in MNRA

Instabilities in the dissolution of a porous matrix

A reactive fluid dissolving the surrounding rock matrix can trigger an instability in the dissolution front, leading to spontaneous formation of pronounced channels or wormholes. Theoretical investigations of this instability have typically focused on a steadily propagating dissolution front that separates regions of high and low porosity. In this paper we show that this is not the only possible dissolutional instability in porous rocks; there is another instability that operates instantaneously on any initial porosity field, including an entirely uniform one. The relative importance of the two mechanisms depends on the ratio of the porosity increase to the initial porosity. We show that the "inlet" instability is likely to be important in limestone formations where the initial porosity is small and there is the possibility of a large increase in permeability. In quartz-rich sandstones, where the proportion of easily soluble material (e.g. carbonate cements) is small, the instability in the steady-state equations is dominant.Comment: to be published in Geophysical Research Letter

The effects of the functionality of cross-linkable monomers on selected properties of poly(vinyl-chloride)-monomer mixtures after irradiation with fast electrons

Mixtures containing poly resin, plasticizer, stabilizer and various methacrylate monomers were fluxed on a two roll mill, molded, cut into shapes sutiable for testing, irradiated in air with high-energy electrons from a synchronous transformer accelerator and subjected to several tests. The object of the investigation was to determine the effect of the functionality and/or structure of a series of monomers on certain properties of the ploy-monomer mixtures after electron irradiation