Modelling accretion disc and stellar wind interactions: the case of Sgr A

Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disc, the ram and thermal pressures of the disc terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin-Helmholtz instabilities. Because the characteristic wind and stellar velocities are in ∼10⁸ cm s^-1 range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericentre passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of L ≈ 4 × 10³³ erg s^-1 for a stellar mass-loss rate, disc number density, and thermal pressure strength of [Formula: see text], n_d = 10⁵ cm^-3, and α = 0.1, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disc at a distance of ∼3000 gravitational radii from the supermassive black hole.

Engineering the right membranes for electrodes at the biological interface; solvent cast and electropolymerised

A generic approach to adapting many sensors to the rigours of biomedical measurement is to protect appropriate components with selective solute discriminating membrane layers. Our work has utilised external barrier membranes for mass transport control to enzyme electrodes in order to adapt their behaviour to requisite clinical ranges; these membranes have had the additional important effect of reducing surface deposition of colloid and other material on electrode surfaces. Internal membranes, especially those interposed between the enzyme layer and the working electrode of an amperometric sensor, have been found by us to be remarkably important in determining sensor operational stability. This paper will review such membrane materials, including coating technologies, and suggest appropriate directions for future work.

Selective membranes for the construction and optimization of an amperometric oxalate enzyme electrode

Oxalate oxidase has been immobilized within a membrane laminate. Integration of the laminate within an amperometric electrode afforded a sensor for determining oxalate via electrochemical oxidation of H2O2 and subsequent current generation. A study of novel poly(vinyl chloride) (PVC) and cellulose acetate (CA) membranes with various outer/inner membrane configurations made it possible to eliminate direct electrochemical interference from species present in urine, typically ascorbate (60-500 mumol l-1), homovanillic acid (40-60 mumol l-1) and oxalate itself (50-500 mumol l-1). Of the membranes studied, only CA and plasticized PVC were found to screen out the direct oxalate electrochemical signal effectively. Cellulose acetate exhibited the best selectivity ratio for H2O2 over urine interference. An outer 0.05 micron pore radius polycarbonate membrane imparted sensor linearity in the narrow range 2-200 mumol l-1 oxalate, which fell below the normal range of oxalate in urine. The application of unplasticized PVC as an outer membrane allowed the sensor to exhibit linear characteristics which extended beyond the clinically relevant range (> 700 mumol l-1). The Michaelis-Menten constant of the immobilized enzyme was measured and found to be 10(-3) mol l-1.