The Wang–Silk approximation, Q−1≈Q−1stars+Q−1gas, is frequently used for estimating the effective Q parameter in two‐component discs of stars and gas. Here we analyse this approximation in detail, and show how its accuracy depends on the radial velocity dispersions and Toomre parameters of the two components. We then propose a much more accurate but still simple approximation for the effective Q parameter, which further takes into account the stabilizing effect of disc thickness. Our effective Q parameter is a natural generalization of Toomre’s Q, and as such can be used in a wide variety of contexts, e.g. for predicting star formation thresholds in galaxies or for measuring the stability level of galactic discs at low and high redshifts.

Cu(In,Ga)Se2 and CdTe heterojunction solar cells grown on rigid (glass) or flexible foil substrates require p‐type absorber layers of optimum optoelectronic properties and n‐type wide‐bandgap partner layers to form the p–n junction. Transparent conducting oxide and specific metal layers are used for front and back electrical contacts. Efficiencies of solar cells depend on various deposition methods as they control the optoelectronic properties of the layers and interfaces. Certain treatments, such as addition of Na in Cu(In,Ga)Se2 and CdCl2 treatment of CdTe have a direct influence on the electronic properties of the absorber layers and efficiency of solar cells. Processes for the development of superstrate and substrate solar cells are reviewed. Copyright © 2004 John Wiley & Sons, Ltd.

Observations of turbulent velocity dispersions in the HI component of galactic discs show a characteristic floor in galaxies with low star formation rates and within individual galaxies the dispersion profiles decline with radius. We carry out several high resolution adaptive mesh simulations of gaseous discs embedded within dark matter haloes to explore the roles of cooling, star-formation, feedback, shearing motions and baryon fraction in driving turbulent motions. In all simulations the disc slowly cools until gravitational and thermal instabilities give rise to a multiphase medium in which a large population of dense self-gravitating cold clouds are embedded within a warm gaseous phase that forms through shock heating. The diffuse gas is highly turbulent and is an outcome of large scale driving of global non-axisymmetric modes as well as cloud-cloud tidal interactions and merging. At low star-formation rates these processes alone can explain the observed HI velocity dispersion profiles and the characteristic value of ∼ 10 km s −1 observed within a wide range of disc galaxies. Supernovae feedback creates a significant hot gaseous phase and is an important driver of turbulence in galaxies with a star-formation rate per unit area 10 −3 M ⊙ yr −1 kpc −2 .

In this paper, we propose a Q stability parameter that is more realistic than those commonly used, and is easy to evaluate [see Eq. (19)]. Using our Q N parameter, you can take into account several stellar and/or gaseous components as well as the stabilizing effect of disc thickness, you can predict which component dominates the local stability level, and you can do all that simply and accurately. To illustrate the strength of Q N , we analyse the stability of a large sample of spirals from The H i Nearby Galaxy Survey (THINGS), treating stars, H i and H 2 as three distinct components. Our analysis shows that H 2 plays a significant role in disc (in)stability even at distances as large as half the optical radius. This is an important aspect of the problem, which was missed by previous (two-component) analyses of THINGS spirals. We also show that H i plays a negligible role up to the edge of the optical disc; and that the stability level of THINGS spirals is, on average, remarkably flat and well above unity.

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