On water ice formation in interstellar clouds

A model is proposed for the formation of water ice mantles on grains in interstellar clouds. This occurs by direct accretion of monomers from the gas, be they formed by gas or surface reactions. The formation of the first monolayer requires a minimum extinction of interstellar radiation, sufficient to lower the grain temperature to the point where thermal evaporation of monomers is just offset by monomer accretion from the gas. This threshold is mainly determined by the adsorption energy of water molecules on the grain material; for hydrocarbon material, chemical simulation places this energy between 0.5 and 2 kcal mol⁻¹, which sets the (true) visible extinction threshold at a few magnitudes. However, realistic distributions of matter in a cloud will usually add to this an unrelated amount of cloud core extinction, which can explain the large dispersion of observed (apparent) thresholds. Once the threshold is crossed, all available water molecules in the gas are quickly adsorbed, because the grain cools down and the adsorption energy on ice is higher than on bare grain. The relative thickness of the mantle, and, hence, the slope of  τ₃( A _v)  depend only on the available water vapour, which is a small fraction of the oxygen abundance. Chemical simulation was also used to determine the adsorption sites and energies of O and OH on hydrocarbons and study the dynamics of formation of water molecules by surface reactions with gaseous H atoms, as well as their chances to stick in situ.     

Diffuse Galactic light at high Galactic latitude: nature and interpretation

The hypothesis of an extended red emission (ERE) in diffuse Galactic light (DGL) has been put forward in 1998 by Gordon, Witt & Friedmann who found that scattered starlight was not enough to explain the amount of DGL in the R band, in some high Galactic latitude directions. This paper re-investigates, for high Galactic latitudes, the brightnesses and colours of DGL, integrated star and galaxy light (ISGL), and of the total extrasolar light (ISGL+DGL) measured by Pioneer. Under the traditional assumption that DGL is forward scattering of background starlight by interstellar dust on the line of sight, ISGL and Pioneer have very close colours, as it is found by Gordon, Witt & Friedmann. Pioneer observations at high |b| thus accept an alternative and simple interpretation, with no involvement of ERE in DGL.     

Low-frequency damping of a moored semisubmersible obtained from simulated extinction tests and mean wave drift forces

Low-frequency damping of a moored semisubmersible drilling platform was obtained from numerical extinction tests simulated in still water and in regular waves and from mean wave drift forces calculated at zero forward speed. The influence of drag forces was represented by the modified Morison equation. The platform as used for the 18th ITTC Comparative Mooring Study was analyzed in irregular beam waves. The computed time series of sway as well as the corresponding sway response spectrum compared favourably with model test measurements, demonstrating that this procedure to determine low-frequency damping can be effective.     

钝顶螺旋藻C-藻蓝蛋白和多管藻R-藻红蛋白的分离纯化及摩尔消光系数的测定

１９９３年１１月～１９９４年１月用简单的羟基磷灰石柱层析法从多管藻中分 离纯化了Ｒ－藻红蛋白和从钝顶螺旋藻中纯化了Ｃ－藻蓝蛋白，它们的纯度（指可见 光部分的最大吸收与 ２８０ｎｍ处吸收值之比）可分别达到 ６（Ｒ－藻红蛋白）和 ５． ５（Ｃ－ 藻蓝蛋白）。由于不同藻种的同种藻胆蛋白的摩尔消光系数不同，所以应用凯氏定 氮法并结合可见光的吸收值测定了上述两种藻胆蛋白的摩尔消光系数，钝顶螺旋 藻Ｃ－藻蓝蛋白为１．８５３ × １０６ｍｏｌ－１ｃｍ－１（６２０ｎｍ），多管藻Ｒ－藻红蛋白为１．７９６ × １０６ｍｏｌ－１ｃｍ－１（４９８ｎｍ），为藻胆蛋白的浓度测定提供了方便。     

Circumstellar dust and circumstellar extinction curves

We have calculated the circumstellar extinction curves produced by dust grains which absorb and scatter the stellar radiation in the shells of pre-main-sequence stars. A Monte Carlo method was used to model the radiative transfer in non-spherical shells. The dependence on the particle size distribution and the dust shell parameters has been examined.The application of the theoretical results to explain the extinction and polarization of the Herbig Be star HD 45677 shows that the dust shell is not disk-like and that very small grains are absent in it.