On the origin of warps and the role of the intergalactic medium

There is still no consensus as to what causes galactic discs to become warped. Successful models should account for the frequent occurrence of warps in quite isolated galaxies, their amplitude as well as the observed azimuthal and vertical distributions of the H  i layer. Intergalactic accretion flows and intergalactic magnetic fields may bend the outer parts of spiral galaxies. In this paper we consider the viability of these non-gravitational torques to take the gas off the plane. We show that magnetically generated warps are clearly flawed because they would wrap up into a spiral in less than two or three galactic rotations. The inclusion of any magnetic diffusivity to dilute the wrapping effect causes the amplitude of the warp to damp. We also consider the observational consequences of the accretion of an intergalactic plane-parallel flow at infinity. We have computed the amplitude and warp asymmetry in the accretion model, for a disc embedded in a flattened dark matter halo, including self-consistently the contribution of the modes with azimuthal wavenumbers   m = 0  and   m = 1  . Since the m = 0 component, giving a U-shaped profile, is not negligible compared to the m = 1 component, this model predicts quite asymmetric warps, maximum gas displacements on the two sides in the ratio 3 : 2 for the preferred Galactic parameters, and the presence of a fraction ∼3.5 per cent of U-shaped warps, at least. The azimuthal dependence of the moment transfer by the ram pressure would produce a strong asymmetry in the thickness of the H  i layer and asymmetric density distributions in z , in conflict with observational data for the warp in our Galaxy and in external galaxies. The amount of accretion that is required to explain the Galactic warp would give gas scaleheights in the far outer disc that are too small. We conclude that accretion of a flow with no net angular momentum cannot be the main and only cause of warps.     

The generation of ionization-shock front oscillations by a variable radiation flux

The effect of a time-varying radiation flux incident on an ionization front on the generation of ionization-shock front oscillations in the interstellar medium is analyzed analytically and numerically. We take into account both variations in the flux of ionizing radiation directly from the source that produces the ionization front and the absorption of energetic photons by the post-front plasma. Based on our calculations, we show that the time dependence of the radiation flux can be an additional factor (apart from small inhomogeneities in the interstellar medium) that contributes to the amplification of oscillations and to the kinetic energy input to the observed turbulent motions in H II regions.     

The Shadow of Comet Hale–Bopp in Lyman-Alpha

For a few months around perihelion, thecentral part of the Hale–Bopp hydrogencloud has been optically thick to thesolar Lyα radiation, and hassignificantly reduced the solar flux availablefor the resonance glow of interstellarhydrogen beyond the comet. This shadowing effecton the interstellar gas is the first everobserved comet shadow. It is modeled andcompared with SWAN observations. Shadowmodelling will help to constrain the cometwater production and radiative transfer effectsin the interstellar ionisation cavity.     

H I distribution in the region of the supernova remnant G78.2+2.1

Based on RATAN-600 21-cm line observations with an angular resolution of 2.4′ over a wide range of radial velocities, we analyze the neutral-hydrogen distribution in the region of the SNR G78.2+2.1. In addition to an H I shell at low radial velocities immediately surrounding the radio remnant, we detected an extended expanding H I shell, ≈3° in diameter, at a radial velocity of ?25 km s^?1, which closely coincides in coordinates and angular sizes with the outer X-ray shell discovered by Lozinskaya et al. (2000). The Hα emission studied by these authors in the SNR region also has a secondary peak at radial velocities from ?45 to ?20 km s^?1. Since the radial velocities of these two objects differ significantly, their distances can be assumed to differ as well; i.e., a chance projection of two distinct objects is observed.     

E (B-V) determination from an UV-visual two-colour diagram: O and B stars in the catalogue of stellar ultraviolet fluxes

We present here, for the O and B type stars in the Catalogue of Stellar Ultraviolet Fluxes, an approach which does not require a precise knowledge of spectral type and luminosity class for derivingE(B-V) colour excesses. The method is based on the use of an UV-visual two-colour diagram; galactic variations in the interstellar extinction law are analyzed and fully taken into account. Our results have been compared with those derived by using the differences between observed and intrinsic colours for stars with known spectral classification. The very good agreement in a large number of cases (94 per cent) demonstrates that our approach permits the derivation of reliable colour excess values for early type stars even if only a rough spectral classification is available.     

水平分层弹性粘弹性介质的静力响应

本文首先利用传播矩阵法构造出了水平分层弹性介质在表载、体力及内部力源联合作用下的一般模型。然后利用“对应原理”将弹性问题的一般模型推广到粘弹性问题的一般模型——粘弹性分层介质传播矩阵基本方程。最后用粘弹性分层介质传播矩阵方程对一个实际地学问题做了试算,计算结果说明,用传播矩阵法处理粘弹性问题是非常有效的。     

2002年12月25日乌恰5.8级地震的长、中、短期跟踪预测和反思

在乌恰地区地震活动呈现长期平静异常的背景下,通过多种方法对2002年12月25日乌恰5．8级地震进行了1-3年的中长期预测。应用“模糊地震活动度”和“空区参数法”两种方法进行了1年和半年的中期预测,结合对该地区小震活动动态的分析与震例分析,尤其是预警地震的分析判断,提出了3个月的短期预测意见,较好地预测了这次地震。最后对预测情况进行了正反两个方面的总结和反思,提出了七点看法。     

The effect of random recharge on uniform steady free-surface flow in heterogeneous porous formations

The effect of parametric uncertainty in recharge rate and spatial variability of hydraulic conductivity upon free-surface flow is investigated in a stochastic framework. We examine the three-dimensional free-surface gravitational flow problem for sloped mean uniform flow in a randomly heterogeneous porous medium under the influence of random recharge. We develop analytic solutions for the variance of free-surface position, head, and specific discharge on the free surface. Additionally, we obtain semi-analytic solutions for the statistical moments of head and specific discharge beneath the free-surface. Statistical moments are derived using a first-order approximation and then compared with their parallel in an unbounded medium. The effect of recharge mean and variability on the statistical moments is analyzed. Results can be applied to more complex flows, slowly varying in the mean.