球模仪测试冻土松弛模量的非线性Kelvin解答及其试验研究

冻土的松弛模量是进行蠕变力学分析的关键参数。但在理论方面, 尚未基于球模仪试验建立较完善的松弛模量表征公式。基于半空间黏弹性理论和分数阶微积分理论, 获得了松弛模量公式的非线性Kelvin模型解答并进行了材料参数影响分析。在给出恒载条件下, 可以用单轴蠕变试验的分析结果评估基于球形压痕试验的冻土松弛模量来预测准确性。随后分别以冻结细砂的球模仪试验曲线、 冻结砂和冻结砂质黏土的单轴蠕变曲线为例, 预测了各条试验曲线和冻土的松弛模量, 并通过其他试验实测数据进行了检验。结果表明： 该松弛模量表征公式的材料参数最少, 随各材料参数变化具有较佳单调性; 基于球形模板试验, 非线性Kelvin解答能对冻结细砂的松弛模量和位移曲线做出合理可靠的预测。非线性Kelvin模型能很好地吻合单轴蠕变试验曲线, 基于单轴蠕变试验的冻结砂松弛模量随时间变化规律和数值范围与球模仪试验分析结果较一致, 冻结砂质黏土的松弛模量随应力和温度的变化规律再现了已有研究结论。     

On the effects of resolution in dissipationless cosmological simulations

We present a study of numerical effects in dissipationless cosmological simulations. The numerical effects are evaluated and studied by comparing the results of a series of 64³-particle simulations of varying force resolution and number of time-steps, performed using three of the N -body techniques currently used for cosmological simulations: the Particle–Mesh (PM), the Adaptive Particle–Particle–Particle–Mesh (AP³M) and the newer Adaptive Refinement Tree (ART) codes. This study can therefore be interesting both as an analysis of numerical effects and as a systematic comparison of different codes.
We find that the AP³M and the ART codes produce similar results given that convergence is reached within the code type. We also find that numerical effects may affect the high-resolution simulations in ways that have not been discussed before. In particular, our study revealed the presence of two-body scattering, the effects of which can be greatly amplified by inaccuracies in time integration. This process appears to affect the correlation function of matter, the mass function, the inner density of dark matter haloes and other statistics at scales much larger than the force resolution, although different statistics may be affected in a different fashion. We discuss the conditions for which strong two-body scattering is possible and discuss the choice of the force resolution and integration time-step. Furthermore, we discuss recent claims that simulations with force softening smaller than the mean interparticle separation are not trustworthy and argue that this claim is incorrect in general, and applies only to the phase-sensitive statistics. Our conclusion is that, depending on the choice of mass and force resolution and the integration time-step, a force resolution as small as 0.01 of the mean interparticle separation can be justified.     

Oldmetal‐rich globular cluster populations: Peak color and peak metallicity trends with mass of host spheroids

We address the problem of the factors contributing to a peak color trend of old metal‐rich globular cluster (MRGC) populations with mass of their hosts, early‐type galaxies and spheroidal subsystems of spiral ones (spheroids). The colormass trend is often converted to a metallicity‐mass trend under the assumption that age effects are small or negligible. While direct estimates of the ages of MRGC populations neither can rule out nor reliably support the populations' age trend, key data on timing of the formation of spheroids and other indirect evidence imply it in the sense: the more massive spheroid the older on average its MRGC population. We show that the contribution of an allowable age trend of the MRGC populations to their peak color trend can achieve up to ∼50% or so. In this event the comparable value of the color trend, ∼30%, is due to alpha‐element ratio systematic variations of the order of Δ[α /Fe] ≈ 0.1 to 0.2 dex because of a correlation between the [α /Fe] ratios and age. Hence a systematic variation of exactly [Fe/H] ratios may turn out to be less significant among the contributors, and its range many times lower, i.e. of the order of Δ[Fe/H]∼0.1 or even none, than the corresponding range deduced by assuming no age trend. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectroscopic studies of southern-hemisphere Cepheids: Three Cepheids in Crux (BG Cru,R Cru,and T Cru)

This paper is devoted to spectroscopic studies of three bright Cepheids (BG Cru, R Cru, and T Cru) and continues the series of our works aimed at determining the atmospheric parameters and chemical composition of southern-hemisphere Cepheids. We have studied 12 high-resolution spectra taken with the 1.9-m telescope of the South African Astronomical Observatory and the 8-m VLT telescope of the European Southern Observatory in Chile. The atmospheric parameters and chemical composition have been determined for these stars. The averaged atmospheric parameters are: T _eff = 6253 ± 30 K, log g = 2.15, V _t = 4.30 km s^?1 for BG Cru; T _eff = 5812 ± 22 K, log g = 1.65, V _t = 3.80 km s^?1 for R Cru; and T _eff = 5588 ± 21 K, log g = 1.70, V _t = 4.30 km s^?1 for T Cru. All these Cepheids exhibit a nearly solar metallicity ([Fe/H] = +0.04 dex for BG Cru, +0.06 dex for R Cru, and +0.08 dex for T Cru); the carbon, oxygen, sodium, magnesium, and aluminum abundances suggest that the objects have already passed the first dredge-up. The abundances of other elements are nearly solar. An anomalous behavior of the absorption lines of metals (neutral atoms and ions) in the atmosphere of the small-amplitude Cepheid BG Cru is pointed out. The main components in these lines split up into additional blue and red analogs that are smaller in depth and equivalent width and vary with pulsation phase. Such splitting of the absorption lines of metals (with the hydrogen lines being invariable) is known for the classical Cepheid X Sgr. The calculated nonlinear pulsation model of BG Cru with the parameters L = 2000 L _⊙, T _eff = 6180 K, and M = 4.3M _⊙ shows that this small-amplitude Cepheid pulsates in the first overtone and is close to the blue boundary of the Cepheid instability strip. According to the model, the extent of the Cepheid’s atmosphere is relatively small. Therefore, no spectroscopic manifestations of shock waves through variability are possible in this Cepheid and the observed blue and red components in metal absorption lines can be explained solely by the presence of an extended circumstellar envelope around BG Cru.     

Supersonic motions of galaxies in clusters

We study motions of galaxies in galaxy clusters formed in the concordance Λ cold dark matter cosmology. We use high-resolution cosmological simulations that follow the dynamics of dark matter and gas and include various physical processes critical for galaxy formation: gas cooling, heating and star formation. Analysing the motions of galaxies and the properties of intracluster gas in a sample of eight simulated clusters at z = 0, we study the velocity dispersion profiles of the dark matter, gas and galaxies. We measure the mean velocity of galaxy motions and gas sound speed as a function of radius and calculate the average Mach number of galaxy motions. The simulations show that galaxies, on average, move supersonically with the average Mach number of ≈1.4, approximately independent of the cluster-centric radius. The supersonic motions of galaxies may potentially provide an important source of heating for the intracluster gas by driving weak shocks and via dynamical friction, although these heating processes appear to be inefficient in our simulations. We also find that galaxies move slightly faster than the dark matter particles. The magnitude of the velocity bias,   b _v ≈ 1.1  , is, however, smaller than the bias estimated for subhaloes in dissipationless simulations. Interestingly, we find velocity bias in the tangential component of the velocity dispersion, but not in the radial component. Finally, we find significant random bulk motions of gas. The typical gas velocities are of order ≈20–30 per cent of the gas sound speed. These random motions provide about 10 per cent of the total pressure support in our simulated clusters. The non-thermal pressure support, if neglected, will bias measurements of the total mass in the hydrostatic analyses of the X-ray cluster observations.