Planetary nebulae and ALMA

Our understanding of the late evolution of intermediate mass stars (∼1–8M_⊙) through the planetary nebula phase is undergoing major developments. Observations at infrared and millimeter wavelengths have revealed important components of neutral gas and dust in the nebulae that directly trace their formation from mass-loss on the Asymptotic Giant Branch. At the same time, high resolution imaging, especially with the Hubble Space Telescope, has revealed a surprising array of structures in the nebulae: multiple arcs, tori, jets, and myriads of small scale fragments. None of these are fully understood, and all involve the neutral gas component. This paper highlights recent observations of these structures and discusses the open questions, with an emphasis on those areas where observations with ALMA are likely to make important contributions.     

Low velocity shock-cloud encounters II.

We report on a comparative study of nearby shocked clouds with and without star formation, based on IRAS, HI(21cm), CO(1-0) NH₃ (and other molecular line) observations. The dark clouds L1780 (no star formation) and L1251 (high SFE) are discussed here. Their density and velocity structure are compared with the predictions of the HD model of Horváth & Tóth (1995), Paper I.     

Observations and modeling of infrared and millimeter molecular lines towards GL2591

We have observed C₂H₂ and HCN rovibrational transitions near 13µm in absorption against GL2591. We also have observed rotational transitions at 0.6-3 mm of CS, HCN, H₂CO, and HCO⁺. Analysis of the rotational lines, which arise in the extended cloud around the source, shows that no single density model can explain all the data. Models with density and temperature gradients do much better; in particular models withn(r) r ^–1.5 can reproduce the observed pattern of emission line strengths. The abundances show significant depletion compared to models of gas-phase chemistry. The rovibrational data were analyzed in comparison to the absorption line analysis of CO by Mitchellet al. (1989). Our data are consistent with the C₂H₂ and HCN absorption arising in the same warm (200 K) and hot (1010 K) components seen in CO, but we see little evidence for the cold (38 K) component seen in CO. The rovibrational lines from higher states (J 21) indicate that the hot HCN deviates from LTE, leading to a density of about 3 × 10⁷ cm^–3. Comparison of the two data sets shows that the rovibrational absorption of HCN, rather than arising in the extended envelope, must come from a region with a small angular extent. A model in which early-time gas phase abundances are preserved on grain mantles and released at high temperature can explain the data.     

Magnetic fields in protoplanetary disks

Magnetic fields likely play a key role in the dynamics and evolution of protoplanetary disks. They have the potential to efficiently transport angular momentum by MHD turbulence or via the magnetocentrifugal acceleration of outflows from the disk surface. Magnetically-driven mixing has implications for disk chemistry and evolution of the grain population, and the effective viscous response of the disk determines whether planets migrate inwards or outwards. However, the weak ionisation of protoplanetary disks means that magnetic fields may not be able to effectively couple to the matter. I examine the magnetic diffusivity in a minimum solar nebula model and present calculations of the ionisation equilibrium and magnetic diffusivity as a function of height from the disk midplane at radii of 1 and 5 AU. Dust grains tend to suppress magnetic coupling by soaking up electrons and ions from the gas phase and reducing the conductivity of the gas by many orders of magnitude. However, once grains have grown to a few microns in size their effect starts to wane and magnetic fields can begin to couple to the gas even at the disk midplane. Because ions are generally decoupled from the magnetic field by neutral collisions while electrons are not, the Hall effect tends to dominate the diffusion of the magnetic field when it is able to partially couple to the gas, except at the disk surfaces where the low density of neutrals permits the ions to remain attached to the field lines. For a standard population of 0.1 μm grains the active surface layers have a combined column Σ_active≈2 g cm⁻² at 1 AU; by the time grains have aggregated to 3 μm, Σ_active≈80 g cm⁻². Ionisation in the active layers is dominated by stellar X-rays. In the absence of grains, X-rays maintain magnetic coupling to 10% of the disk material at 1 AU (i.e. Σ_active≈150 g cm⁻²). At 5 AU the Σ_active≈Σ_total once grains have aggregated to 1 μm in size.     

水化钠蒙脱石微观物理力学参数的分子动力学模拟

钠蒙脱石是一种层状黏土矿物, 可使用分子动力学方法来模拟这一矿物的水化性质和微观物理力学参数。本文通过建立所含水分子数不同的钠蒙脱石模型, 分析了能量优化前后钠蒙脱石晶胞参数和能量的变化, 研究了不同含水量和温度条件下钠蒙脱石体积与密度的变化情况, 使用径向分布函数研究了钠蒙脱石的水化性质, 得到了水分子数不同情况下钠蒙脱石的物理力学参数。结果表明, 与优化前相比, 钠蒙脱石晶胞形状在能量优化后变得更加不规则, 优化后, 钠蒙脱石中的扭曲能有所上升, 其他形式能量和总能量都有不同程度地下降, 随着水分子数的不断增多, 钠蒙脱石体积逐渐增大, 密度逐渐减小, 环境温度上升时, 钠蒙脱石体积增大, 密度减小, 钠蒙脱石的水化性质、物理力学参数与水分子数有关。     

利用钾长石粉体水热合成13X沸石分子筛的晶化过程

实验研究了以钾长石粉体为原料水热合成13X沸石分子筛的晶化过程,确定了晶化过程的诱导期、晶化期和沸石晶体的平均生长速率。以钾长石焙烧熟料为前驱物合成13X沸石,反应混合物首先转变为铝硅酸钠（钾）凝胶,再逐步水热晶化为13X沸石;在反应物未完全转变为铝硅酸钠（钾）凝胶之前,13X沸石已开始结晶。13X沸石晶体的生成主要发生在凝胶相内部,是凝胶结构逐步趋于有序化的过程。13X沸石晶体生成是由凝胶相内的[TO₄]四面体相互连接,互套构成笼状结构。推测13X沸石形成机理为：硅酸根离子和铝酸根（+铁酸根）离子发生聚合反应,生成次级结构单元双六元环;双六元环进一步缩合,生成方钠石型笼;最后,方钠石型笼进一步相互联结,生成13X型沸石分子筛的硅铝骨架结构。     

鱼类生长激素的分子生物学和应用研究的进展

依据１９８５－１９９５年国际有关资料对鱼类生长激素的分子生物学及其研究的最新成果与进展进行综合评述。研究表明，已完成鳗鲡等９种鱼类生长激素的氨基酸组成与全序列分析及大麻哈鱼等２０种鱼类的生长激素基因的分离与克隆，有些已在工程菌或动物细胞中的高效表达；鱼类生长激素基因的调控机制与生长激素的活性部位研究正在深入进行；将外源的生长上入鱼体或将外源的生长激素基因导入受体鱼，以期促进生长，增加抗性，是生长激     

海湾扇贝4次引种后代的表型特征和遗传分化

分析了不同时期引入到我国的4个繁育历史清晰、未发生种质混杂的海湾扇贝养殖群体成体的壳形态指标和群体遗传结构。壳表型参数分析表明,海湾扇贝北部亚种(Northernbay scallop,Argopecten irradians irradians)的浙江养殖群体(ZJ)、加拿大新布朗斯威克群体(NB)、马萨诸塞-弗吉尼亚混合群体(M-V)及海湾扇贝南部亚种(Southern bay scallop,Arg-opecten irradians concentricus)的北卡罗来那群体(NC)等不同群体间在壳形态方面存在显著差异南部亚种的NC群体壳高与壳长基本相等[壳高与壳长的比值(H/L)达0.99,壳型宽壳宽和壳长的比值(W/L)达到0.59];而北部亚种3个群体的壳长均大于壳高,其中ZJ和NB群体的壳形态尤其偏长,其H/L分别是0.92和0.90,NB群体的壳宽显著小于其他群体,W/L仅为0.36。RAPD分析结果表明,ZJ、NB、M-V、NC群体的多态位点比例分别为31.82%、29.55%、28.79%和31.82%,平均杂合度分别为0.1078、0.1134、0.0966和0.1197,表明在我国独立繁育了19代的ZJ养殖群体仍然保持了与其他群体相近的遗传多样性水平。4个群体间的遗传分化系数(Gst)为0.1892,表明其中18.92%的变异来自不同群体间的遗传差异,说明4个群体在遗传上存在着较大的分化。从遗传距离分析,ZJ与NB较近,两者间的遗传距离为0.0319,而两者与M-V间的遗传距离相对较远,分别为0.0442和0.0524;NC与M-V间的遗传距离为0.0368。这些结果表明,从原产地引种到国外时间较早的ZJ、NB等2个群体与较晚引进我国的M-V群体之间的遗传距离已超过了M-V群体(北部亚种)与NC群体(南部亚种)之间的遗传距离,说明独立繁育了近20代后,ZJ群体和NB群体均已产生了较大的遗传分化,这种分化可能有利于海湾扇贝的品种培育。     

应变-水位相关斜率异常与地震的关系研究

利用回归分析求解乌鲁木齐台同一钻孔内应变和水位的相关斜率,并用筛选法方差判定其异常,研究相关斜率和地震的关系。结果表明： ①４ 个元件同时出现相关斜率异常时有震率高;②相关斜率正异常映震效果好;③相关斜率属短临异常范畴;④相关斜率异常映震能力具有地区差异性。     

AFM observations and simulations of jarosite growth at the molecular scale: probing the basis for the incorporation of foreign ions into jarosite as a storage mineral

 The growth of K-jarosite was evaluated using scanning electron microscopy (SEM), atomic force microscopy (AFM), and molecular simulations. SEM micrographs show crystals with almost quadratic and, to a minor extent, triangular faces. These were identified to be {0 1 2} and (0 0 1) faces, respectively, using an X-ray single-crystal diffractometer. These results are in agreement with molecular simulations that show that the dipole-free {0 1 2} face is the most stable surface, followed by the (0 0 1) surface, that can lower its dipole moment perpendicular to the surface during relaxation. On the {0 1 2} face, the most stable step directions were identified. From these, an idealized equilibrium growth island can be constructed when the corner energies are minimized by the successive removal of corner ions until a stoichiometric growth island is obtained. Such an equilibrium growth island is the basis for developing a spiral growth model that explains the mechanism of spiral formation as observed using AFM. Furthermore, reactive sites that are potential candidates for the incorporation of foreign ions can be located. Received: 2 January 2001 / Accepted: 30 April 2001