塔里木河上游胡杨群落及种群特征分析

基于样地调查方法对塔里木河上游阿拉尔、新其满、英巴扎断面河岸胡杨群落特征及胡杨种群结构与侧枝生长量进行了研究和分析。结果表明:①塔里木河上游河岸胡杨群落物种组成较简单,共有16种植物,分属10科16属,其中乔木1种、灌木和草本分别为6种和9种。3个断面发现的物种种数分别为6种、8种和12种,共有种为胡杨(Populus euphratica)、多枝柽柳(Tamarix ramosissma)和铃铛刺(Halimodendron halodendron)。②各断面胡杨群落物种多样性指数较低,物种丰富度指数、Simpson指数和Shannon-Winner指数大小顺序为英巴扎,新其满,阿拉尔。③胡杨种群主要以幼龄个体为主,种群树高、胸径为底宽上窄增长型结构。④1998—2002年各断面胡杨枝轮生长量波动较大,2003—2007年生长量呈下降趋势。     

CO observations of semiregular variables

O-rich SRa and SRb variables have been observed in the¹²CO(1-0) and (2-1) lines. A total of 40 stars were observed and 19 detections can be reported. The majority of the objects are weak in CO and have envelopes with small expansion velocities between 1.9 to 15.6 km/s. No correlation is found between period and expansion velocity for the short period objects whereas a trend may be present for the long period ones.Supported by the AustrianFWF, project number P 9638-AST. Based on observations at ESO, Chile and Onsala Space Observatory, Chalmers Tekniska Högskola, Sweden. This research has made use of SIMBAD, CDS, Strasbourg, France.     

Evolution of Population II stars

The current knowledge of the evolution of Population II stars, as observed in galactic halos and globular clusters, is outlined. The recent theoretical results provided by an improved physical understanding of the stars are reported, with a particular emphasis upon those stellar evolutionary phases which are the keystones of the interpretation of globular cluster stars. Within the up–to–date theoretical scenario, the luminosity of the turn–off stars and horizontal branch stars, which are of fundamental importance for distance and age determinations, turns out to be fainter and brighter, respectively, in comparison with previous theoretical computations. The predicted absolute magnitude of RR Lyrae stars is consistent with the bright values suggested from the long distance scale (Sandage 1993), but the slope of the relation between and metallicity ([Fe/H]) agrees with that suggested from the short distance scale (see, e.g., Carney et al. 1992, Clementini et al. 1995), at least with [Fe/H]-1.3. As to the globular cluster ages, the new computations provide younger ages ( 10 Gyr–13 Gyr), weakening the conflict with current cosmological estimates. The recent results derived from the fitting of HIPPARCOS parallaxes of field subdwarfs to the fiducial main sequence of globular clusters are consistent with the up–to–date theoretical models. However, the direct determinations of , which are based on the HIPPARCOS proper motions and trigonometric parallaxes of field RR Lyrae stars, give fainter absolute magnitudes than those obtained from the improved evolutionary models. This seems to suggest that the problem of the globular cluster distance and age is still open. Received 6 April 1998     

A new, complete sample of faint B stars in the Galactic halo

A new sample of 31 faint B and A0 stars is reported, 30 of which comprise a complete sample within the limits ( U − V )<0 and 10.0< B ≲18.0. The sample is based on low- and intermediate-resolution spectrophotometry of colour-excess objects selected in the US survey. Atmospheric parameters for the stars are derived through the use of synthetic colours, Balmer-line strengths, and model-atmosphere fitting. The atmospheric parameters and preliminary metallicity estimates indicate that most of the stars are distributed along the blue horizontal branch, with low metallicities ([Fe/H]∼−1.0) and with both the first and second Newell gaps present. However, nine of the B/A0 stars can be identified as candidate main-sequence stars, based on evidence of high metallicities ([Fe/H]∼0) and/or derived effective temperatures and surface gravities which place them close to the main-sequence relation. The completeness characteristics of the sample are discussed, and its surface density is compared to that of other recently isolated B-star samples. The sample exhibits a shallow integral number-count slope. This new sample will help provide increased statistical coverage of the B-star population in the Galactic halo through its relatively faint magnitude-completeness limits and its relatively red colour-completeness limit.     

The absolute magnitude of field metal-poor horizontal branch stars

Hipparcos satellite parallaxes for 22 metal-poor field horizontal branch stars with V ₀<9 are used to derive their absolute magnitude. The weighted mean value is M_V =+0.69±0.10 for an average metallicity of [Fe/H]=−1.41; a somewhat brighter average magnitude of M_V =+0.60±0.12 for an average metallicity of [Fe/H]=−1.51 is obtained by eliminating HD 17072, which might be on the first ascent of the giant branch rather than on the horizontal branch. The present values agree with the determinations based on proper motions and application of the Baade–Wesselink method to field RR Lyraes; they are 0.1–0.2 mag fainter than those based on calibration of cluster distances obtained by using local subdwarfs and on alternative distance calibrators for the Large Magellanic Cloud (LMC). The possibility that there is a real difference between the luminosity of the horizontal branch for clusters and the field is briefly commented on.     

The density distribution of the Galactic stellar halo as traced by blue horizontal branch stars

Data of blue horizontal branch (BHB) stars and RR Lyrae variable stars from the literature are combined with unpublished observations of BHB stars in five fields. A flattened power law is used to model the spatial distribution of the horizontal branch stars. Completeness of the data sample and contamination by blue stragglers and metal-rich main-sequence A stars are considered, and taken into account. Using a maximum-likelihood method, the following best-fitting parameters are obtained: a power-law index α=−3.2±0.3 and an axial ratio of q =0.52±0.11 for the isodensity surfaces. From the fit a value for the local density of BHB stars of ρ₀=26⁺²⁰₋₁₁ kpc⁻³ is found. The values of the three parameters are in complete agreement with recent determinations by other authors.     

Numerical simulation of the upper ocean currents in South China Sea

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Decadal Current Variations in the Southwestern Japan/East Sea

Absolute geostrophic velocities were calculated along TOPEX/Poseidon (T/P) groundtracks located in the Ulleung Basin of the southwestern Japan/East Sea (JES) from a combined analysis of nearly a decade of T/P data and two years of pressure-gauge-equipped inverted echo sounder (PIES) data obtained during the United States Office of Naval Research’s JES Program. Geostrophic velocities have been calculated daily for the Ulleung Basin from June 1999 to July 2001 from a three-dimensional mapping of temperature and salinity produced by PIES data interpreted via the Gravest Empirical Mode (GEM) technique combined with the Navy’s Modular Ocean Data Assimilation System (MODAS). These velocities were then used to convert T/P velocity anomalies to absolute velocities for the T/P time period of 1993 to 2002. Current intensities and variabilities associated with the East Korean Warm Current, Ulleung Warm Eddy, and Offshore Branch are examined. Spatial and temporal variations of the sea surface circulation are strong. Intensification of the currents generally occurred during the fall season. The flow pattern in individual years differed greatly from year to year and differed from climatology in important qualitative ways.     

长江口水下三角洲沉积物粒度组成及其在减沙背景下的响应

近年来长江口水下三角洲对入海泥沙减少的响应问题越来越受到关注。通过分析长江水下三角洲百年沉积速率的分布特征及位于水下三角洲泥质沉积区不同位置4个浅孔的粒度、~(210)Pb和~(137)Cs数据,探讨其对流域入海水沙变化及水动力变化的响应。研究结果显示,1954年后长江口北支萎缩是导致CDZS10孔所在区域沉积物粒度较其他3孔明显粗的主要原因。而CDZS10孔沉积物上粗下细则是由20世纪80年代三峡蓄水后长江入海泥沙显著下降、海洋动力明显增强引起的。另外,北支萎缩及三峡水库调蓄作用导致泥质区南移,并向杭州湾和嵊泗列岛延伸发展。     

Experimental investigation on the flow induced vibration of an equilateral triangle prism in water

A series of flow induced vibration (FIV) experiments for an equilateral triangle prism elastically mounted in a water channel are performed with different system stiffness at constant damping and mass. An amplitude variation coefficient is proposed to describe FIV stationarity in the present study. The FIV of the prism can be divided into three primary regions based on the amplitude and frequency responses, which are the vortex induced vibration (VIV) branch, the transition branch from VIV to galloping, and the galloping branch. The transition branch occurs at the reduced velocity in the range of 7.8 < U_r = U/(f_n,air·D) < 10.4, accompanied with a relatively rapid increase in amplitude and a precipitous drop in frequency and vibration stationarity. In addition, the reduced velocity where the transition region is initiated is independent of the system stiffness. The maximum amplitude reaches 3.17 D in the galloping branch. The ratio of the response frequency to the natural frequency of the prism in air remains locked to approximately 0.65 throughout the fully developed galloping branch. Large amplitude responses in an infinite range of flow velocities, excellent vibration stationarity and steady vibration frequencies, which are characteristics of the galloping of the prism, have a positive impact on improving energy conversion.