Surface inhomogeneities and line variability on DF Tau

We have mapped surface inhomogeneities on the classical T Tauri star DF Tau, using the Li  i doublet at 670.8 nm, the Ca  i lines at 612.2 nm and 643.9 nm and a calcium and iron blend at 646.3 nm. We find compelling evidence that there are hotspots with temperatures of more than 5000 K. Two of the hotspots produce line-profile deformations that can be traced as they move through the cross-correlated profiles. When one of the hotspots crosses the stellar disc, redshifted absorption components appear in the Na D lines. As these redshifted absorption features are usually tracers for mass-infall we interpret this hotspot as an accretion shock close to the stellar surface.   Parts of the surface of DF Tau are covered with a hot chromosphere that is visible in the Ca  ii infrared triplet lines and the narrow component of He  i . We find no correlation between the veiling and the lines that originate from the hot chromosphere, suggesting that the veiling and the chromospheric emission are produced in physically distinct regions.     

Large-amplitude periodic variations in the angular diameter of R Leonis

We report the first direct detection of long-term periodic diameter variations in a Mira variable. Angular diameter measurements of the 313-d period variable R Leonis at 833 nm and 940 nm obtained between 1996 February and 1997 June using the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WHT) show a cyclic modulation of the apparent stellar diameter by approximately 35 per cent. The agreement between these new data and archival measurements from 1992 January suggests coherence in the modulation over a 5-yr period. Our data are consistent with recent models which suggest that, in photometric bands with only weak to moderate molecular contamination, periodic variations in stellar diameter of order 50 per cent can be maintained. The measurements indicate that the apparent stellar diameter was largest at visual phase 0.5 and that any phase shifts between the visual light curve and those at 833 and 940 nm were at most 0.05. The large offset (∼ 0.25) between the phase of the observed diameter maximum and that predicted for the photospheric continuum diameter variations suggests that our observations are more sensitive to the changing temperature structure of the outer atmosphere than to the deeper continuum-forming layers.     

A low frequency radio telescope at mauritius for a southern sky survey

A new, meter-wave radio telescope has been built in the north-east of Mauritius, an island in the Indian Ocean, at a latitude of -20.14‡. The Mauritius Radio Telescope (MRT) is a Fourier Synthesis T-shaped array, consisting of a 2048 m long East-West arm and an 880 m long South arm. In the East-West arm 1024 fixed helices are arranged in 32 groups and in the South arm 16 trolleys, with four helices on each, which move on a rail are used. A 512-channel digital complex correlation receiver is used to measure the visibility function. At least 60 days of observing are required for obtaining the visibilities up to 880 m spacing. The Fourier transform of the calibrated visibilities produces a map of the area of the sky under observation with a synthesized beam width 4′ × 4.6′ sec(δ + 20.14‡) at 151.5 MHz. The primary objective of the telescope is to produce a sky survey in the declination range –70‡ to –10‡ with a point source sensitivity of about 200 mJy (3a level). This will be the southern sky equivalent of the Cambridge 6C survey. In this paper we describe the telescope, discuss the array design and the calibration techniques used, and present a map made using the telescope.     

Chromatic and spectroscopic signatures of microlensing events as a tool for the gravitational imaging of stars

The detection of microlensing events from stars in the Large Magellanic Cloud and in the Galactic bulge raises important constraints on the distribution of dark matter and on galactic structure, although some events may be the result of a new type of intrinsic variability. When lenses are relatively close to the sources, we predict that chromatic and spectroscopic effects are likely to appear for a significant fraction of the microlensing events. These effects are due to the differential amplification of the limb and the centre of the stellar disc, and present a systematic dependence with wavelength and time that provides an unambiguous signature of a microlensing event (as opposed to a new type of intrinsic stellar variability). We present detailed predictions of the effects, using realistic model atmospheres. The observations of these effects provide a direct constraint on stellar atmospheres, allowing a three-dimensional reconstruction or imaging of its structure, a unique tool with which to test the current models of stellar atmospheres.     

玲珑金矿床矿物晶格畸变的成因

利用高分辨透射电子显微镜,结合能谱仪观察了２０ｎｍ左右的胶态Ａｕ,在黄铁矿的［００１］方向析出。固体原子象显示出Ａｕ的附近由于晶格畸变而空隙度较大。     

二维介质中的空间变量分离速度成像技术

二维介质速度成像的新方法是基于地震波的旅行时间，由射线路径沿线的速度分布特征所决定的，将二维介质中的速度分布用两个分别以二维空间坐标为自变量的勒让德多项式的线性组合来描述。根据走时积分方程，即可建立地震波走时与其射线路径沿线的速度分布之间的对应关系，进而重建出地震波射线覆盖区域的速度结构。该成像技术是一个迭代过程，且适用于折射波及反射波。     

大地电磁场成像方法综述与新进展

对电磁场成像技术进行了系统总结，分析阐述了大地电磁场成像技术的研究现状与存在问题，并对各种方法的特点进行了比较和归纳，进而在Zhdanov等对偏移电磁场的研究和成像技术基础上提出了改进的有限差分法大地电磁场偏移成像技术，该方法提高了差分方程的精度和成像分辨率，并对客观选取背景电导率、两种极化模式联合成像、多参数和再次成像以及全频段成像和成像结合反演技术进行了研究。     

地下岩层裂隙的几种勘察研究方法

本文介绍了地下岩层裂隙的4种勘察研究方法:钻孔岩芯法、超声成像法、钻孔电视法、钻孔流量测井法。经过多项工程实践,我们认为超声成像法对钻孔孔壁扫描获得裂隙产状、开度等参数是研究地下岩层裂隙的有效方法。钻孔流量测井法也可研究裂隙,计算裂隙开度。     

Electric imaging and laboratory resistivity testing for geotechnical investigation of Pusan clay deposits

Large-scale reclamation works for new land development in the Nakdong River plain have been extensively carried out on soft clays. Several geotechnical characteristics of the clays could not be well evaluated, partly due to easy disturbance during drilling and sampling. Consequently, geophysical methods, seen as nondestructive testing tools, have been applied in geotechnical investigation of Pusan clays for the first time. In this study, the 2D electric imaging technique was employed to map the thick soft clay deposits in four reclamation sites. The Pusan clay deposit was very well mapped. Electric resistivity of Pusan clays was measured on over 50 core samples in the laboratory, and then correlated with other geotechnical parameters such as salinity, organic content, water content, plasticity, unit weight and sampling depth. Additionally, electric resistivity of about 20 natural clays collected worldwide was measured and compared to that of the Pusan clays as an initial effort in creating a database of clay electric resistivity to help further application of electric imaging in geotechnical investigation of clayey soils.