A 98 per cent spectroscopically complete sample of the most powerful equatorial radio sources at 408 MHz     

P. N. Best  H. J. A. Röttgering  M. D. Lehnert 《Monthly notices of the Royal Astronomical Society》1999,310(1):223-254

A new sample of very powerful radio galaxies is defined from the Molonglo Reference Catalogue, according to the criteria S _408 MHz>5 Jy, −30°≤ δ ≤10° and | b |≥10°. The sample is selected to have similar properties to the northern 3CR revised sample, and to be visible to a combination of existing northern telescopes such as the Very Large Array radio interferometer and large southern hemisphere telescope facilities. The sample contains 178 sources, of which spectroscopic redshifts are available in the literature for 128. For the remaining 50 sources, new radio imaging, optical imaging and spectroscopic observations are presented to identify the host galaxies and determine their redshifts. With these new observations the total sample is 100 per cent optically identified and redshifts are available for 174 (98 per cent) of the sources. The sample consists of one starburst galaxy, one Seyfert galaxy, 127 radio galaxies and 49 quasars. Basic properties of the sample, such as the distributions of the quasar and radio-galaxy populations in redshift and their locations on the radio power versus linear size ( P − D ) diagram, show no significant differences from the revised 3CR sample. The equatorial location and the high spectroscopic completeness of this sample make it a valuable resource for detailed studies of the nature and environments of these important objects with the new generation of southern hemisphere telescopes.  相似文献   

On the class of Oe stars'     

I. Negueruela  I. A. Steele  G. Bernabeu 《Astronomische Nachrichten》2004,325(9):749-760

We present high‐quality spectra of the majority of stars that have been classified as Oe and find that their published spectral types are generally too early, most likely due to infilling of He I lines. As a matter of fact, all stars classified as Oe actually fall inside the range O9–B0 with the important exception of HD 155806 (O7.5 III) and perhaps HD 39680 (difficult to classify, but likely O8.5V). Observations of a sample of objects with published spectral types in the O9–B0 range previously classified as peculiar or emission‐line stars fail to reveal any new Oe star with spectral type earlier than O9.5. Most objects classified as peculiar in “classical” literature show signs of binarity in our spectra, but no spectral anomalies. We conclude that there is likely a real decline in the fraction of Be stars for spectral types earlier than B0, not due to observational bias. The few Oe stars with spectral types earlier than O9.5 deserve detailed investigation in order to provide constraints on the physical reasons of the Be phenomenon. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

基于二次多项式拟合的超短基线声线修正方法     

罗宇  程梦迪  李倩倩  宋熙昭  施剑 《海洋通报》2023,(1):42-47

超短基线定位解算中的距离观测值是指换能器与水下应答器之间的直线距离,而海水声速的不均匀分布导致声波在海水中的实际传播路径为连续弯曲的曲线,需要结合实测声速剖面进行声线修正。根据声速在分层介质中的传播特性,本文提出了一种基于二次多项式拟合的声线跟踪算法,采用线性插值方法对声速剖面数据进行合理加密并按等深度进行分层,设定每层声速梯度是不断变化的,用二次多项式拟合声速,基于运动学原理建立了完整的数学解算模型。仿真结果表明,该方法修正后的水下目标分布具有明显的收敛性,且优于等梯度声线跟踪算法和等效声速剖面法,显著提高了超短基线水声定位系统的定位精度。  相似文献   

太阳和太阳系等离子体研究专辑:序言          下载免费PDF全文

陈鹏飞  吴德金 《天文学报》2023,(3):23-5

宇宙中超过99.9%的可见物质处于等离子体状态,等离子天体物理是天体物理的重要分支,为理解天体系统的形成、演化及爆发现象提供着重要的理论基础.专辑通过14篇文章系统介绍了中国科学院紫金山天文台等离子天体物理团队在太阳和太阳系等离子体方面的研究成果,希望能帮助读者全面了解太阳与日球等离子体物理研究的重要进展及存在的问题.  相似文献   

基于等效声速剖面法的多波束测深系统声线折射改正技术   总被引：6，自引：2，他引：4  

丁继胜  周兴华  唐秋华  刘忠臣  陈义兰 《海洋测绘》2004,24(6):27-29

多波束测深技术是近几年比较流行的海洋勘测手段之一,其具有全覆盖、无遗漏、高精度和高效率等特点。多波束测深系统的声学原理和海水的不均匀性,使得声波在传播过程中发生声线的折射现象,对波束测点的最终位置的归算带来较大的误差。针对多波束数据后处理方法,介绍了利用等效声速剖面法对多波束测深系统的声线折射进行改正的一种新方法,并通过试验对比,证明了该方法可以提高多波束测深系统的整体测量精度。  相似文献   

ADCP对悬浮沉积物浓度的测量及其误差分析研究   总被引：6，自引：0，他引：6  

兰志刚  龚德俊  李思忍  徐永平  秦枫  姜静波 《海洋科学》2004,28(10):20-23

为实时、连续地测量海水中悬浮沉积物,利用声学多普勒海流剖面仪(ADCP),可以在测流的同时,测量海洋悬浮沉积物浓度。描述了利用后散射强度,估计悬浮沉积物浓度的原理。在现场测试中,利用采水器采集水样,通过水样分析并结合声信号的衰减特征对AI)CP的测量结果进行修正。作者还结合现场测试结果,对这一测量方法的精度进行了分析。现场实验表明,经公式校正后,ADCP测得的后散射强度与实测水样所获得的悬浮沉积物浓度之间有很好的相关性,相关系数达0．88,由此证明,此测量方法具有很好的实用价值和应用前景。  相似文献   

自然火灾碳排放估算模型参数的遥感反演进展   总被引：1，自引：1，他引：1  

吴沁淳  陈方  王长林  李斌  吴薇  刘三超  徐丰 《遥感学报》2016,20(1):11-26

植被燃烧会产生大量的温室气体,为及时了解自然火灾对区域和全球尺度的碳循环和碳平衡模式的影响,有必要研究自然火灾碳排放的大尺度估算方法。结合遥感技术"宏观、快速、实时、客观"获取地表信息的独特优势,对利用遥感技术估算自然火灾碳排放的方法进行综述,概括了自然火灾碳排放使用的估算模型及其需要的主要输入参数,其次分别介绍了模型输入参数的遥感估算方法,并对各方法的优缺点进行分析和评价,最后提出现有方法的改进建议。  相似文献   

分米波毫秒级太阳射电爆发的几个形态     

胡汉明  龚元芳  尚琼珍  吕松泉  李永生 《天文研究与技术》1998,(1)

介绍使用１０ｍ天线上２１ｃｍ波段ｍｓ级连续快速记录系统,观测到分米波太阳射电快速活动的几种形态、特征、尖峰辐射及其共生现象。  相似文献   

The design of visco‐acoustic weighted L1‐error frequency–space wavefield extrapolators          下载免费PDF全文

Syed Abdul Salam  Wail A. Mousa 《Geophysical Prospecting》2018,66(2):300-310

Seismic imaging is an important step for imaging the subsurface structures of the Earth. One of the attractive domains for seismic imaging is explicit frequency–space (f – x) prestack depth migration. So far, this domain focused on migrating seismic data in acoustic media, but very little work assumed visco‐acoustic media. In reality, seismic exploration data amplitudes suffer from attenuation. To tackle the problem of attenuation, new operators are required, which compensates for it. We propose the weighted L ₁ ‐error minimisation technique to design visco‐acoustic f – x wavefield extrapolators. The L ₁ ‐error wavenumber responses provide superior extrapolator designs as compared with the previously designed equiripple L ₄ ‐norm and L_∞‐norm extrapolation wavenumber responses. To verify the new compensating designs, prestack depth migration is performed on the challenging Marmousi model dataset. A reference migrated section is obtained using non‐compensating f – x extrapolators on an acoustic dataset. Then, both compensating and non‐compensating extrapolators are applied to a visco‐acoustic dataset, and both migrated sections are then compared. The final images show that the proposed weighted L ₁ ‐error method enhances the resolution and results in practically stable images.  相似文献   

Imaging shallow structure with active-source surface wave signal recorded by distributed acoustic sensing arrays          下载免费PDF全文

Zhenghong Song  Xiangfang Zeng  Clifford H. Thurber  Hebert F. Wang  Dante Fratta 《地震科学（英文版）》2018,31(4):208-214

Distributed acoustic sensing (DAS) is one recently developed seismic acquisition technique that is based on fiber-optic sensing. DAS provides dense spatial spacing that is useful to image shallow structure with surface waves. To test the feasibility of DAS in shallow structure imaging, the PoroTomo team conducted a DAS experiment with the vibroseis truck T-Rex in Brady’s Hot Springs, Nevada, USA. The Rayleigh waves excited by the vertical mode of the vibroseis truck were analyzed with the Multichannel Analysis of Surface Waves (MASW) method. Phase velocities between 5 and 20 Hz were successfully extracted for one segment of cable and were employed to build a shear-wave velocity model for the top 50 meters. The dispersion curves obtained with DAS agree well with the ones extracted from co-located geophones data and from the passive source Noise Correlation Functions (NCF). Comparing to the co-located geophone array, the higher sensor density that DAS arrays provides help reducing aliasing in dispersion analysis, and separating different surface wave modes. This study demonstrates the feasibility and advantage of DAS in imaging shallow structure with surface waves.  相似文献