The Variability and Radial Velocity of Planetary Nebula Central Stars

The extremely accurate estimates of stellar variability and radial velocity in the Gaia Data Release 3(Gaia DR3)have enabled us to examine the close binarity and radial velocity(RV)of central stars(CSs)of planetary nebulae(PNe).This study is twofold:(1)searching for new close binary CS candidates to better understand how binarity affects the formation and evolution of PNe;and(2)extending the sample size of known RVs of PNe in order to understand their kinematics and the dynamics of the Milky Way.As a target sample,we used all true,possible,and likely PNe available in the literature.Then,we looked for their matched Gaia DR3 sources that provide measurements of variability and RV.As a result,we detected the first large collection of trustworthy photometric variability of 26 symbiotic stars and 82 CSs.In this CS group,there are 24 sources already classified as true close binary CSs in the literature.Hence,we discovered 58 new close binary CS candidates.This close binary(CB)sample represents more than half of what is currently available in the literature.In addition,we identified the radial velocities for 51 PNe.To our knowledge,24 of these were measured for the first time.The RV measurements predicted by Gaia,based on the Doppler shift of the CS absorption lines,and those derived from nebular emission lines,show satisfactory agreement except for a few extremely high-velocity PNe.

Effects of rotation and tidal distortions on the shapes of radial velocity curves of polytropic models of pulsating variable stars

Anharmonic oscillations of rotating stars have been studied by various authors in literature to explain the observed features of certain variable stars. However, there is no study available in literature that has discussed the combined effect of rotation and tidal distortions on the anharmonic oscillations of stars. In this paper, we have created a model to determine the effect of rotation and tidal distortions on the anharmonic radial oscillations associated with various polytropic models of pulsating variable stars. For this study we have used the theory of Rosseland to obtain the anharmonic pulsation equation for rotationally and tidally distorted polytropic models of pulsating variable stars. The main objective of this study is to investigate the effect of rotation and tidal distortions on the shapes of the radial velocity curves for rotationally and tidally distorted polytropic models of pulsating variable stars. The results of the present study show that the rotational effects cause more deviations in the shapes of radial velocity curves of pulsating variable stars as compared to tidal effects.

Effect of interaction of the various modes on the radial velocity curves of the polytropic models of rotationally and tidally distorted pulsating variable stars

In our previous work, we developed a model to study the effects of rotation and/or tidal distortions on anharmonic radial oscillations and hence on the radial velocity curves of the polytropic models of pulsating variable stars.We considered the first three modes(fundamental and the next two higher modes) for the polytropic models of index 1.5 and 3.0 in that work.In the present paper, we are further extending our previous work to study the effect of the interaction of various modes on anharmonic radial oscillations and hence on radial velocity curves of the rotationally and/or tidally distorted polytropic models of pulsating variable stars.For this purpose, we have considered the following cases:(i) fundamental mode(ii) fundamental and the first mode,(iii) fundamental and the next two modes and finally(iv) fundamental and the next three higher modes of pulsation in our study.The objective of this paper is also to investigate whether the interaction of various modes affects the results of our previous study or not.The results of this study show that the interaction of the fundamental mode with higher modes appreciably changes the shape of the radial velocity curve of rotationally distorted and rotationally and tidally distorted polytropic models of pulsating variable stars.

Radial velocity measurements from LAMOST medium-resolution spectroscopic observations: a pointing towards the Kepler field

Radial velocity is one of the key measurements in understanding the fundamental properties of stars, stellar clusters and the Galaxy. A plate of stars in the Kepler field was observed in May of 2018 with the medium-resolution spectrographs of LAMOST, aiming to test the performance of this new system which is the upgraded equipment of LAMOST after the first five-year regular survey. We present our analysis on the radial velocity measurements(RVs) derived from these data. The results show that slight and significant systematic errors exist among the RVs obtained from the spectra collected by different spectrographs and exposures, respectively. After correcting the systematic errors with different techniques, the precision of RVs reaches ～1.3,～1.0,～0.5 and ～0.3 km s^(-1) at S/Nr = 10, 20, 50 and 100, respectively. Comparing with the RVs of standard stars from the APOGEE survey, our RVs are calibrated with a zero-point shift of～7 km s^(-1). The results indicate that the LAMOST medium-resolution spectroscopic system may provide RVs with a reasonable accuracy and precision for the selected targets.

The relative calibration of radial velocity for LAMOST medium resolution stellar spectra

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST)started a medianresolution spectroscopic(MRS,R~7500)survey since October 2018.The main scientific goals of MRS,including binary stars,pulsators and other variable stars,were launched with a time-domain spectroscopic survey.However,the systematic errors,including the bias induced from wavelength calibration and the systematic difference between different spectrographs,have to be carefully considered during radial velocity measurement.In this work,we provide a technique to correct the systematics in the wavelength calibration based on the relative radial velocity measurements from LAMOST MRS spectra.We show that,for the stars with multi-epoch spectra,the systematic bias which is induced from the exposures on different nights can be corrected well for LAMOST MRS in each spectrograph.In addition,the precision of radial velocity zero-point of multi-epoch time-domain observations reaches below 0.5 km s^(-1).As a by-product,we also give the constant star candidates^(**),which can be the secondary radial-velocity standard star candidates of LAMOST MRS time-domain surveys.

Novel method for radial velocity difference estimation of moving targets with wideband signals

For the frequency difference of arrival （FDOA） esti-mation in passive location, this paper transforms the frequency difference estimation into the radial velocity difference estimation, which is difficult to achieve a high accuracy due to the mismatch between the sampling period and the pulse repetition interval. The proposed algorithm firstly estimates the point-in-time that each pulse arrives at two receivers accurately. Secondly two time of arrival （TOA） sequences are subtracted. And final y the radial ve-locity difference of a target relative to two stations with the least square method is estimated. This algorithm only needs accurate estimation of the time delay between pulses and is not influenced by parameters such as frequency and modulation mode. It avoids transmitting a large amount of data between two stations in real time. Simulation results corroborate that the performance is bet-ter than the arithmetic average of the Cramer-Rao lower bound （CRLB） for monopulse under suitable conditions.

Radial velocity of ocean surface current estimated from SAR Doppler frequency measurements—a case study of Kuroshio in the East China Sea

Ocean currents are a key element in ocean processes and in meteorology,affecting material transport and modulating climate change patterns.The Doppler frequency shift information of the synthetic aperture radar(SAR)echo signal can reflect the dynamic characteristics of the sea surface,and has become an essential sea surface dynamic remote sensing parameter.Studies have verified that the instantaneous Doppler frequency shift can realize the SAR detection of the sea surface current.However,the validation of SAR-derived ocean current data and a thorough analysis of the errors associated with them remain lacking.In this study,we derive high spatial resolution flow measurements for the Kuroshio in the East China Sea from SAR data using a theoretical model of shifts in Doppler frequency driven by ocean surface current.Global ocean multi observation(MOB)products and global surface Lagrangian drifter(GLD)data are used to validate the Kuroshio flow retrieved from the SAR data.Results show that the central flow velocity for the Kuroshio derived from the SAR is 0.4–1.5 m/s.The error distribution between SAR ocean currents and MOB products is an approximate standard normal distribution,with the 90%confidence interval concentrated between–0.1 m/s and 0.1 m/s.Comparative analysis of SAR ocean current and GLD products,the correlation coefficient is 0.803,which shows to be significant at a confidence level of 99%.The cross-validation of different ocean current dataset illustrate that the SAR radial current captures the positions and dynamics of the Kuroshio central flow and the Kuroshio Counter Current,and has the capability to monitor current velocity over a wide range of values.

Comparative Analysis of Two Heavy Snow Processes Based on Doppler Radar Radial Velocity

By using the data of synoptic charts and Datong Doppler radar data, two heavy snow processes in Datong during November 9 - 10, 2009 and on March 14, 2010 were analyzed. The results show that surface current, occluded fronts, high-altitude and low-altitude jet stream were main reasons for the heavy snow processes. Zero velocity curves were like ＂S＂ at elevations of 0.5, 1.5 and 2.4~, and there were a pair of＂ bull＇s-eye＂ structure, showing that heavy snow would occur. As for the two heavy snow processes, the qualitative judgment results based on the area of posi- tive and negative velocity zones were consistent with the quantitative analysis results based on the average divergence well, so we can use radar images to judge features of velocity fields rapidly in practice.

High precision phase-domain radial velocity estimation for wideband radar systems

Radial velocity estimation used in wide-band radar systems is investigated.By analyzing the signal of cross-correlation output of adjacent echoes,it is found that the frequency and phase of the cross-correlation output are related to the target’s radial velocity.Since the precision of the phase estimation is higher than that of the frequency,a phase-based velocity estimator is proposed.However,the ambiguity problem exists in the phase estimators,and thus the estimation of the cross-correlation of adjacent echoes(CCAE)is used to calculate the ambiguity number.The root-mean-square-error(RMSE)of the proposed estimator is derived.Simulation results show that the performance of the proposed method is better than that of the frequency-based estimator.

On the radial velocity calibrations in the LAMOST medium-resolution spectroscopic survey of nebulae

Accurate radial velocity determinations of optical emission lines(i.e.,[NⅡ]λλ6548,6584,Hαand[SⅡ]λλ6717,6731)are very important for investigating the kinematic and dynamic properties of nebulae.The second stage survey program of Large sky Area Multi-Object fiber Spectroscopic Telescope(LAMOST)has started a sub-survey of nebulae(MRS-N)which will spectroscopically observe the optical emission lines of a large sample of nebulae near the Galactic plane.Until now,15 MRS-N plates have been observed from September 2017 to June 2019.Based on fitting the sky emission lines in the red band spectra of MRS-N,we investigate the precision of wavelength calibration and find there are systematic deviations of radial velocities(RVs)from~0.2 to 4 km s^(-1) for different plates.Especially for the plates obtained in March 2018,the systematic deviations of RVs can be as large as~4 km s^(-1),which then go down to~0.2-0.5 kms^(-1) at the end of 2018 and January 2019.An RV calibration function is proposed for these MRS-N plates,which can simultaneously and successfully calibrate the systematic deviations and improve the precision of RVs.

A New Filtering Algorithm Utilizing Radial Velocity Measurement

Pulse Doppler radar measurements consist of range, azimuth, elevation and radial velocity. Most of the radar tracking algorithms in engineering only utilize position measurement. The extended Kalman filter with radial velocity measureneut is presented, then a new filtering algorithm utilizing radial velocity measurement is proposed to improve tracking results and the theoretical analysis is also given. Simulation results of the new algorithm, converted measurement Kalman filter, extended Kalman filter are compared. The effectiveness of the new algorithm is verified by simulation results.

Application of multigrid NLS-4DVar in radar radial velocity data assimilation with WRF-ARW

非线性最小二乘法的集合四维变分同化方法是一种结合了集合卡尔曼滤波和四维变分同化优势的混合同化方法。引入多重网格策略的NLS-4DVar方法不仅可以避免使用伴随模式,而且可以从大尺度到小尺度依次修正误差得到精度更高的分析场。本文将高效的多重网格策略的NLS-4DVar方法应用于雷达径向风数据同化中。通过一组基于ARW-WRF模式的观测系统模拟试验检验该方法对雷达径向风的同化能力和同化效率。试验结果显示,同化雷达径向风数据后,初始场得到明显改进且24小时累计降水预报精度有大幅度提高。与此同时,多重网格策略的NLS-4DVar方法还减少了计算代价,明显提高了计算效率。

Quasi-Radial Modes of Pulsating Neutron Stars: Numerical Results for General-Relativistic Rigidly Rotating Polytropic Models

In this paper we compute general-relativistic polytropic models simulating rigidly rotating, pulsating neutron stars. These relativistic compact objects, with a radius of ~10 km and mass between ~1.4 and 3.2 solar masses, are closely related to pulsars. We emphasize on computing the change in the pulsation eigenfrequencies owing to a rigid rotation, which, in turn, is a decisive issue for studying stability of such objects. In our computations, we keep rotational perturbation terms of up to second order in the angular velocity.

通州—三河地区沉积层高分辨率三维S波速度结构和径向各向异性

北京大都市区位于冀中凹陷与太行山隆起和燕山褶皱带的交汇区,区内被第四系松软沉积层覆盖,历史和现今的地震活动性表明,其面临潜在的巨震风险.地震风险评估和强地面运动模拟依赖准确的沉积层三维S波速度模型.本文基于布设在通州—三河地区前所未有的密集台阵数据,构建了该区域高分辨率三维S波速度结构和径向各向异性模型.首先将919个台站组成的密集台阵划分为若干子台阵,利用修正的背景噪声聚束分析方法,提取各个子台阵的Love波频散曲线,通过深度反演构建了通州—三河地区三维SH波速度模型.结合由基阶和一阶Rayleigh波模式反演构建的三维SV波速度模型,获得了台阵下方的径向各向异性模型.模型显示,大兴断裂在穿过牛堡屯之后,继续向北东向延伸约20 km.大兴凸起以大兴断裂为界,表现出明显不同的构造特征和沉积过程.断裂的北西侧第四系沉积层较浅,推测约为100~400 m;断裂的南东侧第四系沉积层厚度横向变化较大,发育夏垫凹陷和甘棠凹陷.沉积盆地径向各向异性的幅度可达±20%,地表约0~60 m范围有一较强的负径向各向异性(VSH<VSV)覆盖层,推测与该地区垂直地裂缝发育有关.正的径向各向异性(VSH>VSV)是沉积作用造成的水平层序排列的沉积分层的反映.

青藏高原东南缘三维S波速度和径向各向异性及泸定M_(S)6.8和芦山M_(S)7.0地震孕震环境探讨

利用青藏高原东南缘布设的684个流动地震台和150个固定地震台的连续背景噪声波形数据,提取了周期5~50 s的Rayleigh波和Love波相速度频散,并反演获得了该区域下方0~70 km三维S波速度和径向各向异性结构.结果表明:(1)受川滇地块南部高速体的阻挡,东南缘中下地壳内存在两条空间分布独立的低速带.西侧(L1)从川滇地块北部向南延伸至滇西南地块,其中下地壳平均V_(S)小于3.4 km·s^(-1)并表现为正径向各向异性结构,反映了高原中下地壳可能存在部分熔融和韧性变形;东侧低速带(L2)沿着小江断层南北分布,受到红河断层的阻挡,弱物质不太可能进入滇西南地块,该区域下地壳和上地幔顶部均显示较强的正各向异性,推测其更可能是沿小江断层相互驱动的块体在横向挤压过程中引起的地壳部分熔融而导致.(2)芦山M_(S)7.0地震位于龙门山逆冲断层南段,泸定M_(S)6.8地震位于鲜水河走滑断层东南段,两者都发生在地壳浅层高低速异常过渡带上,但其深部孕震环境有所不同.芦山震区西北部中下地壳低速体为负各向异性,推测韧性物质沿着龙门山下方陡倾断层向上运移,上地壳积累应力并通过薄弱区域释放导致了芦山地震的发生;泸定震区为正各向异性的中下地壳韧性变形促进了川滇地块SE向的水平运动,受刚性的四川盆地阻挡,加剧了上地壳发震断层的滑动变形和应力积累,脆性上地壳突然破裂导致了泸定地震的发生.

深海声影区时频谱干涉结构与声源定位

被动声呐探测位于深海声影区的水面舰船辐射噪声时,接收信噪比通常较低,导致声源被动定位方法的性能较差。针对这一问题,提出一种利用接收信号时频谱干涉结构的声源距离和径向速度联合估计方法。首先根据射线声学理论,建立时频谱沿频率轴和时间轴的干涉条纹周期与声源距离和径向速度的关系,然后对接收信号时频谱进行二维傅里叶变换和多频带处理以估计上述干涉条纹周期,最后解算声源距离和径向速度。仿真和海试数据处理结果表明,相比于现有利用接收信号自相关的声源距离估计方法,该文利用时频谱二维傅里叶变换的声源定位方法具有较好的稳健性,比较适用于低信噪比条件下的声源被动定位。

一种多雷达机动目标探测高阶运动特征估计方法

针对单站雷达常因为跟踪模型与目标真实运动状态失配而造成对目标高阶运动特征估计精度较差的问题。从雷达组网结合径向速度增广量测的角度,来提升目标高阶运动特征估计精度。采用序贯无迹卡尔曼滤波算法对多雷达的异步量测信息进行集中式融合。分析了不同测距、测角精度与径向速度增广量测下目标高阶运动特征的估计效果,并利用双传感器的几何精度因子优化雷达站布局以分析雷达站布局对目标高阶运动特征估计效果的影响。通过仿真实验发现,在该雷达组网方式下,雷达测距精度比测角精度对目标高阶运动特征估计效果的影响要大。雷达站布局优化结合径向速度增广量测可以有效地获得目标高阶运动特征的高精度估计。

稳流器结构对导流式消防水炮流动特性的影响

为了探究一种新的最佳星形稳流器结构形式,选取六肋式星形稳流器,设计了5种不同长度且交叉排列的星形稳流器.基于CFD对安装有5种不同结构形式的星形稳流器的导流式消防水炮内流体的流动特性展开了数值模拟研究,分析了不同稳流器布置形式对导流式消防水炮内流体速度分布均匀性、轴线速度以及湍动能的影响,并对不同布置形式下的压力降进行了分析.数值研究结果表明,稳流器可有效改善消防水炮内轴线速度分布并降低消防水炮的进出口湍动能差,分段叉排布置的稳流器可显著提升稳流效果.其中,数量为2、长度为40 mm且交叉布置的稳流器组合最有利于提升消防水炮的综合性能.同体积条件下,通过缩短单个稳流器长度,增加稳流器叉排数量的分段错位布置提升消防水炮的性能作用有限.过多数量的叉排布置会造成逆流以及较大的压力损失.研究结果对星形稳流器在消防水炮内的优化设计具有一定借鉴作用.

Derivative Spectroscopy and its Application at Detecting the Weak Emission/Absorption Lines

The development of spectroscopic survey telescopes like Large Sky Area Multi-Object Fiber Spectroscopic Telescope(LAMOST),Apache Point Observatory Galactic Evolution Experiment and Sloan Digital Sky Survey has opened up unprecedented opportunities for stellar classification.Specific types of stars,such as early-type emission-line stars and those with stellar winds,can be distinguished by the profiles of their spectral lines.In this paper,we introduce a method based on derivative spectroscopy(DS)designed to detect signals within complex backgrounds and provide a preliminary estimation of curve profiles.This method exhibits a unique advantage in identifying weak signals and unusual spectral line profiles when compared to other popular line detection methods.We validated our approach using synthesis spectra,demonstrating that DS can detect emission signals three times fainter than Gaussian fitting.Furthermore,we applied our method to 579,680 co-added spectra from LAMOST Medium-Resolution Spectroscopic Survey,identifying 16,629 spectra with emission peaks around the Hαline from 10,963 stars.These spectra were classified into three distinct morphological groups,resulting in nine subclasses as follows.(1)Emission peak above the pseudo-continuum line(single peak,double peaks,emission peak situated within an absorption line,P Cygni profile,Inverse P Cygni profile);(2)Emission peak below the pseudo-continuum line(sharp emission peak,double absorption peaks,emission peak shifted to one side of the absorption line);(3)Emission peak between the pseudo-continuum line.

基于组网多普勒雷达径向速度场的福建近海台风中心定位研究

本文探讨了一种基于组网多普勒雷达径向速度零速度线(区)对福建近海台风中心定位的方法,该方法利用了台风中心位于雷达径向速度产品中零速度线(区)上的特征,理论上可以将两部(或以上)多普勒雷达零速度线(区)的重合点(或区域)认定为台风中心,并利用空间密度聚类法和线性回归分析对台风零速度线(区)弯曲、不连续以及重合点不唯一等情况做进一步中心定位判定。研究发现,该方法时空分辨率高、定位精准,可以对发展形态不规则、速度模糊、临近登陆或登陆后的台风实现定位,对台风实时业务定位和路径预报等具有一定的参考价值,同时具有简单易行、适用范围广的优势。