Formation of Carbon-Enhanced Metal-Poor RR Lyrae Stars     

《Chinese Astronomy and Astrophysics》2020,44(1):61-69

Carbon-enhanced metal-poor (CEMP) stars are considered to be related to the first generation of stars, and responsible for the chemical evolution of the early Galaxy. More than half of them are in binaries, and could be explained by the binary evolution, but the formation channel of them is still not fully understood. Among the hundreds of CEMP stars, there are nine CEMP RR Lyrae stars identified, and at least seven of which are very likely not binaries. The usual binary star evolution channel is difficult to produce such a single star, particularly that of carbon enrichment. One way in which such a single star might be produced is the merger of a helium white dwarf with a Hertzsprung gap (HG) star. We use a stellar evolution program to calculate the models of the merger remnants, and find that the models can reproduce the observed distribution of these CEMP single RR Lyrae stars in terms of surface temperature, gravity, and carbon abundance. Hence, it is extremely possible that the helium white dwarf and HG star merger model is one of the formation channels of the metal-poor carbon-rich RR Lyrae stars.  相似文献   

西南大西洋阿根廷滑柔鱼资源时空分布研究   总被引：2，自引：0，他引：2  

侯庆联  陈新军  汪金涛 《海洋科学》2019,43(5):103-109

通过了解西南大西洋阿根廷滑柔鱼(Illexargentinus)资源丰度年间变化规律,从而对阿根廷滑柔鱼的资源可持续开发与管理打下基础。作者根据2012～2017年西南大西洋阿根廷滑柔鱼生产统计数据,利用灰色关联评价等数理方法对西南大西洋资源渔场的时空分布特征进行了分析。结果表明,2012～2017年间西南大西洋阿根廷滑柔鱼渔场重心主要在经度上分布于58°W～63°W海域,纬度主要集中在41°～44°S、47°～48°S两个区域,高平均网次产量(10 t/网)的海域主要在57°W～67°W、41°S～42°S和57°W～67°W、44°S～50°S海域。灰色关联度表明:6年间,以2015年滑柔鱼资源状况最好,其次是2014年,2012年最差,2013年、2016年和2017年处于中间水平。利用灰色关联分析的阿根廷滑柔鱼资源丰度变化趋势可用于其资源的可持续开发与管理。  相似文献   

江苏海岸中部近岸冬季潮汐和潮流特征     

王逸涵  王韫玮  于谦  蔡辉  高抒 《海洋科学》2019,43(10):66-74

南黄海西侧的江苏海岸近岸区域,素以地形复杂、潮流强劲、悬沙输运剧烈著称,但是较长期的同步潮位和潮流观测数据仍然缺乏,尤其是在近岸(20 km)浅水(20 m)区域。2014年1月在大丰港附近开展了连续潮位和潮流观测,获得的数据揭示了一系列特征。此地潮汐潮流为正规半日潮,浅水分潮显著。平均潮差为3.05 m,最显著的两个分潮为M2和S2分潮,振幅分别为1.45 m和0.52 m。潮流最显著的半日分潮M2分潮和最显著的浅水分潮M4分潮在沿岸方向上振幅分别为0.84m/s和0.12m/s,在跨岸方向上振幅分别为0.24 m/s和0.01 m/s,沿岸方向占绝对优势。潮波的沿岸传播介于前进波和驻波之间,驻波的特征稍强。M2分潮潮流椭圆最大流(长轴)方向为南偏东7.4°。存在冬季沿岸向北的余流,垂向平均值的大小为2.2 cm/s。以上潮汐潮流特征为该区域海洋物质输运研究提供了基础资料。  相似文献   

Research on the Methods of Performance Monitoring and Evaluation for Active Hydrogen Maserstwo     

《Chinese Astronomy and Astrophysics》2019,43(2):292-304

Active hydrogen maser is the main frequency standard for establishing and maintaining the time scale. It has the characteristics of high short-term stability and low phase noise. At present, it plays an increasingly important role in the international atomic time (TAI) and various local time scales. Firstly, in combination with the internal state parameters of an active hydrogen maser, the correlation between the internal state parameters and the comparison data of a hydrogen maser is analyzed, and a method for the performance monitoring of a hydrogen maser is proposed. Secondly, according to the characteristics of hydrogen maser performance, a method for evaluating the performance of a hydrogen maser is given. The hydrogen maser performance includes mainly two aspects, namely the frequency stability and predictability. The performances of two types of active hydrogen masers (CH1-75 and MHM-2010) are evaluated by this method. The correlation analysis of the atomic clock state parameters and comparison data shows that the state parameter monitoring can effectively predict the variation of the hydrogen maser performance. The evaluation results of atomic clock frequency stability and predictability show that the atomic clock with a higher medium-and-long term frequency stability has a better predictability. There are two methods for the predictability evaluation, one is based on the data published by BIPM (Bureau International des Poids et Measures), and another one is based on the quadratic model. Both methods are consistent with the weights published by BIPM. Therefore, the two methods can be used as a quantitative method to evaluate the predictability of a hydrogen maser.  相似文献   

基于历史TLE的空间目标轨道预报误差演化规律研究     

许晓丽  熊永清 《天文学报》2019,60(4):26-38

北美防空司令部(North American Aerospace Defense Command, NORAD)发布的双行根数(Two Line Element, TLE)是广大航天工作者最常用的轨道根数,与其对应的轨道模型是SGP4/SDP4 (Simplified General Perturbation Version 4/Simplified Deep-space Perturbation Version 4)解析模型.由于TLE中并没有包含相应的轨道精度信息,编目轨道的应用范围受到很大的限制.基于Space-Track网站发布的历史TLE数据和配套的SGP4/SDP4动力学模型,采用定轨标预报的方法统计并生成了大量目标轨道的预报误差,通过对预报轨道的时间区间划分给出了每个目标的预报误差随预报时间变化的拟合系数,并进一步对不同类型轨道预报误差的演化规律和特征进行了分类讨论,给出了4种轨道类型目标的轨道预报误差随时间演化的平均解析模型,为拓展双行根数的应用提供有价值的参考.  相似文献   

Study of Star/Galaxy Classification Based on the XGBoost Algorithm     

《Chinese Astronomy and Astrophysics》2019,43(4):539-548

Machine learning has achieved great success in many areas today. The lifting algorithm has a strong ability to adapt to various scenarios with a high accuracy, and has played a great role in many fields. But in astronomy, the application of lifting algorithms is still rare. In response to the low classification accuracy of the dark star/galaxy source set in the Sloan Digital Sky Survey (SDSS), a new research result of machine learning, eXtreme Gradient Boosting (XGBoost), has been introduced. The complete photometric data set is obtained from the SDSS-DR7, and divided into a bright source set and a dark source set according to the star magnitude. Firstly, the ten-fold cross-validation method is used for the bright source set and the dark source set respectively, and the XGBoost algorithm is used to establish the star/galaxy classification model. Then, the grid search and other methods are used to adjust the XGBoost parameters. Finally, based on the galaxy classification accuracy and other indicators, the classification results are analyzed, by comparing with the models of function tree (FT), Adaptive boosting (Adaboost), Random Forest (RF), Gradient Boosting Decision Tree (GBDT), Stacked Denoising AutoEncoders (SDAE), and Deep Belief Nets (DBN). The experimental results show that, the XGBoost improves the classification accuracy of galaxies in the dark source classification by nearly 10% as compared to the function tree algorithm, and improves the classification accuracy of sources with the darkest magnitudes in the dark source set by nearly 5% as compared to the function tree algorithm. Compared with other traditional machine learning algorithms and deep neural networks, the XGBoost also has different degrees of improvement.  相似文献   

耦合土地利用回归与人口加权模型的PM2.5暴露风险评估     

邹雨轩  吴志峰  曹峥 《地球信息科学学报》2019,21(7):1018-1028

PM_2.5已成为人群健康的重要威胁之一,科学精准的暴露评估是PM_2.5风险防控的前提,为提升PM_2.5暴露精准评估,本文利用土地利用数据、道路数据、气象数据等构建PM_2.5土地利用回归反演模型,实现了2013年12月1日-2014年2月8日（冬季）广佛都市区PM_2.5时空动态演变监测,在此基础上将PM_2.5反演结果与人口密度数据耦合,分别从PM_2.5污染浓度与人口加权PM_2.5浓度2个方面,评估广佛都市区PM_2.5污染暴露风险。研究结果表明：① 土地利用回归模型能够较好的反映研究区域内PM_2.5的空间分布特征,R²大于0.78;② 2013年12月1日-2014年2月8日,广佛都市区PM_2.5浓度平均值呈现波动变化趋势,研究时段内,最高平均浓度为97.91 μg/m³ （12月29日-1月11日）,最低平均浓度为53.40 μg/m³ （1月26日-2月8日）,全时段PM_2.5浓度超WHO健康标准的面积占比达99.8%;③ 广佛都市区PM_2.5的空间分布具有异质性规律,其高值区分别位于广州市天河区、越秀区、番禺区北部、花都区北部及佛山市禅城区、南海区中部、三水区中部,低值区主要位于广州市白云区、番禺区东南部及佛山市顺德区南部。人口加权暴露风险存在2个高值中心,分别位于广州市和佛山市的主城区;④ 耦合人口加权模型前后,广佛都市区PM_2.5暴露风险高风险区空间分布发生变化,未考虑人口加权模型时,广佛深高值区较为分散,主要位于南海区、天河区、越秀区、禅城区,考虑人口加权模型后,高值区更加集中于广州市和佛山市的主城区。  相似文献   

非等间距GM(1,1)模型的总体最小二乘算法及其病态问题     

陶武勇  花向红  鲁铁定  陈西江  张伟 《大地测量与地球动力学》2019,39(1):45-50

在非等间距GM(1,1)模型中，系数矩阵中有无误差的常数项和有误差的随机项，并且系数矩阵与观测向量误差同源，即系数矩阵与观测向量中有相同的元素存在，这些相同元素应该有相同的改正数，为此本文推导了一种适合非等间距GM(1,1)模型求解的总体最小二乘算法。同时，考虑到非等间距GM(1,1)模型中存在病态问题时影响总体最小二乘计算结果的稳定性，提出对系数矩阵常数列乘以某一常数的方法，以改善病态问题。  相似文献   

鄂尔多斯地块周缘垂直形变场负位错反演与应变积累研究     

张希  刘立炜  郝明  白卓立 《大地测量与地球动力学》2019,39(6):551-556

利用鄂尔多斯地块及其周缘1970～2014年的垂直形变速率场资料，借助负位错反演研究该区域长期应变积累。结果表明，地块东北缘山西断陷带中北段年均能量积累增量、剪应力强度都较高，西南缘六盘山断裂与渭河断裂西段次之；山西断陷带中南段至晋陕交界处年均剪应力强度较高且显示一定程度的能量积累；西秦岭构造区尤其西秦岭北缘断裂西段、晋冀蒙交界区也反映一定程度的能量积累特性。  相似文献   

Three-dimensional finite element simulation of fracture propagation in rock specimens with pre-existing fissure(s) under compression and their strength analysis     

Ramin Pakzad  Shanyong Wang  Scott W. Sloan 《国际地质力学数值与分析法杂志》2020,44(10):1472-1494

A FORTRAN program, consistent with the commercially available finite element (FE) code ABAQUS, is developed based on a three-dimensional (3D) linear elastic brittle damage constitutive model with two damage criteria. To consider the heterogeneity of rock, the developed FORTRAN program is used to set the stiffness and strength properties of each element of the FE model following a Weibull distribution function. The reliability of the program is assessed against available experimental results for granite cylindrical specimens with a throughgoing, flat and inclined fissure. The calibration procedure of the material parameters is explained in detail, and it is shown that the compressive to tensile strength ratio can have a substantial influence on the failure response of the specimens. Numerical simulations are conducted for models with different levels of heterogeneity. The results show a smaller load bearing capacity for models with less homogeneity, representing gradual coalescence of fully damaged elements forming throughout the models during loading. The maximum load bearing capacity is studied for various combinations of inclination angles of two centrally aligned, throughgoing and flat fissures of equal length embedded in cylindrical models under uniaxial and multiaxial loading conditions. The key role of the compressive to tensile strength ratio is highlighted by repeating certain simulations with a lower compressive to tensile strength ratio. It is proven that the peak loads of the rock models with sufficiently small compressive to tensile strength ratios containing two throughgoing fissures of equal length are similar, provided that the minimum inclination angles of the models are the same. The results are presented and discussed with respect to the existing experimental findings in the literature, suggesting that the numerical model applied in this study can provide useful insight into the failure behaviour of rock-like materials.  相似文献