基于最大熵估计的空间物体星像复原

空间物体的光学观测中存在诸多的图像退化因素,影响了目标的成像过程,降低了探测精度,给高精度的定位计算带来困难.使用基于最大熵估计的图像复原方法,处理了退化严重的实测图像.通过比较处理前后空间目标的定位精度,分析图像复原方法对于数据处理的影响.实验结果表明,使用最大熵估计图像复原方法,降低了图像退化的影响,显著提高了退化星像的定位精度.     

人造卫星观测中重叠像的分离

在人造卫星观测中,由于观测目标相对背景恒星的运动,观测目标成点状像而背景恒星的像则被拉长.该现象极易造成观测目标与背景恒星发生重叠,从而给卫星跟踪观测或是进行同步目标探测造成困难.本文利用图像处理的方法对重叠的目标像和恒星像进行分离,取得了较为理想的效果.     

基于图像叠加的GEO目标探测方法

为了满足地球同步轨道(Geosynchronous Orbit,GEO)空间目标的探测要求,克服长时间曝光CCD像元出现饱和溢出的情况,提出使用多帧连续曝光图像叠加的方法,增加图像的存储范围,同时提升目标的信噪比,保证系统的探测能力.实验结果表明,利用10帧连续图像叠加的方法,可以有效消除像元饱和的情况,提升目标信噪比约3.2倍,提升探测能力近似2.5星等,且序列图像的底片常数精度可靠,用底片常数的均值计算目标位置,精度符合要求.结果验证了使用图像叠加方法探测GEO目标的可行性.     

空间碎片光学观测中若干问题研究

<正>地基光学观测是探测空间碎片的重要手段.本文从目标搜索方案的制定、目标质心提取、目标精密定位以及目标关联4方面入手,研究提升设备探测能力、提高目标观测精度的方法.首先,为了满足对GEO(Geosynchronous Orbit,地球同步轨道)空间碎片探测的要求,克服长时间曝光CCD像元饱和溢出的问题,使用多帧连续曝光图像叠加的方法,增加图像的宽容度,同时保证系统的探测能力.实验表明,叠加10帧连续图像,有效消除了像元饱和的情况,提升目标信噪比约3.2倍,提升探测能力约2.5 mag,使用底片常数的均值计算目标位置,精度符合要求.使用星像几何形态检测     

基于冗余递减匹配的星图识别方法

空间目标光学观测过程中,当望远镜的指向误差与跟踪误差显著时,很难使用邻域匹配的方法完成背景恒星的配准.基于星对角距冗余递减匹配的思想,实现了一种背景恒星的高效识别方法.利用多种条件下的仿真星图以及实测图像进行了试验,结果表明,方法提高了识别成功率,且耗时少,可以准确、快速地完成星图匹配工作.     

面向空间碎片观测的望远镜指向误差校正方法

空间碎片光学观测图像的处理过程中,受望远镜指向误差影响,背景恒星易出现错误匹配或无法匹配的情况,给底片模型的计算与目标的精密定位带来困难.从空间碎片观测图像的特点出发,提出了一种基于特征恒星的多邻域迭代匹配法.实测数据的试验结果表明,该方法消除了望远镜跟踪误差带来的影响,大幅提高了背景星图匹配的效率与正确率,且时间开销较少,有利于数据的实时处理.     

基于先验信息的空间碎片探测方法

提出了一种基于先验信息的空间碎片图像探测方法.该方法通过先验信息,在图像中碎片星像的邻域设置波门,计算波门内的局部背景阈值,辅以相关的判据识别目标.随后使用矩方法计算碎片质心相对波门中心的偏离值,通过线性平移计算碎片质心在整幅图像中的位置.实验表明:该方法复杂度低、便于实现、实时性好,可以高效、准确地探测空间碎片,比较精确地确定碎片的质心位置.     

恒星系统中暗弱伴星的高分辨率成像探测

天文图像的高分辨率重建技术日益成为天文观测的一种手段.随着云南天文台丽江高美古2.4m望远镜的建立,基于2.4m望远镜利用高分辨率图像重建技术,对恒星系统中可能存在的暗弱伴星进行探测将成为一个现实的课题.本文拟通过实验和理论计算来论证,基于2.4m望远镜,利用天文图像的高分辨率重建技术,对恒星系统中可能存在的暗弱伴星在一定亮度范围内进行探测的可行性.     

空间碎片观测中拖长星像的测光

提出空间碎片观测中对拖长背景恒星的测光算法,介绍了该方法的基本原理.利用迭代法对目标图像进行局部阈值分割提取,得到的二值图像通常包含多个连通域.最后对各连通域块检测判断,计算出目标的星等,达到测光的目的.通过与天文图像和数据处理软件IRAF的测光结果对比,此算法的误差在0.02mag左右,可以达到测光的要求.     

基于图像位移叠加方法探测暗弱小行星

为了提高对太阳系内暗弱小天体的探测能力,将位移叠加方法应用于时序光学图像的处理,提高了暗弱小行星的识别效率.通过试位法预估动目标的视运动速度,确定动目标的存在性,进一步根据目标星象特征判据(信噪比和星象伸长率)迭代确定动目标的精确位置.将位移叠加方法运用在近地天体望远镜同一天区连续曝光的多幅图像的处理中,成功探测到暗于21 mag的单帧不可见动目标,验证了图像位移叠加方法探测暗弱动目标的可行性.     

Use of Mathematic Morphological Operators for Processing the Smeared Images of A Camera without Shutter

In the observation of space objects with a full-frame transfer CCD camera, the camera shutter is often removed, and therefore the smeared images are produced, which affect seriously the object detection and location. Based on the difference of geometrical morphology between star images and smears, a mathematical morphology method is proposed to process the smeared images. By comparing the position accuracies of stars and space objects before and after processing, it is veri?ed that the morphological method can effectively remove the smears from astronomical images, and can signi?cantly improve the detection rates and position accuracies of stars and moving objects. The result indicates that this is an effective method to eliminate the smearing effect.     

Database of molecular masers and variable stars

We present the database of maser sources in H2 O, OH and Si O lines that can be used to identify and study variable stars at evolved stages. Detecting the maser emission in H2 O, OH and Si O molecules toward infrared-excess objects is one of the methods for identifing long-period variables(LPVs, including miras and semiregulars), because these stars exhibit maser activity in their circumstellar shells. Our sample contains 1803 known LPV objects. Forty-six percent of these stars(832 objects) manifest maser emission in the line of at least one molecule: H2 O, OH or Si O. We use the database of circumstellar masers in order to search for LPVs which are not included in the General Catalogue of Variable Stars(GCVS). Our database contains 4806 objects(3866 objects without associations in GCVS) with maser detection in at least one molecule. Therefore it is possible to use the database in order to locate and study the large sample of LPV stars. The database can be accessed at http://maserdb.net.     

Automatic removal of false image stars in disk-resolved images of the Cassini Imaging Science Subsystem

Taking a large number of images, the Cassini Imaging Science Subsystem(ISS) has been routinely used in astrometry. In ISS images, disk-resolved objects often lead to false detection of stars that disturb the camera pointing correction. The aim of this study was to develop an automated processing method to remove the false image stars in disk-resolved objects in ISS images. The method included the following steps:extracting edges, segmenting boundary arcs, fitting circles and excluding false image stars. The proposed method was tested using 200 ISS images. Preliminary experimental results show that it can remove the false image stars in more than 95% of ISS images with disk-resolved objects in a fully automatic manner,i.e., outperforming the traditional circle detection based on Circular Hough Transform(CHT) by 17%. In addition, its speed is more than twice as fast as that of the CHT method. It is also more robust(no manual parameter tuning is needed) when compared with CHT. The proposed method was also applied to a set of ISS images of Rhea to eliminate the mismatch in pointing correction in automatic procedure. Experiment results showed that the precision of final astrometry results can be improve by roughly 2 times that of automatic procedure without the method. It proved that the proposed method is helpful in the astrometry of ISS images in a fully automatic manner.     

FAUST observations of ultraviolet sources in the directions of NGC 4038–39 and 6752

Analysis of ultraviolet (UV) observations with the FAUST shuttle-borne telescope toward the Antennae and NGC 6752 celestial regions resulted in the detection of 46 and 221 candidate sources respectively, for a signal-to-noise ratio of 8. We discuss the source detection process and the identification of UV sources with optical counterparts. Using correlations with existing catalogues, we present reliable identifications for approximately 60 per cent of the sources. We find that most identified objects are B, A and F stars. The remaining identified objects are galaxies, a white dwarf in a binary system, and two K-type stars. Nearly all of the remaining unidentified objects have assigned optical counterparts but, lacking additional information, we give these only as best estimates. With help from new diagnostic diagrams, we suggest that these unclassified objects are main-sequence (or giant) stars within the local spiral arm or halo; or other hot evolved objects within the local spiral arm. We discuss the nature of the objects found and compare our results with those predicted from spectral and Galactic models.     

Main-sequence magnetic CP stars: II. Physical parameters and chemical composition of the atmosphere

This paper continues a series of reviews dedicated to magnetic CP stars. The occurrence frequency of CP stars among B5–F0-type main-sequence stars is shown to be equal to about 15–20%. The problems of identification and classification of these objects are addressed. We prefer the classification of Preston, which subdivides chemically peculiar stars into the following groups: Am, λ Boo, Ap/Bp, Hg-Mn, He-weak, and He-strong stars. The main characteristic features of objects of each group are briefly analyzed. The rotation velocities of CP stars are shown to be about three times lower than those of normal stars of the same spectral types (except for λ Boo and He-strong objects). The rotation periods of CP stars range from 0.5 to 100 days, however, there is also a small group of objects with especially long (up to several tens of years) variability periods. All kinds of peculiar stars can be found in visual binaries, with Am-and Hg-Mn-type stars occurring mostly in short-period binaries with P < 10 days, and the binary rate of these stars is close to normal. The percentage of binaries among magnetic stars (20%) is lower than among normal stars. A rather large fraction of CP1-and CP2-type stars was found to occur in young clusters (with ages smaller than 10⁷ years). Photometric and spectral variability of peculiar stars of various types is discussed, and it is shown that only objects possessing magnetic fields exhibit light and spectral variations. The chemical composition of the atmospheres of CP stars of various types is considered. The abundances of various elements are usually determined by comparing the line profiles in the observed spectrum with those of the synthetic spectra computed for various model atmospheres. Different mechanisms are shown to contribute to chemical inhomogeneity at the star’s surface, and the hypothesis of selective diffusion of atoms in a stable atmosphere is developed. Attention is also paid to the problems of the determination of local chemical composition including the stratification of elements. Some of the coolest SrCrEu peculiar stars are found to exhibit fast light variations with periods ranging from 6 to 15 min. These variations are unassociated with rotation, but are due to nonradial pulsations. The final part of the the review considers the fundamental parameters of CP stars. The effective temperatures, luminosities, radii, and masses of these objects are shown to agree with the corresponding physical parameters of normal main-sequence stars of the same spectral types.     

Chemically peculiar stars in the orion OB1 association. I. occurrence frequency, spatial distribution, and kinematics

A total of 85 CP stars of various types are identified among 814 members of the OriOB1 association. The fraction of CP stars decreases with age for different cluster subgroups: from 15.1% in the youngest subgroup (b) to 7.7% in the oldest one (a). Individual comments are provided for each of the 85 stars, where we analyze the physical parameters and distance of the objects. All the 23 Am stars identified as a result of this study are found to have heliocentric distances between 100 and 300 pc and appear not to be members of the OrionOB1 association, but foreground objects. We identified 59 Bp stars, which account for 13.4% of the total number of B-type stars in the association. The fraction of peculiar B-type stars in the OriOB1 association is found to be twice higher than that of peculiar A-type stars. The same is true for field stars. The association contains 22 magnetic stars, out of which 21 are Bp stars and only one is an Ap star. Seventeen of these stars are objects with anomalous helium lines. Magnetic stars show a well-defined tendency to concentrate in the central region of the association (in Orion’s Belt), which contains most of these objects. No significant differences are found between the field strengths in the B-type stars of the association and Bp-type field stars, although there is a noticeable trend for He-rich stars to have stronger fields compared to He-weak stars. We identified 17 binaries, which make up 20% of the total number of peculiar stars studied, which is the standard ratio for CP stars. Except for one HgMn star (HD35548), the radial velocities and proper motions of our identified objects are consistent with the corresponding parameters of normal B-type stars.     

Chemical composition of the He-w stars HD 37058, 212454, and 224926

Our spectrophotometric analysis of the atmospheres of HD 37058, HD 212454, and HD 224926 shows these objects to be typical He-w stars with close-to-zero microturbulence velocities, very different magnetic fields, and wide scatter of chemical anomalies. However, one of the main manifestations of separation is that helium moves from the outer layers of the atmosphere into the star’s interior.Our analysis of the stars HD 212454 and 224926 with Be<100 G shows that despite their weak magnetic fields they have the same degree of chemical anomaly as highly magnetized stars. Chemical composition varies over a wide range for stars with the same magnitude of magnetic field. We find the conditions in the temperature interval 13000–16000 K to be the most favorable for the formation of He-w type stars. Helium underabundance is the strongest near the maximum of the distribution and it is observed in stars with weak as well as strong fields. Because of the scatter mentioned above the degree of chemical anomalies is not strictly related to the magnitude of the magnetic field, although the field has an appreciable effect on the formation of chemical inhomogeneities at the star’s surface. Its influence is minimal in stars with very weak magnetic fields and the presence of strong chemical anomalies indicates that microturbulence in these stars is sufficiently weak even without the effect of the magnetic field. It is plausible to assume that the anomalies arise due to slow rotation.The temperature dependences of rotation velocity vsini for stars with weak magnetic fields show no apparent trends associated with the magnitude of magnetic field. The rotation velocities vsini of almost all stars are lower than those of normal stars, except for HD 131120, 142096, 142990, and 143669, which rotate with the same velocity or even faster than normal stars. These objects do not obey the general rule and their example shows that stable atmospheres can also be found among fast rotators and that magnetic field takes no part in the spin-down of CP stars. We believe that CP stars inherited their slow rotation from protostellar clouds.     

AGB stars in the Magellanic Clouds

The stars that will be detectable in the Magellanic Clouds by the DENIS and 2MASS near infrared surveys are enumerated. All thermally-pulsing AGB stars will be observable in I, J, H and K, along with the top two magnitudes of both the early-AGB and the first giant branch. All carbon stars will be visible, and normal (N type) C stars will be easily distinguished by their large J-K colours. However, it will not be possible to separate faint, warm C stars from K and M stars using the photometry alone. Photometry of AGB stars in clusters will allow an accurate evaluation of the AGB tip luminosities as a function of initial mass. Random phase K magnitudes of LPVs and Cepheids should provide a better measure of the LMC tilt and distortions in the SMC. The K survey should turn up 100 to 150 objects undergoing superwind mass loss, these objects being OH/IR stars and the dust-enshrouded C star equivalents of OH/IR stars. It is shown that crowding should not be a problem even in the LMC bar.     

The domain of γ Doradus variables in the Hertzsprung–Russell diagram

70 new γ Doradus candidates were identified from Hipparcos photometry, which represents more than a doubling of the presently known number. Selecting the objects with good evidence for multiperiodicity, it is found that these stars, together with bona fide members of the class, occupy a well-defined region in a colour–magnitude diagram. This domain corresponds to a range of 7200–7700 K on the zero-age main sequence (ZAMS) and 6900–7500 K one magnitude above it, which partly overlaps with the instability strip of δ Scuti stars.
For the first time, γ Doradus stars can be discussed as a group. They can be found over a significant fraction of the main sequence lifetimes for objects in the relevant temperature range. An upper limit on the surface metallicity of γ Doradus stars is apparent, which may guide the search for their pulsation driving mechanism. The importance of possible objects exhibiting both γ Doradus and δ Scuti-type pulsations is discussed.     

A high galactic latitude survey of far-ultraviolet excess objects

Optical spectra have been obtained for a selection of objects included in a catalog of far ultraviolet bright, high galactic latitude objects detected with a balloon-borne survey telescope. The observed objects provide a sample of subdwarf O and B stars, white dwarfs, and binary systems including a hot subluminous member. Model atmospheres analysis of the subdwarf sample is used to determine the temperature, gravity, and helium to hydrogen ratio of the individual objects. A smooth distribution of objects is found on the gravity versus temperature diagram near the theoretical location of the extended horizontal branch. A break between the helium rich and helium poor objects occurs at 40,000 K. Suspected binary objects were found and analyzed to determine the temperature and gravity of the hot subluminous member in each system. The number of subdwarf stars contained in binaries is determined to be from 65% to 100%. The number versus ultraviolet magnitude distribution of the subdwarf B sample is modeled to obtain a midplane density of 3.3 10(-6) pc-3 and a population scale height of 240 pc. The proportion of white dwarfs that experience the subdwarf phase of evolution is found to be 0.94%.