Comparison of the four catalogues by pairs,the Hipparcos,new Hipparcos,EOC‐2 and IL,by using μδ for the common 1120 stars

We present comparison results of our Independent Latitude (IL) catalogue of μ_δ determinations for 1120 bright stars with the Hipparcos, new Hipparcos and Earth Orientation Catalogue (EOC‐2) values. Also, we took into consideration the EOC3 and EOC4 (recent versions of EOC catalogues). Our μ_δ values are based on zenith telescope observations from seven Independent Latitude (IL) observatories. The IL measures are spanning a time baseline of up to 90 years which is the key advantage to the accurate determination of μ_δ. The short interval of the Hipparcos satellite observations is a disadvantage for a good accuracy of stellar proper motion, especially in the case of double and multiple stars. For this reason many astrometric catalogues have appeared after the publication of the Hipparcos including our IL catalogue. These catalogues are an appropriate combination of the Hipparcos satellite and ground‐based data which yields more accurate stellar coordinates and/or their proper motions. Among various types of ground‐based observations the latitude and universal time variations obtained from several million observations of stars reduced to the Hipparcos reference system were used for this purpose. These observations were obtained during almost the entire last century and were originally used to determine the Earth Orientation Parameters. It is also possible to use these data in the inverse task of checking the accuracy of stellar coordinates and/or their proper motions listed in the Hipparcos Catalogue. Such latitude and universal time variations data are the basis of the EOC and IL catalogues. In this paper, we computed the differences in μ_δ values between pairs of catalogues and analyzed the results to characterize the μ_δ errors for the four catalogues with a special focus on our IL catalogue. The standard errors of μ_δ for IL stars observed over more than 20 years are mostly smaller than or equal to the Hipparcos errors, and close to the accuracy level of the EOC‐2 (EOC‐3, EOC‐4) and the new Hipparcos. The resulting investigations of errors of differences of μ_δ, show that all four catalogues have relatively small random and systematic errors which are close to each other meaning that the corresponding μ_δ values have a high accuracy. Our sample also contains detected double and multiple stars for which the effects of the orbital and proper motions are difficult to separate. The differences of μ_δ values for these stars generally exceed those obtained for single stars. Also, these discrepancies could be attributed to effect of possible, still unrecognized, astrometric binaries. These investigations about the proper motions and double stars are in line with the activity of the IAU Working Group on Astrometry by Small Ground‐Based Telescopes. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

岁差常数改正及 FK5和Hipparcos自行系统

许多研究结果表明,FK5和Hipparcos自行系统的关系,与VLBI和LLR所测定的岁差常数改正值不相符合．利用建立在FK5系统上的PPM和ACRS自行数据的分析,通过多个子样本的考察,发现不论由PPM或ACRS自行资料,都无法给出一致的岁差改正值和分点运动改正值．从而表明,FK5自行内部的系统差是产生这种问题的主要因素．     

The XPM Catalogue: absolute proper motions of 280 million stars

We combined data from the Two-Micron All Sky Survey (2MASS) and USNO-A2.0 catalogues in order to derive the absolute proper motions of about 280 million stars distributed all over the sky excluding a small region near the Galactic Centre, in the magnitude range  12 < B < 19 mag  . The proper motions were derived from the 2MASS Point Sources and USNO-A2.0 catalogue positions with a mean epoch difference of about 45 years for the Northern hemisphere and about 17 years for the Southern one. The zero-point of the absolute proper motion frame (the 'absolute calibration') was specified with the use of about 1.45 million galaxies from 2MASS. Most of the systematic zonal errors inherent in the USNO-A2.0 catalogue were eliminated before the calculation of proper motions. The mean formal error of absolute calibration is less than 1 mas yr⁻¹. The XPM Catalogue will be available via CDS in Strasbourg during 2010. The generated catalogue contains the International Celestial Reference System positions of stars for the J2000 epoch, original absolute proper motions, as well as   B , R , J , H   and K magnitudes. A comparison of the proper motions obtained in this work with the data of other recent catalogues of quasars was fulfilled.     

Determination of proper motions for AC stars: First results

Proper motions of the stars of the Astrographic Catalogue are being derived, using the Hubble Space Telecope Guide Star Catalogue as second epoch. Results on the San Fernando and Cordoba AC zones are presented. Identification with GSC stars (i.e. determination of proper motion) was successful for 97 percent of all AC stars. Comparison of the proper motions thus derived with those of Preliminary PPM South shows that the accuracy is about 0.8 to 0.9 arcsec per century. Thus we can derive proper motions for about 4 million stars, with an accuracy higher than that of the SAO Catalogue.     

Comparison of three catalogues of absolute proper motions of stars in the M33 field

Comparing absolute proper motions of stars obtained by different methods from observations with the Tautenburg Schmidt telescope (134/200/400), the Kiev long focus astrograph (40/550) and the Pulkovo normal astrograph (33/350) the external accuracy of the Tautenburg absolute proper motions was examined. It agrees well with the Tautenburg internal accuracy which is known from earlier investigations as 0″.7 per century. That accuracy was now achieved for all AGK3 stars in the M33 field including the bright stars from 7^m to 10^m.     

The proper-motion signal of unresolved binaries in the Hipparcos catalogue

We present an investigation of the differences between quasi-instantaneous stellar proper motions from the Hipparcos catalogue and long-term proper motions determined by combining Hipparcos and the Astrographic Catalogue. Our study is based on a sample of about 12000 stars of visual magnitude from 7 to 10 in two declination zones on the northern and equatorial sky. The distribution of the proper-motion differences shows an excess of large deviations. This is caused by the influence of orbital motion of unresolved binary systems. The proper-motion deviations provide statistical evidence for 360 astrometric binaries in the investigated zones, corresponding to about 2400 such binaries in the entire Hipparcos catalogue, in addition to those already known. In order to check whether the observed deviations are compatible with standard assumptions on the basic parameters of binary stars, we model the impact of orbital motion on the observed proper motions in a Monte Carlo simulation. We show that the simulation yields an acceptable approximation of the observations, if a binary frequency between 70% and 100% is assumed, i.e.if most of the stars in the sample are assumed to have a companion. Thus Hipparcos astrometric binaries confirm that the frequency of non-single stars among field stars is very high. We also investigate the influence of the mass function for the secondary component on the result of the simulation. A constant mass function and mass functions with moderate increase towards low masses lead to results, which are compatible with the observed proper-motion effects. A high preponderance of very-low-mass or substellar companions as produced, for example, by a M^—1 power law is not in agreement with the frequency of proper-motion deviations in our sample of stars.     

Astrometric detection of binary companions and planets: Acceleration of proper motion

Apparent acceleration of proper motion is one of the observable manifestations of orbital motion in binary stars. Owing to the increasing accuracy of astrometric measurements, it may also be a method to detect binarity of stars. This paper presents some analytical expressions for the effects of binary motion on proper motions when the orbital period is at least several times the span of observations. We estimate orbit dimensions and distances at which low‐mass companions and planets may be detected around main‐sequence stars, using preliminary estimates of precision for the AMEX, GAIA and SIM space missions.     

利用统计方法判断中天观测数据的参考星表

中天观测数据在建立太阳系高精度行星历表时具有重要作用.在处理由JPL(Jet Propulsion Laboratory)以及IMCCE(Institut De Mecanique Celeste Et De Calcul Des Ephemerides)提供的中天数据时发现,有若干组数据的参考星表没有被明确指出。对于这些信息不完整的数据,无从改正数据由参考星表而引起的系统差。课题研究的主要目的是利用DE421历表确定存在缺陷的这类观测数据的参考星表。通过DE421给出行星在GCRS(Geocentric Celestial Reference System)中的相应位置,并把此位置转换到参考假定的参考星表时行星的理论视位置。通过引入的两个统计量分析原始数据与理论位置的差异显著性,检验假定的数据参考星表是否恰当.结果显示,此方法能够有效区分数据的参考星表,能够确定信息不完整的数据组缺少的参考星表。最后,所有中天数据被转换到GCRS坐标系,以便在历表研制中使用。     

Proper motions of 59 766 stars absolutized using galaxies in 149 sky fields (PUL2)

The PUL2 catalog has been photographically compiled in Pulkovo according to Deutch's plan. The catalog contains the mean coordinates of stars in the ICRS system at epoch J2000.0 and their original absolute proper motions. The photographic observations were performed with a normal astrograph. The first and second epochs of the photographic plates are 1937–1965 and 1969–1986, respectively. The PUL2 fields uniformly cover the northern sky. The mean difference between the epochs is 24 years. At least three pairs of plates are available for each field. There are one-hour and five-minute exposures on all plates. One pair of plates was taken with a diffraction grating. Only bright reference stars were measured on the pairs of plates with a grating. Based on a reduction model with six constants and using faint (\(15\mathop m\limits_. 2\)) reference stars, we determined the relative proper motions of the stars. We used ～700 galaxies for absolutization. The mean errors in the relative proper motions of the PUL2 stars are 5.5 mas yr^?1 (milliarcseconds per year) in μ_α cos δ and 5.9 mas yr^?1 in μ_δ. When using galaxies, the mean absolutization error is 7.9 mas yr^?1 in both coordinates. By comparing the PUL2 and HIPPARCOS catalogs, we determined the components of the residual rotation vector ω for HIPPARCOS relative to the extragalactic (equatorial) coordinate system: ω_x,y,z=(?0.98, ?0.03, ?1.66)±(0.47, 0.38, 0.42) mas yr^?1. The mean error of one absolute proper motion of a bright PUL2 star in external convergence is 9 mas yr^?1 in both coordinates.     

Rotation curve of the Galactic outer disk derived from radial velocities and UCAC3 proper motions of carbon stars

The availability of astrometric data and radial velocities of carbon stars near the Galactic plane enables us to investigate the kinematics of the Milky Way,especially the rotation curve.The recently published Third U.S.Naval Observatory CCD Astrograph Catalog (UCAC3) provides the opportunity to test this problem using three-dimensional velocity in order to obtain more reliable rotation curves.We intend to study the Galactic rotation curve up to 15 kpc using the radial velocities and proper motions of carbon stars.The motivation for using UCAC3 is to provide high precision proper motions which have hardly been used in determining the rotation velocity of tracers.Seventy-four carbon stars and carbon-rich Mira variables toward the anti-center direction (90°＜(e)＜ 270°,|b| ＜ 6°) are picked up from the literature then matched with UCAC3 carbon star candidates to obtain their proper motions.A rigorous geometrical method is employed to compute the rotation velocity of each object.Taking carbon stars as tracers,we find a fiat rotation curve of 210 ± 12kms-1assuming/to = 8.0 kpc for the gaiactocentric distance and V0 = 220 km s-1 for the rotation velocity of the Sun.Due to the uncertainties of distances,the rotation velocities are more dispersed if tangential velocities enter the calculation,compared to those derived from radial velocities only.However,the whole rotation curve shows coherence with previous results.Increasing observation and study of carbon stars would be desirable in order to provide more homogeneous data for the kinematical study of the Galactic disk.     

Photometric detection of high proper motions in dense stellar fields using difference image analysis

The difference image analysis (DIA) of the images obtained by the Optical Gravitational Lensing Experiment (OGLE-II) revealed a peculiar artefact in the sample of stars proposed as variable by Woźniak in one of the Galactic bulge fields: the occurrence of pairs of candidate variables showing anti-correlated light curves monotonic over a period of 3 yr. This effect can be understood, quantified and related to the stellar proper motions. DIA photometry supplemented with a simple model offers an effective and easy way to detect high proper motion stars in very dense stellar fields, where conventional astrometric searches are extremely inefficient.     

Apparent proper motions of radio sources from geodetic VLBI data

According to the most recent geodetic VLBI data, some of the radio sources that define the fundamental celestial reference frame are astrometrically unstable. In contrast to the stellar proper motions described by a linear function, the apparent proper motions of quasars are more complex. Therefore, they are difficult to approximate by a particular model. Being disregarded, the positional instability of the defining quasars can lead to a bias in the estimates of other parameters from observations, for example, the nutation angle corrections.     

Inaccuracies in the spectral classification of stars from the Tycho-2 Spectral Type Catalogue

We compare the spectroscopic and trigonometric parallaxes of common stars from the Tycho-2 Spectral Type and Hipparcos Catalogues. This comparison has revealed that the distance estimations for the overwhelming majority of stars by both methods yield very similar results. However, there is a small fraction of stars for which the distance estimates differ significantly. It is these stars that are the subject of our study. We have been able to find the causes of these differences.     

整体平差方法在CCD天体测量中的应用

回顾了基于底片重叠观测的整体平差方法的发展历史和应用概况，分析了该方法应用于底片资料处理没有取得预期效果的原因，介绍了小视场CCD重叠观测的整体平差研究进展．分析与实测资料归算表明，整体平差方法可以在CCD资料处理中充分发挥作用，在保证长焦距、小底片比例尺的同时，通过对CCD重叠观测的整体平差(相当于扩大观测视场)，可覆盖更多的参考星，进而达到改善局部参考架、提高归算结果精度的目的。     

The main sequence from F to K stars of the solar neighbourhood in SDSS colours

For an understanding of Galactic stellar populations in the SDSS filter system well defined stellar samples are needed. The nearby stars provide a complete stellar sample representative for the thin disc population. We compare the filter transformations of different authors applied to the main sequence stars from F to K dwarfs to SDSS filter system and discuss the properties of the main sequence. The location of the mean main sequence in colour‐magnitude diagrams is very sensitive to systematic differences in the filter transformation. A comparison with fiducial sequences of star clusters observed in g ′, r ′, and i ′ show good agreement. Theoretical isochrones from Padua and from Dartmouth have still some problems, especially in the (r – i) colours. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

J 2000.0 positions and proper motions of 257 stars in the central part of the Praesepe astrometric standard region

We given the J2000.0 position and proper motions of 257 stars in the central 1.5° × 1.5° area of the Praesepe astrometric standard region with accuracies of 0.005˜0.10 arcsec for the position in each direction and 0.0002˜0.006 arcsec/yr for the proper motions in each direction.     

利用奇异谱分析提高极移中长期预报精度

由于空间大地观测数据传输耗时及处理过程复杂, 导致极移测量值的获取存在时延, 无法满足对高精度的极移预报值有重大需求的应用领域. 针对极移复杂的时变特性, 提出一种基于奇异谱分析(singular spectrum analysis, SSA)的预报方法. 首先用SSA分离提取极移时序中的高频组分与低频组分; 其次建立最小二乘(least square, LS)外推与自回归(AutoreGressive, AR)模型对极移高频和低频组分进行组合预报. 结果表明, SSA方法能够准确地分离和提取极移低频和高频组分, 对低频和高频组分组合预报可以显著改善极移的中长期(30--365d)预报精度, 与国际地球自转和参考系服务局(International Earth Rotation and Reference Systems Service, IERS)提供的A公报中的极移预报值相比, SSA方法对极移PMX分量(本初子午线方向)和PMY分量(西90$^\circ$子午线方向)的中长期预报精度改进最高分别可达45.97%和62.44%. 研究结果验证了SSA方法对极移中长期预报改进的有效性.