The Establishment, Realization, And Transformation Of New Astronomical Reference Systems

Astrometry encompasses all that is necessary to provide the positions and motion-s of celestial bodies, including techniques, instrumentations, processing and analysis of observational data, reference systems and so on. Astrometry, which is undergoing fundamental change, is fundamental to all other fields of astronomy. The celestial ref-erence system based on bright stars is placed by extragalactic reference frame realized by Very Long Baseline Interferometry (VLBI) at radio wavelength; photographic plates are replaced by CCDs; the role of space-based astrometry is becoming more and more important; the resulting astrometric accuracies of tenths of arcseconds are replaced by milliarcsecond and will be replaced by microarcsecond accuracy in the future.To meet the theoretical needs of high-accuracy astrometry, the International As-tronomical Union (IAU) has adopted a series of resolutions which have improved and developed the definition of reference systems, time scales, and the model of the Earth rotation. The most important resolutions of fundamental astronomy were passed in2000and in2006. After reviewing these resolutions (Chapter2), which are regarded as the basis of our works, we have investigated several problems related to astronomical reference systems and optical astrometry in the Galaxy.In Chapter3we review the features of the IAU2006/2000precession-nutation model, which is the essential part of the Earth’s rotation, as well as the possible the-oretical improvements that have been proposed by various authors. We then compare the IAU models with the most accurate time series of VLBI celestial pole offsets. With different empirical models to be fitted to celestial pole offsets we found that the IAU2006/2000precession is very powerful in predicting the position of the celestial in-termediate pole (CIP) and that the long-term difference between the IAU models and VLBI data arises from small deviations in the linear and18.6-year terms. We also looked at the potential of data from optical observations that has very long time span. The accuracy of optical celestial pole offsets is about100times worse than the VLBI, thus prevent us to obtain robust improvement of the IAU precession-nutation model.In Chapter4, the Galactic aberration induced by the acceleration of the solar sys-tem barycenter is studied in order to investigate its systematic influence on the ICRS realization and the Earth orientation parameters (EOP). We first compute the global ro-tation of the ICRS resulting from the Galactic aberration effect on the apparent proper motions of the ensemble of extragalactic objects. Then we evaluate the influence of the Galactic aberration on the EOP using CIO based ICRS-to-ITRS coordinate transfor-mation. Numerical evaluations of the effect are performed with the ICRF1and ICRF2catalogs over short and long time intervals. We show that the effect of the Galactic aberration strongly depends on the distribution of the sources that are used to realize the ICRS. We also show that this rotation has no component around the axis pointing to the Galactic center and has a zero amplitude in the case of uniform distribution of sources. The effect on the ICRS and EOP increases with time and is not negligible after several tens of years. With high accuracy astrometry and the increasing length of the available VLBI observation time series, this effect should be considered, particularly in constructing the next realization of the ICRS.The improvement of the Galactic coordinate system (GalCS) is presented in Chap-ter5. The Galactic coordinate system is important in studies of the Galactic structure, kinematics, and dynamics. Its original definition was authorized by the IAU in1958without updates until today. We review the transformations of the GalCS between var-ious fundamental reference systems (FK4, FK5, and ICRS) and found some improper use of the coordinate system. Then we consider an improved definition of the GalC-S based on modern observations in long wavelength, such as the infrared sky survey,2MASS catalogs, and the radio catalog, SPECFIND v2.0. The new GalCS is related to the ICRS without any complicated transformations of reference systems and the incli-nation of the optimal Galactic plane has shown to be0.5°larger than the conventional value for the J2000.0Galactic plane:this shows that redefinition of the GalCS may be necessary. Several ways of defining the GalCS are proposed in this chapter.In Chapter6, we compare two of the most important astrometric catalogs in cur-rent years, PPMX and UCAC3, which are considered as extensions of the Hipparcos reference frame. Extensive analysis of these two large catalogs have been made in or- der to determine the local and overall systematic biases. The regional and magnitude dependent differences in stellar position and proper motion are comparable to random errors, and are much larger in the northern hemisphere. The global orientation bias vec-tor ε between the two systems is also significant (up to17mas), which show overall differences of PPMX and UCAC3catalogs and reference systems they realize. On the other hand, the term of the global rotation vector ω is small (tenths of mas per year). Because of plate dependent errors in the UCAC3catalog, we suggest that positions and proper motions of UCAC3stars in northern hemisphere (δ>-20°) should be used with caution in astrometric data reduction if they are used as reference stars.As the application of astrometric data, we study in Chapter7the Galactic rotation curve up to15kpc using the radial velocities and proper motions of carbon stars derived from the UCAC3catalog.74carbon stars and carbon-rich mira variables toward the anti-center direction (90°<l<270°,|b|<6°) are picked from literatures and then matched with UCAC3carbon star candidates to obtain their proper motions. A rigorous geometrical method (without circular assumption in the motion of stars) is employed to compute the rotation velocity of each object. Taking carbon stars as tracers, we find a flat rotation curve of210±12km s-1assuming R0=8.0kpc for the galactocentric distance and V0=220kms-1for the rotation velocity of the Sun, and this shows coherence with previous results using other tracers such as21-cm HI emission.In the next ten years, the forthcoming space astrometric satellite, Gaia, and new VLBI network, VLBI2010, will bring far-reaching influence in astrometry and fun-damental astronomy. With microarcsecond accuracy in the observations of celestial objects’(stars, solar system objects, extragalactic radio sources) position and motion and in monitoring the Earth rotation, it is expected to have new reference systems for astronomical research, better understandings in the Earth rotation, solar system dynam-ics, structure and evolution of the Milky way, and many other scientific achievements.