A Research on the New Astronomical Reference Systems

Abstract

The research of astrometry encompasses all that is necessary to provide the positions and motions of celestial bodies, including observational techniques, instrumentations, processing and analysis of the observational data, reference systems and so on. In the latest decades, great progress has been made in astrometry, including the new theoretical basis supported by general relativity; the application of Very Long Baseline Interferometry (VLBI) to establish the fundamental reference frame; the great success of the space astrometric satellite Hipparcos; the use of CCD as the main signal detector which replaced the photometric plates; the contribution of the Lunar Laser Ranging (LLR), Satellite Laser Ranging (SLR), and Global Positioning System (GPS) to establishing the reference systems and measuring the Earth's rotation. With these developments, the astrometric accuracy will be improved from current 1 mas to 1 microarcsec in the near future.

In order to meet the needs of the high-accuracy astrometry, the International Astronomical Union (IAU) has adopted a series of resolutions for improving the definitions of reference systems, time scales, and the model of the Earth's rotation. The most important ones regarding fundamental astronomy were adopted in 2000 and 2006, respectively. After reviewing these resolutions, which are regarded as the basis of our studies, we investigate several problems on the establishment, realization, and transformation of the new astronomical reference systems.

(1) We review the features of the IAU 2006/2000 precession-nutation model, which is the essential part on the Earth's rotation, as well as the possible theoretical improvements. We then compare the IAU models with the most accurate time series of VLBI celestial pole offsets. The IAU precession model is very powerful in predicting the position of the celestial intermediate pole (CIP), and the long-term differences between the IAU models and VLBI data are shown to arise from small deviations in the linear 18.6-year terms. We also review the potential of the data from the optical observations with a very long time span. The accuracy of the optical celestial pole offsets is about 100 times lower than with VLBI, thus preventing robust improvement of the IAU precession-nutation model.

(2) The Galactic aberration induced by the acceleration of the solar system rotating around the Galactic center is studied to examine its systematic influences on the International Celestial Reference System (ICRS) realization and the Earth orientation parameters (EOPs). It is found that the effect of the Galactic aberration strongly depends on the distribution of the sources that are used to realize the ICRS, and that this rotation has no component around the axis pointing to the Galactic center, and it has a zero amplitude in the case of uniform distribution of sources. Its influence on the ICRS and EOPs increases with time and will not be negligible after tens of years. With high-accuracy astrometry and the increasing length of the available VLBI observation time series, it should be taken into account, particularly in constructing the next realization of the ICRS.

(3) The original definition of the Galactic coordinate system (GalCS) was authorized by the IAU in 1958 without updates until now. We review the transformations of the GalCS between various fundamental reference systems (FK4, FK5, and ICRS), and find some improper uses of the coordinate system. Then we consider an improved definition of the GalCS based on modern observational data in long wavelengths, such as the infrared sky survey 2MASS catalogs, and the radio catalog SPECFIND 2.0. The new GalCS is directly related to the ICRS without any complicated transformations between reference systems. The optimal Galactic plane is shown to be deviated from the J2000.0 Galactic plane by 0.5°. This indicates that the redefinition of the GalCS is necessary. Several ways of defining the GalCS are proposed in this chapter.

(4) We compare two of the most important astrometric catalogs in recent years, PPMX and UCAC3, which are considered as the extensions of the Hipparcos reference frame. Extensive analyses of these two catalogs have been made in order to determine the local and overall systematic biases. The regional and magnitude dependent differences in stellar positions and proper motions are found to be comparable with the random errors, and are much larger in the northern hemisphere. Considering the plate dependent errors in the UCAC3 catalog, we suggest that the positions and proper motions of the UCAC3 stars in the northern hemisphere (δ>-20°) should be used with caution as the reference stars.

(5) As an application of the astrometric data, we study the Galactic rotation curve up to 15 kpc by using the radial velocities and proper motions of carbon stars derived from the UCAC3 catalog. We select 74 carbon stars toward the anti-center direction from the literature, and then match them with the UCAC3 carbon star candidates to obtain their astrometric parameters. A rigorous geometrical method is employed to compute the rotational velocity of each object. Taking the carbon stars as the tracers, we find a flat rotation curve with the velocity of (210±12) km\cdot s^{-1} up to 15 kpc, in agreement with the previous results obtained with 21-cm HI line.

In the next ten years, the forthcoming space astrometric satellite, Gaia, and the new VLBI network, VLBI2010, will make a strong impact on astrometry and fundamental astronomy. With the microarcsec accuracy in the observations of the positions and motions of celestial objects (stars, solar system objects, extragalactic radio sources), and in monitoring the Earth's rotation, we are looking forward to seeing the establishment of new reference systems for astronomical researches, better understandings in the Earth's rotation, solar system dynamics, structure and evolution of the Milky Way, and many other scientific achievements.