自行与银河系研究的前沿

Proper Motions and the Frontier of Studies on the Galaxy

简述了精确测定相对自行的方法,特别介绍了在用2～3个历元的底片和CCD观测结果推导恒星自行的过程中如何消除光学视场畸变、星等差和色差的具体办法;并介绍了用星系把相对自行推算绝对自行的方法。还介绍了用自行资料研究银河系结构和演化的一些前沿课题,其中包括星团研究、与银河系兼并的矮星系的发现、暗物质的检测、外星行星的探测和银河系中心黑洞的质量估算等。最后评价了自行在研究银河系中的重要性,论述了我国研制4m光学/近红外望远镜的重要意义。

The methods of determinations of precise proper motions by using the observations with photographic plates and CCD at the 2-3 epochs are described. The detailed technique to eliminate Optical Field Angle Distortion （OFAD）, magnitude equation, colour equation is given. In order to obtain the absolute proper motions referred to faint galaxies the procedure for determining the zero correction to relative proper motions is presented. For compiling a catalogue referred to the galaxies as the reference frame we should pay attention to the following 5 points：（1） The direct method will be adopted for determining the absolute proper motions referred to faint galaxies with observational data of POSS Ⅰ, POSS Ⅱ and new surveys. （2） The saturation always happens on photographic plates and the measuring error is relatively large for bright stars. It should be considered whether 2-3 order terms or high order terms in the plate model, corresponding to the reference stars down to 8.5 mat or all stars, are adopted. （3） The magnitude equations for stars and galaxies is different due to brightening of galaxies about -0.7 mag. The magnitude equation is not obtained for fainter stars if there is no reference stars. It should be derived by extrapolation or adding a term of magnitude equation in plate model. （4） The colour correction should be considered because QSO are blue celestial bodies and galaxies are red one. （5） The selection of galaxies is the first important work in constitution of extragalactic reference frame for determining the absolute proper motions. The systematic errors such as magnitude and colour equation etc. should be also considered and the moving average methods such as infinitely Overlapping circles or subplate will be adopted to eliminate accidental errors. There has been close relation between kinematics of the Galaxy and cosmology and the astro- metric parameters, specially, proper motions. The five front subjects in this field are described as follows： （1） Studying stellar cluster. In the past it was necessary to wait 20-30 years for determining proper motions but this study down to magnitude 22 or to even fainter magnitudes can be carried out only in 2 years time span with new technology such as adaptive optics, high quality CCD cameras, multi-object spectrograph. Open clusters （OCs） like a probe to study the structure and evolution of the Galactic disk. We expect that the radial velocities and metal abundances for 600 OCs will be obtained in the next 4 years by using LAMOST. （2） Discovering dwarf galaxies and tidal streams. Now many astronomers devote to search tidal stream as Sgr tidal stream and ＂tidal tail＂ in globular clusters. In addition, Majewski proposed to measure the solar motion with Sgr debris plane by SIM PlantQuest for determining the R0 and rate of LSR in 2006. This method would not only overcome traditional difficulties with working in the highly dust-obscured and crowded Galactic center, but be independent of R0 and any assumptions that the reference lies in the center of the Mike Way potential. （3） Searching for dark matter in the Galactic disk. 38 new cool white dwarfs were detected with reduced proper motion by Oppenheimer in 1994. The unseen matter can be indirectly detected by micro-gravitational lensing such as MACHO and EROS projects. L and T dwarf, companion star of white dwarf and M dwarf, were discovered in 1988 and 1995. A greatly contributing action in discoveries of L and T dwarfs has been made by using 2MASS, Sloan, DENIS survey etc.. Gaia can only observe these stars of about 400 due to the limiting magnitude of Gaia. New discoveries will be made by 4-meter-class telescopes on ground. In addition, we expect that SIRTP and JWST can detect the cooler stars than T dwarf. （4） Detecting the exoplanets. Besides discovering exoplanets by ground-based observations with radial velocity and astrometric method the satellite of CoRoT and Kepler was launched in 2006 and 2009 respectively. LAMOST will make its contribution in this field soon. （5） Measuring the masses of black holes and stars. Through high precision astrometric observations for 15 years a supermassive black hole at the Galactic Center is confirmed and the stellar orbits of 15 stars in the system of S-stars are well determined. Many relativistic events such as prograde precession, retrograde precession to be induced by a so far unseen, extended mass component, gravitational redshiff, Doppler boosting and multiple images etc. will be observed in the near future. Finally, it is emphasized that the Galactic structure and evolution can be fully understood only by combining astrometric and astrophysical parameters. And we should attach importance to fabrication of a 4 meter optical/infrared telescope in our country for studying these subjects as mentioned above.