太阳系质心加速度的确定及其对ICRS的影响

徐明辉

(中国科学院上海天文台 上海 200030)

博士学位论文摘要选登

太阳系质心加速度的确定及其对ICRS的影响

徐明辉†

(中国科学院上海天文台 上海 200030)

自1998年1月1日开始,国际天球参考系由一组遥远的射电源实现,取代了传统的恒星参考架.河外源的距离在亿光年尺度,实现了在空间固定的天球参考架.光行差效应由观测者的运动引起,与天体的距离无关.由于太阳系质心在空间的运动存在着微小的加速度,该加速度引起长期光行差发生漂移,使得在太阳系质心处观测到的河外射电源的位置有长期的偏折.空间固定的参考架为该观测现象提供了观测资料,进而可以确定太阳系质心在空间的运动加速度.

国际天球参考架中的河外射电源的观测主要由甚长基线干涉测量(VLBI)完成.VLBI是在20世纪60年代开始发展,70年代兴起的一种几何测量技术.它通过地面上的一组全球分布的望远镜观测同一颗河外射电源,通过信号干涉得到同一信号波前到达不同望远镜的时间差.该技术能揭示地表、地球内部和地球在空间运动变化的规律,还能精确地测定射电源的方向.VLBI的常规观测至今已累积了30多年的观测资料,利用该数据资料建立的天球参考架的轴向稳定性达到了10µas水平.这为太阳系质心加速度的确定提供了可靠的数据.

在天体测量中,天球参考系的实现可以采用动力学和运动学方法,在采用运动学方法建立时,需要对引起天体产生自行的系统性效应进行精确的理论描述.本文利用VLBI观测确定太阳系质心加速度,这是首次基于天体测量学方法从实测中分析质心运动加速度.在当前,最重要的因素必定是长期光行差漂移.本文的工作对天球参考系的讨论有着重大的意义.

在处理方向观测时,需要对引起方向变化的物理因素、几何因素和地球在空间的运动等效应进行自洽又完整的分析.本文较详细地讨论了自行、光行差和视差之间的区别与联系,基于光行差的概念探讨了太阳系质心加速度的估计原理.太阳系质心加速度对VLBI观测量的影响是微弱的,这对VLBI数据分析提出了很高的要求:选取最优的数据集,选用标准的物理影响模型,选取合理的参数化方法和采用正确的框架定义.在进行综合解之前,需要对每次观测进行独立的分析,对解算方案进行优化.本文从误差方程出发对VLBI数据分析方法进行了归纳和总结.

将质心加速度建模后,在VLBI数据分析中求解得到的太阳系质心加速度在银道坐标系中表示为(7.47±0.46,0.17±0.57,3.95±0.47)mm·s−1·yr−1.结果证实目前累积的VLBI观测数据对加速度参数的约束能力在亚毫米每秒每年,加速度结果相对于其误差是显著的.引起射电源产生自行的因素有: 1.太阳系质心加速度产生的视自行;2.射电源结构的变化;3.太阳系质心在空间运动产生的视差动; 4.射电源在空间的本动.对于加速度矢量的确定来说,后3者是主要的误差源.本文采用不同的解算方法来检测估计的结果的稳定性和可靠性,根据视差动模型和光行差模型同时估计质心运动速度和加速度,从而定量地考察视差动以及其他与天体距离有关的因素对加速度估计结果的影响.分析结果表明加速度矢量结果稳定.

太阳系质心加速度对天球参考系的影响主要有:

1.在不改变坐标系原点的前提下,处理长期光行差漂移效应的可行方法是将其归为天体的运动,即天体的视自行;

2.天球参考架不再与时间无关,天球参考架由天体的历元坐标和长期光行差漂移引起的视自行构成;

3.天体将存在最大幅度为5.8µas·yr−1的自行,在处理较长时间段和高精度的观测时需要考虑该效应,如目前的VLBI观测;

4.由于ICRF2定义源在天球上的分布并不均匀,太阳系质心加速度使ICRF2产生绕天极约25 µas·cent−1的整体旋转;

5.天球参考架存在着变形.

本文工作的主要贡献在于:

1.建立了太阳系质心加速度直接估计方法,通过VLBI数据分析得到了高精度结果;

2.发现了太阳系质心在银纬方向上显著的加速度分量;

3.分析了太阳系质心加速度对天球参考架的影响;

4.提出了历元参考架的概念,并将其实现.

Since 1998 January 1,instead of the traditional stellar reference system,the International Celestial Reference System(ICRS)has been realized by an ensemb le of extragalactic radio sources that are located at hundreds ofm illions of light years away(ifwe accept their cosmological distances),so that the reference frame realized by extragalactic radio sources is assumed to be space-fixed.The acceleration of the barycenter of solar system(SSB), which is the origin of the ICRS,gives rise to a system atical variation in the directions of the observed radio sources.This phenomenon is called the secular aberration drift.As a result,the extragalactic reference frame fixed to the space provides a reference standard for detecting the secular aberration drift,and the acceleration of the barycenter with respect to the space can be determ ined from the observations of extragalactic radio sources.

In this thesis,we aim to determ ine the acceleration of the SSB from astrometric and geodetic observations obtained by Very Long Baseline Interferometry(VLBI),which is a technique using the telescopes globally distributed on the Earth to observe a radio source simultaneously,and with the capacity of angular positioning for com pact radio sources at 10-m illiarcsecond level.The method of the global solution,which allows the acceleration vector to be estim ated as a global parameter in the data analysis,is developed.Through the formal error given by the solution,thism ethod shows directly the VLBI observations’capability to constrain the acceleration of the SSB,and demonstrates the significance level of the result.In the next step,the im pact of the acceleration on the ICRS is studied in order to obtain the correction of the celestial reference frame(CRF)orientation.

This thesis begins with the basic background and the general frame of this work.Abrief review of the realization of the CRF based on the kinem atical and the dynam ical methods is presented in Chapter 2,alongwith the definition of the CRF and its relationship with the inertial reference frame.Chapter 3 is divided into two parts.The first part describes various effects that m odify the geometric direction of an ob ject,especially the parallax,theaberration,and thepropermotion.Then thederivativemodeland theprincip le of determ ination of the acceleration are introduced in the second part.The VLBI data analysism ethod,including VLBIdata reduction(solving the ambiguity,identifying the clock break,and determ ining the ionospheric effect),theoretical delay m odel,param eterization, and datum definition,is discussed in detail in Chapter 4.

The estimation of the acceleration by more than 30-year VLBI observations and the results are then described in Chapter 5.The evaluation and the robust check of our results by different solutionsand the comparison to that from another research group are performed. The error sources for the estimation of the acceleration,such as the secular parallax caused by the velocity of the barycenter in space,are quantitatively studied by simulation and data analysis in Chap ter 6.The twom ain im pacts of the acceleration on the CRF,the apparent propermotion with themagnitude of theµas·yr−1leveland the global rotation in the CRF due to the un-uniformed distribution of radio sourceson the sky,are discussed in Chapter 7. The definition and the realization of the epoch CRF are presented aswell.The futurework concerning the exp lanation of the estimated acceleration and potential research on several main problem s in modern astrometry are discussed in the last chapter.

The A cceleration of the Barycen ter of Solar System Ob tained from VLB IObservations and Its Im pact on the ICRS

XU M ing-hui

(Shanghai A stronom ica l Observatory,Chinese Academ y of Scien ces,Shanghai 200030)

10.15940/j.cnki.0001-5245.2016.02.012

†2015-06-08获得博士学位,导师:上海天文台王广利研究员;mhxu@shao.ac.cn