徐明輝

(中國科學(xué)院上海天文臺 上海 200030)

博士學(xué)位論文摘要選登

太陽系質(zhì)心加速度的確定及其對ICRS的影響

徐明輝?

(中國科學(xué)院上海天文臺 上海 200030)

自1998年1月1日開始,國際天球參考系由一組遙遠(yuǎn)的射電源實(shí)現(xiàn),取代了傳統(tǒng)的恒星參考架.河外源的距離在億光年尺度,實(shí)現(xiàn)了在空間固定的天球參考架.光行差效應(yīng)由觀測者的運(yùn)動引起,與天體的距離無關(guān).由于太陽系質(zhì)心在空間的運(yùn)動存在著微小的加速度,該加速度引起長期光行差發(fā)生漂移,使得在太陽系質(zhì)心處觀測到的河外射電源的位置有長期的偏折.空間固定的參考架為該觀測現(xiàn)象提供了觀測資料,進(jìn)而可以確定太陽系質(zhì)心在空間的運(yùn)動加速度.

國際天球參考架中的河外射電源的觀測主要由甚長基線干涉測量(VLBI)完成.VLBI是在20世紀(jì)60年代開始發(fā)展,70年代興起的一種幾何測量技術(shù).它通過地面上的一組全球分布的望遠(yuǎn)鏡觀測同一顆河外射電源,通過信號干涉得到同一信號波前到達(dá)不同望遠(yuǎn)鏡的時間差.該技術(shù)能揭示地表、地球內(nèi)部和地球在空間運(yùn)動變化的規(guī)律,還能精確地測定射電源的方向.VLBI的常規(guī)觀測至今已累積了30多年的觀測資料,利用該數(shù)據(jù)資料建立的天球參考架的軸向穩(wěn)定性達(dá)到了10μas水平.這為太陽系質(zhì)心加速度的確定提供了可靠的數(shù)據(jù).

在天體測量中,天球參考系的實(shí)現(xiàn)可以采用動力學(xué)和運(yùn)動學(xué)方法,在采用運(yùn)動學(xué)方法建立時,需要對引起天體產(chǎn)生自行的系統(tǒng)性效應(yīng)進(jìn)行精確的理論描述.本文利用VLBI觀測確定太陽系質(zhì)心加速度,這是首次基于天體測量學(xué)方法從實(shí)測中分析質(zhì)心運(yùn)動加速度.在當(dāng)前,最重要的因素必定是長期光行差漂移.本文的工作對天球參考系的討論有著重大的意義.

在處理方向觀測時,需要對引起方向變化的物理因素、幾何因素和地球在空間的運(yùn)動等效應(yīng)進(jìn)行自洽又完整的分析.本文較詳細(xì)地討論了自行、光行差和視差之間的區(qū)別與聯(lián)系,基于光行差的概念探討了太陽系質(zhì)心加速度的估計(jì)原理.太陽系質(zhì)心加速度對VLBI觀測量的影響是微弱的,這對VLBI數(shù)據(jù)分析提出了很高的要求:選取最優(yōu)的數(shù)據(jù)集,選用標(biāo)準(zhǔn)的物理影響模型,選取合理的參數(shù)化方法和采用正確的框架定義.在進(jìn)行綜合解之前,需要對每次觀測進(jìn)行獨(dú)立的分析,對解算方案進(jìn)行優(yōu)化.本文從誤差方程出發(fā)對VLBI數(shù)據(jù)分析方法進(jìn)行了歸納和總結(jié).

將質(zhì)心加速度建模后,在VLBI數(shù)據(jù)分析中求解得到的太陽系質(zhì)心加速度在銀道坐標(biāo)系中表示為(7.47±0.46,0.17±0.57,3.95±0.47)mm·s?1·yr?1.結(jié)果證實(shí)目前累積的VLBI觀測數(shù)據(jù)對加速度參數(shù)的約束能力在亞毫米每秒每年,加速度結(jié)果相對于其誤差是顯著的.引起射電源產(chǎn)生自行的因素有: 1.太陽系質(zhì)心加速度產(chǎn)生的視自行;2.射電源結(jié)構(gòu)的變化;3.太陽系質(zhì)心在空間運(yùn)動產(chǎn)生的視差動; 4.射電源在空間的本動.對于加速度矢量的確定來說,后3者是主要的誤差源.本文采用不同的解算方法來檢測估計(jì)的結(jié)果的穩(wěn)定性和可靠性,根據(jù)視差動模型和光行差模型同時估計(jì)質(zhì)心運(yùn)動速度和加速度,從而定量地考察視差動以及其他與天體距離有關(guān)的因素對加速度估計(jì)結(jié)果的影響.分析結(jié)果表明加速度矢量結(jié)果穩(wěn)定.

太陽系質(zhì)心加速度對天球參考系的影響主要有:

1.在不改變坐標(biāo)系原點(diǎn)的前提下,處理長期光行差漂移效應(yīng)的可行方法是將其歸為天體的運(yùn)動,即天體的視自行;

2.天球參考架不再與時間無關(guān),天球參考架由天體的歷元坐標(biāo)和長期光行差漂移引起的視自行構(gòu)成;

3.天體將存在最大幅度為5.8μas·yr?1的自行,在處理較長時間段和高精度的觀測時需要考慮該效應(yīng),如目前的VLBI觀測;

4.由于ICRF2定義源在天球上的分布并不均勻,太陽系質(zhì)心加速度使ICRF2產(chǎn)生繞天極約25 μas·cent?1的整體旋轉(zhuǎn);

5.天球參考架存在著變形.

本文工作的主要貢獻(xiàn)在于:

1.建立了太陽系質(zhì)心加速度直接估計(jì)方法,通過VLBI數(shù)據(jù)分析得到了高精度結(jié)果;

2.發(fā)現(xiàn)了太陽系質(zhì)心在銀緯方向上顯著的加速度分量;

3.分析了太陽系質(zhì)心加速度對天球參考架的影響;

4.提出了歷元參考架的概念,并將其實(shí)現(xiàn).

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獲得博士學(xué)位,導(dǎo)師:上海天文臺王廣利研究員;mhxu@shao.ac.cn