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Instability of the celestial reference frame and effect on UT1 V.E. Zharov
Physics Faculty and Sternberg Astronomical Institute of Moscow State University, Moscow, Russia
XXVII IAU GENERAL ASSEMBLY Rio de Janeiro JD 6 Time and Astronomy Thursday 6 & Friday 7 - August 2009


International Celestial Reference System (ICRS) and International Celestial Reference Frame (ICRF)
Definition of stability of the ICRS. The coordinate axis of this system fixed with respect to the very distant extragalactic radio sources and thus their expected proper motions should be negligibly small. Realization of stability of the ICRF. No-Net-Rotation condition for corrections for the a priori coordinates of the "defining" sources


First step to the ICRF-2

The problem of "defining" or "stable" or "best" radio source selection is now one of the key problems for ICRF improvement


Goals of our work
The ICRF extragalactic radio sources show real apparent motions that should be included in future catalogs. Physical reason of such motion is precession of jet 1. 2. Investigate stability of system based on new approach Estimate effect on EOP

Software ARIADNA (Zharov, 2009) was used. Solutions used in work: sai2008a.eops, sai2008a.eopi (ICRF, IAU1980+EOPC04) sai2008c.eops, sai2008c.eopi (ICRF_corr, IAU1980+EOPC04)


Estimation of the motion parameters
1. Time series of the sources coordinates were used for analysis 2. Variations of each coordinate were fitted by polynomial function


Motion of 1404+286 (OQ208) - 1


Motion of 1404+286 (OQ208) - 2


Blandford-Rees model of source
Radio emission originates both from the the quasi-steady jet itself (emission region or "core" extends from rmin to rmax and rmax~f-1) and from behind strong shock waves which are formed behind dense condensations that are accelerated to relativistic speeds by the flow


Types of motion
1. Linear motion with constant velocity z = 1, V = 100 km/s

µ

0.01 µas/y

1044+719 (defining) z=1.150, scale = 8.286 kpc/" |µ| (43 ± 2) µas/y, VT 1.2c (apparent)


Motion of 1044+719


Motion of the defining source 0014+813
2. Precession motion of steady jet + clouds accelerated by the jet


Statistic of motion

1.4 1.2 1.0 0.8



0.30 0.25 0.20

µ µ

proper motion, mas/year
10 100 1000

0.6

0.15 0.10 0.05 0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 10 100 1000

coordinates, mas

0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4

Nobs

No b s


Difference radio and optical positions
140 120 100 80



N

60 40 20 0 -20 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

261 QSO from ICRF with known red shift z and from catalog with optical positions There is systematical difference 10-40 µas (Assafin et al., 2003) This difference can be explained by model of QSO




Physical distance between radio and optical centers of emission
60

50

Mean size is 300 ­ 500 pars ec s It means that in centers of radio emission will move relative the optical one with velocity 1-10 µas/year

40

N

30

20

10

0 0 500 1000 1500 2000 2500 3000

parsec


Rotation of the ICRF

- 3 2 1 r r s (t) = 3 1 - 1 s (t 0 ) , - 1 2 1 rr s (t), s (t 0 ) are the unit vectors of a source for moments t and t 0 = J2000.0


Rotation of the ICRF (1)


Rotation of the ICRF (2)


Rotation of the ICRF (3)


Variations of the EOP due to rotation of the ICRF
Difference between solutions : sai2008a.eops (ICRF) and sai2008c.eops (ICRFcorr+motions) sai2008a.eopi (ICRF) and sai2008c.eopi (ICRFcorr+motions)


Effect on the EOP






Difference of solutions and the EOPC04



Conclusions
Motion of radio source can be explained by simple physical models of motion of the emission region ("core"). Catalog of sources must contain both their coordinates and apparent motion terms. Rotation of the ICRF is transformed to secular variations of EOP.


Thank you!