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         Gazette IERS Gazette IERS Gazette IERS Gazette IERS Gazette
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         No 25, 23 December 1997    /
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         Title:   Adoption of ICRS by the International Astronomical Union
         Authors: M. Feissel, C. Ma, F. Arias, F. Mignard


   At its 23rd General Assembly in August 1997, the International Astronomical 
Union (IAU) decided that 

- as of 1 January 1998, the IAU celestial reference system shall be the 
  International Celestial Reference System (ICRS) as specified in the 1991 
  IAU Resolution on Reference Systems and as defined by the IERS;

- the corresponding fundamental reference frame shall be the International
  Celestial Reference Frame (ICRF) constructed by the relevant IAU Working 
  Group;

- the Hipparcos Catalogue shall be the primary realization of the ICRS at 
  optical wavelengths;

- IERS should take measures, in conjunction with the appropriate IAU Working 
  Group, to maintain the ICRF and its ties to celestial reference frames at 
  other wavelengths.

   The new IAU celestial reference system replaces the Fundamental Katalog 
No 5 (FK5) that gave star coordinates in a reference system based primarily 
on the dynamics of the solar system. This change is deeper than a mere 
replacement of a  reference  system by a better one. It implies a drastic 
modification in the underlying concepts and has practical consequences at 
various levels for the users needing the best astrometric accuracy.  

   The ICRF consists of the equatorial coordinates of 212 compact and well 
observed extragalactic objects (mainly quasars), determined by Very Long 
Baseline radio Interferometry (VLBI) (Ma et al. 1998). Coordinates are also 
available in the ICRS for 294 compact sources that are candidates for future 
densification of the frame when their positions are improved through further  
observations. Lesser quality positions for 102 other sources are provided to 
be used possibly for the tie of celestial frames at other wavelengths or  
similar purposes. Detailed information on ICRF is available in printed form 
(Ma and Feissel 1997) and via ftp and World Wide Web (hpiers.obspm.fr). 

   Thanks to the observation by Hipparcos of the complete list of FK5 sources,
the offset of the pole and right ascension origins of the FK5 in the ICRS are 
known within +/- 0.002". They are smaller than 0.03".

   The meaning and consequences of the IAU decisions are outlined by Feissel 
and Mignard (1998). The major practical impact for astronomy may be summarized 
as follows. 

- For all uses with accuracy requirement less stringent than 0.05", the 
  adoption of ICRS has no significant effect.

- There is no epoch attached to the system, i.e., future updates of the ICRF 
  will not change the ICRS defining directions. Changes of stellar positions 
  between two epochs are derived by allowing for proper motion. The new 
  positions are still referred to the ICRS.

- Precession-nutation models include two constants to refer the celestial
  motion of the pole to the ICRS pole rather than to the mean pole at J2000.0. 
  These constant terms may be re-evaluated in the future by IERS together with 
  new estimations of the location of the mean pole (see IERS Gazette No 18, 
  16 July 1997).
 
- The directions of celestial objects in the ICRS are consistent with the 
  terrestrial coordinates in the ITRS by the use of the IERS Earth orientation 
  parameters (universal time, polar motion, precession-nutation).    

   The IAU Division 1 (Fundamental Astronomy) has set up an ICRS Working Group, 
in charge of studying all consequences of the adoption of the ICRS, and 
proposing solutions. The ICRS Working Group is chaired by Francois Mignard 
(mignard@obs-azur.fr). 

   The maintenance of the ICRF by the IERS includes two major contributions, 
both to be implemented in cooperation with the ICRS Working Group:

- the organisation of VLBI observations and analyses to monitor the 
  direction and the structure of the radio sources, 

- the astrometric and statistical assessment of the source coordinates in 
  the ICRS, and the monitoring of the tie of celestial reference frames at  
  other wavelengths. 

The corresponding tasks are shared between the VLBI Coordinating Centre and 
the ICRS Section of the Central Bureau.

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References:
 
Feissel, M., Mignard, F., 1998: "The adoption of ICRS on 1 January 1998: 
Meaning and Consequences", Astron. Astrophys. Letter to the Editor. Accepted.

Ma, C., Feissel, M., 1997: Definition and Realization of the International 
Celestial Reference System by VLBI Astrometry of Extragalactic Objects, 
IERS Technical Note No 23, Observatoire de Paris.

Ma, C., Arias E.F., Eubanks T.M, Fey A.L, Gontier A.-M, Jacobs C.S., Sovers 
O.J., Archinal B.A., Charlot P., 1998: The International Celestial Reference 
Frame as realized by Very Long Baseline Interferometry, Astron. J., Submitted.
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