W. Thuillot (1) & M. Stavinschi (2)

(1) Institut de mécanique céleste et de calcul des éphémérides, Paris Observatory

(2) Astronomical Institute of the Romanian Academy, Bucharest

Introduction

The aim of this paper is to call for observers and to describe the objectives for a follow-up program of ground-based astrometric observations to compliment certain observations made by the space project GAIA. This program could, in the long term, relate to all the observational aspects of the GAIA project: photometry, spectroscopy and astrometry. But this paper relates to only the astrometric aspect for solar system objects and intends to call for astrometrists to join a dedicated network to carry out these observations.

Context and problems

The GAIA project is an astrometric, spectroscopic and photometric survey of stellar, galactic and extra-galactic objects. It will provide many measurements of positions of objects of the solar system at an unequalled precision. From 10^5 to 10^6 objects of the solar system, mainly asteroids but also KBO and comets, will be observed up to magnitude 20.

The expected precision is about 10 µas (microarcseconds) for the stellar objects with magnitude brighter than 15. It is about 200 µas for those of magnitude 20. This precision will be however degraded for the nonstellar and fast-moving objects, such as Near Earth Objects (NEOs). It could then be about several mas. One would expect to find many previously unknown objects belonging to the majority of the identified families, and in particular the NEOs.

The observation process of GAIA will not however make it possible to program in advance the confirmation observations of these objects. The faintest or fastest objects are then likely to be lost, in the same way they are sometimes lost following their detection by the large surveys currently in operation.

In order to ensure the maximum efficiency of the observations of detection by GAIA it is thus needed to organize observations "on alert" to check and follow-up from the ground. Several means make it possible to organize these observations, in particular the diffusion of alarms by Internet on mailing lists (Minor Planet Mailing List for example) or the maintenance of an official page of targets. Whichever means used, even if they reach a great number of potential observers, would not ensure that good reactivity to alarms would occur. The constitution of a formalized network of dedicated observers appears necessary.

Towards a dedicated network

The Solar System Working Group of GAIA (SSWG), coordinated by F. Mignard (Observatoire de la Cote d'Azur, France) and V. Zappala (Torino Observatory), undertakes exploratory studies for the observation programme of the solar system by GAIA. Within this framework it plans to constitute a network for ground-based observations to follow-up GAIA observations with the principal aim to confirm new target detections by GAIA and to carry out any astrometric measurements necessary for the improvement of the orbital elements and the cataloguing of these objects.

Robotic telescopes

This network could very effectively be based on the use of robotic telescopes. Several projects are already in progress or under development and some will be operational during the GAIA mission, currently envisaged from 2009 to 2015 (see for example Hessman, 2002). In France, the ARAGO project (PI M. Boër, CESR/OMP Toulouse, France) whose speed of pointing will make it possible to observe on alarm for multiple programs, appears perfectly well suited to this type of task. Mainly dedicated to the study of the transitory events, Gamma Ray Bursts in particular, this project has included for this year a contribution for GAIA follow-up observations within the whole of the programs which it is envisaged to carry out. Other robotic telescopes would obviously be welcome.

Conventional telescopes

Robotic telescopes should make it possible to immediately carry out part of the observations after an alarm is given by GAIA. However, these projects will be certainly far from providing an adequate cover of the sky in longitude and latitude, and their number will be insufficient to combat the weather risks. So part of this network will have to consist of traditional telescopes.

Telescopes of the 1-2m class will be particularly well suited, mainly due to access to these telescopes being more flexible than that for larger telescopes. Also, CCD cameras on these telescopes are generally well suited for accurate astrometry of fast and eventually faint objects. Nevertheless, smaller telescopes could also be used for the brighter objects.

Constraints

The constraints imposed by the objective of the GAIA follow-up network will mainly be the following ones:

It will be necessary to have a possibility of access to the telescopes "on alert" by a local observer. The process of observation of GAIA will allow that alarm to be given approximately 24 hours after the detection of an uncatalogued target. It will be accompanied by a provisional ephemeris for the object allowing a sufficiently precise pointing for a ground-based telescope and to limit the difficulties of detection for a few consecutive nights. It is however difficult, at this date, to estimate the number of objects that will not have been discovered by 2009, thus indicating what will be the frequency of access to these telescopes.

It will be necessary to have sensitive enough CCD cameras to detect, if possible, objects as faint as magnitude 20, and image sizes corresponding to less than 1" on the sky.

It will be necessary to have a field of view of about 10' across, giving sufficient density of catalogued stars to ensure a reduction of precise stellar astrometry. During the mission, if a preliminary catalogue from GAIA is available, smaller fields could be usable.

It will be desirable that certain sites are of sufficient quality (high altitude) to reach observations with small solar elongation. GAIA will detect objects with a solar elongation of 35 degrees.

Actions to be carried out

Constituting a network of observers can be done starting from existing structures. One of these structures is the working group of IAU Commission 8 "Future developments in ground-based astrometry", coordinated by M. Stavinschi and J. Kovalevsky.

This working group has made a census of the characteristics of telescopes and carried out publicity campaigns towards potential observers by electronic mail in order to set up a beginning of the network by 2004 (Thuillot and Stavinschi, 2002). A questionnaire is still available on the web page of the IAU Commission 8 Working Group (http://www.astro.ro/wg2.htm) in order to establish contact with potential observers and to collect useful parameters about their instrumentation.

Once a network has been organized, it will then be possible, during the years 2005-2006, to think about a precise protocol for the acquisition of data as well as for the data processing. It will be necessary to test the astrometric performances on representative targets, to set up the operations of data recovery, to develop and check a "pipeline" of astrometric reductions and then to prepare some experiments on full-scale alarms in order to be perfectly operational by 2009.

Conclusion

The GAIA project will be launched in 2010. This project will be a huge astrometric, spectroscopic and photometric survey of stellar, galactic and extra-galactic objects.

It will provide many positional measurements of objects of the solar system and numerous new objects will be detected. A follow-up program will be necessary to ensure these objects are not lost and to improve their ephemerides. Organizing a network requires time and different steps have to be done. We call observers interested in this network, and particularly astrometric observers, to join the network now to help follow-up GAIA.

Boër, M. et al.: 2002, ARAGO: a robotic observatory for the variable sky, SPIE

Hessman, F.V. : 2002, serveur MONET, Links to Robotic Telescope Projects : http://alpha.uni-sw.gwdg.de/~hessman/MONET/links.html

Mignard, F: 2002, Observations of Solar System objects with GAIA. I. Detection of NEOS, A&A 393,727

Perryman, M. et al.: 2001, GAIA: composition, formation and evolution

of the Galaxy, A&A 369, 339

Thuillot, W., Stavinschi M.: 2002, Follow-up GAIA: call for a dedicated network, http://www.astro.ro/wg2.htm

Further information can be obtained from W. Thuillot (IMCCE/Paris observatory) at the address : thuillot@imcce.fr
 

If you are interested by this project, please could you fill in the questionnaire given here and return it by email to: thuillot@imcce.fr or magda@aira.astro.ro

1.Name(s) and contact address(es) of potential observers (postal and electronic addresses)

2.Site of observation (name, coordinates and elevation)

3.Telescope characteristics (diameter, focal length)

4.Camera characteristics (size of pixels, scale, limiting magnitude)

5.Do you intend to upgrade your system in the next few years?

6.Access to the telescope:
 

1.Time delay estimated to observe after an alert is given

2.Authority (name and address) with which an agreement could be established in order to help with observing or accessing the instruments.

7.Comments