Methodology: precise astrometry from the ground

The stars members of an association were all born from the same molecular cloud with its own original momentum. At the end of the formation process, all the members move together with space motions similar to the parent cloud, making it an extremely effective method of identification. Field stars indeed have random proper motions (i.e., motions across the plane of the sky) while background galaxies have no measurable proper motion. Any object displaying a proper motion similar to the group is therefore most likely a group member.

To identify the members of young nearby associations, we will measure the proper motion for millions of sources around the association. Objects moving at the same speed and in the same direction as the group and having spectral properties consistent with the group will be genuine members.


Measuring proper motions for millions of sources with an accuracy sufficient to evaluate their membership is a major technical challenge. Because the nearest  young associations are far away (between 3 and 15x1015 km!), their memebrs move very slowly on the plane of the sky. The fastest move by 20 to 50 millarcsecond per year. A milliarcsecond is about the size of a 10 cent coin atop the Eiffel Tower as seen from New York. Detecting and measuring such motions requires high quality images and a long time baseline.

By combining archival visible and near-infrared images obtained 15 to 20 years ago  with new images, we can derive proper motion and multi-wavelength photometry for millions of stars in young (<100Myr) nearby (<500pc) associations and clusters.

© H. Bouy

The first step consists in downloading and pre-processing thousands of wide-field images from all major astronomical facilities around the world. Then, for each instrument, we compute a very fine astrometric calibration to get rid of the instrumental distortions.

The cartoon on the right summarizes the various steps of this astrometric analysis:

1. First the input catalogs are read...

2. and then sorted by instrument.

3. then the pixel scales and position angle of each instrument are computed by cross-matching with an external catalog (usually 2MASS).

4. the high order distortion correction is computed using the overlapping plates method. This method takes advantage of the fact that a given star will fall at several positions in the detector.

5. once the distortions are corrected...

6. a photometric solution is computed for every photometric instruments

7. and the proper motion of all the sources are then computed by fitting a linear motion to the multi-epoch measurements, taking into account outliers and uncertainties.

8. finally, the results, including the astrometry and multi-wavelength photometry, are written in a table.

Using this method we achieve a precision of a 10 to 25 milliarcsecond per epoch. By combining many epochs covering a long time baseline, we can then beat further the systematics and derive proper motions with an accuracy better than 1 mas/yr.

© H. Bouy

© Last Update: 02-05-2017 by H. Bouy