6th Gaia CU9 validation plenary

‘To contribute usefully to the scientific progress, one must sometimes not disdain from undertaking simple verifications’ (Foucault, 1847).

Despite the precautions taken when building the Gaia data processing, completely avoiding errors in a one billion+ source catalogue, with many intricate data for each object, is an impossible task. Before the publication of the Gaia archive, an in-depth validation of its contents has thus to be undertaken by the Gaia DPAC. It relies on methods and tools developed in the CU9 Work Package 940 in charge of the scientific validation of the catalogue.

Ensuring there are no systematic errors will lead to check for suspect spatial (or epoch, colour, magnitude, ...) variations, which could be due to observational or processing artefacts. The validation process will thus address the statistical distribution of the astrometric parameters, typically the unbiasedness of e.g. the parallaxes and proper motions, which are the main outcomes of the mission, and which may be subject to many perturbations. It will also scrutinize the photometric and spectroscopic data, assisted by the important classification task performed in Gaia, and, when available, external data.

Most of the validation work is being done with 8 work packages:

• WP942: Internal consistency and more complex scenarios
• WP943: Comparing models with data.
• WP944: Confrontation with external archives.
• WP945: Statistical tools
• WP946: Time series and variability
• WP947: Clusters as validation tools
• WP948: Solar System Objects

Besides the monthly telecons with the work package managers, it has been decided to have two plenary validation meetings per year.

• The first meeting in Vienna in July 2014 described the work done in the various Work Packages.
• The CU9(940) and GENIUS (T5) workshop in Meudon on March 2-4, 2015 rather intended to enter more into the details of all the validation procedures, by familiarising with the validation development and runtime environments, studying and improving current methods, running tests on existing or simulated catalogues.
• The Barcelona meeting on Sept. 9, 2015 allowed to analyse the first validation results, either done in CU3 or in the validation group.
• The Paris meeting on May 26-27, 2016, presented the validation work done within the CUs and CU9 Work Packages and prepared for a re-run (i.e. re-doing validation after corrections) before the Gaia DR1 publication.
• The Sitges meeting on Jan 26, 2017 started to plan the resources needed for DR2

This Paris meeting, which is the sixth plenary, will:

• present the validation work done within the CUs and CU9 Work Packages on the preliminary DR2 data
• discuss about the problems found, what should be: accepted (i.e. documented) or corrected (objects deleted, e.g. artefacts)
• prepare for the validation of the final DR2 data, i.e. the tests which appeared missing on the preliminary data.