GEPI

Supervisors :

• Carine Babusiaux (Observatoire de Paris)
• Frédéric Arenou (Observatoire de Paris)

The Gaia mission of the European Space Agency (ESA) aims to map our galaxy with an unprecedented astrometric precision. It is therefore very important that the data that will be published be rigorously validated to ensure an optimal quality in the Catalogue. These validations are done by one of the teams of the coordination unit CU9 of the Gaia DPAC Consortium (Data Processing and Analysis Consortium) commissioned by ESA of the Gaia catalogue production. As part of this thesis, we implemented all the necessary infrastructure to validate the Gaia catalogue by comparison with external catalogues. This last manages all the interactions with the global environment of validations and with the Gaia database. Then we developed a set of statistical tests to validate the data from the first Gaia catalogue (DR1). These tests relate in particular to the homogeneity of data on the sky, the quality of the positions and of photometry of all the stars of DR1 (more than a billion stars, V < 20) as well as that of the parallaxes and proper motions for Tycho-Gaia Astrometric Solution (TGAS) stars, around two million stars in common in Gaia and Tycho-2 catalogues (V < 12).

These DR1 statistical tests are operational and were applied to the data before their publication. This has improved the data (thus the quality of the catalogue) as well as allowed to characterize the statistical properties. This characterisation is essential for a correct scientific exploitation of the data. The first Gaia catalogue was released the 14th September 2016.

Among the objects that Gaia observes, there is a population of stars particularly interesting, the Red Clump (RC) stars, widely used for distance indicators. We developed and tested two methods to model the colour-colour (CC) and effective temperature - colour relations in all photometric bands, from the ultraviolet to the near infrared: 1. using theoretical models, and 2. empirically, based on a Monte Carlo Markov Chain (MCMC) method. To do so, we have very carefully selected samples of stars with a good photometric quality, a good metallicity determined by spectroscopy, an homogeneous effective temperature and a low interstellar extinction. These calibrations now allow us to characterize the RC in the Gaia G band.

From these CC and temperature-colour relations, we then developed a method that allows to derive intrinsic magnitudes, temperatures and extinctions of the RC stars. Estimates of colours and extinctions are tested on stars with spectroscopically measured effective temperatures and an extinction determined by the measurement of Diffuse Interstellar Bands (DIB). These intrinsic properties of RC stars will allow to characterize the Gaia RC and calibrate, within the Gaia G band, the absolute magnitude of this standard candle, first essential step of determining distances in the Universe.