The first Gaia data release, scheduled for end of the summer 2016, is going to revolutionise the study of Galactic structure and evolution. As part of its preparation, the Data Processing and Analysis Consortium (DPAC) Coordination Unit 9 is in charge of guaranteeing that the data has passed a set of quality tests. These tests ensure that the published data are valid for scientific exploitation. The Besançon Galaxy Model (Robin et al. 2003, 2012), which is integrated in the Gaia Object Generator (GOG, Luri et al. 2014), is being used as a tool for Gaia data validation tests. A new run of GOG, to be executed by the Barcelona team, is expected for February 2016. It will provide a simulation of the full sky as seen by Gaia, using, at its heart, the new Galactic Thin Disc Hess Diagram presented in the above figure. This simulation will be used for Gaia data validation, in the data archive, and for scientific exploitation.  

Thanks to the very precise distances that Gaia will provide, the physical parameters of the stars, such as brightness or temperature, will be better determined. Thus, stellar evolution will be better understood. The evolution of a star, over time, depends on its nature. During its life, a star's brightness and temperature change, and as a consequence its position in the Hess Diagram changes too. Notice that the diagram x-axis makes reference to the star temperature, and the y-axis makes reference to its brightness. The animated view shows, for each age interval, the number of stars per cubic parsec in the Solar Neighbourhood. Notice that the signatures of stellar evolution can be clearly seen. The presented diagram was simulated using the Besançon Galaxy Model (Robin et al. 2003) with the Galactic Thin Disc component from Czekaj et al. (2014). The model machinery incorporates spatial density distributions to extend the stellar population densities in the Solar Neighbourhood through the whole galaxy. It translates the Hess Diagram for each Galactic component (thin disc, thick disc, halo, etc.) to a full-sky simulation with its Gaia observables (parallax, apparent magnitude, and colours). These simulations, together with the first Gaia data release, will provide an excellent tool to test key hypotheses on fundamental Galactic parameters. As an example, notice how the shifting of the Turn Off with age can be clearly seen. Its study, related to the Galactic Thin Disc age and stellar formation bursts, is one of the challenges of Gaia.

This Hess Diagram represents a big improvement in the generation of important Galaxy tracers, such as giant stars at different stages of evolution (i.e., Red Clump), and also in pre-main sequence stars at low masses for the youngest stellar populations. White Dwarfs are treated separately and thus not included here. The above mentioned diagram areas are statistically robust thanks to the rescale of 400 runs of the Solar Neighbourhood Sphere, with a total of about 925 million simulated stars.

By improving the stellar population synthesis of the Milky Wayin such a way, we are striking double: we have a better tool to validate Gaia data releases (specially the Tycho-Gaia Astrometric Solution) and a valuable starting point for the scientific exploitation of the Gaia data. 

Note: DPAC will be releasing its own estimates of stellar parameters, starting in Gaia-DR2 (see CU8 paper by Bailer-Jones et al. 2013).

Credits: ESA/Gaia/DPAC/CU2/R. Mor (ICCUB-IEEC, Barcelona University), A.C. Robin (OSU THETA and Institut UTINAM, Besançon), F. Figueras (ICCUB-IEEC, Barcelona University)

[Published: 11/02/2016]