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A new heart for the Gaia Object Generator

The first Gaia data release is imminent and it is going to revolutionize the study of the Galactic structure and evolution. As part of its preparation, the Data Processing and Analysis Consortium (DPAC) Coordinate Unit 9 is in charge to guarantee 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, as its heart, the new Galactic Thin Disc Hess Diagram presented in the figure. This simulation will be used for Gaia data validation, data archive and scientific exploitation.
Thanks to the good distances that Gaia will provide, the physical parameters of the stars, such as brightness or temperature, will be better determined, thus the stellar evolution will be better understood. The evolution of a star, over time, depends on its nature. During its live the star brightness and temperature changes, 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 the stellar evolution are clearly and beautifully 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). This simulations, together with the first Gaia data release, will provide an excellent tool to test key hypothesis on the fundamental Galactic parameters. As an example, see how the shifting of the Turn Off with age can be clearly seen. Its study, related with 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 youngest stellar populations. The White Dwarfs are treated separately, 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.
To sum up, by improving in such a way the stellar population synthesis of the Milky 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.

GIF - 194.8 ko
The new Galactic Thin Disc Hess Diagram ready to be integrated in the Gaia Object Generator (Luri et al. 2014). It shows the stellar density in the Solar Neighbourhood distributed in temperature and brightness, and splitted in age intervals from 0 to 10 Gyrs. See how the signature of the stellar evolution are clearly and beautifully seen. It was simulated using the Besançon Galaxy Model (Robin et al. 2003) with the Galactic Thin Disc treatment from Czekaj et al. (2014). This 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 youngest populations. White Dwarfs are treated separately, thus not included here.

More information on ESA website