Gaia broad band photometry^⋆

¹ Departament d’Astronomia i Meteorologia, Universitat de Barcelona, c/ Martí i Franquès 1, 08028 Barcelona, Spain
e-mail: carme@am.ub.es; carrasco@am.ub.es
² Institut de Ciències del Cosmos, ICC-UB, c/ Martí i Franquès 1, 08028 Barcelona, Spain
³ Research and Scientific Support Department of the European Space Agency, European Space Research and Technology Centre, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
⁴ Institut d’Estudis Espacials de Catalunya (IEEC), Edif. Nexus, C/ Gran Capità 2-4, 08034 Barcelona, Spain
⁵ Niels Bohr Institute, Copenhagen University Juliane Maries Vej 30, 2100 Copenhagen Ø, Denemark
⁶ INAF, Padova Observatory, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Science Operations Department of the European Space Agency, European Space Astronomy Centre, Villanueva de la Cañada, 28692 Madrid, Spain

Received: 20 July 2010
Accepted: 3 August 2010

Abstract

Aims. The scientific community needs to be prepared to analyse the data from Gaia, one of the most ambitious ESA space missions, which is to be launched in 2012. The purpose of this paper is to provide data and tools to predict how Gaia photometry is expected to be. To do so, we provide relationships among colours involving Gaia magnitudes (white light G, blue G_BP, red G_RP and G_RVS bands) and colours from other commonly used photometric systems (Johnson-Cousins, Sloan Digital Sky Survey, Hipparcos and Tycho).

Methods. The most up-to-date information from industrial partners has been used to define the nominal passbands, and based on the BaSeL3.1 stellar spectral energy distribution library, relationships were obtained for stars with different reddening values, ranges of temperatures, surface gravities and metallicities.

Results. The transformations involving Gaia and Johnson-Cousins V − I_C and Sloan DSS g − z colours have the lowest residuals. A polynomial expression for the relation between the effective temperature and the colour G_BP − G_RP was derived for stars with T_eff ≥ 4500 K. For stars with T_eff < 4500 K, dispersions exist in gravity and metallicity for each absorption value in g − r and r − i. Transformations involving two Johnson or two Sloan DSS colours yield lower residuals than using only one colour. We also computed several ratios of total-to-selective absorption including absorption A_G in the G band and colour excess E(G_BP–G_RP) for our sample stars. A relationship involving A_G / A_V and the intrinsic (V − I_C) colour is provided. The derived Gaia passbands have been used to compute tracks and isochrones using the Padova and BASTI models. Finally, the performances of the predicted Gaia magnitudes have been estimated according to the magnitude and the celestial coordinates of the star.

Conclusions. The provided dependencies among colours can be used for planning scientific exploitation of Gaia data, performing simulations of the Gaia-like sky, planning ground-based complementary observations and for building catalogues with auxiliary data for the Gaia data processing and validation.