Gaia photometry for white dwarfs^⋆,⋆⋆

¹ Departament d’Astronomia i Meteorologia, Institut del Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), c/ Martí i Franquès, 1, 08028 Barcelona, Spain
e-mail: carrasco@am.ub.es; carme@am.ub.es; xluri@am.ub.es
² Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
e-mail: s.catalan-ruiz@warwick.ac.uk
³ Hubble Fellow, Space Telescope Science Institute, 700 San Martin Drive, Baltimore, MD 21218, USA
e-mail: tremblay@stsci.edu
⁴ Centre for Astrophysics Research, University of Hertfordshire, Hatfield, AL10 9AB, UK
e-mail: r.napiwotzki@herts.ac.uk
⁵ Université de Franche-Comté, Institut Utinam, UMR CNRS 6213, OSU Theta, BP 1615, 25010 Besançon Cedex, France
e-mail: annie@obs-besancon.fr
⁶ Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich, 52425 Jülich, Germany
e-mail: p.kowalski@fz-juelich.de

Received: 19 October 2012
Accepted: 17 March 2014

Abstract

Context. White dwarfs can be used to study the structure and evolution of the Galaxy by analysing their luminosity function and initial mass function. Among them, the very cool white dwarfs provide the information for the early ages of each population. Because white dwarfs are intrinsically faint only the nearby (~ 20 pc) sample is reasonably complete. The Gaia space mission will drastically increase the sample of known white dwarfs through its 5–6 years survey of the whole sky up to magnitude V = 20–25.

Aims. We provide a characterisation of Gaia photometry for white dwarfs to better prepare for the analysis of the scientific output of the mission. Transformations between some of the most common photometric systems and Gaia passbands are derived. We also give estimates of the number of white dwarfs of the different galactic populations that will be observed.

Methods. Using synthetic spectral energy distributions and the most recent Gaia transmission curves, we computed colours of three different types of white dwarfs (pure hydrogen, pure helium, and mixed composition with H/He = 0.1). With these colours we derived transformations to other common photometric systems (Johnson-Cousins, Sloan Digital Sky Survey, and 2MASS). We also present numbers of white dwarfs predicted to be observed by Gaia.

Results. We provide relationships and colour–colour diagrams among different photometric systems to allow the prediction and/or study of the Gaia white dwarf colours. We also include estimates of the number of sources expected in every galactic population and with a maximum parallax error. Gaia will increase the sample of known white dwarfs tenfold to about 200 000. Gaia will be able to observe thousands of very cool white dwarfs for the first time, which will greatly improve our understanding of these stars and early phases of star formation in our Galaxy.