Volume 569, September 2014
|Number of page(s)||16|
|Section||Galactic structure, stellar clusters and populations|
|Published online||16 September 2014|
Binary white dwarfs in the halo of the Milky Way⋆
Department of Astrophysics/IMAPPRadboud University Nijmegen,
PO Box 9010, 6500 GL
2 Institute for Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
3 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
4 Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
Accepted: 30 June 2014
Aims. We study single and binary white dwarfs in the inner halo of the Milky Way in order to learn more about the conditions under which the population of halo stars was born, such as the initial mass function (IMF), the star formation history, or the binary fraction.
Methods. We simulate the evolution of low-metallicity halo stars at distances up to ~3 kpc using the binary population synthesis code SeBa. We use two different white dwarf cooling models to predict the present-day luminosities of halo white dwarfs. We determine the white dwarf luminosity functions (WDLFs) for eight different halo models and compare these with the observed halo WDLF of white dwarfs in the SuperCOSMOS Sky Survey. Furthermore, we predict the properties of binary white dwarfs in the halo and determine the number of halo white dwarfs that is expected to be observed with the Gaia satellite.
Results. By comparing the WDLFs, we find that a standard IMF matches the observations more accurately than a top-heavy one, but the difference with a bottom-heavy IMF is small. A burst of star formation 13 Gyr ago fits slightly better than a star formation burst 10 Gyr ago and also slightly better than continuous star formation 10−13 Gyr ago. Gaia will be the first instument to constrain the bright end of the field halo WDLF, where contributions from binary WDs are considerable. Many of these will have He cores, of which a handful have atypical surface gravities (log g < 6) and reach luminosities log (L/L⊙) > 0 in our standard model for WD cooling. These so called pre-WDs, if observed, can help us to constrain white dwarf cooling models and might teach us something about the fraction of halo stars that reside in binaries.
Key words: Galaxy: halo / stars: luminosity function, mass function / white dwarfs / binaries: close
Appendices are available in electronic form at http://www.aanda.org
© ESO, 2014
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