White dwarf population
White dwarfs are the end-product of the evolution of low-intermediate mass stars. The evolution of white dwarfs is driven by a simple cooling process, which has a duration of the order of the age of the Galaxy. Thus, they have imprinted a memory of the various episodes that the Galaxy has been subject to over its history, constituting useful objects to probe its structure and evolution. One of the fundamental tools for studying the properties of the white dwarf population is the luminosity function, which is the number of white dwarfs per volume in a magnitude bin. Different surveys, e.g. the Sloan Digital Sky Survey, SDSS, have provided accurate data that has allowed to improve considerable the knowledge of the thin disk white dwarf luminosity function. However, the thin disc luminosity function is a complicated convolution of the star formation history of the Galaxy, the initial mass function, the initial-final-mass relationship, secular evolution of the thin disk scale height and other effects. A deconvolution is thus extremely difficult. On the other hand, the interpretation of the thick disc/halo luminosity function is much more straightforward. The formation history of the halo and thick disc was probably complex, but observations of field stars and globular clusters indicate that most stars are very old. So we can assume that both halo and thick disc were formed over a short period of time with no further star formation afterwards. The study of this population is particularly interesting as it allows us to study some fundamental properties directly, like the initial mass function.
Wide binaries containing white dwarfs
The evolution of white dwarfs can be described as a cooling process, which is relatively well understood at the present moment (Salaris et al. 2000). The total age of a white dwarf can be expressed as the sum of its cooling time and the pre-white dwarf lifetime of its progenitor. If the age of the companion of the white dwarf can be determined with relative accuracy, we use the binary system to better understand the white dwarf member, in our case, to improve the initial-final mass relationship of white dwarfs, specially at the low-mass domain. On the contrary, if the companion of the white dwarf is a low-mass star and no age indicators are available, the white dwarf member itself is used to constrain the age of the system. In the latter case, this makes possible a calibration between the activity and age of the main-sequence star.
WFCAM --- WFCAM - The UKIRT Wide Field Camera.
WTS --- WTS - WFCAM Transit Survey.
Alhambra --- Alhambra Survey.