Wide binary stars in the Galactic field : a statistical approach

This thesis focuses on the statistical properties of wide binary (WB) star systems in the Galactic field. With projected separations larger than 200 AU and, consequently, having very low binding energies, WB are sensitive probes of the Galactic gravitational potential making them an interesting tool to constrain the dark matter (DM) content in the Milky Way Galaxy. For the present study we select a homogeneous sample covering about 675 square degrees in the direction of the Northern Galactic Pole. It contains nearly 670000 main sequence stars with apparent magnitudes between 15 and 20.5 mag and spectral classes later than G5. The data were taken from the Sloan Digital Sky Survey. We construct the two-point correlation function (2PCF) for angular separations between 2 and 30 arcseconds. The resulting clustering signal is modelled by means of the Wasserman-Weinberg technique. We show that the distribution of semi-major axis is consistent with the canonical …pik law and infer that about 10% of all stars in the solar neighbourhood belong to a WB system. The 2PCF method is, however, seriously limited by the noise from optical pairs, especially for the widest systems, which would provide the most stringent constraints of the DM's nature. To reduce the noise from optical pairs and to increase the sensitivity of the analysis at larger separations, we include distance information from photometric parallaxes. Introducing a novel weighting procedure based on the binding probability of a double star, we infer the distribution of colours and mass ratios, which were carefully corrected for observational selection effects. About 4000 WBs were taken into account statistically, whose components have masses between 0.2 and 0.85 solar masses. We find that the WB colour distribution is in accord with the colour distribution of single field stars. However, pairs with a mass difference exceeding 0.5 solar masses seem to be systematically underrepresented as compared to a random pairing of field stars. Our results are broadly in agreement with prior studies but a direct comparison is often difficult or even impossible. We compiled a 'ranked list' of WB candidates that will prove to be useful for follow-up studies. Due to lack of information about the relative velocities of the pairs, the limitation concerning the widest pairs could not be entirely overcome. A further drawback of our approach is clearly the need for a sophisticated modelling to allow for selection effects. The method, however, successfully circumvents the limitations of studies based on proper motions. The novel procedure presented in this thesis can therefore be regarded as complementary to common proper motion studies, and constitutes a viable approach to study the statistical properties of WBs in the Galactic field.