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Wide Binary Stars in the Galactic Field - A Statistical Approach

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This thesis focuses on the statistical properties of wide binary (WB) star systems in the Galactic field. For the present study we select a homogeneous sample covering about 675 square degrees in the direction of the NGP. It contains nearly 670,000 MS stars with apparent magnitudes between 15 and 20.5 mag and spectral classes later than G5. The data were taken from the SDSS. We construct the two-point correlation function (2PCF) for angular separations between 2 and 30 arcsecs. The resulting clustering signal is modeled by means of the Wasserman-Weinberg technique. We show that the distribution of semi-major axis is consistent with the canonical Oepik law and infer that about 10% of all stars in the solar neighbourhood belong to a WB system. 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 4,000 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 difficult and often impossible. The novel procedure presented in this thesis can 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.

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