Why are there so few cool Ap stars? The statistical, magnetic and spectroscopic properties of the brightest cool A and F type classical Ap stars

Organised magnetic fields have been found to exist in about 5% of main sequence F, A and B stars, with typical strengths ranging from a few hundred to a few tens of thousands of gauss. These magnetic fields lead to important chemical peculiarities in the atmospheres of these stars which define their spectral classification (the so-called Ap/Bp stars). There is an important decrease in the fraction of magnetic stars as one moves toward the cooler A and F type stars. This thesis investigates the magnetic and spectroscopic characteristics of these cooler magnetic stars, in an attempt to understand their statistical properties.;Secondly, the magnetic and spectroscopic characteristics of an observational sample of 36 cool classified Ap stars observed at the Telescope Bernard-Lyot using the MuSiCoS spectropolarimeter are reported and analyzed.;Ten of the undetected stars are discussed regarding some peculiarities in their Least-Squares Deconvolved line profiles, leading to suggestions as to why the Ap/Bp spectral classification might be wrong. The 23 null detections obtained questions the Ap/Bp nature of many of these stars, suggesting the statistical incidence of cool Ap stars may be significantly lower than the value calculated in this thesis. (Abstract shortened by UMI.).;First, I redetermine the statistical incidence of chemically peculiar stars using the Bright Star Catalogue (Hoffleit et al., 1995) and the General Catalogue of Ap and Am stars (Renson et al., 1991) and compare with a previous statistical study performed by Wolff (1968).