Using mean polarization profiles to study stellar magnetic fields

Approximately 10--20% of moderate mass main sequence stars show marked chemical peculiarities. Characteristic chemical over- and underabundance patterns are well correlated with several physical attributes; most notably the existence or non-existence of large-scale ordered magnetic fields. The details of the origins of both the fields and abundance patterns are topics of continuing study, as are detailed descriptions of field structures in magnetic stars and the possible existence of magnetic fields in the canonically "non-magnetic" stars. We expect that mean spectral line profiles, calculated from entire stellar spectra in polarized and unpolarized light, will be useful in studying these topics.;Circular spectropolarimetric observations of 74 survey stars were obtained using the MuSiCoS spectropolarimeter in an attempt to detect magnetic fields. The sample observed includes normal B, A and F stars, emission-line B and A stars, Am stars, HgMn stars, lambda Boo stars and magnetic Ap stars. Using the multi-line analysis technique known as "Least-Squares Deconvolution" (LSD) to extract mean unpolarized (Stokes I) and circularly polarized (Stokes V) signatures from each spectrum, we find absolutely no evidence for magnetic fields in the normal, Am and HgMn stars, with considerably smaller upper limits on longitudinal field measurements than previously obtained for these objects. We conclude that if any magnetic fields exist in the photospheres of these stars, these fields are not ordered as in the magnetic Ap stars, nor do they resemble the fields of active late-type stars.;We also detect for the first time a field in the A2pSr star HD 108945 and make new precise measurements of longitudinal fields in five previously known magnetic Ap stars, but do not detect fields in five other stars classified as Ap SrCrEu.;Also presented is the first ever exploratory investigation into both the applicability of the LSD technique in analyzing stellar spectra, as well as an examination of how LSD implements the scaling in amplitude of both circularly and linearly polarized line profiles with line depth, Lande factor and magnetic field. The influence of rotation velocity on profile amplitude was also explored. This study shows that circularly polarized spectra are well suited to analysis by the LSD technique over a wide range of rotation velocities and magnetic fields and the scaling relations assumed in LSD theory are approximately correct. For linearly polarized profiles, the assumptions inherent in the LSD averaging procedures are only a good approximation for stars within limited regimes of rotation velocity and magnetic field and the scalings which arise from the LSD theory do not apply over much of parameter space.