High-precision magnetic field measurements of Ap and Bp stars

In this paper we describe a new approach for measuring the mean longitudina l magnetic field and net linear polarization of Ap and Bp stars. As was dem onstrated by Wade et al., least-squares deconvolution (LSD; Donati et al.) provides a powerful technique for detecting weak Stokes V, Q and U Zeeman s ignatures in stellar spectral lines. These signatures have the potential to apply strong new constraints to models of stellar magnetic field structure . Here we point out two important uses of LSD Stokes profiles. First, they can provide very precise determinations of the mean longitudinal magnetic f ield. In particular, this method allows one frequently to obtain 1 sigma er ror bars better than 50 G, and smaller than 20 G in some cases. This method is applicable to both broad- and sharp-lined stars, with both weak and str ong magnetic fields, and effectively redefines the quality standard of long itudinal field determinations. Secondly, LSD profiles can in some cases pro vide a measure of the net linear polarization, a quantity analogous to the broad-band linear polarization recently used to derive detailed magnetic fi eld models for a few stars (e.g. Leroy et al.). In this paper we report new high-precision measurements of the longitudinal fields of 14 magnetic Ap/B p stars, as well as net linear polarization measurements for four of these stars, derived from LSD profiles.