Measuring the surface magnetic fields of magnetic stars with unresolved Zeeman splitting

Aims. High dispersion, archival spectra of magnetic Ap stars with resulted Zeeman components in Stokes I are used to derive a simple relation that can be utilised to estimate the mean surface field strengths of stars with v sin I 10 km s(-1). Methods. For each star, the mean surface field, as measured from the observed splitting in be H at 6149 A, is compared to the differential broadening of spectral lines with large and small Lande factors in order to produce a relation to estimate the field strengths or magnetic stars with unresolved Zeeman patterns. Results. The method is shown to be tellable for rotational velocities up to about 50 km s(-1) for field strengths down to about 5 kG. Conclusions. These results should allow for better constraints to be placed on the mean surface magnetic fields of Ap stars where Zeeman patterns are unresolved.