Accurate magnetic field measurements of Vega-like stars and Herbig Ae/Be stars

We obtained accurate circular spectropolarimetric observations of a sample of Vega-like and Herbig Ae/Be stars with FORS1 at the VLT in an attempt to detect their magnetic fields. No magnetic field could be diagnosed in any Vega-like star. The most accurate determination of a magnetic field, at 2.6 sigma level, was performed for the Vega-like star iota Cen, for which we measured [Bz] = -77 +/- 30G. In the prototype of Vega-like stars, the star beta Pictoris, which shows conspicuous signs of chromospheric activity, a longitudinal magnetic field is measured only at similar to 1.5s level. We diagnosed a longitudinal magnetic field for the first time at a level higher than 3 sigma for the two Herbig Ae stars HD31648 and HD 144432 and confirm the existence of a previously detected magnetic field in a third Herbig Ae star, HD 139614. Finally, we discuss the discovery of distinctive Zeeman features in the unusual Herbig Ae star HD 190073, where the Ca II doublet displays several components in both H and K lines. From the measurement of circular polarization in all Balmer lines from H-beta to H-8, we obtain [Bz] = + 26 +/- 34 G. However, using only the Ca II H and K lines for the measurement of circular polarization, we are able to diagnose a longitudinal magnetic field at 2.8 sigma level, [Bz] = + 84 +/- 30 G.