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 FORS 1 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 σ level, was performed for the Vega-like star ι Cen, for which we measured $\left=-77\pm30$ G. In the prototype of Vega-like stars, the star β Pictoris, which shows conspicuous signs of chromospheric activity, a longitudinal magnetic field is measured only at ~1.5 σ level. We diagnosed a longitudinal magnetic field for the first time at a level higher than 3 σ for the two Herbig Ae stars HD 31648 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 $\ion{Ca}{ii}$ doublet displays several components in both H and K lines. From the measurement of circular polarization in all Balmer lines from Hβ to H8, we obtain $\left=+26\pm34$ G. However, using only the $\ion{Ca}{ii}$ H and K lines for the measurement of circular polarization, we are able to diagnose a longitudinal magnetic field at 2.8 σ level, $\left=+84\pm30$ G.