  Herbig Ae stars are young A-type stars in the pre-main sequence evolutionary
phase with masses of ~1.5-3 M_o. They show rather intense surface activity
(Dunkin et al. 1997) and infrared excess related to the presence of
circumstellar disks. Because of their youth, primordial magnetic fields
inherited from the parent molecular cloud may be expected, but no direct
evidence for the presence of magnetic fields on their surface, except in one
case (Donati et al. 1997), has been found until now. Here we report
observations of optical circular polarization with FORS 1 at the VLT in the
three Herbig Ae stars HD 139614, HD 144432 and HD 144668. A definite
longitudinal magnetic field at 4.8 sigma level, <B_z>=-450+-93 G, has been
detected in the Herbig Ae star HD 139614. This is the largest magnetic field
ever diagnosed for a Herbig Ae star. A hint of a weak magnetic field is found
in the other two Herbig Ae stars, HD 144432 and HD 144668, for which magnetic
fields are measured at the ~1.6 sigma and ~2.5 sigma level respectively.
Further, we report the presence of circular polarization signatures in the Ca
II K line in the V Stokes spectra of HD 139614 and HD 144432, which appear
unresolved at the low spectral resolution achievable with FORS 1. We suggest
that models involving accretion of matter from the disk to the star along a
global stellar magnetic field of a specific geometry can account for the
observed Zeeman signatures.
