Herbig Ae stars are young A-type stars in the pre-main sequence evolutionary phase with masses of \sim 1.5–3 M _ { \odot } .
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 , \left < { \cal B } _ { z } \right > = - 450 \pm 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 \sim 1.6 \sigma and \sim 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 .