Letter to the Editor

Magnetic fields in Herbig Ae stars^*

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: shubrig@eso.org
² Central Astronomical Observatory of the Russian Academy of Sciences at Pulkovo, 196140 Saint-Petersburg, Russia
³ Isaac Newton Institute of Chile, St.-Petersburg Branch, Russia

Received: 9 August 2004
Accepted: 12 October 2004

Abstract

Herbig Ae stars are young A-type stars in the pre-main sequence evolutionary phase with masses of ~1.5–3 . They show rather intense surface activity (Dunkin et al. [CITE], MNRAS, 290, 165) 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. [CITE], MNRAS, 291, 658), 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 σ level,  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 σ and ~2.5 σ level respectively. Further, we report the presence of circular polarization signatures in the  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.