Characterising the magnetic fields of the Herbig Ae/Be stars HD97048, HD150193, HD176386, and MWC480

Abstract

Context. Our knowledge of the presence and the role of magnetic fields in intermediate-mass pre-main-sequence stars remains very poor.

Aims. We present the magnetic properties of four Herbig Ae/Be stars that have not been previously studied in detail.

Methods. Our results for the three Herbig Ae/Be stars HD 97048, HD 150193, and HD 176386 are based on multi-epoch low-resolution spectra obtained in spectropolarimetric mode with FORS 2 mounted on the VLT. New high-resolution polarimetric spectra were obtained for MWC 480 with the SOFIN spectrograph installed at the Nordic Optical Telescope. We discuss 41 FORS 2 low-resolution observations of the Herbig Ae/Be stars HD 97048, HD 150193, and HD 176386 and determine their rotational periods. Using stellar fundamental parameters and the longitudinal magnetic field phase curves, we place constraints on the magnetic field geometry. Three high-resolution circularly polarised SOFIN spectra obtained for MWC 480 were measured using the moment technique where wavelength shifts between right- and left-hand side circularly polarised spectra are interpreted in terms of a longitudinal magnetic field ⟨Bz⟩.

Results. Our search for periodicities resulted in P = 0.693 d for HD 97048, P = 1.317 d for HD 150193, and P = 0.899 d for HD 176386. The magnetic field geometry can likely be described by a centred dipole with a polar magnetic field strength Bd of several hundred Gauss. The longitudinal magnetic-field measurements of MWC 480 reveal the presence of a strong kG field, which was undetected in our previous low-resolution polarimetric observations with FORS 1. A weak magnetic field was detected in the circumstellar components of the Ca ii H&K lines and the Na i D lines, indicating a complex interaction between the stellar magnetic field and the circumstellar environment.