Research Note

HARPS spectropolarimetry of three sharp-lined Herbig Ae stars: New insights ^⋆,⋆⋆

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: sjarvinen@aip.de
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁴ Niels Bohr Institute & Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5, 1350 Copenhagen, Denmark
⁵ Pulkovo Observatory, 196140 Saint-Petersburg, Russia
⁶ Saint Petersburg State University, Universitetski pr. 28, 198504 Saint Petersburg, Russia
⁷ Observatório Nacional/MCTI, Rua General José Cristino 77, CEP 20921-400 Rio de Janeiro, RJ, Brazil

Received: 6 July 2015
Accepted: 14 October 2015

Abstract

Aims. Recently, several arguments have been presented that favour a scenario in which the low detection rate of magnetic fields in Herbig Ae stars can be explained by the weakness of these fields and rather large measurement uncertainties. Spectropolarimetric studies involving sharp-lined Herbig Ae stars appear to be a promising approach for the detection of such weak magnetic fields. These studies offer a clear spectrum interpretation with respect to the effects of blending, local velocity fields, and chemical abundances, and allow us to identify a proper sample of spectral lines appropriate for magnetic field determination.

Methods. High-resolution spectropolarimetric observations of the three sharp-lined (vsini< 15 km s^-1) Herbig Ae stars HD 101412, HD 104237, and HD 190073 have been obtained in recent years with the HARPS spectrograph in polarimetric mode. We used these archival observations to investigate the behaviour of their longitudinal magnetic fields. To carry out the magnetic field measurements, we used the multi-line singular value decomposition (SVD) method for Stokes profile reconstruction.

Results. We carried out a high-resolution spectropolarimetric analysis of the Herbig Ae star HD 101412 for the first time. We discovered that different line lists yield differences in both the shape of the Stokes V signatures and their field strengths. They could be interpreted in the context of the impact of the circumstellar matter and elemental abundance inhomogeneities on the measurements of the magnetic field. On the other hand, due to the small size of the Zeeman features on the first three epochs and the lack of near-IR observations, circumstellar and photospheric contributions cannot be estimated unambiguously. In the SVD Stokes V spectrum of the SB2 system HD 104237, we detect that the secondary component, which is a T Tauri star, possesses a rather strong magnetic field ⟨B_z⟩ = 129 ± 12 G, while no significant field is present in the primary component. Our measurements of HD 190073 confirm the presence of a variable magnetic field and indicate that the circumstellar environment may have a significant impact on the observed polarization features.