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

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 (v sin i < 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.

Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412

“…the presence of small shifts in the LSD Stokes V profiles obtained for the neutral Fe and Ti ii lists indicates that we probably observe a contamination by the surrounding warm circumstellar (CS) matter in the form of a wind. Such a scenario was already discussed by Järvinen et al (2015) and would be in agreement with the previous finding of Hubrig et al (2009) who reported a strong contamination of the UVES spectra of this star by weak lines of neutral and ionized iron (see their Fig. 6), where the lines of neutral iron are by far more numerous than those of ionized iron.…”

“…The absence of δ Scuti-like pulsations was already discussed in the previous work of Järvinen et al (2015) where the authors verified that no changes in the line profile shape or radial velocity shifts are present in the obtained spectra on a short-time scale. This step was of importance since pulsations are known to have an impact on the analysis of the presence of a magnetic field and its strength (e.g.…”

“…The results of our measurements of the mean longitudinal magnetic field based on different samples are comparable within the measurement accuracies using the LSD method. Järvinen et al (2015) revealed significant differences between the appearance of Zeeman features using the sample of Fe ii lines and Fe i lines formed at T eff = 8 300 K. This difference disappears if the LSD profiles are calculated for the sample of Fe i lines at T eff = 10 000 K. Järvinen et al discuss that the differences between the measurements using iron line samples can be caused by temperature inhomogeneities or by a contamination by the surrounding warm CS matter. If temperature spots are indeed present on the surface of Herbig Ae stars, then it could be suggested that Fe i lines are formed in somewhat cooler spots.…”

“…Hubrig et al 2014). Järvinen et al (2015) already mentioned the fact that rather large differences in the shape of Zeeman features are observed between the Fe i and Fe ii samples on the first four epochs. In the LSD Stokes V profile obtained for Fe i lines, they observed a distinct feature appearing as a second maximum in the red wing of the Zeeman profile.…”

“…In contrast to low-resolution spectropolarimetry, the high-resolution polarimetric spectra allow us to study in detail surface abundance inhomogeneities and, in particular, how different elements with different abundance distributions across the stellar surface sample the magnetic field. The same spectra were used by Järvinen et al (2015), where magnetic field measurements using iron lines were presented along with the magnetic field measurements of two other sharp-lined Herbig Ae stars, HD 104237 and HD 190073. The emphasis in that study of HD 101412 was however put on the investigation of the impact of the circumstellar matter on the observed line profiles.…”