M. A. Pogodin

The magnetic field in the photospheric and circumstellar components of Herbig Ae stars

Aims.We investigate the photospheric and circumstellar (CS) magnetic field components separately in seven Herbig Ae stars. Methods: The study is based on low-resolution (REœ 2000 and 4000) spectropolarimetric data collected from 2003 to 2005 at the Very Large Telescope (ESO, Chile) with the multi-mode instrument FORS1. Results: We show that the spectropolarimetric results strongly depend on the level of CS contribution to the stellar spectra. We have improved the determination accuracy of magnetic fields up to the 7sigma level in the two Herbig Ae stars HD 139614 and HD 144432, observed in 2005 when these objects were at a low level state in their CS activity. We have established that at a higher level state of CS activity the polarisation signatures are mainly related to the CS matter. The presence of CS polarisation signatures formed in the stellar wind supports the assumption that the magnetic centrifuge is one of the main mechanisms of wind acceleration. Conclusions: .We conclude that the most effective way to investigate the magnetism of Herbig Ae stars is to monitor their spectropolarimetric behaviour at different stages of CS activity. Obviously, higher-resolution spectropolarimetric observations would extend the sample of spectral lines which can be used to measure magnetic fields at different levels in the stellar atmosphere and CS envelope. These observations will give a more complete insight into the magnetic topology in Herbig Ae stars. Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO programme Nos. 072.D-0377, 073.D-0464, 074.C-0463, and 075.D-0507).

Rotationally modulated variations and the mean longitudinal magnetic field of the Herbig Ae star HD 101412

Despite the importance of magnetic fields to a full understanding of the properties of accreting Herbig Ae/Be stars, these fields have scarcely been studied until now over the rotation cycle. One reason for the paucity of these observations is the lack of knowledge of their rotation periods. The sharp-lined young Herbig Ae star HD 101412 with a strong surface magnetic field has become in the past few years one of the most well-studied targets among the Herbig Ae/Be stars. We present our multi-epoch polarimetric spectra of this star acquired with FORS 2 to search for a rotation period and constrain the geometry of the magnetic field. Methods: We measured longitudinal magnetic fields for 13 different epochs distributed over 62 days. These new measurements and our previous measurements of the magnetic field in this star were combined with available photometric observations to determine the rotation period. Results: We find the rotation period to be P = 42.076+/-0.017 d.

A new phase of activity of the Herbig Be star HD 200775 in 2001: Evidence for binarity

The results of high-resolution spectroscopy of the Herbig Be star HD 200775 obtained within the framework of a cooperative observing programme in 2000–2002 are presented. A new high-activity phase of the object’s Ha line occurred in the middle of 2001 in full agreement with a 3.68-year periodicity predicted by Miroshnichenko et al. (1998). A complicated picture of the Ha line profile variability near the activity maximum phase turned out to be very similar to that observed during the previous one in 1997. Variations of the radial velocity with the activity phase are detected in He I, Si II, and S II photospheric lines. The observed phenomena are interpreted in the framework of a model in which the star, together with its gaseous envelope, is a component of an eccentric binary system. A preliminary orbital solution is derived, and the system’s parameters are estimated from the radial velocity curves of the Ha emission line. We find that the orbital eccentricity is e ~ 0.3, the mean companion separation is ~1000 R?, and the secondary companion is most likely to be a ~3.5 M, pre-main sequence object. We emphasize the importance of coordinated spectroscopic and interferometric observations at different phases of the object’s activity for further understanding the properties of the system.

Spectroscopic behaviour of the unusual Ae star HD 190073

The results of high-resolution spectroscopy of the peculiar Ae star HD 190073 obtained within the framework of a cooperative observing programme in 1994-2002 are presented. The temporal behaviour of the Hα ,H β ,H γ ,H δ ,H e λ 5876 A, Na  D and other circumstellar line profiles are investigated in detail. Special attention has been paid to the analysis of the deep multicomponent blueshifted Ca  H and K absorption lines. It has been found that the fine structure of their profiles is variable on timescales from months to decades. The analysis of the circumstellar spectrum of HD 190073, rich in shell-like and emission lines with narrow absorption cores, allows us to conclude that all absorption lines and cores are likely to be of photospheric origin. The emission lines are variable in time and demonstrate signs of a stellar wind as well as a dense equatorial circumstellar disk. As a preliminary hypothesis, we propose that a global magnetic field of a specific topology can be responsible for the formation of stable latitudinal stratification of the outflowing gas resulting in appearance of the complex structure of the Ca  H and K line profiles. We emphasise that a measurement of the stellar magnetic field and an investigation of its detailed configuration would be an important step in understanding the nature of HD 190073.

Magnetic survey of emission line B-type stars with FORS 1 at the VLT ⋆

We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission-line B-type star υ Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B-type stars and He-strong/He-weak stars. Nine Be stars have been studied with magnetic field time series obtained over ∼1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi-mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD 62367, μ Cen, o Aqr, and e Tuc. The strongest longitudinal magnetic field, 〈Bz〉 = 117 ± 38 G, was detected in the Be star HD 62367. Among the Be stars studied with time series, one Be star, λ Eri, displays cyclic variability of the magnetic field with a period of 21.12 min. The binary star υ Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field 〈Bz〉 = –102 ± 10 G at MJD 54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B-type stars out of the observed fourteen cluster members (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterising the magnetic fields of the Herbig Ae/Be stars HD 97048, HD 150193, HD 176386, and MWC 480

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.

Evidence for weak magnetic fields in early-type emission stars

We report the results of our study of magnetic fields in a sample of 15 Be stars using spectropolarimetric data obtained at the European Southern Observatory with the multi-mode instrument FORS 1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four stars, HD56014, HD148184, HD155806, and HD181615. We note that for HD181615 the evolutionary status is not obvious due to the fact that it is a binary system currently observed in the initial rapid phase of mass exchange between the two components. Further, we notify the possible presence of distinct circular polarisation features in the circumstellar components of Ca II H&K in three stars, HD58011, HD117357, and HD181615, hinting at a probable presence of magnetic fields in the circumstellar mass loss disks of these stars. We emphasize the need for future spectropolarimetric observations of Be stars with detected magnetic fields to study the temporal evolution of their magnetic fields and the correlation of magnetic field properties with dynamical phenomena taking place in the gaseous circumstellar disks of these stars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spectroscopic signatures of magnetospheric accretion in Herbig Ae/Be stars - I. The case of HD 101412

Models of magnetically-driven accretion and outflows reproduce many observational properties of T Tauri stars. This concept is not well established for the more massive Herbig Ae/Be stars. We intend to examine the magnetospheric accretion in Herbig Ae/Be stars and search for rotational modulation using spectroscopic signatures, in this first paper concentrating on the well-studied Herbig Ae star HD101412. We used near-infrared spectroscopic observations of the magnetic Herbig Ae star HD101412 to test the magnetospheric character of its accretion disk/star interaction. We reduced and analyzed 30 spectra of HD101412, acquired with the CRIRES and X-shooter spectrographs installed at the VLT (ESO, Chile). The spectroscopic analysis was based on the He I lambda 10,830 and Pa gamma lines, formed in the accretion region. We found that the temporal behavior of these diagnostic lines in the near-infrared spectra of HD101412 can be explained by rotational modulation of line profiles generated by accreting gas with a period P = 20.53+-1.68 d. The discovery of this period, about half of the magnetic rotation period P_m = 42.076 d previously determined from measurements of the mean longitudinal magnetic field, indicates that the accreted matter falls onto the star in regions close to the magnetic poles intersecting the line-of-sight two times during the rotation cycle. We intend to apply this method to a larger sample of Herbig Ae/Be stars.

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

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 promising 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. High-resolution spectropolarimetric observations of the three sharp-lined Herbig Ae stars HD101412, HD104237, and HD190073 have been obtained in recent years with the HARPS. 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 method. We discovered that different line lists for HD101412 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. 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 Stokes V spectrum of HD104237, we detect that the secondary component, a T Tauri star, possesses a rather strong magnetic field =129G, while no significant field is present in the primary component. Our measurements of HD190073 confirm the presence of a variable magnetic field and indicate that the circumstellar environment may have a significant impact on the observed polarization features.

Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars

We present the results of a new magnetic field survey of Herbig Ae/Be and A debris disk stars. They are used to determine whether magnetic field properties in these stars are correlated with the mass-accretion rate, disk inclinations, companion(s), Silicates, PAHs, or show a more general correlation with age and X-ray emission as expected for the decay of a remnant dynamo.