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Register a new userMagnetic topologies of the Herbig Ae/Be stars
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Authors
E. Alecian
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Our recent discoveries of magnetic fields in a small number of Herbig Ae/Be (HAeBe) stars, the evolutionary progenitors of main sequence A/B stars, raise new questions about the origin of magnetic fields in the intermediate mass stars. The favoured fossil field hypothesis suggests that a few percent of magnetic pre-main sequence A/B stars should exhibit similar magnetic strengths and topologies to the magnetic Ap/Bp stars. In this talk I will present the methods that we have used to characterise the magnetic fields of the Herbig Ae/Be stars, as well as our first conclusions on the origin of magnetism in intermediate-mass stars.
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We report on the status of our spectropolarimetric studies of Herbig Ae/Be stars carried out during the last years. The magnetic field geometries of these stars, investigated with spectropolarimetric time series, can likely be described by centred dipoles with polar magnetic field strengths of several hundred Gauss. A number of Herbig Ae/Be stars with detected magnetic fields have recently been observed with X-shooter in the visible and the near-IR, as well as with the high-resolution near-IR spectrograph CRIRES. These observations are of great importance to understand the relation between the magnetic field topology and the physics of the accretion flow and the accretion disk gas emission.
H_alpha spectropolarimetry on Herbig Ae/Be stars shows that the innermost regions of intermediate mass (2 -- 15 M_sun) Pre-Main Sequence stars are flattened. This may be the best evidence to date that the higher mass Herbig Be stars are embedded in circumstellar discs. A second outcome of our study is that the spectropolarimetric signatures for the lower mass Herbig Ae stars differ from those of the higher mass Herbig Be stars. Depolarisations across H_alpha are observed in the Herbig Be group, whereas line polarisations are common amongst the Herbig Ae stars in our sample. These line polarisation effects can be understood in terms of a compact H_alpha source that is polarised by a rotating disc-like configuration. The difference we detect between the Herbig Be and Ae stars may be the first indication that there is a transition in the Hertzsprung-Russell Diagram from magnetic accretion at spectral type A to disc accretion at spectral type B. However, it is also possible that the compact polarised line component, present in the Herbig Ae stars, is masked in the Herbig Be stars due to their higher levels of H_alpha emission.
We have found Herbig Ae/Be star candidates in the western region of the Magellanic Bridge. Using the near infrared camera SIRIUS and the 1.4 m telescope IRSF, we surveyed about 3.0 deg x 1.3 deg (24 deg < RA < 36 deg, -75 deg < Dec. < -73.7 deg) in the J, H, and Ks bands. On the basis of colors and magnitudes, about 200 Herbig Ae/Be star candidates are selected. Considering the contaminations by miscellaneous sources such as foreground stars and early-type dwarfs in the Magellanic Bridge, we estimate that about 80 (about 40%) of the candidates are likely to be Herbig Ae/Be stars. We also found one concentration of the candidates at the young star cluster NGC 796, strongly suggesting the existence of pre-main-sequence (PMS) stars in the Magellanic Bridge. This is the first detection of PMS star candidates in the Magellanic Bridge, and if they are genuine PMS stars, this could be direct evidence of recent star formation. However, the estimate of the number of Herbig Ae/Be stars depends on the fraction of classical Be stars, and thus a more precise determination of the Be star fraction or observations to differentiate between the Herbig Ae/Be stars and classical Be stars are required.
Accretion is the prime mode of star formation, but the exact mode has not yet been identified in the Herbig Ae/Be mass range. We provide evidence that the the maximum variation in mass-accretion rate is reached on a rotational timescale, which suggests that rotational modulation is the key to understanding mass accretion. We show how spectropolarimetry is uniquely capable of resolving the innermost (within 0.1 AU) regions between the star and the disk, allowing us to map the 3D geometry of the accreting gas, and test theories of angular momentum evolution. We present Monte Carlo line-emission simulations showing how one would observe changes in the polarisation properties on rotational timescales, as accretion columns come and go into our line of sight.
We present mid IR spectro-photometric imaging of a sample of eight nearby ($D leq 240$pc) Herbig Ae/Be stars. The spectra are dominated by photospheric emission (HR6000), featureless infrared excess emission (T~Cha), broad silicate emission feature (HR5999) and the infrared emission bands (HD 97048, HD 97300, TY~CrA, HD 176386). The spectrum of HD179218 shows both silicate emission and infrared emission bands (IEB). All stars of our sample where the spectrum is entirely dominated by IEB have an extended emission on scales of a few thousand AU ($sim 10$). We verify the derived source extension found with ISOCAM by multi--aperture photometry with ISOPHT and compare our ISOCAM spectral photometry with ISOSWS spectra.
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