Circular polarization and variability in the spectra of Herbig Ae/Be stars. 1: The Fe II 5018 A and He I 5876 A lines of AB Aurigae

Details Circular polarization and variability in the spectra of Herbig Ae/Be stars. 1: The Fe II 5018 A and He I 5876 A lines of AB Aurigae Circular polarization and variability in the spectra of Herbig Ae/Be stars. 1: The Fe II 5018 A and He I 5876 A lines of AB Aurigae

Oct 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...278..187c&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 278, no. 1, p. 187-198

Astronomy and Astrophysics

Astrophysics

42

Circular Polarization, Emission Spectra, Pre-Main Sequence Stars, Stellar Atmospheres, Stellar Spectra, Variable Stars, Helium, Iron, Stellar Magnetic Fields, Stellar Models, Stellar Rotation

Scientific paper

We present high resolution spectroscopic and spectro-polarimetric observations of the Fe II 5018 and He I 5876 A lines in the pre-main sequence Herbig Ae star AB Aur. No signal was detected in the circular polarization (Stokes V) profile of the magnetically sensitive Fe II 5018 A line, yielding upper limits of the order of 1 kG for the photospheric magnetic field. This upper limit is still much higher than the equipartition magnetic field at the photosphere, estimated to be 60 G. Both lines are spectacularly variable on a night-to-night basis. In particular, the Fe II 5018 A line appears as a photospheric absorption line in one of our spectra, but as a variable emission line, usually composed of a flat-topped broad emission and a roughly triangular emission, in the other spectra. The He I 5876 A line often includes a red absorption component and a blue emission component, but also often appears entirely in emission with a varible asymmetry. We estimate the regions of formation of the two lines, with a very simplified treatment of ionization and excitation processes, and within the framework of existing models of the wind and chromosphere of AB Aur. We find that the Fe II 5018 A line is formed in the bulk of the expanding chromosphere, while the He I 5876 A line is formed at the very base of the wind/chromosphere complex. Although the data presented here are not sufficient to fully interpret such a complex variability, we suggest the idea that it could be linked to the co-rotation of structures in the wind and chromosphere of AB Aur. The photospheric appearance of the Fe II 5018 A line in one of our spectra may be due to a temporary change in the chromospheric physical conditions, like an increase of the temperature, followed by a return to 'normal' conditions, with a time scale shorter than one day.

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