CIRCULAR-POLARIZATION AND VARIABILITY IN THE SPECTRA OF HERBIG AE BE STARS .1. THE FE-II 5018-ANGSTROM AND HE-I 5876-ANGSTROM LINES OF AB AURIGAE/

We present high resolution spectroscopic and spectro-polarimetric obse rvations of the Fe II 5018 and He I 5876 angstrom lines in the pre-mai n sequence Herbig Ae star AB Aur. No signal was detected in the circul ar polarization (Stokes V) profile of the magnetically sensitive Fe II 5018 angstrom line, yielding upper limits of the order of 1 kG for th e photospheric magnetic field. This upper limit is still much higher t han the equipartition magnetic field at the photosphere, estimated to be 60 G. Both lines are spectacularly variable on a night-to-night bas is. In particular, the Fe II 5018 angstrom line appears as a photosphe ric absorption line in one of our spectra, but as a variable emission line, usually composed of a flat-topped broad emission and a roughly t riangular emission, in the other spectra. The He I 5876 angstrom line often includes a red absorption component and a blue emission componen t, but also often appears entirely in emission with a variable asymmet ry. We estimate the regions of formation of the two lines, with a very simplified treatment of ionization and excitation processes, and with in the framework of existing models of the wind and chromosphere of AB Aur. We find that the Fe II 5018 angstrom line is formed in the bulk of the expanding chromosphere, while the He I 5876 angstrom line is fo rmed at the very base of the wind/chromosphere complex. Although the d ata presented here are not sufficient to fully interpret such a comple x variability, we suggest the idea that it could be linked to the co-r otation of structures in the wind and chromosphere of AB Aur. The phot ospheric appearance of the Fe II 5018 angstrom line in one of our spec tra may be due to a temporary change in the chromospheric physical con ditions, like an increase of the temperature, followed by a return to ''normal'' conditions, with a time scale shorter than one day.