Astron. Astrophys. 345, 884-904 (1999)

6. Conclusion

The main results of the MUSICOS 96 campaign on AB Aur are as follows:

• the photospheric lines exhibit a blue component in absorption with a velocity modulated with a 34 hr period, which we interpret as the rotation period of the stellar surface.

• the He I D3 line has a blueshifted emission component, whose velocity is modulated with a period near 45 hr. This modulation may be due to stream structures in an equatorial wind, the stellar equator rotating more slowly than higher latitude regions. Alternatively, this line may be formed in a wind from a rotating disk, at a distance of about 1.6 R_*. In this case, we find that the disk rotation must be significantly slower than keplerian.

• both sets of lines have a red component, which is variable, with some indication of periodicity near 45 hrs. This component may be formed in downflows onto the stellar pole, originating either from the equatorial wind, or from the accretion disk in accreting columns.

The two alternatives presented in this paper to explain the observed variability have in common the presence of a wind, originating either from the equatorial regions of the star, or from a circumstellar inhomogeneous rotating disk. Also common to both models is the need for a highly structured photosphere involving significant radial velocity fields, as well as the presence of downflows onto the stellar pole.

In the equatorial wind model, we need to invoke surface differential rotation of the order of 25%, with the pole rotating faster than the equator, to explain the period difference between the modulation of the photospheric lines and that of the He I and other wind lines. This type of differential rotation is naturally expected in Herbig stars, which suffer a vigorous rotational braking because of their wind, preferentially at the equator. The wind azimuthal structuration is certainly produced by a surface magnetic field, yet undetected in AB Aur, but recently measured in another Herbig Ae star, HD 104237 (Donati et al. 1997).

In the disk wind model, the accretion disk needs to lose its angular momentum near the star surface, which is to be expected because of the unavoidable interaction between the star and the disk. The exact nature of this interaction is still unexplored, but it may provide an explanation for the large radiative losses observed in the heated regions above the photosphere. However, the observed symmetric and unshifted forbidden O I lines in AB Aur and other Herbig stars seem difficult to reconcile with the idea of an accretion disk capable of driving a wind at a mass loss rate of yr^-1, and therefore necessarily optically thick. Such an accretion disk, by masking the receding part of the wind, would lead to blueshifted components in these forbidden lines, which are not observed.

Clearly, a lot of work remains to be done, both theoretically and observationally, to further study these two models, that we have just outlined in this paper.

© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999
helpdesk.link@springer.de