Assuming rapid rotation and using a self-consistent description for the transport of angular momentum and of chemicals by meridional circulation and shear instabilities (cf. Zahn 1992, Talon & Zahn 1997), Talon et al. (1997) successfully explained the C and N anomalies observed in some B stars.
At the same time, it was shown (Matias & Zahn 1997) that this description applied to the transport of angular momentum in the Sun is incomplete, leading to large gradients which are not observed. Another transport mechanism must thus be invoked in low mass stars.
At this point, 2 questions remain: firstly, the nature of that transport mechanism has to be determined unambiguously and secondly, the location of the transition between the regime which is relevant for massive stars and the one which is relevant for low mass stars has to be identified.
In this paper, we addressed that second question. We presented numerical calculations of Li destruction due to rotational mixing using the same description as Talon et al. used for more massive stars and the same free parameters. We showed that this clearly reproduces the hot side of the Li dip. Let us recall that the destruction of lithium is then due solely to rotational mixing enhanced by the spin down of the outer layers. Stars hotter than 7000 K also undergo rotational mixing, but it is much milder due to the weak differential rotation.
The rise of Li abundances on the right side of the dip is not explained within this framework. We propose that it is linked to the appearance of another transport mechanism for angular momentum which reduces the magnitude of the meridional circulation and shears, leading to the observed diminution of Li destruction on the red side of the Li dip. This mechanism is known to occur in the Sun where it is responsible for the flat rotation profile.
© European Southern Observatory (ESO) 1998
Online publication: June 26, 1998