10. Pulsation and turbulence
In their discussion of metallicism and pulsations, Kurtz and collaborators have made the point that pulsations involve rapid motion to a substantial depth in the star. They argue that velocity of the displacement generated by the pulsation remains high throughout the region in which the abundance anomalies are thought to be formed (referring for example to the eigenfunctions displayed in Fig. 8.2 of Cox 1980). They speculate that the high velocities might generate turbulent mixing and therefore inhibit the occurrence of metallicism.
Although the velocity of the radial displacement might be large, turbulence is expected to be generated not by the speed of the displacement itself but rather by velocity gradients: a fast but uniform displacement will not become turbulent. The scale on which the displacement occurs is much greater than other relevant scales, e.g. the pressure scale height. The gradient of the displacement velocity is small over those scales and one would not expect turbulence to be necessarily generated as a result.
Our understanding of the extent to which pulsations are laminar or generate turbulence is too incomplete for us to speculate further on the link between the turbulence assumed in Am stars and the observed pulsations.
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000