Meridional circulation and CNO anomalies in red giant stars

Abstract

The possibility that meridional circulation due to internal rotation might lead to the mixing of CNO-processed material from the vicinity of the hydrogen shell into the envelope of a red giant star has been investigated and found to be generally consistent with the observational data on weak G band and CN-strong stars and on stars with low 12 C/ 13 C ratios. CNO processing of the red giant envelope by meridional circulation begins on the upper subgiant branch or lower giant branch when the hydrogen shell reaches the hydrogen discontinuity produced by the con-vective envelope at the time of deepest penetration on the subgiant branch. The main-sequence angular velocity w MS needed for substantial CNO processing of the envelope has been computed by assuming that a star retains its interior angular momentum into the red giant phase. If the specific angular momentum is constant within the convective envelope of a red giant star, w MS is ~ 10" 4 rad s" 1. Such an angular velocity is plausible if one attributes the observed slow rotation rates of the lower main-sequence stars to a spin-down of the outer convective layers. Prohibitively large values of o> MS are required if the entire convective envelope rotates instead as a solid body. This difficulty can be readily overcome if the inner part of the convective envelope departs from solid-body rotation. Reasons for such a departure are discussed. The distributions of the CNO elements near the hydrogen shell have been computed for several red giant models. The results show that there is a region of significant extent above the hydrogen shell within which carbon has been converted to nitrogen by the CN cycle. Closer to the hydrogen shell there is a region within which oxygen has been converted to nitrogen by the ON cycle. The gradient in the mean molecular weight ^ within the carbon-depleted region is insufficient to inhibit the circulation currents for all values of the heavy-element abundance Z. The circulation currents are only likely to penetrate into the oxygen-depleted region for small values of Z due to a large n gradient which otherwise exists in this region. The larger CN variations in globular-cluster giants might be due to more extensive ON processing of the mixed material, particularly for the extreme CN-strong stars in o> Cen. The envelopes of Population I giants only undergo CN processing, result ing in smaller CN variations. The lack of an observed correlation between the carbon depletions and nitrogen enhancements along the red giant branch of M92 might be caused by varying amounts of ON processing of the mixed material due to a range in the initial main-sequence angular velocity. Meridional circulation would also reduce the envelope 12 C/ 13 C ratio. Mass loss does not significantly affect the present results.