Vol 11, No 11 (2011) / Lai

Chemical mixing by turbulent convection in the overshooting region below the convective envelope of RGB stars

Xiang-Jun Lai, Yan Li

Abstract

Based on the turbulent convection model (TCM), we investigate chemical mixing in the bottom overshooting region of the convective envelope of intermediate mass stars, focusing on its influence on the formation and extension of blue loops in the Hertzsprung-Russell (HR) diagram. A diffusive mixing model is adopted during the Red Giant Branch (RGB) phase. The properties of the blue loop are changed by modification of the element profiles above the H-burning shell, which results from the incomplete mixing in the bottom overshooting region when the stellar model evolves up along the RGB. Such modification of the element profiles will lead to an increase of opacity in the region just above the H-burning shell and a decrease of opacity in the outer homogeneous convection zone, which will result in a quick decrease of the H-shell nuclear luminosity LH when the stellar model evolves from the RGB tip to its bottom and, finally, a much weaker and smaller convection zone will be obtained in the stellar envelope. This helps to form a longer blue loop. The extension of the blue loop is very sensitive to the parameters (CX and αTCM ) of the diffusive mixing model and of the TCM. The results mainly show that: 1) comparing the results of the classical model with the mixing-length theory, the lengths of the obtained blue loops with different combinations of the values of CX and αTCM are all increased and the length of the blue loop increases with the values of parameters CX and αTCM; 2) the diffusive mixing model can significantly extend the time of stellar models lingering on the blue side of the HR diagram, even though the length of the blue loop for the 7M star has a less prominent difference between the classical and diffusive mixing model; 3) both the observations referring to the location of the Cepheid instability strip and the number ratio NB /NR of blue to red evolved stars in the Galactic open clusters can confine the two parameters in a range of 0.5 ≤ αTCM ≤ 0.9 and 10−5CX ≤ 10−4 for the model of 5M . However, for the case of the 7M star, there seems to be no such definite range to even only account for the observed number ratio NB /NR . In any case, our results based on the diffusive mixing model are on the whole in accordance with not only other theoretical ones but also the observations.

Keywords


stars: convection — star: interiors — star: HR diagram–blue loops– Cepheids — Galactic open clusters

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