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Chemical mixing by turbulent convection in the overshooting region below the convective envelope of RGB stars
Lai Xiangjun; Li Yan
2011
Source PublicationResearch in Astronomy and Astrophysics
ISSN1674-4527
Volume11Issue:11Pages:1351
AbstractBased 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 α_TCM) 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 N_B/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 10-5 ≤ Cx ≤ 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 N_B/N_R. 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.
Language英语
Document Type期刊论文
Identifierhttp://ir.bao.ac.cn/handle/114a11/42327
Collection中国科学院国家天文台
Affiliation中国科学院国家天文台
First Author AffilicationNational Astronomical Observatories, Chinese Academy of Sciences
Recommended Citation
GB/T 7714
Lai Xiangjun,Li Yan. Chemical mixing by turbulent convection in the overshooting region below the convective envelope of RGB stars[J]. Research in Astronomy and Astrophysics,2011,11(11):1351.
APA Lai Xiangjun,&Li Yan.(2011).Chemical mixing by turbulent convection in the overshooting region below the convective envelope of RGB stars.Research in Astronomy and Astrophysics,11(11),1351.
MLA Lai Xiangjun,et al."Chemical mixing by turbulent convection in the overshooting region below the convective envelope of RGB stars".Research in Astronomy and Astrophysics 11.11(2011):1351.
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