Darun XIONG

**Darun XIONG** (1932-), born in Nanchang, Jiangxi Province, is an academician of Chinese Academy of Science. Academician Xiong is the director of Academic Committee and the head of Division of Astrophysics of Purple Mountain Observatory. He is also the chief professor of research group of stellar structure, evolution and pulsations. His research area is stellar convection theory and related aspects of stellar structure, evolution and oscillations. Academician Xiong's main academic contribution is the establishment of a new statistical theory of non-local convection to replace the traditional mixing-length theory. His theory is based on hydrodynamic equations and turbulence theory, therefore it can depict stellar convection more precisely.

1.He independently developed a statistical theory of time-dependent convection , and then generalized it to treat nonradial stellar oscillations, It is now the only feasible convection theory to treat nonradial stellar oscillations.

2.Using the time-dependent convection theory, he calculated the oscillations of variable stars and successfully explained the existence of the red edge of instability strip, which has been the most important unresolved problem of pulsations of variable stars since the1970s.

3.He independently developed a statistical theory of non-local convection, and then generalized it to treat the evolution of chemically inhomogeneous stars. It is now the only non-local convection theory that can deal with chemical ununiformity.

4.Using the non-local convection theory, he calculated the evolution of massive stars and solved the well-known semi-convection contradiction. The new evolutionary tracks involve a higher luminosity, and the main sequence band is wider than those indicated by traditional theories. This is favorable to resolve the contradiction between the observational and theoretical distributions of the luminous stars in the H-R diagram. It may also solve the well-known Cepheid mass discrepancy problem.

5. Using the non-local convection theory, he calculated the structure of the solar convection zone. The temperature distribution, the turbulent velocity and the turbulent temperature fields agree well with the observation. His theory also predicts that going through the boundary of the convectively unstable zone, the turbulent velocity-temperature correlation changes its sign, while the mixing-length theory contradicts the observation. Moreover, the new theory gives accordant bottom boundary of the convection zone with that given by helioseismology, and it may be used to explain the lithium and beryllium depletions.

K.G. Petrovay once said it is the best model of solar convection zone.

Academician Xiong's work on stellar convection theory as well as on stellar structure, evolution and pulsations is innovative. He developed the original statistical theory of nonlocal time-dependent convection, and got the first nonlocal model of solar convection zone as well as the first non-local stellar evolution model. Academician Xiong won a first-class natural science award of the Chinese Academy of Science in 1989 and a second-class national natural science award in 1991. His theory is called "Xiong's convection theory" by international colleagues and has been highly estimated and frequently cited.

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