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Researchers Find "strange" Mixing Phenomenon in Upper Main-sequence Star
Author: | Update time:2020-08-20           | Print | Close | Text Size: A A A

On August 11, The Astrophysical Journal published new results about element mixing beyond central convective core in upper main-sequence star reported by WU Tao from Yunnan Observatories, Chinese Academy of Sciences, and other collaborators. Based on the observations of Kepler Telescope during 4 years, they precisely constrained the profiles of the inner diffusion coefficient and the elements of SPB Star KIC 8324482 with the analysis of asteroseismology.

Turbulent motion widely exists in celestial bodies and around our daily lives. It is a significant physical process for transporting matters and energies and dominantly affects stellar structure and evolutions, especially for the upper main-sequence intermediate-mass and massive stars.

Owing to the buoyancy force, the turbulent bubble moves in the convective zone. On the convective boundary, buoyancy force disappears, while velocity is still larger than zero. Therefore, the turbulent bubble penetrates convective boundary and further goes into the radiation equilibrium region to transport energies and matters outward convective zone, until velocity decays to zero finally. Such extra mixing is so-called as convective overshooting. Fully and partly mixing is usually used to describe the mixing of matters in the convective overshooting region.

Dr. WU and collaborators analyzed the shapes of the period spacing variations of g-mode oscillations from observations and calculations. They found that the diffusion coefficient of KIC 8324482 suddenly drops about 11 orders of magnitude on the convective boundary. It stays at a very low level outside the convective core. Convective overshooting in the core does not exist or to be ultra-weak in KIC 8324482, when interpreting the asteroseismically determined diffusion coefficient with the above two described canonical methods.

However, asteroseismic analysis indicates that KIC 8324482 has a large convective core. The mass of the convective core is about 1.3 Msun and radius to be 0.5 Rsun. This means KIC 8324482 should have a considerable overshooting zone beyond the central convective core, according to the theory of convection and the previous researches.

Besides, they analyzed the observed oscillation power spectra with the auto-correlation method and found that KIC 8324482 is a slow rotator. Its rotational period is about 59.35 days. The corresponding surface rotational velocity is about 2.61 km/s on the equatorial plane. A very fast rotation is necessary for the theoretical model, when interpreting the low diffusion coefficient beyond the central convective core as rotation induced mixing (differential rotation). It should be about 20-30 times as asteroseismically determined.

All in all, the profile of diffusion coefficient in SPB star KIC 8324482 constrained by asteroseismology cannot be accurately described by the now available methods for both of convective overshooting and rotation induced mixing. Therefore, developing new descriptions or modifying now available methods is necessary.


WU Tao, Yunnan Observatories, CAS

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