Recently, LI Zhi and LI Yan from Yunnan Observatories of the Chinese Academy of Sciences, used the k-omega model to investigate the convective overshooting in the extreme horizontal-branch stars (EHB). The mass at the outer boundary of the convective shell can exceed 0.2 Msun which is close to the convective core masses derived by asteroseismology, and the core breathing pulse in the late stages of core helium burning may be the key to the formation of White Dwarf (WDs) with a higher oxygen content. The work was recently published in The Astrophysical Journal.
Subdwarf B (sdB) stars are core helium-burning stars and are located on the extreme horizontal branch (EHB) in the Hertzsprung–Russell (HR) diagram. They are hot and compact. The hydrogen envelope of sdB stars is extremely thin, which cannot sustain the hydrogen-shell burning. Many sdB stars exhibiting stellar pulsations have been detected, they can be divided into pressure (p)-mode and gravity (g)-modes according to their oscillation characteristics. A few pulsating sdB stars are hybrid pulsators, which show both p-modes and g-modes. Asteroseismology has allowed the determination of the convective core masses of g-mode pulsators, but the derived masses of the convective core (0.20- 0.28 Msun) are generally much larger than what the stellar models without overshooting predict (about 0.1 Msun).
The researchers use the k-omega model that has been incorporated into MESA (the Modules of Experiments in Stellar Astrophysics) to treat the convective overshooting mixing. The k-omega model predicts a larger convection region, and there are an additional complete-mixing region and a partial-mixing region outside the convective core. The convective core can be divided into two parts when this minimum value is smaller than the adiabatic temperature gradient. The convective core continues to expand until its boundary reaches the inner edge of the convective shell located above at last, which merges the two convection zones into one convective core. Finally, the mass at the outer boundary of the convective shell can be increased to 0.303 Msun, which can reach the convective core mass explored from asteroseismology.
In the final stage, “core breathing pulses” occurred two or three times. Helium was injected into the convective core by overshoot mixing and increased the lifetime of sdB stars. When the core breathing pulse occurs, fresh helium is injected into the convective core, and most helium nuclei are captured by carbon to produce oxygen. The oxygen content in the central core of our g-mode sdB models is about 80% by mass, which is very close to the oxygen content deduced with the asteroseismic method for the WD KIC 08626021. The high amounts of oxygen deduced from asteroseismology may be evidence supporting the existence of core breathing pulses.
Contact:
LI Zhi, YNAO, CAS
lizhi@ynao.ac.cn
