Abstract:

 A low mass star (about 1.6 M⊙) can evolve to the tip of the RGB phase through the stellar evolution code (MESA code). After igniting the helium flash, the stars lose a lot of mass and remain a total mass around 0.5M⊙. The star has a central helium burning core and has a tiny (≤0.02M⊙) hydrogenrich envelope. It is called as the zero age Btype subdwarf star model. For different total mass and different hydrogenrich envelope, these models have different effective temperature and gravity. They distribute with small range from the zero age extreme horizontal branch to the termination age extreme horizontal branch. Their evolutionary track is very similar. The internal structure can be derived from the theoretical evolutionary model. It is found that p-mod appears in the models with high effective temperature whereasgmod appears in the models with low effective temperature. Both of them can exist between the high temperature and the low temperature. Radial(l=0)and nonradial(from l=1 up to l=5)oscillation modes of pmod have the periods of 60-500s and nonradial oscillation modes of gmod has the periods of 1800-5000s. Some pulsation quantities, such as BruntVisl frequency, Lamb frequency, eigenfrequency and inertial parameter can be investigated when the adiabatic linearity pulsation code can be used. The characters of the pulsation for the sdb star have been investigated.

Key words: hot B sub-dwarf star; evolution; stellar structure; pulsation; p-modes; g-modes