Recently, based on the stellar evolution model, LIN Guifang, SU Jie, LI Yan from Yunnan Observatories of the Chinese Academy of Sciences and FU Jianning from Beijing Normal University, studied the influence of the inner chemical profile of white dwarf on the pulsation periods. Using the asteroseismology, they detected the inner carbon/oxygen profile of a DA-type pulsating white dwarf HS 0507+0434B. The paper was recently published in The Astrophysical Journal.

HS 0507+0434B is a pulsating white dwarf with good observational data. Some scholars have done astroseismological analysis on it before. However, the previous research is limited to the measurement of the basic parameters of the white dwarf (i.e. star mass, effective temperature, hydrogen/helium shell mass, and surface gravitational acceleration, etc.). Although accurate and reliable stellar parameters have been obtained, there is a lack of in-depth exploration of its internal structure. There is a large deviation between the periods of individual theoretical calculations and the observed ones in the previous models, which has not been well explained.

Through the analysis of the white dwarf model, they found that the inner carbon/oxygen profile of the white dwarf has different effects on different pulsation modes. The inner carbon/oxygen profile has an influence on all modes extending into the inner core. And it has little effect on the pulsation mode (the so-called "trapping mode") in which the propagation region is confined in the hydrogen/helium shell. The large deviation between the theoretical periods and the observed ones in the previous models just reflects the difference between the theoretical carbon/oxygen profile and the real situation.

In response to these problems, researchers used the nine modes identified from the light curves of HS 0507+0434B to constrain the inner chemical profile of the white dwarf. By adjusting parametric carbon/oxygen profile of the white dwarf based on stellar evolution models, they got a best-fitting model which best matches the observational periods.

The optimal model obtained by this work not only obtains the basic stellar parameters which are consistent with the previous research, but also significantly improves the fit between the theoretical periods and the observed ones.

Based on the chemical profile of the evolutionary models, it is inferred that the core of HS 0507+0434B has a higher oxygen mass fraction, and a smaller overshooting zone in the inner core. This may imply that there are some uncertainties in some physical processes (i.e. thermonuclear reaction rate, convection and overshoot, element diffusion) during the helium burning for HS 0507+0434B.

Contact:

LIN Guifang, YNAO, CAS

linguifang@ynao.ac.cn