Asteroseismology of the DBV star CBS 114

Abstract

Asteroseismology is an unique and powerful tool to detect the internal structure of stars. CBS 114 is the sixth known pulsating DBV star. It was observed by Handler, Metcalfe, \& Wood at South African Astronomical Observatory over 3 weeks in 2001. Then, it was observed by Metcalfe et al. at Bohyunsan Optical Astronomy Observatory and McDonald Observatory respectively for 7 nights in 2004. Totally 2 triplets, 4 doublets, and 5 singlets were identified. The frequency splitting values are very different, from 5.2 $\mu$Hz to 11.9 $\mu$Hz, which may reflect differential rotations. We evolve grids of white dwarf models by \texttt{MESA}. Cores, added with He/C envelopes, of those white dwarf models are inserted into \texttt{WDEC} to evolve grids of DBV star models. With those DBV star models, we calculate eigenperiods. Those calculated periods are used to fit observed periods. A best-fitting model is selected. The parameters are $T_{eff}$ = 25000 K, $M_{*}$ = 0.740 $M_{\odot}$, and log($M_{He}/M_{*}$) = -4.5. With the massive stellar mass, the effective temperature is close to previous spectroscopic result. In addition, kinetic energy distributions are calculated for the best-fitting model. We find that the observed modes with large frequency splitting values are fitted by the calculated modes with much kinetic energy distributed in C/O core. After preliminary analysis, we suggest that the C/O core may rotate at least 2 times faster than the helium layer for CBS 114.