2002MNRAS.332..399B

We study the structural characteristic of the variable DA white dwarf G117-B15A by applying the methods of asteroseismology. For such a purpose, we construct white dwarf evolutionary models considering a detailed and up-to-date physical description as well as several processes responsible for the occurrence of element diffusion. We have considered several thicknesses for the outermost hydrogen layer, whereas for the inner helium-, carbon- and oxygen-rich layers we considered realistic profiles predicted by calculations of the white dwarf progenitor evolution. The stellar masses we have analysed cover the mass range of 0.50≤M ∗/M_☉ ≤0.60.

The evolution of each of the considered model sequences was followed down to very low effective temperatures; in particular, from 12500K on we computed the dipolar, linear, adiabatic oscillations with radial order k=1,...,4. We find that asteroseismological results are not univocal regarding mode identification for the case of G117-B15A. However, our asteroseismological results are compatible with spectroscopic data only if the observed periods of 215.2, 271.0 and 304.4s are due to dipolar modes with k =2,3,4, respectively.

Our calculations indicate that the best fit to the observed period pattern of G117-B15A corresponds to a DA white dwarf structure with a stellar mass of 0.525M_☉, with a hydrogen mass fraction log(M_H/M*)≳-3.83 at an effective temperature T_eff~11800K. The value of the stellar mass is consistent with that obtained spectroscopically by Koester & Allard.