Three groups of DAV star models are evolved with time-dependent element diffusion by WDEC. The core compositions of these models are directly from white dwarf models evolved by MESA, which are results of thermonuclear burning. Based on these DAV star models, we try to study the dispersion of period spacing. The thickness of hydrogen atmosphere can seriously affect the deviation degree of minimal period spacings. The minimal period spacings dominate the dispersion of period spacing. The thinner the hydrogen atmosphere, basically, the more dispersive the period spacing. Standard deviations are used to analyze the dispersion of period spacing. Studying the dispersion of period spacing on a DAV star KUV03442+0719 preliminarily, we suggest that log(M H /M * ) is from -8.5 to -5.5. In addition, modes partly trapped in C/O core are found based on those DAV star models. The identified modes and average period spacings indicate that KUV03442+0719 may be the first star to 'observe' modes partly trapped in C/O core.

, and by Provencal et al. in 2008. We review the observations together and obtain 34 independent frequencies. According to the frequency splitting and the asymptotic period spacing law, we identify 6 l = 1 modes, 4 l = 2 modes, 5 l = 3 modes, 10 l = 1 or 2 modes. Grids of white dwarf models are generated by WDEC with H, He, C, O diffusion in a four-parameter space. The core compositions are directly from white dwarf models generated by MESA. The best-fitting model has M * = 0.710 M , T eff = 12 200 K, log(M He /M * ) = −2.5, log(M H /M * ) = −7.0, log g = 8.261, and φ = 3.185 s. There are four, two, and one modes identified as trapped in H envelope for observed l = 1, 2, and 3 modes, respectively. Trapped modes jump the queue of uniform period spacing.Key words: asteroseismology -stars: individual: EC14012−1446 -white dwarfs. I N T RO D U C T I O NAbout 80 per cent of white dwarfs show DA spectral class. A DA white dwarf star consists of a hydrogen atmosphere covering an intermediate helium layer and a carbon/oxygen core. According to existence of partial hydrogen ionization and subsurface convection zone, a DA star will pulsate when its effective temperature (T eff ) is between 10 850 and 12 270 K (Castanheira, Kepler & Costa 2007). With buoyancy acting as the restoring force, DAV stars are pulsating in g modes. Asteroseismology is a unique tool to detect the inner structure of DAV stars, which requires enough observed frequencies, reliable mode identifications, and realistic stellar models.EC14012−1446, also called WD1401−147, was first identified as a DAV star by Stobie et al. (1995). They observed it for four nights, and then derived five independent frequencies from the ob- Mode identification is an important task for asteroseismological studies. An eigenmode can be characterized by three indices (k, l, m), which are, respectively, the radial order, the spherical harmonic degree, and the azimuthal number. Frequencies of pulsation modes with definite spherical harmonic degrees play a role as tick marks on a ruler for theoretical modelling of pulsating white dwarfs. In particular, stellar rotation can split a pulsation frequency into several ones. The approximate formula between frequency splitting (δν n, l ) and rotational period (P rot ) is derived by Brickhill (1975) aswhere m can be taken from −l to l, leading to totally 2l+1 different values. According to equation (1), modes with l = 1 form a triplet and modes with l = 2 form a quintuplet. Therefore, if triplets or quintuplets are derived from observations of a pulsating star, they can be reliably identified as rotational splitting of l = 1 or l = 2 modes, respectively. For EC14012−1446, Stobie et al. (1995) did not find any rotational splitting phenomenon. Later, based on those independent frequencies they had obtained, Handler et al. (2008) declared the discovery of two triplets with an average frequency splitting of 9.55 μHz. However, Provencal et al. (2012) did not confirm the discovery of Handler et al.

We present analysis of a new pulsating helium-atmosphere (DB) white dwarf, EPIC 228782059, discovered from 55.1 days of K2 photometry. The long-duration, high-quality light curves reveal 11 independent dipole and quadruple modes, from which we derive a rotational period of 34.1 ± 0.4 hr for the star. An optimal model is obtained from a series of grids constructed using the White Dwarf Evolution Code, which returns M * = 0.685 ± 0.003M ⊙, T eff = 21,910 ± 23 K, and log g = 8.14 ± 0.01 dex. These values are comparable to those derived from spectroscopy by Koester & Kepler (20,860 ± 160 K, and 7.94 ± 0.03 dex). If these values are confirmed or better constrained by other independent works, it would make EPIC 228782059 one of the coolest pulsating DB white dwarf stars known, and would be helpful for testing different physical treatments of convection, and to further investigate the theoretical instability strip of DB white dwarf stars.