Asteroseismology of DAV star EC14012−1446, mode identification and model fittings

Abstract

, 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.

The rate of period change in DAV stars

“…The element diffusion scheme of Thoul, Bahcall & Loeb (1994) is added into WDEC by Su et al (2014). The method has been used many times recently to evolve DAV stars (Chen & Li 2014a, Chen & Li 2014b, Su et al 2014, Chen 2016a) and DBV stars (Chen 2016b). In this paper, we studied the rate of period change in DAV stars based on the DAV star models evolved in the method.…”

Asteroseismology of the DBV star CBS 114

“…This method has been used by us to evolve grids of DAV star models to study EC14012-1446 (Chen & Li 2014). For DBV stars, we improve the method by extracting the He/C envelope from white dwarf models evolved by MESA.…”

Application of a screened Coulomb potential in fitting the DAV star HS 0507+0434B

“…The oxygen abundance equals 1.0 minus the carbon abundance in the core. The seed models are exactly same with Table 4 in Chen & Li (2014a). Then, the DAV star models have core compositions of quasi-nuclear-burning results.…”

“…It is an open source software. Chen & Li (2014a) added the core compositions of white dwarfs evolved by MESA to WDEC to evolve grids of white dwarf models. The thermonuclear burning core is more physical than previous artificial core in WDEC.…”