The ZZ Ceti star G 185-32: new insight from asteroseismology

Abstract

One of the brightest pulsating DA white dwarfs (ZZ Ceti stars), G 185-32, shows 19 periods in its power spectrum, from 71 s to 651 s. Its short periods are among the shortest ones observed in a ZZ Ceti star. The one at 141.9 s shows an amplitude which does not vary with wavelength, in contrast with what the linear theory predicts for = 1 or = 2 non-radial g-modes. The mass and T eff of G 185-32 are well constrained by spectroscopic observations and parallax measurement. The aim of the present paper is to take advantage of this rich observational background to derive the fundamental parameters of the star from asteroseismology. We computed a grid of realistic models for G 185-32 and the periods of their adiabatic = 1 and = 2 non-radial g-modes. We determined the model which fits best the observations. We identify 14 periods as real modes and determine their , k, and m values. We show that the remaining 5 periods are linear combinations.We find that G 185-32 oscillates mostly according to = 2 modes. We present a new hypothesis to account for the peculiar behaviour of the 141.9 s mode's amplitude and suggest that it is a true mode interfering with higher degree modes, involving a possible triplet resonance. We determine the main parameters of G 185-32: its total mass M * = 0.638 (±0.007) M , its hydrogen mass fraction M H = 1.70 (±0.10) × 10 −4 M * , which implies a helium mass fraction of ≈10 −2 M * , its T eff = 12 280 (±)80 K, and estimate its rotation period to be 14.5 h.

The Ratio of Helium‐ to Hydrogen‐Atmosphere White Dwarfs: Direct Evidence for Convective Mixing

“…We finally note that our results are compatible with the asteroseismological analyses of ZZ Ceti stars, which suggest that most ZZ Ceti stars have fairly thick hydrogen layers of the order of M H /M tot ∼ 10 −6 and higher (see, e.g., Fontaine et al 1992;Bergeron et al 1993;Bradley 1998;Brassard & Fontaine 2006;Pech & Vauclair 2006). Only a small fraction (about 15%) of all ZZ Ceti stars should have thin layers according to our results.…”

Seismological studies of ZZ Ceti stars - I. The model grid and the application to individual stars

“…They proposed the 141.9-s mode as an = 3. Pech & Vauclair (2006) suggest that G185−32 pulsates mainly with = 2 modes. The 72.5-s mode was labelled by them as = 2, k = 1 and used as a reference mode, even though it has never been observed as the dominant mode.…”

White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Line Widths: Kepler Observations of 27 Pulsating DA White Dwarfs through K2 Campaign 8

“…References. -(0) This work; (1) Giammichele et al (2016); (2) Dolez et al (2006); (3) Fu et al (2013); (4) Su et al (2014); (5) Pfeiffer et al (1996); (6) Bognár et al (2016); (7) Bradley (2001); (8) Castanheira et al (2013); (9) Kepler et al (1995); (10) Pech & Vauclair (2006); (11) Fu et al (2007); (12) Bond et al (1996); (13) Charpinet et al (2009); (14) Vauclair et al (2002); (15) Kawaler et al (1995); (16) Østensen et al (2011); (17) Hermes et al (2017b); (18) Bell et al (2015); (19) Greiss et al (2014); (20) icantly simplifies mode identification and affords us the opportunity to probe internal rotation for 20 of the 27 DAVs we present here. For example, Figure 1 shows the five dipole (ℓ = 1) modes in EPIC 201802933 (SDSSJ1151+0525) identified from their splittings.…”