A large oxygen-dominated core from the seismic cartography of a pulsating white dwarf

Abstract

White-dwarf stars are the end product of stellar evolution for most stars in the Universe 1 . Their interiors bear the imprint of fundamental mechanisms that occur during stellar evolution 2,3 . Moreover, they are important chronometers for dating galactic stellar populations, and their mergers with other white dwarfs now appear to be responsible for producing the type Ia supernovae that are used as standard cosmological candles 4 . However, the internal structure of white-dwarf stars-in particular their oxygen content and the stratification of their cores-is still poorly known, because of remaining uncertainties in the physics involved in stellar modelling codes 5,6 . Here we report a measurement of the radial chemical stratification (of oxygen, carbon and helium) in the hydrogendeficient white-dwarf star KIC08626021 (J192904.6+444708), independently of stellar-evolution calculations. We use archival data 7,8 coupled with asteroseismic sounding techniques 9,10 to determine the internal constitution of this star. We find that the oxygen content and extent of its core exceed the predictions of existing models of stellar evolution. The central homogeneous core has a mass of 0.45 solar masses, and is composed of about 86 per cent oxygen by mass. These values are respectively 40 per cent and 15 per cent greater than those expected from typical white-dwarf models. These findings challenge present theories of stellar evolution and their constitutive physics, and open up an avenue for calibrating white-dwarf cosmochronology 11 .Hydrogen-deficient DB-type white dwarfs are dying stars that have effective temperatures ranging from about 12,000 K to upwards of 35,000 K (ref. 12). They have undergone a late final flash as postasymptotic-giant-branch (post-AGB) stars, which freed them from their thin remaining hydrogen envelope [13][14][15] . Their deeper chemical stratification-as in any typical white dwarf-is otherwise determined by the rate of the nuclear reaction 12 C(α ,γ ) 16 O, which synthesizes oxygen from the ashes of the core-helium-burning phase, by means of mixing processes associated with convection and rotation as well as the number of thermal pulses that occur in evolved AGB stars 16 . The descendant DB white dwarfs must bear the signature of such processes in their core, as well as the imprint of the still-ongoing settling of carbon and oxygen in the surrounding helium-rich envelope.KIC08626021 (GALEX J192904.6+ 444708) is the first pulsating DB white dwarf (also known as the V777 Her class of variable stars) to be moni tored extensively by the Kepler spacecraft for its pulsation properties 8 . This star shows an oscillation spectrum composed of non-radial g-modes, which are potentially sensitive to the deep interior of the star 17 . Using an analysis of 23 months of Kepler highprecision photo metric data 18 , we exploit the eight well secured independent modes, which have periods ranging from 143.2 s to 376.1 s (Fig. 1, Extended Data Table 1 and Methods). These relatively short pulsation periods are-accordin...

On the recent parametric determination of an asteroseismological model for the DBV star KIC 08626021

“…Only when f = 0.087 is adopted, the extent of CBM leads to a larger homogeneous core of 0.354 M ⊙ . This value is still far below the value of 0.45 M ⊙ derived by Giammichele et al (2018) for KIC 08626021. Considering that a value of f = 0.087 = 5× f 0 H P is the local pressure scale height at the formal convective boundary (Herwig et al 1997).…”

“…Such a model would be incompatible with the existence of the early AGB phase and should be already discarded on those grounds. And even with such inconsistently large value of f = 0.174, the mass of the homogeneous CO core is reduced by the first thermal pulse to 0.386M ⊙ (from a value of 0.422M ⊙ at the very end of the HeCB), well below the value of 0.45 M ⊙ derived by Giammichele et al (2018).…”

“…4 we summary our results and conclusions. Figure 1 shows the typical chemical structure of a DBV model with similar parameters to those found by Giammichele et al (2018), namely M WD = 0.58M ⊙ , T eff ∼ 29 000 K and log g = 7.93 cm s −2 , derived from the full computation of the progenitor evolution (upper panel) and the chemical-abundance profiles predicted by the asteroseismic model for the DBV KIC 08626021 (Giammichele et al 2018, bottom panel). The abundance distribution of O, C and He from the core to the outer layers are shown in terms of the outer mass fraction coordinate.…”

“…Based on the parametric approach, Giammichele et al (2018) found an asteroseismic model with an unprecedented precision in their pulsation-period match for the DBV star KIC 08626021, being the derived stellar parameters M WD = 0.570 ± 0.005M ⊙ , T eff = 29 968±198 K, log g = 7.92±0.01 cm s −2 . This pulsating star, located near the blue edge of the instability strip, has been extensively monitored by the Kepler mission, revealing eight independent modes with periods from 143.2 s to 376.1 s (Østensen et al 2011).…”

“…a relative period difference of P obs − P model )/P model < 10 −8 ), well below the observational uncertainties of ∼ 38µs. However, this finding has been put into question by Timmes et al (2018), who showed that the inclusion of neutrino emission, expected in young WDs and not considered by Giammichele et al (2018), impacts the low order g-mode frequencies up to ∼ 70µHz. Additionally, the derived structure parameters, such as a large CO core, a high central O abundance, a well defined C-pure mantle and a thin pure-He envelope pose a challenge to the stellar evolution predictions.…”