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

White-dwarf stars are the end product of stellar evolution for most stars in the Universe. Their interiors bear the imprint of fundamental mechanisms that occur during stellar evolution. 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. 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. Here we report a measurement of the radial chemical stratification (of oxygen, carbon and helium) in the hydrogen-deficient white-dwarf star KIC08626021 (J192904.6+444708), independently of stellar-evolution calculations. We use archival data coupled with asteroseismic sounding techniques 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.

Kepler detected gravity-mode period spacings in a red giant star.

Stellar interiors are inaccessible through direct observations. For this reason, helioseismologists made use of the Sun's acoustic oscillation modes to tune models of its structure. The quest to detect modes that probe the solar core has been ongoing for decades. We report the detection of mixed modes penetrating all the way to the core of an evolved star from 320 days of observations with the Kepler satellite. The period spacings of these mixed modes are directly dependent on the density gradient between the core region and the convective envelope.

Asteroseismology of HD 129929: core overshooting and nonrigid rotation.

We have gathered and analyzed 1493 high-quality multicolor Geneva photometric data taken over 21 years of the B3Vstar HD 129929. We detect six frequencies, among which appear the effects of rotational splitting with a spacing of approximately 0.0121 cycles per day, which implies that the star rotates very slowly. A nonadiabatic analysis of the oscillations allows us to constrain the metallicity of the star to Z epsilon [0.017,0.022], which agrees with a similar range derived from spectroscopic data. We provide evidence for the occurrence of core convective overshooting in the star, with alpha(ov) = 0.10 +/- 0.05, and we rule out rigid rotation.