Journey to the Center of a White-Dwarf Star

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Representation of the vibrations propagating through the white-dwarf star KIC 08

Representation of the vibrations propagating through the white-dwarf star KIC 08626021. Illustration: S. Charpinet

A stellar core that challenges predictions: here is the result of the first cartography of the interior of a white-dwarf star performed by an international team that includes  Valérie Van Grootel and Marc-Antoine Dupret , two researchers belonging to the STAR Institute at Université de Liège.  This discovery will provide insights on the different physical mechanisms involved in the evolution of stars and our Sun. This result is published in Nature.

By exploiting data from NASA's Kepler spacecraft on the pulsating star KIC 08626021, an international team of astrophysicists led by a young researcher from the « Institut de Recherche en Astrophysique et Planétologie » (IRAP) of Toulouse, unraveled the cartography of the internal composition of a white dwarf, the distant successor of a star just like our Sun. The luminosity oscillations observed at the surface of this star were deciphered with « asteroseismic » techniques, which are similar to methods used by geophysicists to study the structure of our planet with the seismic waves caused by Earthquakes.
« White-dwarf stars are the interior relics of approximately 97% of the stars in the Universe », says Valérie Van Grootel, F.R.S.-FNRS Research Associate at the STAR Institute of ULiège and co-author of this study. These stellar fossils retain prints of past physical processes like nuclear burning and episodes of convective mixing - phenomena which are quite uncertain in the actual modeling of stellar evolution theories. « To have a glimpse at the internal composition of these stars will allow scientists to better elucidate the physical processes at play during early stages of stellar evolution, and hence constrain stellar evolution theory» explains Van Grootel.
During the phase in which they cool down, white dwarfs undergo episodes of instability where they start to pulsate. « The deep vibrations generated are the keys to unveiling the interior of these stellar remnants », says Marc-Antoine Dupret, associate professor at ULiège and also co-author of this study. The internal chemical stratification of a white-dwarf star creates a unique signature on the modulation of the light coming out of the star, which, once deciphered, allows one to obtain a cartography of its full structure.

From KIC08626021, a white dwarf scarcely larger than the Earth, 1375 light-years away from the boundaries of the Swan and Lyra constellations, only a very faint flash arrives. Yet tiny vibrations - star tremors - could be measured with great precision on its surface. These earthquakes unveil for the first time the mysteries escaped from the very heart of these stellar fossils (Crédit: Stéphane Charpinet).
What can we learn from unfolding all the layers of these stars’ Not only that their inner core is bigger and richer in oxygen than predicted, but we are also able to get the distribution profiles of all the main chemical elements present. This exciting discovery provides a precise benchmark to better calibrate the physical processes of nuclear burning and mixing processes taking place in most of the stars - particularly during the pre-white dwarf phases. A precise knowledge of the internal chemical composition of white dwarfs is also necessary to accurately use them as « cosmological chronometers » for dating stellar populations of our Galaxy.

Scientific reference

«  A l arge oxygen-dominated core from the seismic cartography of a pulsating white dwarf  »

N. Giammichele, S. Charpinet, G. Fontaine, P. Brassard, E.M. Green, V. Van Grootel, P. Bergeron, W. Zong, & M.-A. Dupret. DOI : 10.1038/NATURE25136

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