Unveiling the internal chemical stratification in white-dwarf stars
Gilles Fontaine
Université de Montréal

White-dwarf stars represent the final products of the evolution of more than 95% of all stars, and their internal constitution bears the imprints of several mechanisms that occur during the previous phases of stellar evolution in general. In particular, it is expected that the internal chemical stratification of a typical white dwarf is shaped, over time, by the still uncertain rate of the 12C(alpha,gamma)16O thermonuclear reaction, by convection, semi-convection, overshooting, other mixing processes such as rotational mixing, and by thermal pulses that occur primarily on the Asymptotic Giant Branch. All of these notoriously suffer from major uncertainties and are not securely modeled in current stellar evolution codes, thus illustrating the numerous shortcomings that still plague our treatments of the physics involved. Here we report the unraveling of the internal radial chemical stratification (oxygen-carbon-helium) in the pulsating hydrogen-deficient white-dwarf star KIC08626021 on the basis of asteroseismic sounding and independently of stellar evolution calculations. Using a novel approach to describe and test the chemical distribution inside the core and envelope of a pulsating white dwarf, we uniquely recover a model whose theoretical pulsation periods match perfectly the high precision observations of that star obtained with the Kepler satellite. Our seismic calibration of the oxygen-carbon-helium internal stratification in a representative white-dwarf star implies to revisit stellar evolution theory and its constitutive physics in the light of this new constraint. It also has repercussions on techniques using white dwarfs to derive stellar population ages.

Date:	Jeudi, 1 février 2018
Time:	11:30
Where:	Université de Montréal
	Pavillon Roger-Gaudry, Local D-460
Contact:	Björn Benneke