Cepheids are pulsating stars, they have stable periodic variations of their radius and luminosity. These stars are known to have a precise relation between their pulsation period P and their intrinsic luminosity (or absolute magnitude) Mabs. This relation was discovered by Leavitt in 1912.
Thanks to it, astronomers can calculate the distance of a Cepheid star simply by watching it: doing so, they get an apparent magnitude Mapp and a period P, which gives them the absolute magnitude Mabs. They then get the distance of the star which is dependant on Mapp − Mabs.
That is the reason why Cepheids are used to calculate distances in the close universe (mostly the Local Group).

The mecanism responsible for these periodic pulsations was first suggested by Eddington in 1926. It is called κ-mecanism, from the opacity κ.

Hot Cepheids (stars with high surface temperature) are well known and explained. But Cepheids from the blue edge of the Herzsprung-Russel diagram have a convection zone near their surface which interacts with the κ-mecanism. Those interactions have long been neglected or well approximated, for lack of adapted computer power.

 

In his thesis work, Thomas Gastine performed the first direct numerical simulations with convection-pulsations coupling in a simple case: the Cepheid is simplified into a cartesian 1D or 2D box and the global problem comes down to the propagation of acoustic modes.

 

After giving the two essential conditions for instability to survive, Gastine showed that purely radiative unstable cases reached a non linear saturation. But when convection is added, this saturation is sometimes lost : a strong coupling appears between convection and pulsations so that convection kills the acoustic mode propagation.

Gastine's and his advisor Boris Dintrans' hypothesis is that convection has lesser effect in three dimensions simulations, due to reduce size of convection plumes. Saturation could then be maintained in 3D.

 

My thesis consisted in establishing highly parallel 3D simulations with hydrodynamical HERACLES code developped at CEA, mostly by Édouard Audit, to study this hypothesis.

 

My work won the Mésochallenges GENCI 2014 prize.