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Warwick Astronomy PhD projects

3D Model Atmospheres of White Dwarfs

The large majority of stars become white dwarfs following the successive phases of nuclear burning. We study these remnants in order to solve open questions related to the stellar formation history in the disk, the halo, and clusters of the Milky Way, and the fate of planetary systems. In addition to being probes of fundamental astrophysical relations, white dwarfs are fascinating compact matter laboratories. This project will be based on 3D radiation-hydrodynamics simulations of helium-rich white dwarf atmospheres, representing about 25% of the observed population.

The computation of 3D model atmospheres for stars has just reached its maturity in recent years. These simulations are based on the fundamental fluid mechanics equations, and do not assume the commonly used 1D analytical model of convection (mixing-length theory) with numerous free parameters. The first grid of 3D simulations using the CO5BOLD code have recently been computed for pure-hydrogen atmosphere white dwarfs. The new 3D simulations with effectively no free parameters have been employed to model the observed hydrogen Balmer lines in spectroscopic analyses. White dwarfs with radiative and convective atmospheres have derived mean masses that are now the same, in much better agreement with our understanding of stellar evolution.

 


Snapshot of a 3D white dwarf simulation at effective temperature Teff = 12,000 K and log g = 8 (cgs units) for a pure-hydrogen atmosphere. Left: temperature structure for a slice in the horizontal-vertical xz plane through a box with coordinates x,y,z (in km). The temperature is color coded from 60,000 (red) to 7000 K (blue). The arrows represent relative convective velocities. Right: emergent integrated intensity at the top of the horizontal xy plane. The root-mean-square intensity contrast with respect to the mean intensity is 18.8%, illustrating the significant deviation from the commonly used 1D approximation which corresponds to no horizontal contrast.


Within your PhD, you will be involved in the computation of CO5BOLD 3D simulations for helium-atmosphere white dwarfs. It expected that much can be learned about the physics of convection since these will be the first simulations of helium-dominated stellar atmospheres. The second step will involve the calculation of 3D spectra and the comparison with observations of white dwarfs, such as those from the SDSS (Sloan Digital Sky Survey), to assess about 3D effects on the fundamental stellar parameters (mass and age). These improved models will also be particularly useful to look at white dwarfs where the accretion of rocky asteroids is observed, which is a special expertise of the astrophysics group at Warwick. The helium-atmosphere white dwarfs are particularly interesting since the metals sink very rapidly at the bottom of the convection zone, and the accretion events must be very recent.

Supervisor: Pier-Emmanuel Tremblay

Please fill in our PhD enquiries form if you are interested in studying for a PhD in Astronomy at Warwick.