Hydrodynamic simulations of WD-WD mergers and the origin of RCB stars
arxiv(2024)
Abstract
We study the properties of double white dwarf (DWD) mergers by performing
hydrodynamic simulations using the new and improved adaptive mesh refinement
code Octo-Tiger. We follow the orbital evolution of DWD systems of mass ratio
q=0.7 for tens of orbits until and after the merger to investigate them as a
possible origin for R Coronae Borealis (RCB) type stars. We reproduce previous
results, finding that during the merger, the Helium WD donor star is tidally
disrupted within 20-80 minutes since the beginning of the simulation onto the
accretor Carbon-Oxygen WD, creating a high temperature shell around the
accretor. We investigate the possible Helium burning in this shell and the
merged object's general structure. Specifically, we are interested in the
amount of Oxygen-16 dredged-up to the hot shell and the amount of Oxygen-18
produced. This is critical as the discovery of very low Oxygen-16 to Oxygen-18
ratios in RCB stars pointed out the merger scenario as a favorable explanation
for their origin. A small amount of hydrogen in the donor may help keep the
Oxygen-16 to Oxygen-18 ratios within observational bounds, even if moderate
dredge-up from the accretor occurs. In addition, we perform a resolution study
to reconcile the difference found in the amount of Oxygen-16 dredge-up between
smoothed-particle hydrodynamics and grid-based simulations.
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