A leading formation scenario for R Coronae Borealis ( RCB ) stars invokes the merger of degenerate He and CO white dwarfs ( WD ) in a binary .
The observed ratio of ^ { 16 } \mathrm { O } / ^ { 18 } \mathrm { O } for RCB stars is in the range of 0.3-20 much smaller than the solar value of \sim 500 .
In this paper , we investigate whether such a low ratio can be obtained in simulations of the merger of a CO and a He white dwarf .
We present the results of five 3-dimensional hydrodynamic simulations of the merger of a double white dwarf system where the total mass is 0.9 M _ { \odot } and the initial mass ratio ( q ) varies between 0.5 and 0.99 .
We identify in simulations with q \lesssim 0.7 a feature around the merged stars where the temperatures and densities are suitable for forming ^ { 18 } \mathrm { O } .
However , more ^ { 16 } \mathrm { O } is being dredged-up from the C- and O-rich accretor during the merger than the amount of ^ { 18 } \mathrm { O } that is produced .
Therefore , on a dynamical time scale over which our hydrodynamics simulation runs , a ^ { 16 } \mathrm { O } / ^ { 18 } \mathrm { O } ratio of \sim 2000 in the “ best ” case is found .
If the conditions found in the hydrodynamic simulations persist for 10 ^ { 6 } seconds the oxygen ratio drops to 16 in one case studied , while in a hundred years it drops to \sim 4 in another case studied , consistent with the observed values in RCB stars .
Therefore , the merger of two white dwarfs remains a strong candidate for the formation of these enigmatic stars .