DO R CORONAE BOREALIS STARS FORM FROM DOUBLE WHITE DWARF MERGERS?

作者：Staff Jan E^*; Menon Athira; Herwig Falk; Even Wesley; Fryer Chris L; Motl Patrick M; Geballe Tom; Pignatari Marco; Clayton Geoffrey C; Tohline Joel E

来源：Astrophysical Journal, 2012, 757(1): 76.

DOI：10.1088/0004-637X/757/1/76

• binaries: close
• stars: abundances
• stars: AGB and post-AGB
• white dwarfs
• Online-only material: color figures

A leading formation scenario for R Coronae Borealis (RCB) stars invokes the merger of degenerate He and CO white dwarfs (WDs) in a binary. The observed ratio of O-16/O-18 for RCB stars is in the range of 0.3-20 much smaller than the solar value of similar to 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 WD. We present the results of five three-dimensional hydrodynamic simulations of the merger of a double WD system where the total mass is 0.9M(circle dot) and the initial mass ratio (q) varies between 0.5and 0.99. We identify in simulations with q less than or similar to 0.7 a feature around the merged stars where the temperatures and densities are suitable for forming O-18. However, more O-16 is being dredged up from the C- and O-rich accretor during the merger than the amount of O-18 that is produced. Therefore, on the dynamical timescale over which our hydrodynamics simulation runs, an O-16/O-18 ratio of similar to 2000 in the %26quot;best%26quot; case is found. If the conditions found in the hydrodynamic simulations persist for 10(6) s the oxygen ratio drops to 16 in one case studied, while in a hundred years it drops to similar to 4 in another case studied, consistent with the observed values in RCB stars. Therefore, the merger of two WDs remains a strong candidate for the formation of these enigmatic stars.

• 出版日期2012-9-20