Making nonmassive O-Ne-Mg rich white dwarfs in cataclysmic binaries

A significant fraction of classical novae are observed to eject material rich in Ne and Mg. These events are believed to occur on O-Ne-Mg white dwarfs which, according to stellar evolution theory, should have masses close to the Chandrasekhar limit. The expected masses of the shells that should be ejected by massive white dwarfs in nova outbursts are much lower than those observed in several Ne-rich novae. We propose a simple alternative mechanism to produce moderate-mass white dwarfs with Ne- and Mg-rich outer shells. A numerical simulation confirms that rapid accretion (∼10^-6 script M sign_⊙ yr^-1) of hydrogen onto a C-O white dwarf - at an early evolutionary stage, before nova outbursts start to occur - leads to a buildup of a massive Ne-Mg-rich outer layer. It is this outer shell that we propose as the source of Ne and Mg in novae. This alternative scenario is shown, in Shara [AJ, 107, 1546 (1994), to naturally account for the high observed abundance of interstellar ²⁶Al.