THE BINARY-SYSTEM L151-81A/B - A POTENTIAL TEST OF ACCRETION THEORY

The common proper motion binary system L151-81A/B consists of two white dwarfs, the hottest (youngest) example of only 21 double degenerates in wide binaries known at present. The brighter of the two stars is a helium-rich white dwarf while its companion is hydrogen-rich. The system is particularly interesting because the cooler (older) hydrogen-atmosphere star has T(eff) = 12,000 +/- 2000 K, nominally placing it within the ZZ Ceti instability strip. We show that the mass of hydrogen accreted onto this object during the companion's preceding AGB mass-loss episode should be "thick," with M(H) approximately 10(-4) to 10(-6) M., if the standard Bondi-Hoyle accretion rate is appropriate. Thus, even for wide binaries, the mass of accreted hydrogen is large enough that double-DB and other double-non-DA white dwarf binaries should be quite rare in the Galaxy. Finally, previous white dwarf pulsation studies suggest that the star should not pulsate if the hydrogen layer is much thicker than approximately 10(-8) M.. Therefore, if the DA is found to pulsate, then the Bondi-Hoyle rate is an overestimate by some 2-4 orders of magnitude; conversely, if the star does not pulsate and is found to lie unambiguously within the instability strip, then the Bondi-Hoyle rate is a reasonable order of magnitude