The survival of 1612-MHz OH masers in asymptotic giant branch plus white dwarf binaries

Abstract

We have constructed a model of the OH distribution in an oxygen-rich outflow from an asymptotic giant branch star possessing a white dwarf companion. We assess the probability of 1612-MHz OH maser emission occurring despite the presence of a luminous accretion disc around the white dwarf. We find that the OH is resistant to photodissociation in all but the least massive winds, as a result of shielding by the dust component of the AGB outflow and self-shielding by H2O and OH molecules. This conclusion only partially supports the suggestion of Lewis that a ~ 40 per cent non-detection rate of OH 1612-MHz masers in potential OH/IR stars is due to photodissociation of OH. Our results are more compatible with the more tightly directed survey by Lewis & Engels of oxygen-rich OH/IR-star colour ‘mimics’, which shows a 1612-MHz detection rate of at least 75 per cent. We also postulate that other effects, such as the disruption of the velocity coherence of the wind, may be as important as photodissociation in inhibiting maser formation in AGB + WD binary systems.