A search for symbiotic behaviour amongst OH/IR colour mimics

Abstract

Recent maser surveys have shown that many potential OH/IR stars have no OH masers in their circumstellar envelopes, despite the modest requirements which should be implicitly met by IRAS colour-selected candidates. It has been suggested that these OH/IR colour mimics must have a degenerate companion which dissociates OH molecules and disrupts the masing action, i.e. that they are related to symbiotic Miras. Coincidentally, there is a paucity of long-period symbiotic Miras and symbiotic OH/IR stars. Phenomenologically, those that are known seem to cluster in the zone where field Miras transform into OH/IR stars. If it could be proven that OH/IR colour mimics contain a degenerate star, and that observable evidence of this star is hidden from view by circumstellar dust whilst it slowly accretes from the wind of its Mira companion, then we have an excellent explanation not only for the existence of OH/IR colour mimics, but also for the low observed frequency of symbiotic OH/IR stars and the common occurrence of very slow novae in long-period symbiotic Miras. Here, we employ radio continuum radiation (which should escape unhindered from within the dust shells) as a simple probe of the postulated hot degenerate companions which would inevitably ionize a region of their surrounding gas. We compare the radio and infrared properties of the colour mimics with those of normal symbiotic Miras, using the strong correlation between radio and mid-IR emission in symbiotic stars. We show that if a hot companion exists then, unlike their symbiotic counterparts, they must produce radiation-bounded nebulae. Our observations provide no support for the above scenario for the lack of observed masers, but neither do they permit a rejection of this scenario.