Modeling of the dust and gas outflows from OH 26.5+0.6: The superwind

We have observed the extreme OH/IR star, OH 26.5+0.6, in the infrared dust continuum and in the submillimeter rotational lines of CO. Mid-infrared images reveal the compact nature of the circumstellar shell (<0.''5). A deep 9.7 mu m absorption feature and an absorption at 18 mu m show that the dust mass-loss rate is very high. However, the low antenna temperatures of CO J = 1-0 and 2-1 lines suggest that the outer part of the circumstellar shell is much more tenuous. In order to resolve this discrepancy, we have observed the J = 3-2 and 4-3 CO rotational transitions. We have developed a model for the circumstellar shell for OH 26.5+0.6 which is consistent with the infrared and submillimeter observations. The dust and gas data are well fitted by a two-shell model, consisting of a dense shell surrounded by a more tenuous shell. The former we identify with the superwind (M = 5.5 x 10(-4) M. yr(-1)), and the latter we identify with mass loss on the asymptotic giant branch (AGB) (M = 10(-6) M. yr(-1)). The transition between the two mass-loss phases is shown to be rather abrupt (Delta t < 150 yr). Depending on the mass of the progenitor, this superwind phase may be the last thermal pulse (for M* < 1.5 M.), or the first of a series of the superwind phases (for up to 8 M.), punctuated by a period of low mass-loss rates, before the star evolves off the AGB.