Imaging the circumstellar envelope of OH 26.5+0.6

Using the Berkeley-Illinois-Maryland-Association (BIMA) Millimeter Array, we were able to map the extreme OH/IR star, OH 26.5+0.6, in the ¹²CO J = 1-0 line transition. The CO emission is partially resolved with a deconvolved source size of 8.5″ × 5.5″. The spectrum shows that the blue-shifted emission is missing, most likely due to interstellar absorption. By modelling the infrared spectral energy distribution, we derive a dust mass loss rate of 1.9 × 10^-6 M_⊙ yr^-1. From this we are able to place an upper limit on the extent of the dusty envelope of 10¹⁶ cm while our BIMA map shows that the CO photodissociation radius extends out to about 7 × 10¹⁶ cm. To best fit the BIMA observations and the higher CO rotational transitions using our full radiative transfer code, we needed to include a second, more tenuous AGB wind, outside the high density superwind to account for the observed flux. From our model, we conclude that up to 80% of the CO flux comes from the unresolved superwind.