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B. M. Lewis (Arecibo Observatory)
Analysis of the J-H v H-K plot for 2MASS counterparts to OH/IR stars in the Arecibo sample, that were available from the second incremental release, shows that most of them have detached shells (Lewis, Kopon, & Terzian in press). Normal shells with apparently continuous steady-state mass-loss, and thus red IRAS colors, are in fact quite rare. The OH/IR star with the reddest mid-IR colors in our sample is IRAS 19396+2338, whose spectral energy distribution has been modeled using the radiative transfer code DUSTY. This model has been used to trace the evolution of the mid-IR colors as a function of the mass-loss history, both when mass-loss turns on abruptly at a constant value, so the shell builds its thickness, and for the case where the fully developed shell abruptly ceases mass-loss, and so evolves along a proto planetary nebular sequence with a central star of constant stellar temperature and luminosity. We find the color changes reproduce the linear sequence exhibited in the J-H v H-K plot, and so can attach a time-scale for these changes. The most likely explanation for the preponderance of detached shells is that they are artifacts of a large amplitude oscillation in the mass-loss rate, as proposed by Simis (2001).
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.