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B. M. Lewis (Arecibo Observatory)
Most OH/IR stars were identified by detecting a 1612 MHz maser at the position of an IRAS source, so most lack accurate positions and near-IR or optical fluxes, with consequent uncertainty as to their luminosities: 2MASS can often remedy this. By using the {~}2" quality MSX positions as an intermediary, a pilot study (AJ 121, 426) identified a K\rm s = 14.2 mag 2MASS source with the heavily obscured hypergiant 19566+3423, which increases its estimated luminosity from {~}68000 to 180,000 L\odot. The currently released 2MASS dataset provides identifications for 134 (32 %) of the OH/IR stars in the Arecibo sky (0\deg \leq{\delta}\leq 38\deg), and near-IR colors for 72: these constitute the present sample.
OH/IR stars exhibit a linear correlation of J-H v H-K, with the most translucent shells having the hottest colors, and the most opaque the coolest. Sevenster (AJ 123, 2772) draws attention to the remapping of the patently detached shells of proto planetary nebulae back along this sequence: these are far more transparent in the near-IR than their red IRAS colors would otherwise suggest, so the oldest PPN have very blue colors. This reasoning can be reversed: by coding our sample relative to a (25-12) {\mu}m color threshold of -0.25, -0.30, -0.35, etc one finds that most OH/IR stars are mapped back to blue colors on a J-H v H-K plot. Thus 10/11 objects with reliable near-IR fluxes and (25-12) {\mu}m > -0.25 have detached shells. The two bluest, and hence oldest detached objects from this set of 11 are 19576+2814 (OH65.7-0.8), which has an LRS type of 39, and 20160+2734 (AU Vul). Indeed this result suggests that classic OH/IR stars with a secularly increasing mass-loss rate are rare in the Arecibo sample. Our best candidate, with the reddest near-IR colors, is 19396+2338.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.