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M. E. DeCesar (Pennsylvania State University), B. M. Lewis (National Astronomy and Ionosphere Center)
Most known OH/IR stars were identified by NIR color-selection from the IRAS Point Source Catalog, with subsequent confirmation by the detection of their 1612 MHz masers. However, more than half of the candidates do not exhibit 1612 MHz masers: these stars are the so-called color "mimics". The NIR colors of OH/IR stars follow a tight locus that is largely determined by the universal nature of differential extinction by dust. Radiative transfer models of circumstellar shells generated by DUSTY show that the NIR colors of an OH/IR star can traverse the entire locus in less than 60 years once mass-loss stops (Lewis BAAS 35, 1358). Hence the 1612 MHz masers will also disappear within a few decades after the cessation of copious mass-loss, suggesting that many of the mimics should correspond to a post-OH/IR star phase in the ongoing development of an AGB star. We have identified the 2MASS counterparts for a complete sample of mimics in the Arecibo sky (0\circ 3/4 d 3/4 38\circ) with | b | > 10\circ to see whether their NIR colors can both inform the identification of OH/IR stars and help elucidate the evolutionary sequence of these stars. Our results show that while OH/IR stars occupy the H-K range from 0.35 to 3.4, 77% of mimics fall between 0.2 and 0.65. An H-K > 0.65 constraint on potential OH/IR star candidates provides a high probability for their confirmation. Moreover, many of the mimics have moderately red NIR colors of (25-12) \mum > -0.6 that point to a past history of copious mass-loss and a likelihood of their having recently exhibited 1612 MHz masers.
This work was made possible by the National Astronomy and Ionosphere Center and the National Science Foundation.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.