The color mimics of OH / IR stars

Abstract

OH / IR star candidates are readily identified by color selection from the IRAS Point Source Catalog; they are confirmed by detecting a 1612 MHz maser. However, ≥40% of color selected sources do not exhibit masers, and most of these do not exhibit water or mainline OH masers either, even though their low resolution IRAS spectra may show the 9.7 μm silicate feature that flags an oxygen-rich status. These IR sources are therefore generally not associated with carbon stars, though most are generated by circumstellar shells. The most likely explanation for these “OH / IR star color mimics” is that mimics have a degenerate companion collecting an accretion disk from their winds, which provides them with an extra local source of UV to dissociate molecules from within their dust shells. In some cases the extra UV is sufficient to excise all molecules from a shell, as happens with symbiotic novae; in some cases it merely reduces their number and the ability of a shell to support a maser.