Photometry of masing and nonmasing OH/IR stars

We report broadband infrared photometry performed on a sample of 60 stars with IRAS colors in the range displayed by Type II OH/IR stars. All of the stars have been tested for 1612 MHz OH maser emission by Eder, Lewis, and Terzian (1988), but emission has been detected in only 27 of them. A morphological comparison is made between the masing and nonmasing groups to discern whether masering is correlated with near- and middle-infrared properties. Both groups have the same scale height perpendicular to the galactic plane and share many characteristics in common with the local Mira population. Where the IRAS colors of the two groups overlap significantly, we find no statistically significant difference in the K - L color distribution. Using theoretical models relating this color and the strength of the 9.7 μm silicate feature to the optical depth of the circumsteilar dust shell, we find that red giant dust shells need a minimum optical depth of τ_9.7 ∼0.1 to support maser emission. Stars with a silicate feature in absorption only occur at low galactic latitudes, whereas stars with the feature in emission are found at all latitudes. We conclude that interstellar extinction is partly responsible for this effect.