ROBERTS 22 - A BIPOLAR NEBULA WITH OH-EMISSION

Roberts 22 is a bipolar reflection nebula illuminated by a hidden A2 Ie star. Most of its energy is radiated at infrared wavelengths. It also shows strong OH maser emission (OH 284.18-0.79) on the 1612 and 1665 MHz transitions, generally similar to the masers associated with M stars having infrared excesses. But the system contains no late-type star. This remarkable assemblage of attributes makes Roberts 22 unique; however, it is probably a key member of the newly-recognized population of bipolar nebulae. From our analysis of the properties of Roberts 22 we question some published interpretations of other bipolar nebulae, in particular the derivation of spectral types for their underlying stars by the assumption of photo-ionization of the gas, and their evolutionary description as proto-planetary nebulae. © 1980 Royal Astronomical Society. Provided by the NASA Astrophysics Data System.