Radio and Infrared Observations of OH/IR Stars

Combined infrared and OH spectral line observations are essential to understanding the nature of OH/IR stars, the properties of their circumstellar envelopes and the OH maser pump mechanism. We have used the Very Large Array of the National Radio Astronomy Observatory to determine very precise radio positions for a selection of such objects near the tangential point and at the galactic center and have subsequently made simultaneous infrared and OH observations of both samples. From the infrared measurements, luminosities of these stars and masses of the thick dust shells which surround them have been derived, while the double-peaked OH emission line profiles have yielded both the stellar radial velocities, v_R, and the expansion velocities of the envelopes, v_e, as well as the maser luminosities. The selection criteria and the observations are described in detail elsewhere (Baud et al. 1984). The results are consistent with these stars lying at the top of the Asymptotic Giant Branch (AGB) of the Hertzsprung-Russell diagram; the dense circumstellar shells suggest that these objects are evolving rapidly through a phase of high mass loss.