OH/IR stars - Late stages of evolution of intermediate-mass stars

Abstract

In this paper we attempt to interpret a large body of radio and infrared data of OH/IR stars in terms of our present understanding of stellar evolution. As a working hypothesis we adopt the view that the (often invisible) stars that produce the thick OH masing and infrared emitting shells of gas and dust are in their last phases of evolution. We first model the galactic longitude distribution of OH maser stars brighter than an apparent 1612 MHz flux density of 1.7 Jy and we define a complete galactic subsample of 160 OH maser stars brighter than an absolute 1612 MHz flux density of 110 Jy (at 1 kpc). Dividing the OH/IR sources in this bright subsample into three age groups according to their ..delta..V values (the velocity separation between the 1612 MHz maser peaks), we have calculated average absolute 1612 MHz flux densities, average distances to the galactic plane, average bolometric luminsities, and average mass loss rates for each group from radio and infrared data of about 20 sources with known distances (the ones with high radial velocities, so-called tanagential-point sources). Because these sources are located at quite large distances (6-8 kpc) in the plane of the Galaxy, wemore » have corrected the infrared fluxes for interstellar extinction using a simple model of the galactic distribution of interstellar dust. Finally, we have calibrated the ages (initial main-sequence masses) of the OH/IR sources in each age group by comparison with open cluster data.« less