Kinematic structure of OH/IR stars

Abstract

The discovery of OH maser emission in late-type stars has created the controversy of the kinematic nature of the sources and for several years it was not clear whether the characteristic doublepeak profile of OH/IR stars is due to rotation, shock waves, expansion or collapse. This problem was finally resolved when quantitative calculations taking into account the physics of masers show that no model other than a uniformly expanding model can produce the double-peak profile. With the development of VLA/VLBI observations for OH/IR stars, the availability of OH intensity distribution from interferometric data in addition to the total flux from single-dish measurements allows a more rigorous testing of the kinematic models of OH/IR stars. It is also possible that the physical parameters responsible for the creation of the maser can be determined with higher accuracy than before. In this paper, a kinematic model is constructed for 1612 MHz OH maser emission in OH/IR stars. The spatial distributions for OH maser intensity are calculated from a model of a sphericallysymmetric uniformly-expanding circumstellar shell. By comparing VLA/VLBI maps of OH/IR stars with model results, the acceptable of combinations of physical parameters M/Ve, range (nH2)ma x , (nH2)min' fOH (=[nOH]/[nH2]) are derived. The theoretical relations between OH shell radius Ro and mass loss rate H and between OH maser luminosity LOH and M are also obtained. These relations are in good agreement with empirical relations established by Bowers et al. (1983) and Baud and Habing (1983). The physical conditions