The Structure of Hydroxyl Masers and Circumstellar Envelopes of Long Period Variable Stars

Abstract

Long period variable stars with envelopes of circumstellar dust and gas often exhibit strong hydroxyl (OH) maser emission. This emission is characterized by two emission line complexes typically separated by 20 km/s. While the OH maser spectra suggest well defined dynamical properties of the circumstellar material, the crucial observations required to determine these properties have been lacking. This thesis is primarily concerned with determining the radial velocity of the central star and the spatial distribution of the maser emission in the circumstellar material. The radial velocity of the central star can be determined by a statistical analysis of spectral line velocities in an ensemble of stars. Such an analysis is presented in Chapter I for optical emission and absorption lines and for radio OH maser emission lines. The results indicate that, contrary to currently accepted theories, the optical absorption line velocities and the high velocity OH emission line velocities are red-shifted with respect to the stellar radial velocity. This argues against models of the OH emission which involve shock fronts or emission from the limb of a spherically expanding circumstellar shell. The spatial distribution of the maser emission can be determined from radio interferometric observations. Spectral-line very long baseline (VLB) interferometric observations of the OH maser emission in long period variable stars such as IRC+F10011, U Ori and R Aql are presented in Chapter II. The primary result of these observations is that the apparent sizes of the OH maser components in long period variable stars are greater than or about 5 x 1015 cm. These large apparent sizes argue strongly against either gravitational collapse or rotation as the dominant dynamical process in the circumstellar envelope. Analysis of data obtained on IRC+10011 and VY CMa (discussed in Chapter III) indicate that the two emission complexes probably are separated by distances less than, or comparable to, the extent of the emission in either complex. These and other findings suggest that OH emission from long period variable stars comes from a series of concentric, inhomogeneous, circumstellar shells expanding from the central star. The origin of these shells is probably a result of condensation of dust followed by radiative acceleration away from the star during the stellar light cycle. A direct Fourier inversion of VLB data is presented in Chapter III for the 1612 MHz OH emission of VY CMa. This is the first attempt at such an analysis procedure for any VLB observations. This work demonstrates that Fourier inversion of spectral-line VLB data, even with very limited u-v coverage, is more efficient and less biased than direct model fitting for the resolution of complex source structures.