Modeling IR spectra of OH/IR stars at different phases

Abstract

We investigate the spectral energy distributions (SEDs) of OH/IR stars (OH127.8+0.0 and OH26.5+0.6) having thick dust envelopes at different pulsation phases. Using new infrared observational data including the Infrared Space Observatory (ISO) data, we determine the new pulsation parameters. The deep silicate absorption features show significant variations depending on the pulsation phase. The variations are mainly due to changes in the properties of dust envelopes around the OH/IR stars. Comparing the results of detailed radiative model calculations with observations, we explore the changes of the relevant parameters of the envelopes and central stars depending on the pulsation phase. We find that when the central luminosity increases from the minimum to maximum phase, the inner radius of the dust shell increases with velocity faster than the outer shell expansion velocity and the dust shell optical depth decreases. During the phase change from the minimum to maximum, we find that dust formation ceases and about a half of the dust grains in the volume difference should have evaporated. During the phase change from the maximum to minimum, we find that the dust formation should be enhanced because the inner radius is decreasing. In the outer radii of the dust shell, the constant dust winds are easily maintained. We expect that the dust evaporation process driven by pulsation could be a mechanism for crystallizing the dust grains in inner regions of the dust shells around OH/IR stars.