Mass Loss from Evolved Stars

Citations Over Time

Abstract

This work consists of two studies of the matter that flows outward from the surface of stars that have reached the late stages of evolution. This process of mass loss is important to the understanding of the continued evolution of the star and the replenishment of the interstellar medium from which the next generation of stars will form. The two major constituents of these circumstellar envelopes are gaseous molecules and solid dust grains, both of which form in the outflow as the matter expands and cools. Each of these components is investigated separately. I present observations of the CO (J = 1-0), 13CO (J = 1-0), HCN (J = 1-0), H13CN (J = 1-0), HC3N (J = 10-9), and HNC (J = 1-0) emission lines from a number of well-known evolved stars. From these data, estimates of mass-loss rates, envelope sizes, molecular abundances, and isotope ratios are derived in an attempt to understand the composition, kinematics, and chemistry of the molecular component of the mass flow. I find a reasonable degree of consistency among the evolved stars studied here and with the well-studied, prototypical object, IRC + 10216. The solid component of the mass flow is investigated by means of broad-band submillimeter observations of the thermal emission from dust in the circumstellar envelope. These observations, at an effective wavelength of 400 μΐη, provide the most direct means for estimating the mass-loss rate in dust and also help to define the longwavelength thermal spectrum of the dusty envelope. The mass-loss rates in dust are compared with the mass-loss rates in gas to estimate the gas-to-dust ratio in the outflowing matter and these values are found to be in general agreement with the interstellar ratio of ~ 100. The analysis of the thermal emission of the circumstellar dust, which includes the 400 μΐη data, provides reasonably strong evidence that the grains in both carbon-rich and oxygen-rich envelopes are not crystalline but are composed of amorphous structures.

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