Book Description

As low- and intermediate mass stars (with masses up to 8-9 solar masses) age, they experience a series of evolutionary changes which culminate in the removal of nearly their entire envelope through extensive mass loss. The ejected material cools down, which allows for the formation of molecules and the nucleation of dust grains in the circumstellar environment (CSE) of the star. Much about the properties and composition of the gas and dust in these CSEs is not well understood. Here, we study the rich CSEs of two unusual evolved stars by analyzing spectral observations in the infrared and using molecular spectroscopy to determine the chemical and physical properties of the circumstellar gas. We first present the mid-infrared spectrum of the carbon-rich pre-planetary nebula SMP LMC 11. The spectrum is rich in molecular bands of a variety of species, including several polyynes and cyanopolyynes and unusually strong absorption from benzene. We also confirm the presence of propyne. We find that this molecular gas resides in a dense torus around the central object. A detailed comparison with results from chemical models indicates that important chemical pathways to benzene and other carbon-rich molecules are currently missing from the models. Next, we analyze the molecular gas in the mid- and near-infrared spectrum of the peculiar evolved binary HR 4049. This system is known to be surrounded by a long-lived circumbinary disk. We find that the disk is gas-rich, hot, massive and much more radially extended than previously thought. A recent and significant increase in CO2 emission may also point to ongoing processing in the disk. Given the column densities of especially CO2 and H2O in the disk, radiative trapping must be an important ingredient in determining the physical structure of the disk and its resulting spectrum. Such effects may well be important in many other circumstellar environments as well. Finally, we can also estimate the mass of the primary, and conclude that it may be too low for the star to have experienced a ǹ̀ormal" evolutionary path.