Infrared Interferometry and Spectroscopy of Circumstellar Envelopes

Abstract

This thesis reports on two experiments designed to reveal fundamentally new information about the inner dust and gas envelopes around mass-losing stars. The mid-infrared Infrared Spatial Interferometer was outfitted with an RF filterbank to allow interferometric observations of molecular absorption features (NH3 and SiH4) with very high spectral resolution ( / ~ 105). These new data permitted the molecular stratification around carbon star IRC +10216 and red supergiant VY CMa to be investigated. For IRC +10216, it was determined that both ammonia and silane form in the dusty outflow significantly beyond both the dust formation and gas acceleration zones ( >~ 20 R* ). More specifically, ammonia was found to form before silane in a region of decaying gas turbulence, while the silane is produced in a region of relatively smooth gas flow much further from the star ( >~ 80 R* ). The depletion of SiS on grains soon after dust formation may fuel silane-producing reactions on the grain surfaces. For VY CMa, a combination of interferometric and spectral observations suggest that NH3 is forming near the termination of the gas acceleration phase in a region of high gas turbulence (~40 R* ). The second half of the thesis describes a novel aperture masking experiment which converted the Keck-I 10-m primary mirror into a separate-element interferometric array. High signal-to-noise images were reconstructed of bright near-infrared sources at the diffraction limit (~ 0.050'' at 2 m m) using VLBI techniques. The inner dust shells of IRC +10216 and VY CMa are shown to be highly clumpy and inhomogeneous, a finding inconsistent with current (simple) models of mass-loss. For IRC +10216, spatial resolution on the scale of the star itself was attained, and proper motion of dust clumps within 10 R* was detected, revealing the dynamics of the outflow directly. Unexpectedly, carbon-rich dust shells around some late- type Wolf-Rayet stars were resolved into highly- collimated, spinning ``pinwheel'' nebulae, formed from the interacting winds of embedded short- period (~1 yr) binaries. Precise orbital parameters and wind velocities are determined from the multi-epoch spiral morphology; important implications on binary and stellar evolution are discussed.