Observability of Scattered-Light Echoes around Variable Stars and Cataclysmic Events

Scattered light echoes from variable and cataclysmic stars offer one of themost effective means to probe the structure and composition of circumtellar andinterstellar media. I build a simple model of light-echo surface brightness byconsidering the source spectrum, and the dust density, geometry, and scatteringefficiency. I use this model to investigate whether echoes should be observablearound short and long-period giants, cataclysmic variables, and supernovae.Only supernovae are expected to illuminate material on both circumstellar andinterstellar scales. Giant and post-AGB stars (e.g. Cepheids and Miras) withhigh mass-loss rates can produce observable echoes within their circumstellarenvelopes. Echoes from novae and dwarf novae are probably detectable only fromnearby material, and only in unusually dense gas. I present characteristicexposure times to image such echoes over a wide range of wavelengths forground-based and {\em Hubble Space Telescope} observations. I apply theseresults to analyze the dust properties of the recently-reported echoes aroundSN 1993J, finding the dust in M81 to have a grain-size distribution andchemical composition consistent with Galactic dust. Optimal observingstrategies for echo detection are also discussed.Comment: 43 pages, 18 figures; Accepted for publication in the A