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

Abstract

Scattered light echoes from variable and cataclysmic stars offer one of the most effective means to probe the structure and composition of circumtellar and interstellar media. I build a simple model of light-echo surface brightness by considering the source spectrum, and the dust density, geometry, and scattering efficiency. I use this model to investigate whether echoes should be observable around short and long-period giants, cataclysmic variables, and supernovae. Only supernovae are expected to illuminate material on both circumstellar and interstellar scales. Giant and post-AGB stars (e.g. Cepheids and Miras) with high mass-loss rates can produce observable echoes within their circumstellar envelopes. Echoes from novae and dwarf novae are probably detectable only from nearby material, and only in unusually dense gas. I present characteristic exposure times to image such echoes over a wide range of wavelengths for ground-based and Hubble Space Telescope observations. I apply these results to analyze the dust properties of the recentlyreported echoes around SN 1993J, finding the dust in M81 to have a grain-size distribution and chemical composition consistent with Galactic dust. Optimal observing strategies for echo detection are also discussed. Subject headings: circumstellar matter — ISM: dust — stars: supernova (SN 1993J) — stars: variables – techniques: photometric 1.