Brilliant Pebbles: a Method for Detection of Very Large Interstellar Grains

A photon of wavelength λ ∼ 1μm interacting with a dust grain of radius ap ∼ 1mm – in other words, a “pebble” – undergoes scattering in the forward direction, largely within a small characteristic diffraction angle θs ∼ λ/ap ∼ 100. Though mm-size dust grains contribute negligibly to the interstellar medium’s visual extinction, the signal they produce in scattered light may be detectable for variable sources. Observations of variable light scattered into small angles allows for a direct measurement of the large grain population while also yielding tomographic information of the interstellar medium’s mass distribution. Subject headings: dust scattering