A Straight and Narrow Ionized Filament

We report the discovery of a extremely narrow, extremely linear, ionizedfilament. The filament is 2.5 degrees long and has an H$\alpha$ surfacebrightness of 0.5 rayleighs. The filament is approximately ``Y'' shaped. Thewidest separation of the two diagonal segments is 5 arcminutes. We discuss fourpossible origins for this feature: (1) an extremely low density, nearby jet,(2) an unusually linear filament associated with some large-scale nearbynebula, perhaps even the Local Bubble, (3) an ionized trail left by mechanicalinput from a star or compact object moving through the ISM, or (4) an ionizedtrail left by photoionization (``Fossil \stromgren Trail'') from a star orcompact object. We favor this last hypothesis, and derive some of the basicproperties for an ionized trail. Regardless of whether this latter hypothesisapplies to this specific filament, the basic properties of such a trail, itslength, width, and brightness, are interesting, predictable, and should beobservable behind some white dwarfs. We suggest future tests for ascertainingthe origin of this filament, and discuss how this structure might be useful toconstrain the thermal and velocity structure of the nearby interstellar medium.Comment: 18 pages, 5 figures, to appear in AJ, 2001; astroph abstract is abridge