1. V. Strelnitski and H. A. Smith, Laboratory for Astrophysics, National Air and Space Museum, Smithsonian Institution, Washington, DC 20560, USA.
2. M. R. Haas, E. F. Erickson, D. J. Hollenbach, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA.
3. S. W. J. Colgan, NASA Ames Research Center, Moffett Field, CA 94035-1000, and Search for Extraterrestrial Intelligence Institute, 2035 Landings Drive, Mountain View, CA 94043, USA.