The Deuterium-to-Hydrogen Ratio in a Low-Metallicity Cloud Falling onto the Milky Way

Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218. Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter Street, Madison, WI 53706. Princeton University Observatory, Princeton, NJ 08544. Current address: Department of Astronomy, University of Massachusetts, Amherst, MA 01003. Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218. Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences, University of Colorado, 389-UCB, Boulder, CO 80309. Department of Astronomy and Astrophysics and the Enrico Fermi Institute, University of Chicago, Chicago, IL