AAS 207th Meeting, 8-12 January 2006
Session 193 Recent Discoveries in the Far Ultraviolet
Special Session, Thursday, 10:00-11:30am, January 12, 2006, Delaware B

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[193.07] The Local Milky Way D/H Ratio

H. W. Moos (Johns Hopkins University)

The FUSE spectral region contains all of the Lyman transitions of D I except for Ly alpha. As a consequence, it can measure interstellar D I column densities over three orders of magnitude; the FUSE team has measured log (D) from values of 13.15 towards HZ 43 A in the Local Bubble (LB) to 16.16 towards HD 90087 at a distance of 2700 pc. Values of D/H determined using FUSE data along with results from Copernicus, IMAPS and HST now exist for interstellar gas along 40+ sightlines. About half of these sightlines probe gas beyond the LB. These measurements show that the D/H ratios in the LB are consistent with a single value of about 16 parts per million (ppm), indicating that the gas in the LB is well mixed. However, the D/H ratios beyond the LB exhibit a wide range of values from 22 ppm down to 5 ppm, with large variations over distances of a few hundred pc. In comparison, high-z visible measurements of the gas in the intergalactic medium, which should be close to the primordial value, indicate D/H= 28 ppm. Likewise, the baryonic densities measured by WMAP coupled with Big Bang Nucleosynthesis models indicate 26-28 ppm for the primordial value. What is the cause of the wide variation in the Milky Way values? Why are the high Milky Way values (~22 ppm) so close to the primordial values? We will present a summary of the measurements in the Milky Way and discuss possible causes of the observed behavior of the D/H ratio. The high Milky Way values may provide an important upper limit for the primordial baryon densities determined by WMAP and the extragalactic D/H measurements. FUSE is a NASA mission with support from CNES and CSA.

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