What Is the Total Deuterium Abundance in the Local Galactic Disk?

Analyses of spectra obtained with the Far Ultraviolet Spectroscopic Explorer(FUSE) satellite, together with spectra from the Copernicus and IMAPSinstruments, reveal an unexplained very wide range in the observeddeuterium/hydrogen (D/H) ratios for interstellar gas in the Galactic diskbeyond the Local Bubble. We argue that spatial variations in the depletion ofdeuterium onto dust grains can explain these local variations in the observedgas-phase D/H ratios. We present a variable deuterium depletion model thatnaturally explains the constant measured values of D/H inside the Local Bubble,the wide range of gas-phase D/H ratios observed in the intermediate regime (logN(H I} = 19.2-20.7), and the low gas-phase D/H ratios observed at largerhydrogen column densities. We consider empirical tests of the deuteriumdepletion hypothesis: (i) correlations of gas-phase D/H ratios with depletionsof the refractory metals iron and silicon, and (ii) correlation with themolecular hydrogen rotational temperature. Both of these tests are consistentwith deuterium depletion from the gas phase in cold, not recently shocked,regions of the ISM, and high gas-phase D/H ratios in gas that has been shockedor otherwise heated recently. We argue that the most representative value forthe total (gas plus dust) D/H ratio within 1 kpc of the Sun is >=23.1 +/- 2.4(1 sigma) parts per million (ppm). This ratio constrains Galactic chemicalevolution models to have a very small deuterium astration factor, the ratio ofprimordial to total (D/H) ratio in the local region of the Galactic disk, whichwe estimate to be f_d <= 1.19 +/-0.16 (1 sigma) or <= 1.12 +/- 0.14 (1 sigma)depending on the adopted light element nuclear reaction rates.Comment: 19 pages, 9 figure