Atomic Deuterium/Hydrogen in the Galaxy

Abstract

Abstract. An accurate value of the deuterium/hydrogen (D/H) ratio in the local interstellar medium (LISM) and a better understanding of the D/H variations with position in the Galactic disk can provide essential information on the primordial D/H ratio in the Galaxy at the time of the protosolar nebula, and the amount of astration and mixing in the Galaxy over time. Recent measurements have been obtained with UV spectrographs on FUSE, HST, and IMAPS using hot white dwarfs, OB stars, and late-type stars as background light sources against which to measure absorption by D and H in the interstellar medium along the lines of sight. Recent analyses of FUSE observations of seven white dwarfs and subdwarfs provide a weighted mean value of D/H = (1.52 ± 0.08) × 10 −5 (15.2 ± 0.8 ppm), consistent with the value of (1.50 ± 0.10) × 10 −5 (15.0 ± 1.0 ppm) obtained from analysis of lines of sight toward nearby late-type stars. Both numbers refer to the ISM within about 100 pc of the Sun, which samples warm clouds located within the Local Bubble. Outside of the Local Bubble at distances of 200 to 500 pc, analyses of far-UV spectra obtained with IMAPS indicate a much wider range of D/H ratios between 0.8 to 2.2 ppm, providing information on inhomogeneous astration in the Galactic disk. Why are Accurate Measurements of the D/H Ratio Important?Measurements of D/H, the number ratio of deuterium in all forms to hydrogen in all forms, are important for at least two reasons. First, an accurate measurement of the primordial ratio, (D/H) prim , counts the number of baryons in the universe to determine the ratio Ω B of the baryon density to the closure density, and tests our assumptions concerning nucleosynthesis during the first 100-1,000 seconds of the universe (e.g., Burles et al., 2001). Deuterium is our best probe of primordial nucleosynthesis because theory predicts that D was formed only in the very early universe, D is the easiest isotope to be destroyed by nuclear reactions in stars (astration), and Ω B is a very sensitive single-valued function of (D/H) prim . The best approximation to (D/H) prim would be an accurate measurement of D/H in gas where there has been little chemical fractionation or star formation, as indicated by very low metal abundances, but such measurements remain difficult.Second, measurements of D/H in different locations in our Galaxy will provide an accurate test of the assumptions underlying Galactic chemical evolution models. A major problem in astrophysics is understanding how galaxies evolve and, in particular, how the chemical element abundances evolve. In broad overview, we know that stars form out of gas clouds and over time they destroy D, create metals, and return some of this deuterium-poor and metal-rich material to the ISM by winds and supernova explosions. The detailed rates for these processes depend on the Space Science Reviews 95/96: 1-12, 2003.

Physical Processes in the Interstellar Medium

“…This holds despite the continuous transformation of gas into stars, and it suggests that Hi is continuously replenished (Hopkins et al, 2008;Prochaska & Wolfe, 2009). In our Galaxy, the presence of deuterium at the solar neighborhood (Linsky, 2003) as well as in the Galactic Center (Lubowich et al, 2000) also points towards a continuous inflow of low-metallicity material. As deuterium is destroyed in stars and as there is no other known source of deuterium in the Milky Way, it must be of cosmological and extragalactic origin (Ostriker & Tinsley, 1975;Chiappini et al, 2002).…”

Short-term Variations in the Galactic Environment of the Sun

“…Component blending between local and distant ISM usually prevent the use of data from distant stars. The H o Lα line is always saturated, even for low column density sightlines, so we use D o as a proxy for H o , with a ratio D o /H o =1.5 ×10 −5 that is valid for local ISM (Vidal-Madjar and Ferlet, 2002, Linsky, 2003, Lehner et al, 2003. These data, combined with radiative transfer models of ionization gradients, give the velocity, composition, temperature, and morphology of the CLIC.…”

The Origin of Cosmic Structures Part 4—Nucleosynthesis

“…We will first look at the deuterium ratio. The gray band in the figure represents the range of values determined by a series of observations along lines of sight out to a distance of 100 pc from the Sun, [21]. From that same reference, for greater distances, the values range from Bubble which has a lower density than the surrounding regions.…”