We determine the D/H ratio in the interstellar medium toward the DO white dwarf PG 0038 + 199 using spectra from the Far Ultraviolet Spectroscopic Explorer ( FUSE ) , with ground-based support from Keck HIRES .
We employ curve of growth , apparent optical depth and profile fitting techniques to measure column densities and limits of many other species ( H _ { 2 } , Na i , C i , C ii , C iii , N i , N ii , O i , Si ii , P ii , S iii , Ar i and Fe ii ) which allow us to determine related ratios such as D/O , D/N and the H _ { 2 } fraction .
Our efforts are concentrated on measuring gas-phase D/H , which is key to understanding Galactic chemical evolution and comparing it to predictions from Big Bang nucleosynthesis .
We find column densities \log N ( \mbox { { H i } } ) = 20.41 \pm 0.08 , \log N ( \mbox { { D i } } ) = 15.75 \pm 0.08 and \log N ( \mbox { H$ { } _ { 2 } $ } ) = 19.33 \pm 0.04 , yielding a molecular hydrogen fraction of 0.14 \pm 0.02 ( 2 \sigma errors ) , with an excitation temperature of 143 \pm 5 K. The high H i column density implies that PG 0038 + 199 lies outside of the Local Bubble ; we estimate its distance to be 297 ^ { +164 } _ { -104 } pc ( 1 \sigma ) .
[ D i + HD ] / [ H i + 2H _ { 2 } ] toward PG 0038 + 199 is 1.91 ^ { +0.52 } _ { -0.42 } \times 10 ^ { -5 } ( 2 \sigma ) .
There is no evidence of component structure on the scale of \Delta v > 8 km s ^ { -1 } based on Na i , but there is marginal evidence for structure on smaller scales .
The D/H value is high compared to the majority of recent D/H measurements , but consistent with the values for two other measurements at similar distances .
D/O is in agreement with other distant measurements .
The scatter in D/H values beyond \sim 100 pc remains a challenge for Galactic chemical evolution .