Two New Low Galactic D/H Measurements from the Far Ultraviolet Spectroscopic Explorer
Abstract
We analyze interstellar absorption observed toward two subdwarf O stars, JL 9 and LS 1274, using spectra taken by the Far Ultraviolet Spectroscopic Explorer (FUSE). Column densities are measured for many atomic and molecular species (H I, D I, C I, N I, O I, P II, Ar I, Fe II, and H2), but our main focus is on measuring the D/H ratios for these extended lines of sight, as D/H is an important diagnostic for both cosmology and Galactic chemical evolution. We find D/H=(1.00+/-0.37)×10-5 toward JL 9 and D/H=(0.76+/-0.36)×10-5 toward LS 1274 (2 σ uncertainties). With distances of 590+/-160 and 580+/-100 pc, respectively, these two lines of sight are currently among the longest Galactic lines of sight with measured D/H. With the addition of these measurements, we see a significant tendency for longer Galactic lines of sight to yield low D/H values, consistent with previous inferences about the deuterium abundance from D/O and D/N measurements. Short lines of sight with H I column densities of logN(HI)<19.2 suggest that the gas-phase D/H value within the Local Bubble is (D/H)LBg=(1.56+/-0.04)×10-5. However, the four longest Galactic lines of sight with measured D/H, which have d>500 pc and logN(HI)>20.5, suggest a significantly lower value for the true local disk gas-phase D/H value, (D/H)LDg=(0.85+/-0.09)×10-5. One interpretation of these results is that D is preferentially depleted onto dust grains relative to H and that longer lines of sight that extend beyond the Local Bubble sample more depleted material. In this scenario, the higher Local Bubble D/H ratio is actually a better estimate than (D/H)LDg for the true local disk D/H, (D/H)LD. However, if (D/H)LDg is different from (D/H)LBg simply because of variable astration and incomplete interstellar medium mixing, then (D/H)LD=(D/H)LDg.
Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE), which is operated for NASA by Johns Hopkins University under NASA contract NAS5-32985.- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2004
- DOI:
- 10.1086/421325
- arXiv:
- arXiv:astro-ph/0403606
- Bibcode:
- 2004ApJ...609..838W
- Keywords:
-
- ISM: Abundances;
- Stars: Individual: Alphanumeric: JL 9;
- Stars: Individual: Alphanumeric: LS 1274;
- Ultraviolet: ISM;
- Astrophysics
- E-Print:
- 35 pages, 6 figures