Deuterium Toward WD1634-573: Results from the Far Ultraviolet Spectroscopic Explorer (FUSE) Mission

TL;DR

This study uses FUSE observations to measure the D/H ratio toward WD1634-573, revealing inconsistencies with expected ISM values and highlighting potential ionization state variations in the local interstellar medium.

Contribution

First measurement of D/H ratio toward WD1634-573 using FUSE data, comparing multiple methods and discussing ionization state implications.

Findings

D/H ratio toward WD1634-573 is approximately 1.6 x 10^-5.

Discrepancy between D/H(O) and D/H(N) suggests ionization state variations.

Ionization states of N and O may differ, affecting D/H measurements.

Abstract

We use Far Ultraviolet Spectrocopic Explorer (FUSE) observations to study interstellar absorption along the line of sight to the white dwarf WD1634-573 (d=37.1+/-2.6 pc). Combining our measurement of D I with a measurement of H I from Extreme Ultraviolet Explorer data, we find a D/H ratio toward WD1634-573 of D/H=(1.6+/-0.5)e-5. In contrast, multiplying our measurements of D I/O I=0.035+/-0.006 and D I/N I=0.27+/-0.05 with published mean Galactic ISM gas phase O/H and N/H ratios yields D/H(O)=(1.2+/-0.2)e-5 and D/H(N)=(2.0+/-0.4)e-5, respectively. Note that all uncertainties quoted above are 2 sigma. The inconsistency between D/H(O) and D/H(N) suggests that either the O I/H I and/or the N I/H I ratio toward WD1634-573 must be different from the previously measured average ISM O/H and N/H values. The computation of D/H(N) from D I/N I is more suspect, since the relative N and H ionization states could conceivably vary within the LISM, while the O and H ionization states will be more tightly coupled by charge exchange.