Hydrogen Column Density Evaluations toward Capella: Consequences on the Interstellar Deuterium Abundance

The deuterium abundance evaluation in the direction of Capella has for a long time been used as a reference for the local interstellar medium (ISM) within our Galaxy. We show here that broad and weak H I components could be present on the Capella line of sight, leading to a large new additional systematic uncertainty on the N(H I) evaluation. We find the D/H ratio toward Capella to be equal to 1.67(±0.3) × 10-5 with almost identical χ2 for all the fits (this range includes only the systematic error; the 2 σ statistical one is almost negligible in comparison). We conclude that D/H evaluations over H I column densities below 1019 cm-2 (even perhaps below 1020 cm-2 if demonstrated by additional observations) may present larger uncertainties than previously anticipated. We mention that the D/O ratio might be a better tracer for D I variations in the ISM as recently measured by the Far Ultraviolet Spectroscopic Explorer.