The Deuterium Abundance in the Local Interstellar Medium

Abstract

As the Galaxy evolves, the abundance of deuterium in the interstellar medium (ISM) decreases from its primordial value: deuterium is ‘astrated’. The deuterium astration factor fD, the ratio of the primordial D abundance (the D to H ratio by number) to the ISM D abundance, is determined by the competition between stellar destruction and infall, providing a constraint on models of the chemical evolution of the Galaxy. Although conventional wisdom suggests that the local ISM (i.e. within ∼1–2 kpc of the Sun) should be well mixed and homogenized on time-scales short compared to the chemical evolution time-scale, the data reveal gas-phase variations in the deuterium, iron and other metal abundances as large as factors of ∼4–5 or more, complicating the estimate of the ‘true’ ISM D abundance and of the deuterium astration factor. Here, assuming that the variations in the observationally inferred ISM D abundances result entirely from the depletion of D on to dust, rather than from unmixed accretion of nearly primordial material, a model-independent, Bayesian approach is used to determine the undepleted abundance of deuterium in the ISM (or a lower limit to it). We find the best estimate for the undepleted, ISM deuterium abundance to be (D/H)ISM≥ (2.0 ± 0.1) × 10−5. This result is used to provide an estimate of (or an upper bound to) the deuterium astration factor, fD≡ (D/H)P/(D/H)ISM≤ 1.4 ± 0.1.