On the Deuterium-to-hydrogen Ratio of the Interstellar Medium

David H. Weinberg
Astrophysical Journal 851, 25 Link to Article [DOI: 10.3847/1538-4357/aa96b2]
Department of Astronomy and Center for Cosmology and AstroParticle Physics, The Ohio State University, Columbus, OH 43210, USA

Observational studies show that the global deuterium-to-hydrogen ratio $({\rm{D}}/{\rm{H}})$ in the local interstellar medium (ISM) is about 90% of the primordial ratio predicted by Big Bang nucleosynthesis. The high ${({\rm{D}}/{\rm{H}})}_{\mathrm{ISM}}$ implies that only a small fraction of interstellar gas has been processed through stars, which destroy any deuterium they are born with. Using analytic arguments for one-zone chemical evolution models that include accretion and outflow, I show that the deuterium abundance is tightly coupled to the abundance of core collapse supernova (CCSN) elements, such as oxygen. These models predict that the ratio of the ISM deuterium abundance to the primordial abundance is ${X}_{{\rm{D}}}/{X}_{{\rm{D}}}^{{\rm{P}}}\approx {(1+{{rZ}}_{{\rm{O}}}/{m}_{{\rm{O}}}^{\mathrm{cc}})}^{-1}$, where r is the recycling fraction, ${Z}_{{\rm{O}}}$ is the ISM oxygen mass fraction, and ${m}_{{\rm{O}}}^{\mathrm{cc}}$ is the population-averaged CCSN yield of oxygen. Using values r = 0.4 and ${m}_{{\rm{O}}}^{\mathrm{cc}}=0.015$ appropriate to a Kroupa initial mass function and recent CCSN yield calculations, solar oxygen abundance corresponds to ${X}_{{\rm{D}}}/{X}_{{\rm{D}}}^{{\rm{P}}}\approx 0.87$, consistent with the observations. This approximation is accurate for a wide range of parameter values, and physical arguments and numerical tests suggest that it should remain accurate for more complex chemical evolution models. The good agreement with the upper range of observed ${({\rm{D}}/{\rm{H}})}_{\mathrm{ISM}}$ values supports the long-standing suggestion that sightline-to-sightline variations of deuterium are a consequence of dust depletion, rather than a low global ${({\rm{D}}/{\rm{H}})}_{\mathrm{ISM}}$ enhanced by localized accretion of primordial composition gas. This agreement limits deviations from conventional yield and recycling values, including models in which most high-mass stars collapse to form black holes without expelling their oxygen in supernovae, and it implies that Galactic outflows eject ISM hydrogen as efficiently as they eject CCSN metals.


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