On the Deuterium-to-Hydrogen Ratio of the Interstellar Medium
  Observations show that the global deuterium-to-hydrogen ratio (D/H) in the
local interstellar medium (ISM) is about 90% of the primordial ratio predicted
by big bang nucleosynthesis. The high (D/H)$_{ISM}$ 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 (D/H)$_{ISM}$ to the primordial abundance is $\approx 1/(1+r Z_O/m_O)$,
where r is the recycling fraction, $Z_O$ is the ISM oxygen mass fraction, and
$m_O$ is the population averaged CCSN yield of oxygen. Using values $r=0.4$ and
$m_O=0.015$ appropriate to a Kroupa (2001) initial mass function and recent
CCSN yield calculations, solar oxygen abundance corresponds to an ISM (D/H)
that is 87\% of the primordial value, consistent with observations. This
approximation is accurate for a wide range of parameter values, and physical
arguments suggest that it should remain accurate for more complex chemical
evolution models, making the deuterium abundance a robust prediction of almost
any model that reproduces the observed ISM metallicity. The good agreement with
the upper range of observed (D/H)$_{ISM}$ values supports the long-standing
suggestion that sightline-to-sightline variations of deuterium are a
consequence of dust depletion, rather than a low global (D/H)$_{ISM}$ enhanced
by localized accretion of primordial composition gas. This agreement limits
deviations from conventional yield and recycling values, and it implies that
Galactic outflows eject ISM hydrogen as efficiently as they eject CCSN metals.
