The Evolution of the Elemental Abundances in the Gas and Dust Phases of the Galaxy

Abstract

We present models for the evolution of the elemental abundances in the gas and dust phases of the interstellar medium (ISM) of our Galaxy by generalizing standard models for its dynamical and chemical evolution. In these models, the stellar birthrate history is determined by the infall rate of primordial gas and by its functional dependence on the mass surface density of the stars and gas. We adopt a two-component Galaxy consisting of a central bulge and an exponential disk with different infall rates and stellar birthrate histories. Condensation in stellar winds, Type Ia and Type II supernovae, and the accretion of refractory elements onto preexisting grains in dense molecular clouds are the dominant contributors to the abundance of elements locked up in the dust. Grain destruction by sputtering and evaporative grain-grain collisions in supernova remnants are the most important mechanisms that return these elements back to the gas phase. Guided by observations of dust formation in various stellar sources, and by the presence of isotopic anomalies in meteorites, we calculate the production yield of silicate and carbon dust as a function of stellar mass. We find that Type II supernovae are the main source of silicate dust in the Galaxy. Carbon dust is produced primarily by low-mass stars in the ~2-5 M ☉ range. Type Ia SNe can be important sources of metallic iron dust in the ISM. We also analyze the origin of the elemental depletion pattern and find that the observed core + mantle depletion must reflect the efficiency of the accretion process in the ISM. We also find that grain destruction is very efficient, leaving only ~10% of the refractory elements in grain cores. Observed core depletions are significantly higher, requiring significant UV, cosmic ray, or shock processing of the accreted mantle into refractory core material. Adopting the current grain destruction lifetimes from Jones et al., we formulate a prescription for its evolution in time. We make a major assumption, that the accretion timescale evolves in a similar fashion, so that the current ratio between these quantities is preserved over time. We then calculate the evolution of the dust abundance and composition at each Galactocentric radius as a function of time. We find that the dust mass is linearly proportional to the ISM metallicity and is equal to about 40% of the total mass of heavy elements in the Galaxy, independent of Galactocentric radius. The derived relation of dust mass with metallicity is compared to the observed Galactic dust abundance gradient, and to the M dust versus log (O/H) relation that is observed in external dwarf galaxies. The dependence of dust composition on the mass of the progenitor star and the delayed recycling of newly synthesized dust by low-mass stars back to the ISM give rise to variations in the dust composition as a function of time. We identify three distinct epochs in the evolution of the dust composition, characterized by different carbon-to-silicate mass ratios. Two such epochs are represented by the Galaxy and the SMC. The third is characterized by an excess of carbon dust (compared to the Milky Way Galaxy), and should be observed in galaxies or star-forming regions in which the most massive carbon stars are just evolving off the main sequence. Our models provide a framework for the self-consistent inclusion of dust in population synthesis models for various pre-galactic and galactic systems, allowing for the calculation of their UV to far-infrared spectral energy distribution at various stages of their evolution.