A Stochastic Approach to Chemical Evolution

Abstract

Observations of elemental abundances in the Galaxy have repeatedly shown an intrinsic scatter as a function of time and metallicity. The standard approach to chemical evolution does not attempt to address this scatter in abundances since only the mean evolution is followed. In this work the scatter is addressed via a stochastic approach to solving chemical evolution models. Three standard chemical evolution scenarios are studied using this stochastic approach; a closed box model, an infall model, and an outflow model. These models are solved for the solar neighborhood in a Monte Carlo fashion. The evolutionary history of one particular region is determined randomly based on the star formation rate and the initial mass function. Following the evolution in an ensemble of such regions leads to the predicted spread in abundances expected, based solely on different evolutionary histories of otherwise identical regions. In this work 13 isotopes are followed including the light elements, the CNO elements, a few $α$-elements, and iron. It is found that the predicted spread in abundances for a $10^5\Msun$ region is in good agreement with observations for the $α$-elements. For CN the agreement is not as good perhaps indicating the need for more physics input for low mass stellar evolution. Similarly for the light elements the predicted scatter is quite small which is in contradiction to the observations of \He{3} in \hii\ regions. The models are tuned for the solar neighborhood so good agreement with \hii\ regions is not expected. This has important implications for low mass stellar evolution and on using chemical evolution to determine the primordial light element abundances in order to test big-bang nucleosynthesis.