The chemistry of population iii supernova ejecta. i. formation of molecules in the early universe

Abstract

We study the formation and destruction of molecules in the ejecta of Population III supernovae (SNe) using a chemical kinetic approach to follow the evolution of molecular abundances from day 100 to day 1000 after explosion. The chemical species included in the study range from simple diatomic molecules to more complex dust precursor species. All relevant molecule formation and destruction processes that are unique to the SN environment are considered. Our work focuses on zero-metallicity progenitors with masses of 20, 170, and 270 M, and we study the effect of different levels of heavy element mixing and the inward diffusion of hydrogen and helium on the ejecta chemistry. We show that the ejecta chemistry does not reach a steady state within the relevant timespan (3 yr) for molecule formation, thus invalidating previous results relying on this assumption. The primary species formed in the harsh SN environment are O2, CO, SiS, and SO. The SiO, formed as early as 200 days after explosion, is rapidly depleted by the formation of silica molecular precursors in the ejecta. The rapid conversion of CO to C2 and its thermal fractionation at temperatures above 5000 K allow for the formation of carbon chains in the oxygen-rich zone of the unmixed models, providing an important pathway for the formation of carbon dust in hot environments where the C/O ratio is less than 1. We show that the fully mixed ejecta of a 170 M progenitor synthesizes 11.3 M of molecules, whereas 20 M and 270 M progenitors produce 0.78 M and 3.2 M of molecules, respectively. The admixing of 10% of hydrogen into the fully mixed ejecta of the 170 M progenitor increases its molecular yield to 47 M. The unmixed ejecta of a 170 M progenitor SN without hydrogen penetration synthesizes 37 M of molecules, whereas its 20 M counterpart produces 1.2 M. This smaller efficiency at forming molecules is due to the large fraction of He + in the outer mass zone of the ejecta. Finally, we discuss the cosmological implication of molecule formation by Pop III SNe in the early universe. © 2009 The American Astronomical Society. All rights reserved.