Low-Temperature Crystallization of Amorphous Silicate in Astrophysical Environments

TL;DR

This paper presents a theoretical model explaining how amorphous silicate grains can crystallize at low temperatures in space through exothermic chemical reactions, expanding the conditions under which crystallization can occur.

Contribution

The study introduces a new model linking stored energy and reaction duration to low-temperature crystallization, applicable to astrophysical environments.

Findings

Crystallization depends on stored energy density and reaction timescale.

Conditions for crystallization are Q > Q_min and τ < τ_cool.

Silicate crystallization can occur under wider astrophysical conditions.

Abstract

We construct a theoretical model for low-temperature crystallization of amorphous silicate grains induced by exothermic chemical reactions. As a first step, the model is applied to the annealing experiments, in which the samples are (1) amorphous silicate grains and (2) amorphous silicate grains covered with an amorphous carbon layer. We derive the activation energies of crystallization for amorphous silicate and amorphous carbon from the analysis of the experiments. Furthermore, we apply the model to the experiment of low-temperature crystallization of amorphous silicate core covered with an amorphous carbon layer containing reactive molecules. We clarify the conditions of low-temperature crystallization due to exothermic chemical reactions. Next, we formulate the crystallization conditions so as to be applicable to astrophysical environments. We show that the present crystallization mechanism is characterized by two quantities: the stored energy density Q in a grain and the duration of the chemical reactions \tau . The crystallization conditions are given by Q > Q_{min} and \tau < \tau _{cool} regardless of details of the reactions and grain structure, where \tau _{cool} is the cooling timescale of the grains heated by exothermic reactions, and Q_{min} is minimum stored energy density determined by the activation energy of crystallization. Our results suggest that silicate crystallization occurs in wider astrophysical conditions than hitherto considered.