Effect of stochastic grain heating on cold dense clouds chemistry

TL;DR

This study investigates how temperature fluctuations of small dust grains, caused by photons and cosmic rays, influence chemical reactions in cold dense clouds, revealing significant effects on molecule formation.

Contribution

It introduces a Monte Carlo simulation of dust grain temperature fluctuations across a size distribution, highlighting their impact on cloud chemistry, especially CO2 and complex organic molecules.

Findings

Surface CO2 abundances increase by over an order of magnitude with grain size distribution.

Temperature spikes enable formation of terrestrial complex organic molecules on small grains.

Cosmic ray secondary photons can overheat small grains, reducing molecule formation by about tenfold.

Abstract

The temperatures of dust grains play important roles in the chemical evolution of molecular clouds. Unlike large grains, the temperature fluctuations of small grains induced by photons may be significant. Therefore, if the grain size distribution is included in astrochemical models, the temperatures of small dust grains may not be assumed to be constant. We simulate a full gas-grain reaction network with a set of dust grain radii using the classical MRN grain size distribution and include the temperature fluctuations of small dust grains. Monte Carlo method is used to simulate the real-time dust grain's temperature fluctuations which is caused by the external low energy photons and the internal cosmic ray induced secondary photons. The increase of dust grains radii as ice mantles accumulate on grain surfaces is also included in our models. We found that surface CO$_2$ abundances in models with grain size distribution and temperature fluctuations are more than one order of magnitude larger than those with single grain size. Small amounts of terrestrial complex organic molecules (COMs) can also be formed on small grains due to the temperature spikes induced by external low energy photons. However, cosmic ray induced secondary photons overheat small grains so that surface CO sublime and less radicals are formed on grains surfaces, thus the production of surface CO$_2$ and COMs decreases by about one order of magnitude. The overheating of small grains can be offset by grain growth so that the formation of surface CO$_2$ and COMs becomes more efficient.