Evidence for ubiquitous carbon grain destruction in hot protostellar envelopes

TL;DR

This study provides evidence that carbon-rich refractory grains are destroyed in hot protostellar envelopes, leading to enhanced nitrogen-containing molecules, which explains the observed chemical composition differences between warm and hot gas regions.

Contribution

It presents observational evidence for widespread destruction of refractory organics in hot protostellar environments, revealing new insights into chemical processes during star formation.

Findings

Nitrogen-containing, oxygen-poor molecules are enhanced in hot gas.

Oxygen-containing molecules show no hot component enhancement.

Results suggest refractory organics are destroyed, affecting C/O and N/O ratios.

Abstract

Earth is deficient in carbon and nitrogen by up to ${\sim}4$ orders of magnitude compared with the Sun. Destruction of (carbon- and nitrogen-rich) refractory organics in the high-temperature planet forming regions could explain this deficiency. Assuming a refractory cometary composition for these grains, their destruction enhances nitrogen-containing oxygen-poor molecules in the hot gas ($\gtrsim 300$K) after the initial formation and sublimation of these molecules from oxygen-rich ices in the warm gas (${\sim}150$K). Using observations of $37$ high-mass protostars with ALMA, we find that oxygen-containing molecules (CH$_3$OH and HNCO) systematically show no enhancement in their hot component. In contrast, nitrogen-containing, oxygen-poor molecules (CH$_3$CN and C$_2$H$_3$CN) systematically show an enhancement of a factor ${\sim} 5$ in their hot component, pointing to additional production of these molecules in the hot gas. Assuming only thermal excitation conditions, we interpret these results as a signature of destruction of refractory organics, consistent with the cometary composition. This destruction implies a higher C/O and N/O in the hot gas than the warm gas, while, the exact values of these ratios depend on the fraction of grains that are effectively destroyed. This fraction can be found by future chemical models that constrain C/O and N/O from the abundances of minor carbon, nitrogen and oxygen carriers presented here.