Isotopic fractionation in interstellar molecules

TL;DR

This review explores isotopic fractionation of hydrogen and nitrogen in interstellar molecules, highlighting recent observations, chemical processes, and potential mechanisms influencing isotopic compositions in star-forming regions.

Contribution

It provides a comprehensive overview of hydrogen and nitrogen isotopic fractionation in low-mass star-forming regions, emphasizing recent observational and theoretical developments.

Findings

Interstellar molecules are significantly enriched in deuterium.

Molecules in prestellar cores show variable 15N enrichment or depletion.

Understanding of 15N fractionation chemistry remains limited.

Abstract

The level of isotopic fractionation in molecules provides insights into their formation environments and how they formed. In this article, we review hydrogen and nitrogen isotopic fractionation in low-mass star-forming regions. Interstellar molecules are significantly enriched in deuterium. The importance of the nuclear spin states of light species on deuterium fractionation and the usefulness of singly and doubly deuterated molecules as chemical tracers are discussed. Observations have revealed that molecules in prestellar cores are enriched in or depleted in 15N depending on molecules. Compared with deuterium fractionation chemistry, our understanding of 15N fractionation chemistry is not well established. We briefly discuss potential 15N fractionation routes, i.e., isotopic-exchange reactions and isotopic selective photodissociation of N2. In addition, the selective freeze-out of 15N atoms onto dust grains around the transition between N atoms and N2 is discussed as a potential mechanism that causes the depletion of 15N in the gas phase.