Molecules in Bipolar Outflows

TL;DR

This paper reviews recent advances in understanding the molecular composition and shock chemistry of bipolar outflows, highlighting progress, limitations, and the unique chemistry of high-velocity components near protostars.

Contribution

It provides a comprehensive review of molecular tracers in bipolar outflows, emphasizing recent progress and current limitations in shock chemistry modeling and observations.

Findings

Rapid progress in characterizing molecular composition of outflows

Limitations due to limited statistical observations

Distinct chemistry observed in high-velocity components

Abstract

Bipolar outflows constitute some of the best laboratories to study shock chemistry in the interstellar medium. A number of molecular species have their abundance enhanced by several orders of magnitude in the outflow gas, likely as a combined result of dust mantle disruption and high temperature gas chemistry, and therefore become sensitive indicators of the physical changes taking place in the shock. Identifying these species and understanding their chemical behavior is therefore of high interest both to chemical studies and to our understanding of the star-formation process. Here we review some of the recent progress in the study of the molecular composition of bipolar outflows, with emphasis in the tracers most relevant for shock chemistry. As we discuss, there has been rapid progress both in characterizing the molecular composition of certain outflows as well as in modeling the chemical processes likely involved. However, a number of limitations still affect our understanding of outflow chemistry. These include a very limited statistical approach in the observations and a dependence of the models on plane-parallel shocks, which cannot reproduce the observed wing morphology of the lines. We finish our contribution by discussing the chemistry of the so-called extremely high velocity component, which seems different from the rest of the outflow and may originate in the wind from the very vicinity of the protostar.