Chemistry of dense clumps near moving Herbig-Haro objects

TL;DR

This paper models the effects of a moving radiation source from Herbig-Haro objects on nearby dense molecular clumps, showing that molecular enhancements can persist longer than previously thought, supporting grain evaporation as a key process.

Contribution

It introduces a dynamic model of radiation interaction with molecular clumps, demonstrating sustained molecular enhancements in a moving source scenario.

Findings

Enhanced molecular emission regions are longer-lived with a moving source.

Methanol and other species can maintain high abundances for ~10,000 years.

The chemical effects are similar to static models, supporting grain evaporation as the main mechanism.

Abstract

Localised regions of enhanced emission from HCO+, NH3 and other species near Herbig-Haro objects (HHOs) have been interpreted as arising in a photochemistry stimulated by the HHO radiation on high density quiescent clumps in molecular clouds. Static models of this process have been successful in accounting for the variety of molecular species arising ahead of the jet; however recent observations show that the enhanced molecular emission is widespread along the jet as well as ahead. Hence, a realistic model must take into account the movement of the radiation field past the clump. It was previously unclear as to whether the short interaction time between the clump and the HHO in a moving source model would allow molecules such as HCO+ to reach high enough levels, and to survive for long enough to be observed. In this work we model a moving radiation source that approaches and passes a clump. The chemical picture is qualitatively unchanged by the addition of the moving source, strengthening the idea that enhancements are due to evaporation of molecules from dust grains. In addition, in the case of several molecules, the enhanced emission regions are longer-lived. Some photochemically-induced species, including methanol, are expected to maintain high abundances for ~10,000 years.