# MHOs toward HMOs: A Search for Molecular Hydrogen emission-line Objects toward High-Mass Outflows

## TL;DR

This study conducted a near-infrared imaging survey of 26 high-mass star-forming regions, detecting 236 MHOs (156 new), and analyzed shock signatures to understand outflow mechanisms and their relation to CO outflows.

## Contribution

It provides a large new catalog of MHOs in high-mass protostellar regions and distinguishes shock emission from fluorescence using flux ratios, enhancing understanding of outflow processes.

## Key findings

- Detected 236 MHOs, 156 new, in 22 regions.
- Most MHOs show flux ratios indicating C-type shocks.
- Over half of the regions show MHO arrangements matching CO outflow features.

## Abstract

We present the results of a narrow-band near-infrared imaging survey for Molecular Hydrogen emission-line Objects (MHOs) toward 26 regions containing high-mass protostellar candidates and massive molecular outflows. We have detected a total of 236 MHOs, 156 of which are new detections, in 22 out of the 26 regions. We use H$_2$ 2.12-$\mu$m/H$_2$ 2.25-$\mu$m flux ratios, together with morphology, to separate the signatures of fluorescence associated with photo-dissociation regions (PDRs) from shocks associated with outflows in order to identify the MHOs. PDRs have typical low flux ratios of ~ 1.5 - 3, while the vast majority of MHOs display flux ratios typical of C-type shocks (~ 6-20). A few MHOs exhibit flux ratios consistent with expected values for J-type shocks (~ 3-4), but these are located in regions that may be contaminated with fluorescent emission. Some previously reported MHOs have low flux ratios, and are likely parts of PDRs rather than shocks indicative of outflows. We identify a total of 36 outflows across the 22 target regions where MHOs were detected. In over half these regions, MHO arrangements and fluorescent structures trace features present in CO outflow maps, suggesting the CO emission traces a combination of dynamical effects, which may include gas entrained in expanding PDRs as well as bipolar outflows. Where possible, we link MHO complexes to distinct outflows and identify candidate driving sources.