12mm line survey of the dense molecular gas towards the W28 field TeV gamma-ray sources

TL;DR

This study uses 12mm Mopra observations to analyze dense molecular gas around the W28 supernova remnant, revealing complex structures, star formation, and potential cosmic-ray interactions linked to TeV gamma-ray sources.

Contribution

It provides new insights into the internal structures and dynamics of dense molecular gas near W28, especially the disrupted cloud associated with TeV gamma-ray emission.

Findings

Detection of broad NH3 emission matching TeV source HESS J1801-233

Identification of dense cores associated with high-mass star formation

Evidence of external disruption of molecular clouds by W28 SNR

Abstract

We present 12mm Mopra observations of dense molecular gas towards the W28 supernova remnant (SNR) field. The focus is on the dense molecular gas towards the TeV gamma-ray sources detected by the H.E.S.S. telescopes, which likely trace the cosmic-rays from W28 and possibly other sources in the region. Using the NH3 inversion transitions we reveal several dense cores inside the molecular clouds, the majority of which coincide with high-mass star formation and HII regions, including the energetic ultra-compact HII region G5.89-0.39. A key exception to this is the cloud north east of W28, which is well-known to be disrupted as evidenced by clusters of 1720MHz OH masers and broad CO line emission. Here we detect broad NH3, up to the (9,9) transition, with linewidths up to 16 km/s. This broad NH3 emission spatially matches well with the TeV source HESS J1801-233 and CO emission, and its velocity dispersion distribution suggests external disruption from the W28 SNR direction. Other lines are detected, such as HC3N and HC5N, H2O masers, and many radio recombination lines, all of which are primarily found towards the southern high-mass star formation regions. These observations provide a new view onto the internal structures and dynamics of the dense molecular gas towards the W28 SNR field, and in tandem with future higher resolution TeV gamma-ray observations will offer the chance to probe the transport of cosmic-rays into molecular clouds.