Study of the molecular clump associated with the high-energy source HESS J1858+020

TL;DR

This study investigates a molecular clump near HESS J1858+020, revealing a young stellar object but no outflows, and suggests gamma-ray emission likely results from cosmic ray interactions with the molecular gas.

Contribution

First detailed molecular analysis of the clump associated with HESS J1858+020, proposing a hadronic origin for gamma-ray emission based on new observational data.

Findings

Density of the clump is between 10^3 and 10^4 cm^-3.

A young high-mass protostar was discovered within the clump.

Gamma-ray emission is likely due to cosmic ray interactions, not outflows.

Abstract

HESS J1858+020 is a weak gamma-ray source lying near the southern border of the SNR G35.6-0.4. A molecular cloud, composed by two clumps, shows signs of interaction with the SNR and with a nearby extended HII region. In particular, the southernmost clump coincides with the center of the HESS source. In this work we study this clump in detail with the aim of adding information that helps in the identification of the nature of the very-high energy emission. We observed the mentioned molecular clump using the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J=3-2, 13CO J=3-2, HCO+ J=4-3 and CS J=7-6 lines with an angular resolution of 22". To complement this observations we analyzed IR and submillimeter continuum archival data. From the 12CO and 13CO J=3-2 lines and the 1.1 mm continuum emission we derived a density of between 10^{3} and 10^{4} cm^-3 for the clump. We discovered a young stellar object (YSO), probably a high mass protostar, embedded in the molecular clump. However, we did not observe any evidence of molecular outflows from this YSO which would reveal the presence of a thermal jet capable of generating the observed gamma-rays. We conclude that the most probable origin for the TeV gamma-ray emission are the hadronic interactions between the molecular gas and the cosmic rays accelerated by the shock front of the SNR G35.6-0.4.