Hadronic acceleration in the young star cluster NGC 6611 inside the M16 region unveiled by Fermi-LAT: constraints on the acceleration efficiency

TL;DR

This study analyzes Fermi-LAT data to investigate gamma-ray emission from the young star cluster NGC 6611, constraining the efficiency of hadronic cosmic ray acceleration to 1-4% within this star-forming region.

Contribution

It provides the first quantitative estimate of cosmic ray acceleration efficiency in a young massive star cluster using gamma-ray observations.

Findings

Detected significant GeV emission associated with NGC 6611.

Interpreted the emission as hadronic processes at the stellar wind termination shock.

Constrained the acceleration efficiency to between 1% and 4%.

Abstract

Context. Young Massive Star Clusters, long considered as potentially important sources of galactic cosmic rays, have recently emerged as gamma-ray emitters up to very high energies. Aims. In order to quantify the contribution of this source class to the pool of Galactic CRs, we need to estimate the typical acceleration efficiency of these systems. Methods. We search for emission in the GeV band, as most of the energy is emitted in this band. We perform an analysis of Fermi-LAT data collected towards the M16 region, a star-forming region also known as the Eagle Nebula, which hosts the Young Massive Star Cluster NGC 6611. We model the acceleration at the stellar wind termination shock and the propagation through the wind-blown bubble to derive the energetics of the process and interpret the GeV observations. Results. We find significant GeV emission in correspondence of a molecular cloud associated to the Young Massive Star Cluster NGC 6611. We interpret this as hadronic emission associated to particle accelerated at the cluster wind termination shock and propagated through the low-density wind-excavated bubble to the cloud. Our modeling allows us to put firm constraints on the acceleration efficiency in NGC 6611, assessing it between $\sim$ 1 % and $\sim$ 4 %.