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

Context. Young massive star clusters were long considered as potentially important sources of galactic cosmic rays (CRs). They have recently emerged as gamma-ray emitters up to very high energies. Aims. To quantify the contribution of these sources to Galactic CRs, we need to estimate their typical acceleration efficiency. Methods. We searched for emission in the GeV band because most of the energy is emitted in this band. We analyzed Fermi-LAT data collected toward the M16 region. This star-forming region is also known as the Eagle nebula and hosts the young massive star cluster NGC 6611. Based on the young age of the cluster, we assumed acceleration at the cluster wind-termination shock and modeled the propagation through the wind-blown bubble to derive the energetics of the process and interpret the GeV observations. Results. For the first time, we found significant GeV emission in correspondence of a molecular cloud associated with the young massive star cluster NGC 6611. We interpret this as hadronic emission associated with particles that are accelerated at the cluster wind-termination shock and propagated through the low-density wind-excavated bubble to the cloud. Our modeling allowed us to place firm constraints on the acceleration efficiency in NGC 6611. We assess that it lies between ∼1% and ∼4%.