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 work investigates whether young massive star clusters contribute significantly to Galactic cosmic rays by quantifying the acceleration efficiency in NGC 6611 within the M16 region. Using 16.5 years of Fermi-LAT GeV gamma-ray data, the authors detect extended emission spatially associated with a wind-blown bubble and an adjacent molecular cloud, arguing a hadronic origin from protons accelerated at the cluster wind termination shock. By applying a Morlino2021-inspired model to the wind termination shock and bubble propagation under three turbulence regimes (Kolmogorov, Kraichnan, Bohm) and two wind-luminosity scenarios, they constrain the acceleration efficiency to ∼1.2%–3.6%, with E_max values ranging from tens to hundreds of TeV depending on the diffusion regime. The study also examines potential leptonic contributions, the implied wind-CR energetics, and the grammage at the source, concluding that stellar winds in young clusters can contribute a non-negligible fraction of Galactic CRs while not dominating the overall CR budget. These results reinforce the role of young stellar systems as hadronic accelerators and provide benchmarks for applying similar analyses to other clusters.

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 %.

Figures (7)

• Figure 1: Left: Test statistics (TS) map in the region around M16 derived from Fermi-LAT observations. The position of the O stars in the cluster NGC 6611 are indicated as blue stars, and its termination and forward shock radii are indicated a solid and dotted blue circles, while a dashed circle indicates the limit of the Hii region. The light-blue, cyan, and pink contours indicate CO gas column densities of $1.5 \times 10^{22}$, $1.7 \times 10^{22}$ and $2.3 \times 10^{22}$ cm$^{-2}$, obtained in the velocity range 15-30 km s$^{-1}$. The magenta circles indicate the position of Fermi sources from the 4FGL catalog Ballet2023 and their relative position uncertainties, evaluated as their 68% confidence radius. Center: molecular gas distribution in the region, as traced by CO line emission, integrated over the line of sight in the velocity range 15 km/s, 30 km/s. The black contours refer to Fermi-LAT test-statistics and represent 3,4, and 5 sigma levels. Right: the velocity-longitude distribution of CO. The map is integrated over the whole latitude range. The star marker indicates the position of NGC 6611.
• Figure 2: Representation of the geometry of the system: in the upper panel, the distribution of gas density as a function of the radial distance from the center of the cluster (black curve), and the radial distribution of accelerated particles at different energies (green solid, dashed, dashed-dotted and dotted lines), considering propagation in Kraichnan-like turbulence. In both cases the highest value for the wind luminosity, $L_{w,2}$, is assumed. In the lower panel, a sketch of the geometry of the system projected on the sky is superimposed to a cutout of an image of the system obtained in otpical and through an H$\alpha$ filter (VPHAS+ survey ).
• Figure 3: Spectral energy distribution (SED) of the source detected by Fermi-LAT in correspondence of NGC 6611/M16. The solid and dashed green lines represent the model of emission in the case of Kraichnan turbulence, and differ in the assumed wind luminosity, $L_w$, as indicated in the figure legend.
• Figure 4: Parameter space explored with the fitting procedure. The darker zones are values with a smaller $\chi^2$, while the shaded regions represent those parameters excluded by the gamma-ray upper limits. The best fit value is indicated as circle, unless it is found in the forbidden parameter region, in which case the new best fit value is indicated as a cross.
• Figure 5: Three different fit results to the spectral energy distribution (SED) of the source detected by Fermi-LAT in correspondence of NGC 6611/M16. The solid and dashed curve differ in the assumed wind luminosity, $L_w$, as indicated in the figure legend. The three different panels refer to the three turbulence spectra assumed: Kolmogorov (red), Kraichnan (green), and Bohm (blue).