Massive Stars as Major Factories of Galactic Cosmic Rays
The paper explores the significant contributions of massive stars, particularly those located in compact stellar clusters, to the local fluxes of Galactic Cosmic Rays (CRs). The research hinges on the analysis of energy and radial distributions of multi-TeV CRs, as inferred from very high energy (VHE) $\gamma$-ray observations directed at the Galactic Center (GC) and young massive star clusters like Cyg OB2 and Westerlund 1.
The findings highlight a striking resemblance in the energy and radial distributions of CRs from these regions, suggesting that the diffuse VHE $\gamma$-ray emissions are sourced from CRs accelerated by ultra-compact stellar clusters at the GC's core. The distinct $1/r$ decrement in CR density from a star cluster underscores continuous CR injection into the interstellar medium over millions of years. The lack of brightness in $\gamma$-ray images near these clusters rules out a leptonic origin for the observed $\gamma$-rays. Furthermore, the $\sim E{-2.3}$ power-law energy spectra of parent protons extend up to 1 PeV, indicating that young massive stars might serve as proton PeVatrons, significantly contributing to the highest energy Galactic CR fluxes.
Notably, this framework posits that supernova remnants (SNRs), historically considered prime CR sources through their acceleration mechanisms, may not sufficiently account for CRs up to the "knee" region of the cosmic ray spectrum (approximately 1 PeV). Observations question the SNRs' capability to function as PeVatrons, as evidenced by the early spectral cutoffs below 10 TeV. This challenges the traditional notion of SNRs being principal CR sources, especially at energies approaching PeV scales. The acceleration theories have yet to definitively prove that young SNRs can efficiently energize particles to PeV levels, propelling the studies toward investigating alternative sources like massive star clusters.
The research thus posits that the winds of massive stars, within these clusters, might efficiently convert kinetic energy into CRs, with conversion efficiencies reaching up to 10%. This perspective aligns with the observation that the mixed gas dynamics facilitated by multiple interacting shocks in stellar clusters can elevate CR protons to PeV energies. Hence, the acceleration mechanisms in these clusters present them as viable candidates for Cradle Regions of Cosmic PeVatrons.
Implications of this study extend both practically and theoretically. Practically, and in terms of future explorations in the VHE domain, this understanding underpins the importance of targeting young stellar clusters for detecting potential CR PeVatron activity. Theoretically, it presents a paradigm shift in understanding the Galactic CR origins, stressing the role of collective stellar activities over individual explosive events like supernovae.
Looking forward, further exploration into the spatial and spectral characteristics of CRs around these clusters could provide deeper insights, particularly through the observational capacities of projects like the Cherenkov Telescope Array (CTA). These studies might solidify or reevaluate the identified role of massive star clusters in CR generation and propagation, further refining our understanding of cosmic ray physics in the astrophysical context.
