Constraining cosmic ray acceleration in young star clusters using multi-wavelength observations

TL;DR

This study uses multi-wavelength simulations to explore cosmic ray acceleration in young star clusters, identifying the reverse shock as the likely CR injection site and demonstrating how observations can constrain CR parameters.

Contribution

It provides a detailed comparison of different CR injection scenarios in star clusters, highlighting the observational signatures and the importance of the reverse shock as the primary acceleration site.

Findings

Gamma-ray luminosity can reach ~5% of mechanical luminosity if shocks in wind regions are primary CR sites.

X-ray luminosity is significantly higher in the forward/reverse shock injection scenario.

Multi-wavelength observations can effectively constrain cosmic ray parameters in star clusters.

Abstract

We use $1$D and $3$D two-fluid cosmic ray (CR) hydrodynamic simulations to investigate the role of CRs in the vicinity of a compact young star cluster. We model a self-gravitating cloud (density profile $\rho \propto r^{-1}$), include important thermal and non-thermal processes, and explore two different CR injection scenarios. We show that if internal shocks in the wind-driving region are the main site for CR acceleration, then the resulting $\gamma$-ray luminosity ($L_{\rm \gamma}$) can reach $\approx 5\%$ of the mechanical luminosity ($L_{\rm w}$), independent of the fraction of wind energy ($\sim 1-20\%$) injected into CRs. In contrast, if the forward/reverse shock of a bubble is the injection site then $L_{\rm \gamma}$ increases linearly with the CR injection fraction, as expected analytically. We find that the X-ray luminosity ($L_{\rm x}$) in the forward/reverse shock injection scenario is $\gtrsim 10^{-3} L_{\rm w}$, which is $\sim 10$ times larger than in the central wind-driving injection case. We predict the corresponding range of the synchrotron radio luminosity. We show how multi-wavelength observations can constrain the CR parameters. Comparing the predicted multi-wavelength luminosities with those of 30 Doradus we identify the reverse shock as the most probable CR injection site, and that thermal conduction is important. We do not find significant dynamical impact of CRs in our models.