# The LHAASO PeVatron Bright Sky: What We Learned
## 2 PeVatron Candidate Source Typologies
### Superbubbles and Young Massive Clusters  
The first interest in the role of superbubbles (SBs, the shell structure originating from multiple stars in OB associations) around Young MSCs (YMSCs, \(M\gtrsim 10^{4}M_{\odot}\)) in the CR context was in the early 1980s [95]. Indeed, they have several possible acceleration sites, since they host hundreds of massive stars with their fast winds [96], have a turbulent interior [97], and have a lot of potential SNRs expanding inside them [98]. The entire bubble can reach dimensions greater than 100 pc, creating expectations of extended sources, which was confirmed by several VHE detections of regions surrounding young MSCs [94; 99], e.g., Cygnus Cocoon [100; 101; 102; 103; 104], Westerlund 2 [105], 30 Dor in the Large Magellanic Cloud [106], the Central Molecular Zone [107; 108], and Westerlund 1 [109] ( complete list in [67]). Moreover, the collective effects of shocks from the SN explosions and the wind termination shocks in PWNe can explain the mismatch of the models with the observed \({}^{22}Ne/^{20}Ne\) ratio [68; 110], solving one of the main issues linked to the role of SNRs in CR acceleration (see [111], for a review).  
The power injected by a single stellar wind is between 10-50% of the power generated by a SNR shock [67], with an average energy per massive star of about \(10^{48}\) erg [4; 46]. Consequently, isolated massive stars fail to supply the required CR energy budget in our galaxy. However, in the most compact clusters (YMSCs with a size of a few pc, also called wind-blowing clusters by [75]), powerful winds from the youngest stars and Wolf Rayet stars can merge into a collective outflow that accelerates CRs in the entire region [46; 75]. The active mechanism is probably the less efficient second-order Fermi acceleration [112] but this lasts up to a million years, a much longer time with respect to the efficient phase of SNRs.  
The long duration of CR injection was also confirmed by the spatial \(\gamma\)-ray distribution detected in Cygnus OB2, Westerlund 1, and Westerlund 2, which implies a CR distribution  
Figure 4: Differential luminosity curves for some of the most commonly studied SNRs at TeV energies. It is evident how even the youngest SNRs cannot reach \(E>50\) TeV (Figure from [94]).  
\(\propto 1/r\)[94], indication of a continuous injection. Moreover, magnetic turbulence with \(B\sim\)100 \(\mu\)G could be reached inside the core [113]. This allows YMSCs to reach E \(>\) PeV with a spectrum in agreement with the observations [114] and to explain the CR spectrum at knee energies and above; even if with an expected and strong dependence on the diffusion coefficient [67; 68; 75; 114]. According to these models, the \"knee\" of the CR particle spectrum can be explained by the transition between the isolated standard SNRs and the SNRs in compact YMSCs (see Figure 3).  
In summary, according to observational and theoretical criteria, YMSCs and Superbubbles may be one of the main contributors to galactic CRs. However, the theoretical models are still incomplete and the higher angular resolution of future IACTs, together with multi-wavelength and multi-messengers observations, will be fundamental for resolving the morphology of these regions.