tokens: acceleration. Our inference above is qualitatively different from these models in the sense that it is independent of SNe event in a star cluster. We also note that LOFAR observations show PeV CRs to be enriched in low-Z nuclei (Buitink et al. 2016 ), consistent with their being accelerated from CNO enriched wind material in SBs (Thoudam et al. 2016 ). It is also possible that the hard component of CRs due to WTS is the one inferred to be present in molecular clouds (de Boer et al. 2017 ). Therefore, WTSs are potential PeVatrons and they should be studied in detail in this regard. 5.5 Decoupling of grammage from ISM Our proposed scenario can also provide the astrophysical framework for the phenomenologically motivated models of CR propagation in which the grammage traversed by CRs are mostly near the source (Blasi Serpico 2009 ; Cowsik Madziwa-Nussinov 2016 ; Eichler 2017 ; Biermann et al. 2018 ). In these models, the diffusion property of CR particles is assumed to be different inside the ‘cocoons’ surrounding the CR acceleration sites than elsewhere in the ISM in order to alleviate problems associated with secondary production. The scenario of CR acceleration in SBs satisfies the basic premises of these models, since most of the CR collisions occur in the outer shell of shocked ISM, whereas CRs are advected by the wind in the inner regions. The grammage suffered by CRs in the shocked ISM region is ∼10 g cm −2 , comparable to the total grammage of GCR (Gabici et al. 2019 ), since the typical residence time is t r ∼ 1 Myr (∼κ/ v 2 , for a diffusion coefficient κ ∼ 10 27 cm 2 s −1 and v ∼ 50 km s −1 , the typical outer shock speed), and typical density in this region is ∼10 cm −3 . Therefore, CR grammage can be decoupled from Galactic residence time, alleviating a number of outstanding problems (Butt 2009 ; Gabici et al. 2019 ). Instead of listing them all, we mention one of the problems here, that of the observed scaling of light elements with metallicity in halo stars (Parizot Drury 1999 ). In the case of isolated SNRs (which accelerates ISM particles), the Li/Be/B abundance is expected to scale as Z 2 , since both CR and the target gas share the same metallicity. In the case of WTS, the metallicity of CRs is independent of that of the ambient ISM, the abundance of spallation products should scale as Z , as observed (Parizot Drury 1999 ). 5.6 Caveats and future directions In this study, we show the first calculations for estimating the energetics of the wind and SN shocks in isolated stars/star clusters. We have used a shock detection algorithm and three different tracers to quantify the fraction of shock energy processed by wind, SN ejecta, and ambient material. Although in the above sections, we have shown that this study has succeeded in revealing answers to various problems in GCRs, below we highlight some of the limitations of our models and future directions. Non-radiative strong shocks: We have considered the Rankine–Hugoniot
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