  Cosmic ray acceleration at the termination shock of compact star clusters has
recently received much attention, mainly because of the detection of gamma ray
emission from some of such astrophysical sources. Here we focus on the
acceleration of nuclei at the termination shock and we investigate the role
played by proton energy losses and spallation reactions of nuclei, especially
downstream of the shock. We show that for a reasonable choice of the mean gas
density in the cavity excavated by the cluster wind, dominated by the presence
of dense clouds, the spectrum of He nuclei escaping the bubble is
systematically harder than the spectrum of hydrogen, in a manner that appears
to be qualitatively consistent with the observed and yet unexplained phenomenon
of discrepant hardening. We also find that, in this scenario, the spallation
reactions of heavier nuclei are likely to be so severe that their spectra
become very hard and with a low normalization, meaning that it is unlikely that
heavy nuclei escaping star clusters can provide a sizeable contribution to the
spectrum of cosmic rays at the Earth. Limitations and implications of this
scenario are discussed.
