Evidence against stellar chromospheric origin of Galactic cosmic rays

Interstellar space is filled with a gas of relativistic ions and electrons — the Galactic cosmic rays. These energetic particles tie interstellar gas to ambient magnetic fields by ionizing the component molecules and atoms, and so play a role in stabilizing molecular clouds against collapse1 and regulating the collapse of protostellar clouds2. The observed energy spectrum of cosmic rays up to ≳1015 eV is consistent with their acceleration by supernova shock waves3, but the original source of cosmic-ray nuclei remains unclear. There has been a widely held belief that the source consists of a solar-like ionized medium4, probably the chromospheres of late-type Sun-like stars5. This model predicts an overabundance of easily ionized elements. Here we show that lead, which is easily ionized, is underabundant in the Galactic cosmic rays in contradiction with this model. Rather, our measurements are consistent with two other possible models: one in which the nuclei originate in interstellar gas, and in entire grains accelerated to about one per cent of the speed of light by supernova shock waves6,7; and another in which the cosmic rays contain an admixture of an exotic, freshly synthesized component8, probably originating in neutrino-driven winds from newly born neutron stars9.