A new mechanism for the crab fingers

Images of the outer filamentary parts of the Crab Nebula, earlier obtained with the Hubble Space Telescope WFPC2, are reconsidered. On inspection of the images it is found that most of the mainly radially oriented fingers possess an internal structure, usually in the form of two or more filaments. For some of the fingers there are clear signs that the filaments are twisted around each other. A mechanism for the fingers taking the internal structure into account is proposed. The mechanism is based on the reshaping of magnetized filaments under the influence of inertia and magnetic forces. When interacting with an expanding shell, driven by the pressure of the synchrotron nebula, part of such a filament may develop into a double helix pointing radially inwards. For this transformation to take place it is required that the magnetic field within the filament is sufficiently twisted and that the filament contains a mass condensation. The finger is identified with the double helix. Similar structures of double helices are found to be present also in other environments.